Firewood: Compaction-Less Database Optimized for Efficiently Storing Recent Merkleized Blockchain State

:warning: Firewood is alpha-level software and is not ready for production use. The Firewood API and on-disk state representation may change with little to no warning.

Firewood is an embedded key-value store, optimized to store recent Merkleized blockchain state with minimal overhead. Firewood is implemented from the ground up to directly store trie nodes on-disk. Unlike most state management approaches in the field, it is not built on top of a generic KV store such as LevelDB/RocksDB. Firewood, like a B+-tree based database, directly uses the trie structure as the index on-disk. Thus, there is no additional “emulation” of the logical trie to flatten out the data structure to feed into the underlying database that is unaware of the data being stored. The convenient byproduct of this approach is that iteration is still fast (for serving state sync queries) but compaction is not required to maintain the index. Firewood was first conceived to provide a very fast storage layer for the EVM but could be used on any blockchain that requires an authenticated state.

Firewood only attempts to store recent revisions on-disk and will actively clean up unused data when revisions expire. Firewood keeps some configurable number of previous states in memory and on disk to power state sync (which may occur at a few roots behind the current state). To do this, a new root is always created for each revision that can reference either new nodes from this revision or nodes from a prior revision. When creating a revision, a list of nodes that are no longer needed are computed and saved to disk in a future-delete log (FDL) as well as kept in memory. When a revision expires, the nodes that were deleted when it was created are returned to the free space.

Firewood guarantees recoverability by not referencing the new nodes in a new revision before they are flushed to disk, as well as carefully managing the free list during the creation and expiration of revisions.

Architecture Diagram

Terminology

• Revision - A historical point-in-time state/version of the trie. This represents the entire trie, including all Key/Values at that point in time, and all Nodes.
• View - This is the interface to read from a Revision or a Proposal.
• Node - A node is a portion of a trie. A trie consists of nodes that are linked together. Nodes can point to other nodes and/or contain Key/Value pairs.
• Hash - In this context, this refers to the merkle hash for a specific node.
• Root Hash - The hash of the root node for a specific revision.
• Key - Represents an individual byte array used to index into a trie. A Key usually has a specific Value.
• Value - Represents a byte array for the value of a specific Key. Values can contain 0-N bytes. In particular, a zero-length Value is valid.
• Key Proof - A proof that a Key exists within a specific revision of a trie. This includes the hash for the node containing the Key as well as all parents.
• Range Proof - A proof that consists of two Key Proofs, one for the start of the range, and one for the end of the range, as well as a list of all Key/Value pairs in between the two. A Range Proof can be validated independently of an actual database by constructing a trie from the Key/Values provided.
• Change Proof - A proof that consists of a set of all changes between two revisions.
• Put - An operation for a Key/Value pair. A put means "create if it doesn't exist, or update it if it does. A put operation is how you add a Value for a specific Key.
• Delete - An operation indicating that a Key should be removed from the trie.
• Batch Operation - An operation of either Put or Delete.
• Batch - An ordered set of Batch Operations.
• Proposal - A proposal consists of a base Root Hash and a Batch, but is not yet committed to the trie. In Firewood's most recent API, a Proposal is required to Commit.
• Commit - The operation of applying one or more Proposals to the most recent Revision.

Roadmap

• [ ] Complete the proof code
• [ ] Complete the revision manager
• [ ] Complete the API implementation
• [ ] Implement a node cache
• [ ] Hook up the RPC

Run

There are several examples, in the examples directory, that simulate real world use-cases. Try running them via the command-line, via cargo run --release --example simple.

Logging

If you want logging, enable the logging feature flag, and then set RUST_LOG accordingly. See the documentation for env_logger for specifics. We currently have very few logging statements, but this is useful for print-style debugging.

Release

See the release documentation for detailed information on how to release Firewood.

CLI

Firewood comes with a CLI tool called fwdctl that enables one to create and interact with a local instance of a Firewood database. For more information, see the fwdctl README.

Test

cargo test --release

License

Firewood is licensed by the Ecosystem License. For more information, see the LICENSE file.