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books/architecture: port database design document (#267)
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Browse source 
* add chapters

* add files, intro

* db abstraction

* backends

* abstraction

* syncing

* serde

* issues

* common/types

* common/ops

* common/service

* service diagram

* service/resize

* service/thread-model

* service/shutdown

* storage/blockchain

* update md files

* cleanup

* fixes

* update for https://github.com/Cuprate/cuprate/pull/290

* review fix
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36
books/architecture/src/SUMMARY.md
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---
- [⚪️ Storage](storage/intro.md)
- [⚪️ Database abstraction](storage/database-abstraction.md)
- [⚪️ Blockchain](storage/blockchain.md)
- [⚪️ Transaction pool](storage/transaction-pool.md)
- [⚪️ Pruning](storage/pruning.md)
- [🟢 Storage](storage/intro.md)
- [🟢 Database abstraction](storage/db/intro.md)
- [🟢 Abstraction](storage/db/abstraction/intro.md)
- [🟢 Backend](storage/db/abstraction/backend.md)
- [🟢 ConcreteEnv](storage/db/abstraction/concrete_env.md)
- [🟢 Trait](storage/db/abstraction/trait.md)
- [🟢 Syncing](storage/db/syncing.md)
- [🟢 Resizing](storage/db/resizing.md)
- [🟢 (De)serialization](storage/db/serde.md)
- [🟢 Known issues and tradeoffs](storage/db/issues/intro.md)
- [🟢 Abstracting backends](storage/db/issues/traits.md)
- [🟢 Hot-swap](storage/db/issues/hot-swap.md)
- [🟢 Unaligned bytes](storage/db/issues/unaligned.md)
- [🟢 Endianness](storage/db/issues/endian.md)
- [🟢 Multimap](storage/db/issues/multimap.md)
- [🟢 Common behavior](storage/common/intro.md)
- [🟢 Types](storage/common/types.md)
- [🟢 `ops`](storage/common/ops.md)
- [🟢 `tower::Service`](storage/common/service/intro.md)
- [🟢 Initialization](storage/common/service/initialization.md)
- [🟢 Requests](storage/common/service/requests.md)
- [🟢 Responses](storage/common/service/responses.md)
- [🟢 Resizing](storage/common/service/resizing.md)
- [🟢 Thread model](storage/common/service/thread-model.md)
- [🟢 Shutdown](storage/common/service/shutdown.md)
- [🟢 Blockchain](storage/blockchain/intro.md)
- [🟢 Schema](storage/blockchain/schema/intro.md)
- [🟢 Tables](storage/blockchain/schema/tables.md)
- [🟢 Multimap tables](storage/blockchain/schema/multimap.md)
- [⚪️ Transaction pool](storage/txpool/intro.md)
- [⚪️ Pruning](storage/pruning/intro.md)
---

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# ⚪️ Blockchain

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# Blockchain
This section contains storage information specific to [`cuprate_blockchain`](https://doc.cuprate.org/cuprate_blockchain),
the database built on-top of [`cuprate_database`](https://doc.cuprate.org/cuprate_database) that stores the blockchain.

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# Schema
This section contains the schema of `cuprate_blockchain`'s database tables.

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# Multimap tables
## Outputs
When referencing outputs, Monero will [use the amount and the amount index](https://github.com/monero-project/monero/blob/c8214782fb2a769c57382a999eaf099691c836e7/src/blockchain_db/lmdb/db_lmdb.cpp#L3447-L3449). This means 2 keys are needed to reach an output.
With LMDB you can set the `DUP_SORT` flag on a table and then set the key/value to:
```rust
Key = KEY_PART_1
```
```rust
Value = {
KEY_PART_2,
VALUE // The actual value we are storing.
}
```
Then you can set a custom value sorting function that only takes `KEY_PART_2` into account; this is how `monerod` does it.
This requires that the underlying database supports:
- multimap tables
- custom sort functions on values
- setting a cursor on a specific key/value
## How `cuprate_blockchain` does it
Another way to implement this is as follows:
```rust
Key = { KEY_PART_1, KEY_PART_2 }
```
```rust
Value = VALUE
```
Then the key type is simply used to look up the value; this is how `cuprate_blockchain` does it
as [`cuprate_database` does not have a multimap abstraction (yet)](../../db/issues/multimap.md).
For example, the key/value pair for outputs is:
```rust
PreRctOutputId => Output
```
where `PreRctOutputId` looks like this:
```rust
struct PreRctOutputId {
amount: u64,
amount_index: u64,
}
```

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# Tables
> See also: <https://doc.cuprate.org/cuprate_blockchain/tables> & <https://doc.cuprate.org/cuprate_blockchain/types>.
The `CamelCase` names of the table headers documented here (e.g. `TxIds`) are the actual type name of the table within `cuprate_blockchain`.
Note that words written within `code blocks` mean that it is a real type defined and usable within `cuprate_blockchain`. Other standard types like u64 and type aliases (TxId) are written normally.
Within `cuprate_blockchain::tables`, the below table is essentially defined as-is with [a macro](https://github.com/Cuprate/cuprate/blob/31ce89412aa174fc33754f22c9a6d9ef5ddeda28/database/src/tables.rs#L369-L470).
Many of the data types stored are the same data types, although are different semantically, as such, a map of aliases used and their real data types is also provided below.
| Alias | Real Type |
|----------------------------------------------------|-----------|
| BlockHeight, Amount, AmountIndex, TxId, UnlockTime | u64
| BlockHash, KeyImage, TxHash, PrunableHash | [u8; 32]
---
| Table | Key | Value | Description |
|--------------------|----------------------|-------------------------|-------------|
| `BlockHeaderBlobs` | BlockHeight | `StorableVec<u8>` | Maps a block's height to a serialized byte form of its header
| `BlockTxsHashes` | BlockHeight | `StorableVec<[u8; 32]>` | Maps a block's height to the block's transaction hashes
| `BlockHeights` | BlockHash | BlockHeight | Maps a block's hash to its height
| `BlockInfos` | BlockHeight | `BlockInfo` | Contains metadata of all blocks
| `KeyImages` | KeyImage | () | This table is a set with no value, it stores transaction key images
| `NumOutputs` | Amount | u64 | Maps an output's amount to the number of outputs with that amount
| `Outputs` | `PreRctOutputId` | `Output` | This table contains legacy CryptoNote outputs which have clear amounts. This table will not contain an output with 0 amount.
| `PrunedTxBlobs` | TxId | `StorableVec<u8>` | Contains pruned transaction blobs (even if the database is not pruned)
| `PrunableTxBlobs` | TxId | `StorableVec<u8>` | Contains the prunable part of a transaction
| `PrunableHashes` | TxId | PrunableHash | Contains the hash of the prunable part of a transaction
| `RctOutputs` | AmountIndex | `RctOutput` | Contains RingCT outputs mapped from their global RCT index
| `TxBlobs` | TxId | `StorableVec<u8>` | Serialized transaction blobs (bytes)
| `TxIds` | TxHash | TxId | Maps a transaction's hash to its index/ID
| `TxHeights` | TxId | BlockHeight | Maps a transaction's ID to the height of the block it comes from
| `TxOutputs` | TxId | `StorableVec<u64>` | Gives the amount indices of a transaction's outputs
| `TxUnlockTime` | TxId | UnlockTime | Stores the unlock time of a transaction (only if it has a non-zero lock time)
<!-- TODO(Boog900): We could split this table again into `RingCT (non-miner) Outputs` and `RingCT (miner) Outputs` as for miner outputs we can store the amount instead of commitment saving 24 bytes per miner output. -->

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# Common behavior
The crates that build on-top of the database abstraction ([`cuprate_database`](https://doc.cuprate.org/cuprate_database))
share some common behavior including but not limited to:
- Defining their specific database tables and types
- Having an `ops` module
- Exposing a `tower::Service` API (backed by a threadpool) for public usage
This section provides more details on these behaviors.

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# `ops`
Both [`cuprate_blockchain`](https://doc.cuprate.org/cuprate_blockchain)
and [`cuprate_txpool`](https://doc.cuprate.org/cuprate_txpool) expose an
`ops` module containing abstracted abstracted Monero-related database operations.
For example, [`cuprate_blockchain::ops::block::add_block`](https://doc.cuprate.org/cuprate_blockchain/ops/block/fn.add_block.html).
These functions build on-top of the database traits and allow for more abstracted database operations.
For example, instead of these signatures:
```rust
fn get(_: &Key) -> Value;
fn put(_: &Key, &Value);
```
the `ops` module provides much higher-level signatures like such:
```rust
fn add_block(block: &Block) -> Result<_, _>;
```
Although these functions are exposed, they are not the main API, that would be next section:
the [`tower::Service`](./service/intro.md) (which uses these functions).

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# Initialization
A database service is started simply by calling: [`init()`](https://doc.cuprate.org/cuprate_blockchain/service/fn.init.html).
This function initializes the database, spawns threads, and returns a:
- Read handle to the database
- Write handle to the database
- The database itself
These handles implement the `tower::Service` trait, which allows sending requests and receiving responses `async`hronously.

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# tower::Service
Both [`cuprate_blockchain`](https://doc.cuprate.org/cuprate_blockchain)
and [`cuprate_txpool`](https://doc.cuprate.org/cuprate_txpool) provide
`async` [`tower::Service`](https://docs.rs/tower)s that define database requests/responses.
The main API that other Cuprate crates use.
There are 2 `tower::Service`s:
1. A read service which is backed by a [`rayon::ThreadPool`](https://docs.rs/rayon)
1. A write service which spawns a single thread to handle write requests
As this behavior is the same across all users of [`cuprate_database`](https://doc.cuprate.org/cuprate_database),
it is extracted into its own crate: [`cuprate_database_service`](https://doc.cuprate.org/cuprate_database_service).
## Diagram
As a recap, here is how this looks to a user of a higher-level database crate,
`cuprate_blockchain` in this example. Starting from the lowest layer:
1. `cuprate_database` is used to abstract the database
1. `cuprate_blockchain` builds on-top of that with tables, types, operations
1. `cuprate_blockchain` exposes a `tower::Service` using `cuprate_database_service`
1. The user now interfaces with `cuprate_blockchain` with that `tower::Service` in a request/response fashion
```
┌──────────────────┐
│ cuprate_database │
└────────┬─────────┘
┌─────────────────────────────────┴─────────────────────────────────┐
│ cuprate_blockchain │
│ │
│ ┌──────────────────────┐ ┌─────────────────────────────────────┐ │
│ │ Tables, types │ │ ops │ │
│ │ ┌───────────┐┌─────┐ │ │ ┌─────────────┐ ┌──────────┐┌─────┐ │ │
│ │ │ BlockInfo ││ ... │ ├──┤ │ add_block() │ │ add_tx() ││ ... │ │ │
│ │ └───────────┘└─────┘ │ │ └─────────────┘ └──────────┘└─────┘ │ │
│ └──────────────────────┘ └─────┬───────────────────────────────┘ │
│ │ │
│ ┌─────────┴───────────────────────────────┐ │
│ │ tower::Service │ │
│ │ ┌──────────────────────────────┐┌─────┐ │ │
│ │ │ Blockchain{Read,Write}Handle ││ ... │ │ │
│ │ └──────────────────────────────┘└─────┘ │ │
│ └─────────┬───────────────────────────────┘ │
│ │ │
└─────────────────────────────────┼─────────────────────────────────┘
│
┌─────┴─────┐
┌────────────────────┴────┐ ┌────┴──────────────────────────────────┐
│ Database requests │ │ Database responses │
│ ┌─────────────────────┐ │ │ ┌───────────────────────────────────┐ │
│ │ FindBlock([u8; 32]) │ │ │ │ FindBlock(Option<(Chain, usize)>) │ │
│ └─────────────────────┘ │ │ └───────────────────────────────────┘ │
│ ┌─────────────────────┐ │ │ ┌───────────────────────────────────┐ │
│ │ ChainHeight │ │ │ │ ChainHeight(usize, [u8; 32]) │ │
│ └─────────────────────┘ │ │ └───────────────────────────────────┘ │
│ ┌─────────────────────┐ │ │ ┌───────────────────────────────────┐ │
│ │ ... │ │ │ │ ... │ │
│ └─────────────────────┘ │ │ └───────────────────────────────────┘ │
└─────────────────────────┘ └───────────────────────────────────────┘
▲ │
│ ▼
┌─────────────────────────┐
│ cuprate_blockchain user │
└─────────────────────────┘
```

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# Requests
Along with the 2 handles, there are 2 types of requests:
- Read requests, e.g. [`BlockchainReadRequest`](https://doc.cuprate.org/cuprate_types/blockchain/enum.BlockchainReadRequest.html)
- Write requests, e.g. [`BlockchainWriteRequest`](https://doc.cuprate.org/cuprate_types/blockchain/enum.BlockchainWriteRequest.html)
Quite obviously:
- Read requests are for retrieving various data from the database
- Write requests are for writing data to the database

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# Resizing
As noted in the [`cuprate_database` resizing section](../../db/resizing.md),
builders on-top of `cuprate_database` are responsible for resizing the database.
In `cuprate_{blockchain,txpool}`'s case, that means the `tower::Service` must know
how to resize. This logic is shared between both crates, defined in `cuprate_database_service`:
<https://github.com/Cuprate/cuprate/blob/0941f68efcd7dfe66124ad0c1934277f47da9090/storage/service/src/service/write.rs#L107-L171>.
By default, this uses a _similar_ algorithm as `monerod`'s:
- [If there's not enough space to fit a write request's data](https://github.com/Cuprate/cuprate/blob/0941f68efcd7dfe66124ad0c1934277f47da9090/storage/service/src/service/write.rs#L130), start a resize
- Each resize adds around [`1,073,745,920`](https://github.com/Cuprate/cuprate/blob/2ac90420c658663564a71b7ecb52d74f3c2c9d0f/database/src/resize.rs#L104-L160) bytes to the current map size
- A resize will be [attempted `3` times](https://github.com/Cuprate/cuprate/blob/0941f68efcd7dfe66124ad0c1934277f47da9090/storage/service/src/service/write.rs#L110) before failing
There are other [resizing algorithms](https://doc.cuprate.org/cuprate_database/resize/enum.ResizeAlgorithm.html) that define how the database's memory map grows, although currently the behavior of `monerod` is closely followed (for no particular reason).

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# Responses
After sending a request using the read/write handle, the value returned is _not_ the response, yet an `async`hronous channel that will eventually return the response:
```rust,ignore
// Send a request.
// tower::Service::call()
// V
let response_channel: Channel = read_handle.call(BlockchainReadRequest::ChainHeight)?;
// Await the response.
let response: BlockchainReadRequest = response_channel.await?;
```
After `await`ing the returned channel, a `Response` will eventually be returned when
the `Service` threadpool has fetched the value from the database and sent it off.
Both read/write requests variants match in name with `Response` variants, i.e.
- `BlockchainReadRequest::ChainHeight` leads to `BlockchainResponse::ChainHeight`
- `BlockchainWriteRequest::WriteBlock` leads to `BlockchainResponse::WriteBlockOk`

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# Shutdown
Once the read/write handles to the `tower::Service` are `Drop`ed, the backing thread(pool) will gracefully exit, automatically.
Note the writer thread and reader threadpool aren't connected whatsoever; dropping the write handle will make the writer thread exit, however, the reader handle is free to be held onto and can be continued to be read from - and vice-versa for the write handle.

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# Thread model
The base database abstractions themselves are not concerned with parallelism, they are mostly functions to be called from a single-thread.
However, the `cuprate_database_service` API, _does_ have a thread model backing it.
When a `Service`'s init() function is called, threads will be spawned and
maintained until the user drops (disconnects) the returned handles.
The current behavior for thread count is:
- [1 writer thread](https://github.com/Cuprate/cuprate/blob/0941f68efcd7dfe66124ad0c1934277f47da9090/storage/service/src/service/write.rs#L48-L52)
- [As many reader threads as there are system threads](https://github.com/Cuprate/cuprate/blob/0941f68efcd7dfe66124ad0c1934277f47da9090/storage/service/src/reader_threads.rs#L44-L49)
For example, on a system with 32-threads, `cuprate_database_service` will spawn:
- 1 writer thread
- 32 reader threads
whose sole responsibility is to listen for database requests, access the database (potentially in parallel), and return a response.
Note that the `1 system thread = 1 reader thread` model is only the default setting, the reader thread count can be configured by the user to be any number between `1 .. amount_of_system_threads`.
The reader threads are managed by [`rayon`](https://docs.rs/rayon).
For an example of where multiple reader threads are used: given a request that asks if any key-image within a set already exists, `cuprate_blockchain` will [split that work between the threads with `rayon`](https://github.com/Cuprate/cuprate/blob/0941f68efcd7dfe66124ad0c1934277f47da9090/storage/blockchain/src/service/read.rs#L400).

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# Types
## POD types
Since [all types in the database are POD types](../db/serde.md), we must often
provide mappings between outside types and the types actually stored in the database.
A common case is mapping infallible types to and from [`bitflags`](https://docs.rs/bitflag) and/or their raw integer representation.
For example, the [`OutputFlag`](https://doc.cuprate.org/cuprate_blockchain/types/struct.OutputFlags.html) type or `bool` types.
As types like `enum`s, `bool`s and `char`s cannot be casted from an integer infallibly,
`bytemuck::Pod` cannot be implemented on it safely. Thus, we store some infallible version
of it inside the database with a custom type and map them when fetching the data.
## Lean types
Another reason why database crates define their own types is
to cut any unneeded data from the type.
Many of the types used in normal operation (e.g. [`cuprate_types::VerifiedBlockInformation`](https://doc.cuprate.org/cuprate_types/struct.VerifiedBlockInformation.html)) contain lots of extra pre-processed data for convenience.
This would be a waste to store in the database, so in this example, the much leaner
"raw" [`BlockInfo`](https://doc.cuprate.org/cuprate_blockchain/types/struct.BlockInfo.html)
type is stored.

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# ⚪️ Database abstraction

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# Backend
First, we need an actual database implementation.
`cuprate-database`'s `trait`s allow abstracting over the actual database, such that any backend in particular could be used.
This page is an enumeration of all the backends Cuprate has, has tried, and may try in the future.
## `heed`
The default database used is [`heed`](https://github.com/meilisearch/heed) (LMDB). The upstream versions from [`crates.io`](https://crates.io/crates/heed) are used. `LMDB` should not need to be installed as `heed` has a build script that pulls it in automatically.
`heed`'s filenames inside Cuprate's data folder are:
| Filename | Purpose |
|------------|---------|
| `data.mdb` | Main data file
| `lock.mdb` | Database lock file
`heed`-specific notes:
- [There is a maximum reader limit](https://github.com/monero-project/monero/blob/059028a30a8ae9752338a7897329fe8012a310d5/src/blockchain_db/lmdb/db_lmdb.cpp#L1372). Other potential processes (e.g. `xmrblocks`) that are also reading the `data.mdb` file need to be accounted for
- [LMDB does not work on remote filesystem](https://github.com/LMDB/lmdb/blob/b8e54b4c31378932b69f1298972de54a565185b1/libraries/liblmdb/lmdb.h#L129)
## `redb`
The 2nd database backend is the 100% Rust [`redb`](https://github.com/cberner/redb).
The upstream versions from [`crates.io`](https://crates.io/crates/redb) are used.
`redb`'s filenames inside Cuprate's data folder are:
| Filename | Purpose |
|-------------|---------|
| `data.redb` | Main data file
<!-- TODO: document DB on remote filesystem (does redb allow this?) -->
## `redb-memory`
This backend is 100% the same as `redb`, although, it uses [`redb::backend::InMemoryBackend`](https://docs.rs/redb/2.1.2/redb/backends/struct.InMemoryBackend.html) which is a database that completely resides in memory instead of a file.
All other details about this should be the same as the normal `redb` backend.
## `sanakirja`
[`sanakirja`](https://docs.rs/sanakirja) was a candidate as a backend, however there were problems with maximum value sizes.
The default maximum value size is [1012 bytes](https://docs.rs/sanakirja/1.4.1/sanakirja/trait.Storable.html) which was too small for our requirements. Using [`sanakirja::Slice`](https://docs.rs/sanakirja/1.4.1/sanakirja/union.Slice.html) and [sanakirja::UnsizedStorage](https://docs.rs/sanakirja/1.4.1/sanakirja/trait.UnsizedStorable.html) was attempted, but there were bugs found when inserting a value in-between `512..=4096` bytes.
As such, it is not implemented.
## `MDBX`
[`MDBX`](https://erthink.github.io/libmdbx) was a candidate as a backend, however MDBX deprecated the custom key/value comparison functions, this makes it a bit trickier to implement multimap tables. It is also quite similar to the main backend LMDB (of which it was originally a fork of).
As such, it is not implemented (yet).

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# `ConcreteEnv`
After a backend is selected, the main database environment struct is "abstracted" by putting it in the non-generic, concrete [`struct ConcreteEnv`](https://doc.cuprate.org/cuprate_database/struct.ConcreteEnv.html).
This is the main object used when handling the database directly.
This struct contains all the data necessary to operate the database.
The actual database backend `ConcreteEnv` will use internally [depends on which backend feature is used](https://github.com/Cuprate/cuprate/blob/0941f68efcd7dfe66124ad0c1934277f47da9090/storage/database/src/backend/mod.rs#L3-L13).
`ConcreteEnv` itself is not too important, what is important is that:
1. It allows callers to not directly reference any particular backend environment
1. It implements [`trait Env`](https://doc.cuprate.org/cuprate_database/trait.Env.html) which opens the door to all the other database traits
The equivalent "database environment" objects in the backends themselves are:
- [`heed::Env`](https://docs.rs/heed/0.20.0/heed/struct.Env.html)
- [`redb::Database`](https://docs.rs/redb/2.1.0/redb/struct.Database.html)

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# Abstraction
This next section details how `cuprate_database` abstracts multiple database backends into 1 API.
## Diagram
A simple diagram describing the responsibilities/relationship of `cuprate_database`.
```text
┌───────────────────────────────────────────────────────────────────────┐
│ cuprate_database │
│ │
│ ┌───────────────────────────┐ ┌─────────────────────────────────┐ │
│ │ Database traits │ │ Backends │ │
│ │ ┌─────┐┌──────┐┌────────┐ │ │ ┌─────────────┐ ┌─────────────┐ │ │
│ │ │ Env ││ TxRw ││ ... │ ├─────┤ │ heed (LMDB) │ │ redb │ │ │
│ │ └─────┘└──────┘└────────┘ │ │ └─────────────┘ └─────────────┘ │ │
│ └──────────┬─────────────┬──┘ └──┬──────────────────────────────┘ │
│ │ └─────┬─────┘ │
│ │ ┌─────────┴──────────────┐ │
│ │ │ Database types │ │
│ │ │ ┌─────────────┐┌─────┐ │ │
│ │ │ │ ConcreteEnv ││ ... │ │ │
│ │ │ └─────────────┘└─────┘ │ │
│ │ └─────────┬──────────────┘ │
│ │ │ │
└────────────┼───────────────────┼──────────────────────────────────────┘
│ │
└───────────────────┤
│
▼
┌───────────────────────┐
│ cuprate_database user │
└───────────────────────┘
```

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# Trait
`cuprate_database` provides a set of `trait`s that abstract over the various database backends.
This allows the function signatures and behavior to stay the same but allows for swapping out databases in an easier fashion.
All common behavior of the backend's are encapsulated here and used instead of using the backend directly.
Examples:
- [`trait Env`](https://github.com/Cuprate/cuprate/blob/2ac90420c658663564a71b7ecb52d74f3c2c9d0f/database/src/env.rs)
- [`trait {TxRo, TxRw}`](https://github.com/Cuprate/cuprate/blob/2ac90420c658663564a71b7ecb52d74f3c2c9d0f/database/src/transaction.rs)
- [`trait {DatabaseRo, DatabaseRw}`](https://github.com/Cuprate/cuprate/blob/2ac90420c658663564a71b7ecb52d74f3c2c9d0f/database/src/database.rs)
For example, instead of calling `heed` or `redb`'s `get()` function directly, `DatabaseRo::get()` is called.
## Usage
With a `ConcreteEnv` and a particular backend selected,
we can now start using it alongside these traits to start
doing database operations in a generic manner.
An example:
```rust
use cuprate_database::{
ConcreteEnv,
config::ConfigBuilder,
Env, EnvInner,
DatabaseRo, DatabaseRw, TxRo, TxRw,
};
// Initialize the database environment.
let env = ConcreteEnv::open(config)?;
// Open up a transaction + tables for writing.
let env_inner = env.env_inner();
let tx_rw = env_inner.tx_rw()?;
env_inner.create_db::<Table>(&tx_rw)?;
// Write data to the table.
{
let mut table = env_inner.open_db_rw::<Table>(&tx_rw)?;
table.put(&0, &1)?;
}
// Commit the transaction.
TxRw::commit(tx_rw)?;
```
As seen above, there is no direct call to `heed` or `redb`.
Their functionality is abstracted behind `ConcreteEnv` and the `trait`s.

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# Database abstraction
[`cuprate_database`](https://doc.cuprate.org/cuprate_database) is Cuprate’s database abstraction.
This crate abstracts various database backends with `trait`s.
All backends have the following attributes:
- [Embedded](https://en.wikipedia.org/wiki/Embedded_database)
- [Multiversion concurrency control](https://en.wikipedia.org/wiki/Multiversion_concurrency_control)
- [ACID](https://en.wikipedia.org/wiki/ACID)
- Are `(key, value)` oriented and have the expected API (`get()`, `insert()`, `delete()`)
- Are table oriented (`"table_name" -> (key, value)`)
- Allows concurrent readers
The currently implemented backends are:
- [`heed`](https://github.com/meilisearch/heed) (LMDB)
- [`redb`](https://github.com/cberner/redb)
Said precicely, `cuprate_database` is the embedded database other Cuprate
crates interact with instead of using any particular backend implementation.
This allows the backend to be swapped and/or future backends to be implemented.
This section will go over `cuprate_database` details.

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# Endianness
`cuprate_database`'s (de)serialization and storage of bytes are native-endian, as in, byte storage order will depend on the machine it is running on.
As Cuprate's build-targets are all little-endian ([big-endian by default machines barely exist](https://en.wikipedia.org/wiki/Endianness#Hardware)), this doesn't matter much and the byte ordering can be seen as a constant.
Practically, this means `cuprated`'s database files can be transferred across computers, as can `monerod`'s.

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# Hot-swappable backends
> See also: <https://github.com/Cuprate/cuprate/issues/209>.
Using a different backend is really as simple as re-building `cuprate_database` with a different feature flag:
```bash
# Use LMDB.
cargo build --package cuprate-database --features heed
# Use redb.
cargo build --package cuprate-database --features redb
```
This is "good enough" for now, however ideally, this hot-swapping of backends would be able to be done at _runtime_.
As it is now, `cuprate_database` cannot compile both backends and swap based on user input at runtime; it must be compiled with a certain backend, which will produce a binary with only that backend.
This also means things like [CI testing multiple backends is awkward](https://github.com/Cuprate/cuprate/blob/main/.github/workflows/ci.yml#L132-L136), as we must re-compile with different feature flags instead.

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# Known issues and tradeoffs
`cuprate_database` takes many tradeoffs, whether due to:
- Prioritizing certain values over others
- Not having a better solution
- Being "good enough"
This section is a list of the larger ones, along with issues that don't have answers yet.

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# Multimap
`cuprate_database` does not currently have an abstraction for [multimap tables](https://en.wikipedia.org/wiki/Multimap).
All tables are single maps of keys to values.
This matters as this means some of `cuprate_blockchain`'s tables differ from `monerod`'s tables - the primary key is stored _for all_ entries, compared to `monerod` only needing to store it once:
```rust
// `monerod` only stores `amount: 1` once,
// `cuprated` stores it each time it appears.
struct PreRctOutputId { amount: 1, amount_index: 0 }
struct PreRctOutputId { amount: 1, amount_index: 1 }
```
This means `cuprated`'s database will be slightly larger than `monerod`'s.
The current method `cuprate_blockchain` uses will be "good enough" as the multimap
keys needed for now are fixed, e.g. pre-RCT outputs are no longer being produced.
This may need to change in the future when multimap is all but required, e.g. for FCMP++.
Until then, multimap tables are not implemented as they are tricky to implement across all backends.

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# Traits abstracting backends
Although all database backends used are very similar, they have some crucial differences in small implementation details that must be worked around when conforming them to `cuprate_database`'s traits.
Put simply: using `cuprate_database`'s traits is less efficient and more awkward than using the backend directly.
For example:
- [Data types must be wrapped in compatibility layers when they otherwise wouldn't be](https://github.com/Cuprate/cuprate/blob/d0ac94a813e4cd8e0ed8da5e85a53b1d1ace2463/database/src/backend/heed/env.rs#L101-L116)
- [There are types that only apply to a specific backend, but are visible to all](https://github.com/Cuprate/cuprate/blob/d0ac94a813e4cd8e0ed8da5e85a53b1d1ace2463/database/src/error.rs#L86-L89)
- [There are extra layers of abstraction to smoothen the differences between all backends](https://github.com/Cuprate/cuprate/blob/d0ac94a813e4cd8e0ed8da5e85a53b1d1ace2463/database/src/env.rs#L62-L68)
- [Existing functionality of backends must be taken away, as it isn't supported in the others](https://github.com/Cuprate/cuprate/blob/d0ac94a813e4cd8e0ed8da5e85a53b1d1ace2463/database/src/database.rs#L27-L34)
This is a _tradeoff_ that `cuprate_database` takes, as:
- The backend itself is usually not the source of bottlenecks in the greater system, as such, small inefficiencies are OK
- None of the lost functionality is crucial for operation
- The ability to use, test, and swap between multiple database backends is [worth it](https://github.com/Cuprate/cuprate/pull/35#issuecomment-1952804393)

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# Copying unaligned bytes
As mentioned in [`(De)serialization`](../serde.md), bytes are _copied_ when they are turned into a type `T` due to unaligned bytes being returned from database backends.
Using a regular reference cast results in an improperly aligned type `T`; [such a type even existing causes undefined behavior](https://doc.rust-lang.org/reference/behavior-considered-undefined.html). In our case, `bytemuck` saves us by panicking before this occurs.
Thus, when using `cuprate_database`'s database traits, an _owned_ `T` is returned.
This is doubly unfortunately for `&[u8]` as this does not even need deserialization.
For example, `StorableVec` could have been this:
```rust
enum StorableBytes<'a, T: Storable> {
Owned(T),
Ref(&'a T),
}
```
but this would require supporting types that must be copied regardless with the occasional `&[u8]` that can be returned without casting. This was hard to do so in a generic way, thus all `[u8]`'s are copied and returned as owned `StorableVec`s.
This is a _tradeoff_ `cuprate_database` takes as:
- `bytemuck::pod_read_unaligned` is cheap enough
- The main API, `service`, needs to return owned value anyway
- Having no references removes a lot of lifetime complexity
The alternative is somehow fixing the alignment issues in the backends mentioned previously.

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# Resizing
`cuprate_database` itself does not handle memory map resizes automatically
(for database backends that need resizing, i.e. heed/LMDB).
When a user directly using `cuprate_database`, it is up to them on how to resize. The database will return [`RuntimeError::ResizeNeeded`](https://doc.cuprate.org/cuprate_database/enum.RuntimeError.html#variant.ResizeNeeded) when it needs resizing.
However, `cuprate_database` exposes some [resizing algorithms](https://doc.cuprate.org/cuprate_database/resize/index.html)
that define how the database's memory map grows.

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# (De)serialization
All types stored inside the database are either bytes already or are perfectly bitcast-able.
As such, they do not incur heavy (de)serialization costs when storing/fetching them from the database. The main (de)serialization used is [`bytemuck`](https://docs.rs/bytemuck)'s traits and casting functions.
## Size and layout
The size & layout of types is stable across compiler versions, as they are set and determined with [`#[repr(C)]`](https://doc.rust-lang.org/nomicon/other-reprs.html#reprc) and `bytemuck`'s derive macros such as [`bytemuck::Pod`](https://docs.rs/bytemuck/latest/bytemuck/derive.Pod.html).
Note that the data stored in the tables are still type-safe; we still refer to the key and values within our tables by the type.
## How
The main deserialization `trait` for database storage is [`Storable`](https://doc.cuprate.org/cuprate_database/trait.Storable.html).
- Before storage, the type is [simply cast into bytes](https://github.com/Cuprate/cuprate/blob/2ac90420c658663564a71b7ecb52d74f3c2c9d0f/database/src/storable.rs#L125)
- When fetching, the bytes are [simply cast into the type](https://github.com/Cuprate/cuprate/blob/2ac90420c658663564a71b7ecb52d74f3c2c9d0f/database/src/storable.rs#L130)
When a type is casted into bytes, [the reference is casted](https://docs.rs/bytemuck/latest/bytemuck/fn.bytes_of.html), i.e. this is zero-cost serialization.
However, it is worth noting that when bytes are casted into the type, [it is copied](https://docs.rs/bytemuck/latest/bytemuck/fn.pod_read_unaligned.html). This is due to byte alignment guarantee issues with both backends, see:
- <https://github.com/AltSysrq/lmdb-zero/issues/8>
- <https://github.com/cberner/redb/issues/360>
Without this, `bytemuck` will panic with [`TargetAlignmentGreaterAndInputNotAligned`](https://docs.rs/bytemuck/latest/bytemuck/enum.PodCastError.html#variant.TargetAlignmentGreaterAndInputNotAligned) when casting.
Copying the bytes fixes this problem, although it is more costly than necessary. However, in the main use-case for `cuprate_database` (`tower::Service` API) the bytes would need to be owned regardless as the `Request/Response` API uses owned data types (`T`, `Vec<T>`, `HashMap<K, V>`, etc).
Practically speaking, this means lower-level database functions that normally look like such:
```rust
fn get(key: &Key) -> &Value;
```
end up looking like this in `cuprate_database`:
```rust
fn get(key: &Key) -> Value;
```
Since each backend has its own (de)serialization methods, our types are wrapped in compatibility types that map our `Storable` functions into whatever is required for the backend, e.g:
- [`StorableHeed<T>`](https://github.com/Cuprate/cuprate/blob/2ac90420c658663564a71b7ecb52d74f3c2c9d0f/database/src/backend/heed/storable.rs#L11-L45)
- [`StorableRedb<T>`](https://github.com/Cuprate/cuprate/blob/2ac90420c658663564a71b7ecb52d74f3c2c9d0f/database/src/backend/redb/storable.rs#L11-L30)
Compatibility structs also exist for any `Storable` containers:
- [`StorableVec<T>`](https://github.com/Cuprate/cuprate/blob/2ac90420c658663564a71b7ecb52d74f3c2c9d0f/database/src/storable.rs#L135-L191)
- [`StorableBytes`](https://github.com/Cuprate/cuprate/blob/2ac90420c658663564a71b7ecb52d74f3c2c9d0f/database/src/storable.rs#L208-L241)
Again, it's unfortunate that these must be owned, although in the `tower::Service` use-case, they would have to be owned anyway.

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# Syncing
`cuprate_database`'s database has 5 disk syncing modes.
1. `FastThenSafe`
1. `Safe`
1. `Async`
1. `Threshold`
1. `Fast`
The default mode is `Safe`.
This means that upon each transaction commit, all the data that was written will be fully synced to disk.
This is the slowest, but safest mode of operation.
Note that upon any database `Drop`, the current implementation will sync to disk regardless of any configuration.
For more information on the other modes, read the documentation [here](https://github.com/Cuprate/cuprate/blob/2ac90420c658663564a71b7ecb52d74f3c2c9d0f/database/src/config/sync_mode.rs#L63-L144).

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# ⚪️ Storage
# Storage
This section covers all things related to the on-disk storage of data within Cuprate.
## Overview
The quick overview is that Cuprate has a [database abstraction crate](./database-abstraction.md)
that handles "low-level" database details such as key and value (de)serialization, tables, transactions, etc.
This database abstraction crate is then used by all crates that need on-disk storage, i.e. the
- [Blockchain database](./blockchain/intro.md)
- [Transaction pool database](./txpool/intro.md)
## Service
The interface provided by all crates building on-top of the
database abstraction is a [`tower::Service`](https://docs.rs/tower), i.e.
database requests/responses are sent/received asynchronously.
As the interface details are similar across crates (threadpool, read operations, write operations),
the interface itself is abstracted in the [`cuprate_database_service`](./common/service/intro.md) crate,
which is then used by the crates.
## Diagram
This is roughly how database crates are set up.
```text
┌─────────────────┐
┌──────────────────────────────────┐ │ │
│ Some crate that needs a database │ ┌────────────────┐ │ │
│ │ │ Public │ │ │
│ ┌──────────────────────────────┐ │─►│ tower::Service │◄─►│ Rest of Cuprate │
│ │ Database abstraction │ │ │ API │ │ │
│ └──────────────────────────────┘ │ └────────────────┘ │ │
└──────────────────────────────────┘ │ │
└─────────────────┘
```

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# storage
# Storage
This subdirectory contains all things related to the on-disk storage of data within Cuprate.
TODO: This subdirectory used to be `database/` and is in the middle of being shifted around.
See <https://architecture.cuprate.org/storage/intro.html> for design documentation
and the following links for user documentation:
The old `database/` design document is in `cuprate-blockchain/` which will eventually be ported Cuprate's architecture book.
- <https://doc.cuprate.org/cuprate_database>
- <https://doc.cuprate.org/cuprate_database_service>
- <https://doc.cuprate.org/cuprate_blockchain>
- <https://doc.cuprate.org/cuprate_txpool>

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# Database
FIXME: This documentation must be updated and moved to the architecture book.
Cuprate's blockchain implementation.
- [1. Documentation](#1-documentation)
- [2. File structure](#2-file-structure)
- [2.1 `src/`](#21-src)
- [2.2 `src/backend/`](#22-srcbackend)
- [2.3 `src/config/`](#23-srcconfig)
- [2.4 `src/ops/`](#24-srcops)
- [2.5 `src/service/`](#25-srcservice)
- [3. Backends](#3-backends)
- [3.1 heed](#31-heed)
- [3.2 redb](#32-redb)
- [3.3 redb-memory](#33-redb-memory)
- [3.4 sanakirja](#34-sanakirja)
- [3.5 MDBX](#35-mdbx)
- [4. Layers](#4-layers)
- [4.1 Backend](#41-backend)
- [4.2 Trait](#42-trait)
- [4.3 ConcreteEnv](#43-concreteenv)
- [4.4 ops](#44-ops)
- [4.5 service](#45-service)
- [5. The service](#5-the-service)
- [5.1 Initialization](#51-initialization)
- [5.2 Requests](#53-requests)
- [5.3 Responses](#54-responses)
- [5.4 Thread model](#52-thread-model)
- [5.5 Shutdown](#55-shutdown)
- [6. Syncing](#6-Syncing)
- [7. Resizing](#7-resizing)
- [8. (De)serialization](#8-deserialization)
- [9. Schema](#9-schema)
- [9.1 Tables](#91-tables)
- [9.2 Multimap tables](#92-multimap-tables)
- [10. Known issues and tradeoffs](#10-known-issues-and-tradeoffs)
- [10.1 Traits abstracting backends](#101-traits-abstracting-backends)
- [10.2 Hot-swappable backends](#102-hot-swappable-backends)
- [10.3 Copying unaligned bytes](#103-copying-unaligned-bytes)
- [10.4 Endianness](#104-endianness)
- [10.5 Extra table data](#105-extra-table-data)
---
## 1. Documentation
Documentation for `database/` is split into 3 locations:
| Documentation location | Purpose |
|---------------------------|---------|
| `database/README.md` | High level design of `cuprate-database`
| `cuprate-database` | Practical usage documentation/warnings/notes/etc
| Source file `// comments` | Implementation-specific details (e.g, how many reader threads to spawn?)
This README serves as the implementation design document.
For actual practical usage, `cuprate-database`'s types and general usage are documented via standard Rust tooling.
Run:
```bash
cargo doc --package cuprate-database --open
```
at the root of the repo to open/read the documentation.
If this documentation is too abstract, refer to any of the source files, they are heavily commented. There are many `// Regular comments` that explain more implementation specific details that aren't present here or in the docs. Use the file reference below to find what you're looking for.
The code within `src/` is also littered with some `grep`-able comments containing some keywords:
| Word | Meaning |
|-------------|---------|
| `INVARIANT` | This code makes an _assumption_ that must be upheld for correctness
| `SAFETY` | This `unsafe` code is okay, for `x,y,z` reasons
| `FIXME` | This code works but isn't ideal
| `HACK` | This code is a brittle workaround
| `PERF` | This code is weird for performance reasons
| `TODO` | This must be implemented; There should be 0 of these in production code
| `SOMEDAY` | This should be implemented... someday
## 2. File structure
A quick reference of the structure of the folders & files in `cuprate-database`.
Note that `lib.rs/mod.rs` files are purely for re-exporting/visibility/lints, and contain no code. Each sub-directory has a corresponding `mod.rs`.
### 2.1 `src/`
The top-level `src/` files.
| File | Purpose |
|------------------------|---------|
| `constants.rs` | General constants used throughout `cuprate-database`
| `database.rs` | Abstracted database; `trait DatabaseR{o,w}`
| `env.rs` | Abstracted database environment; `trait Env`
| `error.rs` | Database error types
| `free.rs` | General free functions (related to the database)
| `key.rs` | Abstracted database keys; `trait Key`
| `resize.rs` | Database resizing algorithms
| `storable.rs` | Data (de)serialization; `trait Storable`
| `table.rs` | Database table abstraction; `trait Table`
| `tables.rs` | All the table definitions used by `cuprate-database`
| `tests.rs` | Utilities for `cuprate_database` testing
| `transaction.rs` | Database transaction abstraction; `trait TxR{o,w}`
| `types.rs` | Database-specific types
| `unsafe_unsendable.rs` | Marker type to impl `Send` for objects not `Send`
### 2.2 `src/backend/`
This folder contains the implementation for actual databases used as the backend for `cuprate-database`.
Each backend has its own folder.
| Folder/File | Purpose |
|-------------|---------|
| `heed/` | Backend using using [`heed`](https://github.com/meilisearch/heed) (LMDB)
| `redb/` | Backend using [`redb`](https://github.com/cberner/redb)
| `tests.rs` | Backend-agnostic tests
All backends follow the same file structure:
| File | Purpose |
|------------------|---------|
| `database.rs` | Implementation of `trait DatabaseR{o,w}`
| `env.rs` | Implementation of `trait Env`
| `error.rs` | Implementation of backend's errors to `cuprate_database`'s error types
| `storable.rs` | Compatibility layer between `cuprate_database::Storable` and backend-specific (de)serialization
| `transaction.rs` | Implementation of `trait TxR{o,w}`
| `types.rs` | Type aliases for long backend-specific types
### 2.3 `src/config/`
This folder contains the `cupate_database::config` module; configuration options for the database.
| File | Purpose |
|---------------------|---------|
| `config.rs` | Main database `Config` struct
| `reader_threads.rs` | Reader thread configuration for `service` thread-pool
| `sync_mode.rs` | Disk sync configuration for backends
### 2.4 `src/ops/`
This folder contains the `cupate_database::ops` module.
These are higher-level functions abstracted over the database, that are Monero-related.
| File | Purpose |
|-----------------|---------|
| `block.rs` | Block related (main functions)
| `blockchain.rs` | Blockchain related (height, cumulative values, etc)
| `key_image.rs` | Key image related
| `macros.rs` | Macros specific to `ops/`
| `output.rs` | Output related
| `property.rs` | Database properties (pruned, version, etc)
| `tx.rs` | Transaction related
### 2.5 `src/service/`
This folder contains the `cupate_database::service` module.
The `async`hronous request/response API other Cuprate crates use instead of managing the database directly themselves.
| File | Purpose |
|----------------|---------|
| `free.rs` | General free functions used (related to `cuprate_database::service`)
| `read.rs` | Read thread-pool definitions and logic
| `tests.rs` | Thread-pool tests and test helper functions
| `types.rs` | `cuprate_database::service`-related type aliases
| `write.rs` | Writer thread definitions and logic
## 3. Backends
`cuprate-database`'s `trait`s allow abstracting over the actual database, such that any backend in particular could be used.
Each database's implementation for those `trait`'s are located in its respective folder in `src/backend/${DATABASE_NAME}/`.
### 3.1 heed
The default database used is [`heed`](https://github.com/meilisearch/heed) (LMDB). The upstream versions from [`crates.io`](https://crates.io/crates/heed) are used. `LMDB` should not need to be installed as `heed` has a build script that pulls it in automatically.
`heed`'s filenames inside Cuprate's database folder (`~/.local/share/cuprate/database/`) are:
| Filename | Purpose |
|------------|---------|
| `data.mdb` | Main data file
| `lock.mdb` | Database lock file
`heed`-specific notes:
- [There is a maximum reader limit](https://github.com/monero-project/monero/blob/059028a30a8ae9752338a7897329fe8012a310d5/src/blockchain_db/lmdb/db_lmdb.cpp#L1372). Other potential processes (e.g. `xmrblocks`) that are also reading the `data.mdb` file need to be accounted for
- [LMDB does not work on remote filesystem](https://github.com/LMDB/lmdb/blob/b8e54b4c31378932b69f1298972de54a565185b1/libraries/liblmdb/lmdb.h#L129)
### 3.2 redb
The 2nd database backend is the 100% Rust [`redb`](https://github.com/cberner/redb).
The upstream versions from [`crates.io`](https://crates.io/crates/redb) are used.
`redb`'s filenames inside Cuprate's database folder (`~/.local/share/cuprate/database/`) are:
| Filename | Purpose |
|-------------|---------|
| `data.redb` | Main data file
<!-- TODO: document DB on remote filesystem (does redb allow this?) -->
### 3.3 redb-memory
This backend is 100% the same as `redb`, although, it uses `redb::backend::InMemoryBackend` which is a database that completely resides in memory instead of a file.
All other details about this should be the same as the normal `redb` backend.
### 3.4 sanakirja
[`sanakirja`](https://docs.rs/sanakirja) was a candidate as a backend, however there were problems with maximum value sizes.
The default maximum value size is [1012 bytes](https://docs.rs/sanakirja/1.4.1/sanakirja/trait.Storable.html) which was too small for our requirements. Using [`sanakirja::Slice`](https://docs.rs/sanakirja/1.4.1/sanakirja/union.Slice.html) and [sanakirja::UnsizedStorage](https://docs.rs/sanakirja/1.4.1/sanakirja/trait.UnsizedStorable.html) was attempted, but there were bugs found when inserting a value in-between `512..=4096` bytes.
As such, it is not implemented.
### 3.5 MDBX
[`MDBX`](https://erthink.github.io/libmdbx) was a candidate as a backend, however MDBX deprecated the custom key/value comparison functions, this makes it a bit trickier to implement [`9.2 Multimap tables`](#92-multimap-tables). It is also quite similar to the main backend LMDB (of which it was originally a fork of).
As such, it is not implemented (yet).
## 4. Layers
`cuprate_database` is logically abstracted into 5 layers, with each layer being built upon the last.
Starting from the lowest:
1. Backend
2. Trait
3. ConcreteEnv
4. `ops`
5. `service`
<!-- TODO: insert image here after database/ split -->
### 4.1 Backend
This is the actual database backend implementation (or a Rust shim over one).
Examples:
- `heed` (LMDB)
- `redb`
`cuprate_database` itself just uses a backend, it does not implement one.
All backends have the following attributes:
- [Embedded](https://en.wikipedia.org/wiki/Embedded_database)
- [Multiversion concurrency control](https://en.wikipedia.org/wiki/Multiversion_concurrency_control)
- [ACID](https://en.wikipedia.org/wiki/ACID)
- Are `(key, value)` oriented and have the expected API (`get()`, `insert()`, `delete()`)
- Are table oriented (`"table_name" -> (key, value)`)
- Allows concurrent readers
### 4.2 Trait
`cuprate_database` provides a set of `trait`s that abstract over the various database backends.
This allows the function signatures and behavior to stay the same but allows for swapping out databases in an easier fashion.
All common behavior of the backend's are encapsulated here and used instead of using the backend directly.
Examples:
- [`trait Env`](https://github.com/Cuprate/cuprate/blob/2ac90420c658663564a71b7ecb52d74f3c2c9d0f/database/src/env.rs)
- [`trait {TxRo, TxRw}`](https://github.com/Cuprate/cuprate/blob/2ac90420c658663564a71b7ecb52d74f3c2c9d0f/database/src/transaction.rs)
- [`trait {DatabaseRo, DatabaseRw}`](https://github.com/Cuprate/cuprate/blob/2ac90420c658663564a71b7ecb52d74f3c2c9d0f/database/src/database.rs)
For example, instead of calling `LMDB` or `redb`'s `get()` function directly, `DatabaseRo::get()` is called.
### 4.3 ConcreteEnv
This is the non-generic, concrete `struct` provided by `cuprate_database` that contains all the data necessary to operate the database. The actual database backend `ConcreteEnv` will use internally depends on which backend feature is used.
`ConcreteEnv` implements `trait Env`, which opens the door to all the other traits.
The equivalent objects in the backends themselves are:
- [`heed::Env`](https://docs.rs/heed/0.20.0/heed/struct.Env.html)
- [`redb::Database`](https://docs.rs/redb/2.1.0/redb/struct.Database.html)
This is the main object used when handling the database directly, although that is not strictly necessary as a user if the [`4.5 service`](#45-service) layer is used.
### 4.4 ops
These are Monero-specific functions that use the abstracted `trait` forms of the database.
Instead of dealing with the database directly:
- `get()`
- `delete()`
the `ops` layer provides more abstract functions that deal with commonly used Monero operations:
- `add_block()`
- `pop_block()`
### 4.5 service
The final layer abstracts the database completely into a [Monero-specific `async` request/response API](https://github.com/Cuprate/cuprate/blob/2ac90420c658663564a71b7ecb52d74f3c2c9d0f/types/src/service.rs#L18-L78) using [`tower::Service`](https://docs.rs/tower/latest/tower/trait.Service.html).
For more information on this layer, see the next section: [`5. The service`](#5-the-service).
## 5. The service
The main API `cuprate_database` exposes for other crates to use is the `cuprate_database::service` module.
This module exposes an `async` request/response API with `tower::Service`, backed by a threadpool, that allows reading/writing Monero-related data from/to the database.
`cuprate_database::service` itself manages the database using a separate writer thread & reader thread-pool, and uses the previously mentioned [`4.4 ops`](#44-ops) functions when responding to requests.
### 5.1 Initialization
The service is started simply by calling: [`cuprate_database::service::init()`](https://github.com/Cuprate/cuprate/blob/d0ac94a813e4cd8e0ed8da5e85a53b1d1ace2463/database/src/service/free.rs#L23).
This function initializes the database, spawns threads, and returns a:
- Read handle to the database (cloneable)
- Write handle to the database (not cloneable)
These "handles" implement the `tower::Service` trait, which allows sending requests and receiving responses `async`hronously.
### 5.2 Requests
Along with the 2 handles, there are 2 types of requests:
- [`ReadRequest`](https://github.com/Cuprate/cuprate/blob/d0ac94a813e4cd8e0ed8da5e85a53b1d1ace2463/types/src/service.rs#L23-L90)
- [`WriteRequest`](https://github.com/Cuprate/cuprate/blob/d0ac94a813e4cd8e0ed8da5e85a53b1d1ace2463/types/src/service.rs#L93-L105)
`ReadRequest` is for retrieving various types of information from the database.
`WriteRequest` currently only has 1 variant: to write a block to the database.
### 5.3 Responses
After sending one of the above requests using the read/write handle, the value returned is _not_ the response, yet an `async`hronous channel that will eventually return the response:
```rust,ignore
// Send a request.
// tower::Service::call()
// V
let response_channel: Channel = read_handle.call(ReadResponse::ChainHeight)?;
// Await the response.
let response: ReadResponse = response_channel.await?;
// Assert the response is what we expected.
assert_eq!(matches!(response), Response::ChainHeight(_));
```
After `await`ing the returned channel, a `Response` will eventually be returned when the `service` threadpool has fetched the value from the database and sent it off.
Both read/write requests variants match in name with `Response` variants, i.e.
- `ReadRequest::ChainHeight` leads to `Response::ChainHeight`
- `WriteRequest::WriteBlock` leads to `Response::WriteBlockOk`
### 5.4 Thread model
As mentioned in the [`4. Layers`](#4-layers) section, the base database abstractions themselves are not concerned with parallelism, they are mostly functions to be called from a single-thread.
However, the `cuprate_database::service` API, _does_ have a thread model backing it.
When [`cuprate_database::service`'s initialization function](https://github.com/Cuprate/cuprate/blob/9c27ba5791377d639cb5d30d0f692c228568c122/database/src/service/free.rs#L33-L44) is called, threads will be spawned and maintained until the user drops (disconnects) the returned handles.
The current behavior for thread count is:
- [1 writer thread](https://github.com/Cuprate/cuprate/blob/9c27ba5791377d639cb5d30d0f692c228568c122/database/src/service/write.rs#L52-L66)
- [As many reader threads as there are system threads](https://github.com/Cuprate/cuprate/blob/9c27ba5791377d639cb5d30d0f692c228568c122/database/src/service/read.rs#L104-L126)
For example, on a system with 32-threads, `cuprate_database` will spawn:
- 1 writer thread
- 32 reader threads
whose sole responsibility is to listen for database requests, access the database (potentially in parallel), and return a response.
Note that the `1 system thread = 1 reader thread` model is only the default setting, the reader thread count can be configured by the user to be any number between `1 .. amount_of_system_threads`.
The reader threads are managed by [`rayon`](https://docs.rs/rayon).
For an example of where multiple reader threads are used: given a request that asks if any key-image within a set already exists, `cuprate_database` will [split that work between the threads with `rayon`](https://github.com/Cuprate/cuprate/blob/9c27ba5791377d639cb5d30d0f692c228568c122/database/src/service/read.rs#L490-L503).
### 5.5 Shutdown
Once the read/write handles are `Drop`ed, the backing thread(pool) will gracefully exit, automatically.
Note the writer thread and reader threadpool aren't connected whatsoever; dropping the write handle will make the writer thread exit, however, the reader handle is free to be held onto and can be continued to be read from - and vice-versa for the write handle.
## 6. Syncing
`cuprate_database`'s database has 5 disk syncing modes.
1. FastThenSafe
1. Safe
1. Async
1. Threshold
1. Fast
The default mode is `Safe`.
This means that upon each transaction commit, all the data that was written will be fully synced to disk. This is the slowest, but safest mode of operation.
Note that upon any database `Drop`, whether via `service` or dropping the database directly, the current implementation will sync to disk regardless of any configuration.
For more information on the other modes, read the documentation [here](https://github.com/Cuprate/cuprate/blob/2ac90420c658663564a71b7ecb52d74f3c2c9d0f/database/src/config/sync_mode.rs#L63-L144).
## 7. Resizing
Database backends that require manually resizing will, by default, use a similar algorithm as `monerod`'s.
Note that this only relates to the `service` module, where the database is handled by `cuprate_database` itself, not the user. In the case of a user directly using `cuprate_database`, it is up to them on how to resize.
Within `service`, the resizing logic defined [here](https://github.com/Cuprate/cuprate/blob/2ac90420c658663564a71b7ecb52d74f3c2c9d0f/database/src/service/write.rs#L139-L201) does the following:
- If there's not enough space to fit a write request's data, start a resize
- Each resize adds around [`1_073_745_920`](https://github.com/Cuprate/cuprate/blob/2ac90420c658663564a71b7ecb52d74f3c2c9d0f/database/src/resize.rs#L104-L160) bytes to the current map size
- A resize will be attempted `3` times before failing
There are other [resizing algorithms](https://github.com/Cuprate/cuprate/blob/2ac90420c658663564a71b7ecb52d74f3c2c9d0f/database/src/resize.rs#L38-L47) that define how the database's memory map grows, although currently the behavior of [`monerod`](https://github.com/Cuprate/cuprate/blob/2ac90420c658663564a71b7ecb52d74f3c2c9d0f/database/src/resize.rs#L104-L160) is closely followed.
## 8. (De)serialization
All types stored inside the database are either bytes already, or are perfectly bitcast-able.
As such, they do not incur heavy (de)serialization costs when storing/fetching them from the database. The main (de)serialization used is [`bytemuck`](https://docs.rs/bytemuck)'s traits and casting functions.
The size & layout of types is stable across compiler versions, as they are set and determined with [`#[repr(C)]`](https://doc.rust-lang.org/nomicon/other-reprs.html#reprc) and `bytemuck`'s derive macros such as [`bytemuck::Pod`](https://docs.rs/bytemuck/latest/bytemuck/derive.Pod.html).
Note that the data stored in the tables are still type-safe; we still refer to the key and values within our tables by the type.
The main deserialization `trait` for database storage is: [`cuprate_database::Storable`](https://github.com/Cuprate/cuprate/blob/2ac90420c658663564a71b7ecb52d74f3c2c9d0f/database/src/storable.rs#L16-L115).
- Before storage, the type is [simply cast into bytes](https://github.com/Cuprate/cuprate/blob/2ac90420c658663564a71b7ecb52d74f3c2c9d0f/database/src/storable.rs#L125)
- When fetching, the bytes are [simply cast into the type](https://github.com/Cuprate/cuprate/blob/2ac90420c658663564a71b7ecb52d74f3c2c9d0f/database/src/storable.rs#L130)
When a type is casted into bytes, [the reference is casted](https://docs.rs/bytemuck/latest/bytemuck/fn.bytes_of.html), i.e. this is zero-cost serialization.
However, it is worth noting that when bytes are casted into the type, [it is copied](https://docs.rs/bytemuck/latest/bytemuck/fn.pod_read_unaligned.html). This is due to byte alignment guarantee issues with both backends, see:
- https://github.com/AltSysrq/lmdb-zero/issues/8
- https://github.com/cberner/redb/issues/360
Without this, `bytemuck` will panic with [`TargetAlignmentGreaterAndInputNotAligned`](https://docs.rs/bytemuck/latest/bytemuck/enum.PodCastError.html#variant.TargetAlignmentGreaterAndInputNotAligned) when casting.
Copying the bytes fixes this problem, although it is more costly than necessary. However, in the main use-case for `cuprate_database` (the `service` module) the bytes would need to be owned regardless as the `Request/Response` API uses owned data types (`T`, `Vec<T>`, `HashMap<K, V>`, etc).
Practically speaking, this means lower-level database functions that normally look like such:
```rust
fn get(key: &Key) -> &Value;
```
end up looking like this in `cuprate_database`:
```rust
fn get(key: &Key) -> Value;
```
Since each backend has its own (de)serialization methods, our types are wrapped in compatibility types that map our `Storable` functions into whatever is required for the backend, e.g:
- [`StorableHeed<T>`](https://github.com/Cuprate/cuprate/blob/2ac90420c658663564a71b7ecb52d74f3c2c9d0f/database/src/backend/heed/storable.rs#L11-L45)
- [`StorableRedb<T>`](https://github.com/Cuprate/cuprate/blob/2ac90420c658663564a71b7ecb52d74f3c2c9d0f/database/src/backend/redb/storable.rs#L11-L30)
Compatibility structs also exist for any `Storable` containers:
- [`StorableVec<T>`](https://github.com/Cuprate/cuprate/blob/2ac90420c658663564a71b7ecb52d74f3c2c9d0f/database/src/storable.rs#L135-L191)
- [`StorableBytes`](https://github.com/Cuprate/cuprate/blob/2ac90420c658663564a71b7ecb52d74f3c2c9d0f/database/src/storable.rs#L208-L241)
Again, it's unfortunate that these must be owned, although in `service`'s use-case, they would have to be owned anyway.
## 9. Schema
This following section contains Cuprate's database schema, it may change throughout the development of Cuprate, as such, nothing here is final.
### 9.1 Tables
The `CamelCase` names of the table headers documented here (e.g. `TxIds`) are the actual type name of the table within `cuprate_database`.
Note that words written within `code blocks` mean that it is a real type defined and usable within `cuprate_database`. Other standard types like u64 and type aliases (TxId) are written normally.
Within `cuprate_database::tables`, the below table is essentially defined as-is with [a macro](https://github.com/Cuprate/cuprate/blob/31ce89412aa174fc33754f22c9a6d9ef5ddeda28/database/src/tables.rs#L369-L470).
Many of the data types stored are the same data types, although are different semantically, as such, a map of aliases used and their real data types is also provided below.
| Alias | Real Type |
|----------------------------------------------------|-----------|
| BlockHeight, Amount, AmountIndex, TxId, UnlockTime | u64
| BlockHash, KeyImage, TxHash, PrunableHash | [u8; 32]
| Table | Key | Value | Description |
|-------------------|----------------------|--------------------|-------------|
| `BlockBlobs` | BlockHeight | `StorableVec<u8>` | Maps a block's height to a serialized byte form of a block
| `BlockHeights` | BlockHash | BlockHeight | Maps a block's hash to its height
| `BlockInfos` | BlockHeight | `BlockInfo` | Contains metadata of all blocks
| `KeyImages` | KeyImage | () | This table is a set with no value, it stores transaction key images
| `NumOutputs` | Amount | u64 | Maps an output's amount to the number of outputs with that amount
| `Outputs` | `PreRctOutputId` | `Output` | This table contains legacy CryptoNote outputs which have clear amounts. This table will not contain an output with 0 amount.
| `PrunedTxBlobs` | TxId | `StorableVec<u8>` | Contains pruned transaction blobs (even if the database is not pruned)
| `PrunableTxBlobs` | TxId | `StorableVec<u8>` | Contains the prunable part of a transaction
| `PrunableHashes` | TxId | PrunableHash | Contains the hash of the prunable part of a transaction
| `RctOutputs` | AmountIndex | `RctOutput` | Contains RingCT outputs mapped from their global RCT index
| `TxBlobs` | TxId | `StorableVec<u8>` | Serialized transaction blobs (bytes)
| `TxIds` | TxHash | TxId | Maps a transaction's hash to its index/ID
| `TxHeights` | TxId | BlockHeight | Maps a transaction's ID to the height of the block it comes from
| `TxOutputs` | TxId | `StorableVec<u64>` | Gives the amount indices of a transaction's outputs
| `TxUnlockTime` | TxId | UnlockTime | Stores the unlock time of a transaction (only if it has a non-zero lock time)
The definitions for aliases and types (e.g. `RctOutput`) are within the [`cuprate_database::types`](https://github.com/Cuprate/cuprate/blob/31ce89412aa174fc33754f22c9a6d9ef5ddeda28/database/src/types.rs#L51) module.
<!-- TODO(Boog900): We could split this table again into `RingCT (non-miner) Outputs` and `RingCT (miner) Outputs` as for miner outputs we can store the amount instead of commitment saving 24 bytes per miner output. -->
### 9.2 Multimap tables
When referencing outputs, Monero will [use the amount and the amount index](https://github.com/monero-project/monero/blob/c8214782fb2a769c57382a999eaf099691c836e7/src/blockchain_db/lmdb/db_lmdb.cpp#L3447-L3449). This means 2 keys are needed to reach an output.
With LMDB you can set the `DUP_SORT` flag on a table and then set the key/value to:
```rust
Key = KEY_PART_1
```
```rust
Value = {
KEY_PART_2,
VALUE // The actual value we are storing.
}
```
Then you can set a custom value sorting function that only takes `KEY_PART_2` into account; this is how `monerod` does it.
This requires that the underlying database supports:
- multimap tables
- custom sort functions on values
- setting a cursor on a specific key/value
---
Another way to implement this is as follows:
```rust
Key = { KEY_PART_1, KEY_PART_2 }
```
```rust
Value = VALUE
```
Then the key type is simply used to look up the value; this is how `cuprate_database` does it.
For example, the key/value pair for outputs is:
```rust
PreRctOutputId => Output
```
where `PreRctOutputId` looks like this:
```rust
struct PreRctOutputId {
amount: u64,
amount_index: u64,
}
```
## 10. Known issues and tradeoffs
`cuprate_database` takes many tradeoffs, whether due to:
- Prioritizing certain values over others
- Not having a better solution
- Being "good enough"
This is a list of the larger ones, along with issues that don't have answers yet.
### 10.1 Traits abstracting backends
Although all database backends used are very similar, they have some crucial differences in small implementation details that must be worked around when conforming them to `cuprate_database`'s traits.
Put simply: using `cuprate_database`'s traits is less efficient and more awkward than using the backend directly.
For example:
- [Data types must be wrapped in compatibility layers when they otherwise wouldn't be](https://github.com/Cuprate/cuprate/blob/d0ac94a813e4cd8e0ed8da5e85a53b1d1ace2463/database/src/backend/heed/env.rs#L101-L116)
- [There are types that only apply to a specific backend, but are visible to all](https://github.com/Cuprate/cuprate/blob/d0ac94a813e4cd8e0ed8da5e85a53b1d1ace2463/database/src/error.rs#L86-L89)
- [There are extra layers of abstraction to smoothen the differences between all backends](https://github.com/Cuprate/cuprate/blob/d0ac94a813e4cd8e0ed8da5e85a53b1d1ace2463/database/src/env.rs#L62-L68)
- [Existing functionality of backends must be taken away, as it isn't supported in the others](https://github.com/Cuprate/cuprate/blob/d0ac94a813e4cd8e0ed8da5e85a53b1d1ace2463/database/src/database.rs#L27-L34)
This is a _tradeoff_ that `cuprate_database` takes, as:
- The backend itself is usually not the source of bottlenecks in the greater system, as such, small inefficiencies are OK
- None of the lost functionality is crucial for operation
- The ability to use, test, and swap between multiple database backends is [worth it](https://github.com/Cuprate/cuprate/pull/35#issuecomment-1952804393)
### 10.2 Hot-swappable backends
Using a different backend is really as simple as re-building `cuprate_database` with a different feature flag:
```bash
# Use LMDB.
cargo build --package cuprate-database --features heed
# Use redb.
cargo build --package cuprate-database --features redb
```
This is "good enough" for now, however ideally, this hot-swapping of backends would be able to be done at _runtime_.
As it is now, `cuprate_database` cannot compile both backends and swap based on user input at runtime; it must be compiled with a certain backend, which will produce a binary with only that backend.
This also means things like [CI testing multiple backends is awkward](https://github.com/Cuprate/cuprate/blob/main/.github/workflows/ci.yml#L132-L136), as we must re-compile with different feature flags instead.
### 10.3 Copying unaligned bytes
As mentioned in [`8. (De)serialization`](#8-deserialization), bytes are _copied_ when they are turned into a type `T` due to unaligned bytes being returned from database backends.
Using a regular reference cast results in an improperly aligned type `T`; [such a type even existing causes undefined behavior](https://doc.rust-lang.org/reference/behavior-considered-undefined.html). In our case, `bytemuck` saves us by panicking before this occurs.
Thus, when using `cuprate_database`'s database traits, an _owned_ `T` is returned.
This is doubly unfortunately for `&[u8]` as this does not even need deserialization.
For example, `StorableVec` could have been this:
```rust
enum StorableBytes<'a, T: Storable> {
Owned(T),
Ref(&'a T),
}
```
but this would require supporting types that must be copied regardless with the occasional `&[u8]` that can be returned without casting. This was hard to do so in a generic way, thus all `[u8]`'s are copied and returned as owned `StorableVec`s.
This is a _tradeoff_ `cuprate_database` takes as:
- `bytemuck::pod_read_unaligned` is cheap enough
- The main API, `service`, needs to return owned value anyway
- Having no references removes a lot of lifetime complexity
The alternative is either:
- Using proper (de)serialization instead of casting (which comes with its own costs)
- Somehow fixing the alignment issues in the backends mentioned previously
### 10.4 Endianness
`cuprate_database`'s (de)serialization and storage of bytes are native-endian, as in, byte storage order will depend on the machine it is running on.
As Cuprate's build-targets are all little-endian ([big-endian by default machines barely exist](https://en.wikipedia.org/wiki/Endianness#Hardware)), this doesn't matter much and the byte ordering can be seen as a constant.
Practically, this means `cuprated`'s database files can be transferred across computers, as can `monerod`'s.
### 10.5 Extra table data
Some of `cuprate_database`'s tables differ from `monerod`'s tables, for example, the way [`9.2 Multimap tables`](#92-multimap-tables) tables are done requires that the primary key is stored _for all_ entries, compared to `monerod` only needing to store it once.
For example:
```rust
// `monerod` only stores `amount: 1` once,
// `cuprated` stores it each time it appears.
struct PreRctOutputId { amount: 1, amount_index: 0 }
struct PreRctOutputId { amount: 1, amount_index: 1 }
```
This means `cuprated`'s database will be slightly larger than `monerod`'s.
The current method `cuprate_database` uses will be "good enough" until usage shows that it must be optimized as multimap tables are tricky to implement across all backends.