chunked_wal/api/
state_machine.rs

1//! State machine abstraction for applying records in a deterministic order.
2
3use std::fmt::Debug;
4
5use crate::ChunkId;
6use crate::WALRecord;
7use crate::WalTypes;
8use crate::types::Segment;
9
10/// A trait representing a state machine of [`WAL`] that can apply records to
11/// modify its state.
12///
13/// The Raft-log follows a Write-Ahead Log (WAL) + State Machine pattern. This
14/// trait defines the state machine component that processes records persisted
15/// in the WAL to build and maintain application state.
16///
17/// # Type Parameters
18/// * `W` - The WAL type set that defines records and checkpoints
19///
20/// [`WAL`]: crate::api::wal::WAL
21pub trait StateMachine<W>
22where W: WalTypes
23{
24    /// The type of error that can occur during record application
25    type Error: std::error::Error + Debug + 'static;
26
27    /// Applies a record that is already persisted in the WAL to the state
28    /// machine, potentially modifying its state.
29    ///
30    /// # Arguments
31    /// * `record` - The record to apply.
32    /// * `chunk_id` - The identifier of the chunk containing this record.
33    /// * `global_segment` - The global offset and size of the record in the log
34    ///   file.
35    fn apply(
36        &mut self,
37        record: &WALRecord<W>,
38        chunk_id: ChunkId,
39        global_segment: Segment,
40    ) -> Result<(), Self::Error>;
41
42    /// Returns the current checkpoint value for WAL storage.
43    ///
44    /// The WAL stores this value at the beginning of each new chunk. Keep it
45    /// small because it may be duplicated across chunks. The framework only
46    /// persists the value; the semantic content belongs to the state-machine
47    /// implementation.
48    fn checkpoint(&self) -> W::Checkpoint;
49}
chunked_wal/api/state_machine.rs

chunked_wal/api/
state_machine.rs
