Crate eventfold 

eventfold

Your application state is a fold over an event log.

eventfold is a lightweight, append-only event log with derived views for Rust. You define events as JSON, write pure reducer functions to fold them into state, and let the library handle persistence, snapshots, and log rotation. No database, no infrastructure — just files in a directory.

Quick Start

use eventfold::{Event, EventLog};
use serde::{Serialize, Deserialize};
use serde_json::json;

#[derive(Default, Clone, Serialize, Deserialize)]
struct Counter { count: u64 }

fn count_reducer(mut state: Counter, _event: &Event) -> Counter {
    state.count += 1;
    state
}

let mut log = EventLog::builder(dir.path())
    .view::<Counter>("counter", count_reducer)
    .open()?;

log.append(&Event::new("click", json!({"x": 10})))?;
log.refresh_all()?;

let state: &Counter = log.view("counter")?;
assert_eq!(state.count, 1);

Core Concepts

• Events are immutable JSON records appended to a log file (app.jsonl).
• Reducers are pure functions fn(State, &Event) -> State that fold events into application state.
• Views are derived state computed by applying a reducer to the event log, with snapshots on disk for incremental performance.

See docs/guide.md for a detailed concepts guide.

Re-exports

pub use snapshot::Snapshot;

Modules

snapshot

Snapshot persistence for derived view state.

Structs

AppendConflict

Conflict details when a conditional append fails.

AppendResult

Result of a successful append operation.

Event

An immutable event record stored in the log.

EventLog

An append-only event log backed by files in a single directory.

EventLogBuilder

Builder for configuring and opening an EventLog.

EventReader

Cheap, cloneable reader for an event log.

EventWriter

Exclusive writer for a single event log file.

View

A derived view over an event log.

Enums

ConditionalAppendError

Error type for conditional append operations.

LockMode

Controls file locking behavior for an EventWriter.

WaitResult

Result of waiting for new events.

Traits

ViewOps

Trait for type-erased view operations during log rotation.

Functions

line_hash

Compute xxh64 hash of raw line bytes (without trailing newline), hex-encoded.

Type Aliases

ReduceFn

A pure function that folds an event into state.