# Database

## Abstractions

* We created a [Database trait abstraction](https://github.com/paradigmxyz/reth/blob/0d9b9a392d4196793736522f3fc2ac804991b45d/crates/interfaces/src/db/mod.rs) using Rust Stable GATs which frees us from being bound to a single database implementation. We currently use MDBX, but are exploring [redb](https://github.com/cberner/redb) as an alternative.
* We then iterated on [`Transaction`](https://github.com/paradigmxyz/reth/blob/0d9b9a392d4196793736522f3fc2ac804991b45d/crates/stages/src/db.rs#L14-L19) as a non-leaky abstraction with helpers for strictly-typed and unit-tested higher-level database abstractions.

## Codecs

* We want Reth's serialized format to be able to trade off read/write speed for size, depending on who the user is.
* To achieve that, we created the [Encode/Decode/Compress/Decompress trais](https://github.com/paradigmxyz/reth/blob/0d9b9a392d4196793736522f3fc2ac804991b45d/crates/interfaces/src/db/table.rs#L9-L36) to make the (de)serialization of database `Table::Key` and `Table::Values` generic.
    * This allows for [out-of-the-box benchmarking](https://github.com/paradigmxyz/reth/blob/0d9b9a392d4196793736522f3fc2ac804991b45d/crates/db/benches/encoding_iai.rs#L5) (using [Criterion](https://github.com/bheisler/criterion.rs) and [Iai](https://github.com/bheisler/iai))
    * It also enables [out-of-the-box fuzzing](https://github.com/paradigmxyz/reth/blob/0d9b9a392d4196793736522f3fc2ac804991b45d/crates/interfaces/src/db/codecs/fuzz/mod.rs) using [trailofbits/test-fuzz](https://github.com/trailofbits/test-fuzz).
* We implemented that trait for the following encoding formats:
    * [Ethereum-specific Compact Encoding](https://github.com/paradigmxyz/reth/blob/0d9b9a392d4196793736522f3fc2ac804991b45d/crates/codecs/derive/src/compact/mod.rs): A lot of Ethereum datatypes have unnecessary zeros when serialized, or optional (e.g. on empty hashes) which would be nice not to pay in storage costs. 
        * [Erigon](https://github.com/ledgerwatch/erigon/blob/12ee33a492f5d240458822d052820d9998653a63/docs/programmers_guide/db_walkthrough.MD) achieves that by having a `bitfield` set on  Table "PlainState which adds a bitfield to Accounts.
        * Akula expanded it for other tables and datatypes manually. It also saved some more space by storing the length of certain types (U256, u64) using the modular_bitfield crate, which compacts this information.
        * We generalized it for all types, by writing a derive macro that autogenerates code for implementing the trait. It, also generates the interfaces required for fuzzing using ToB/test-fuzz:
    * [Scale Encoding](https://github.com/paritytech/parity-scale-codec)
    * [Postcard Encoding](https://github.com/jamesmunns/postcard)
    * Passthrough (called `no_codec` in the codebase)
* We made implementation of these traits easy via a derive macro called [`main_codec`](https://github.com/paradigmxyz/reth/blob/0d9b9a392d4196793736522f3fc2ac804991b45d/crates/codecs/derive/src/lib.rs#L15) that delegates to one of Compact (default), Scale, Postcard or Passthrough encoding. This is [derived on every struct we need](https://github.com/search?q=repo%3Aparadigmxyz%2Freth%20%22%23%5Bmain_codec%5D%22&type=code), and lets us experiment with different encoding formats without having to modify the entire codebase each time.