JAEB - Just Another Event Bus

In-process, snapshot-driven event bus for Tokio applications.

JAEB focuses on correctness and observability for monolith-style event-driven Rust services:

• sync + async handlers behind one subscribe API
• compile-time policy validation (retry policies cannot be used with sync handlers)
• listener priority with FIFO stability for equal priorities
• typed and global middleware
• dead-letter stream with recursion guard
• graceful shutdown with in-flight async drain
• optional metrics (metrics feature) and built-in tracing
• optional standalone macros (macros feature): #[handler] and register_handlers!
• summer-rs integration via summer-jaeb and #[event_listener] macro support summer-jaeb-macros

When to use JAEB

Use JAEB when you need:

• domain events inside one process (e.g. OrderCreated -> projections, notifications, audit)
• decoupled modules with type-safe fan-out
• retry/dead-letter behavior per listener
• deterministic sync-lane ordering with priority hints

JAEB is not a message broker. It does not provide persistence, replay, or cross-process delivery.

Installation

[dependencies]
jaeb = "0.3.6"
tokio = { version = "1", features = ["macros", "rt-multi-thread"] }

With metrics instrumentation:

[dependencies]
jaeb = { version = "0.3.6", features = ["metrics"] }

With standalone handler macros:

[dependencies]
jaeb = { version = "0.3.6", features = ["macros"] }

Quick Start

use std::time::Duration;

use jaeb::{
    DeadLetter, EventBus, EventBusError, EventHandler, HandlerResult, RetryStrategy, SubscriptionPolicy, SyncEventHandler,
};

#[derive(Clone)]
struct OrderCheckoutEvent {
    order_id: i64,
}

struct AsyncCheckoutHandler;

impl EventHandler<OrderCheckoutEvent> for AsyncCheckoutHandler {
    async fn handle(&self, event: &OrderCheckoutEvent) -> HandlerResult {
        println!("async checkout {}", event.order_id);
        Ok(())
    }
}

struct SyncAuditHandler;

impl SyncEventHandler<OrderCheckoutEvent> for SyncAuditHandler {
    fn handle(&self, event: &OrderCheckoutEvent) -> HandlerResult {
        println!("sync audit {}", event.order_id);
        Ok(())
    }
}

struct DeadLetterLogger;

impl SyncEventHandler<DeadLetter> for DeadLetterLogger {
    fn handle(&self, dl: &DeadLetter) -> HandlerResult {
        eprintln!(
            "dead-letter: event={} listener={} attempts={} error={}",
            dl.event_name, dl.subscription_id, dl.attempts, dl.error
        );
        Ok(())
    }
}

#[tokio::main]
async fn main() -> Result<(), EventBusError> {
    let bus = EventBus::new(64)?;

    let retry_policy = SubscriptionPolicy::default()
        .with_priority(10)
        .with_max_retries(2)
        .with_retry_strategy(RetryStrategy::Fixed(Duration::from_millis(50)));

    let checkout_sub = bus
        .subscribe_with_policy::<OrderCheckoutEvent, _, _>(AsyncCheckoutHandler, retry_policy)
        .await?;

    let _audit_sub = bus.subscribe::<OrderCheckoutEvent, _, _>(SyncAuditHandler).await?;
    let _dl_sub = bus.subscribe_dead_letters(DeadLetterLogger).await?;

    bus.publish(OrderCheckoutEvent { order_id: 42 }).await?;
    bus.try_publish(OrderCheckoutEvent { order_id: 43 })?;

    checkout_sub.unsubscribe().await?;
    bus.shutdown().await?;
    Ok(())
}

Architecture

JAEB uses an immutable snapshot registry (ArcSwap) for hot-path reads:

publish(event)
  -> load snapshot (lock-free)
  -> global middleware
  -> typed middleware
  -> async lane (spawned)
  -> sync lane (serialized FIFO, priority-ordered)

• async and sync listeners are separated per event type
• priority is applied per lane (higher first)
• equal priority preserves registration order

API Highlights

• EventBus::builder() for buffer size, timeouts, concurrency limit, and default policy
• default_subscription_policy(SubscriptionPolicy) sets fallback policy for subscribe
• subscribe_with_policy(handler, policy) accepts:
  • SubscriptionPolicy for async handlers
  • SyncSubscriptionPolicy for sync handlers and once handlers
• publish waits for sync listeners and task-spawn for async listeners
• try_publish is non-blocking and returns EventBusError::ChannelFull on saturation

Core policy types:

• SubscriptionPolicy { priority, max_retries, retry_strategy, dead_letter }
• SyncSubscriptionPolicy { priority, dead_letter }
• IntoSubscriptionPolicy<M> sealed trait for compile-time mode/policy safety

Backward-compatible aliases remain available (deprecated):

• FailurePolicy -> SubscriptionPolicy
• NoRetryPolicy -> SyncSubscriptionPolicy
• IntoFailurePolicy -> IntoSubscriptionPolicy

Examples

• examples/basic-pubsub - minimal publish/subscribe
• examples/sync-handler - sync dispatch lane behavior
• examples/closure-handlers - closure-based handlers
• examples/retry-strategies - fixed/exponential/jitter retry configuration
• examples/dead-letters - dead-letter subscription and inspection
• examples/middleware - global and typed middleware
• examples/backpressure - try_publish saturation behavior
• examples/concurrency-limit - max concurrent async handlers
• examples/graceful-shutdown - controlled shutdown and draining
• examples/introspection - EventBus::stats() output
• examples/axum-integration - axum REST app publishing domain events
• examples/macro-handlers - standalone #[handler] + register_handlers!
• examples/macro-handlers-auto - standalone #[handler] auto-discovery with register_handlers!(bus)
• examples/jaeb-demo - full demo with tracing + metrics exporter
• examples/summer-jaeb-demo - summer-rs plugin + #[event_listener]

Run an example:

cargo run -p axum-integration

Feature Flags

When metrics is enabled, JAEB records:

• eventbus.publish (counter, per event type)
• eventbus.handler.duration (histogram, per event type)
• eventbus.handler.error (counter, per event type)
• eventbus.handler.join_error (counter, per event type)

summer-rs Integration

Use summer-jaeb and summer-jaeb-macros for plugin-based auto-registration via #[event_listener].

Macro support includes:

• retries
• retry_strategy
• retry_base_ms
• retry_max_ms
• dead_letter
• priority
• name

Standalone Macros

Enable the macros feature to use #[handler] and register_handlers! without summer-rs.

The #[handler] macro generates a struct named <FunctionName>Handler and an async register(&EventBus) method. Policy attributes are supported:

• retries
• retry_strategy
• retry_base_ms
• retry_max_ms
• dead_letter
• priority
• name

Notes

• JAEB requires a running Tokio runtime.
• Events must be Send + Sync + 'static; async handlers also require Clone.
• The crate enforces #![forbid(unsafe_code)].

License

jaeb is distributed under the MIT License.

Copyright (c) 2025-2026 Linke Thomas

This project uses third-party libraries. See THIRD-PARTY-LICENSES for dependency and license details.