⚡ JAEB — Just Another Event Bus

A lightweight, in-process event bus for Tokio — snapshot-driven dispatch with retry, dead-letter, and middleware support.

✨ Highlights

• 🔀 Sync + Async handlers behind one subscribe API
• 🔁 Retry & Dead Letters with per-listener policies
• 🧩 Typed & Global Middleware pipeline
• 📊 Optional Metrics (Prometheus-compatible via metrics crate)
• 🔍 Built-in Tracing support (trace feature)
• 🛑 Graceful Shutdown with async drain + timeout
• 🏗️ summer-rs Integration for plugin-based auto-registration

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. If you need durable messaging, consider pairing JAEB with an external queue for outbox-style patterns.

Installation

[dependencies]
jaeb = "0.5.0"
tokio = { version = "1", features = ["macros", "rt-multi-thread"] }

With metrics instrumentation:

[dependencies]
jaeb = { version = "0.5.0", features = ["metrics"] }

With tracing:

[dependencies]
jaeb = { version = "0.5.0", features = ["trace"] }

With standalone handler macros:

[dependencies]
jaeb = { version = "0.5.0", features = ["macros"] }

⚡ Quick Start

Full example with sync/async handlers, retry policies, and dead-letter handling:

use std::time::Duration;

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

struct OrderCheckoutEvent {
    order_id: i64,
}

struct AsyncCheckoutHandler;

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

struct SyncAuditHandler;

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

struct DeadLetterLogger;

impl SyncEventHandler<DeadLetter> for DeadLetterLogger {
    fn handle(&self, dl: &DeadLetter, _bus: &EventBus) -> 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::builder().build().await?;

    let retry_policy = AsyncSubscriptionPolicy::default()
        .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?;

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

Detailed usage patterns (builder descriptors, direct subscribe*, manual descriptors, Deps/Dep<T>, and trade-offs) are documented in USAGE.md.

Architecture

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

graph LR
    P[publish] --> S[Load Snapshot]
    S --> GM[Global Middleware]
    GM --> TM[Typed Middleware]
    TM --> AL[Async Lane - enqueued]
    TM --> SL[Sync Lane - serialized FIFO]

Loading

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

API Highlights

• EventBus::builder() for timeouts, concurrency limit, and default subscription policies
• default_subscription_policies(SubscriptionDefaults) sets fallback policies for subscribe
• subscribe_with_policy(handler, policy) accepts:
  • AsyncSubscriptionPolicy for async handlers
  • SyncSubscriptionPolicy for sync handlers and once handlers
• publish waits for sync listeners and async-listener task spawning
• max_concurrent_async(n) caps async handler execution across the whole bus, not per event type; async listeners are scheduled in registration order within a publish, then run asynchronously as tracked tasks

Core policy types:

• AsyncSubscriptionPolicy { max_retries, retry_strategy, dead_letter }
• SyncSubscriptionPolicy { priority, dead_letter }
• SubscriptionDefaults { policy, sync_policy }

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/concurrency-limit - max concurrent async handlers
• examples/graceful-shutdown - controlled shutdown and draining
• examples/introspection - EventBus::stats() output
• examples/fire-once - one-shot / fire-once handler
• examples/panic-safety - panic handling behavior in handlers
• examples/subscription-lifecycle - subscribe/unsubscribe lifecycle
• examples/axum-integration - axum REST app publishing domain events
• examples/axum-integration-macros - axum REST app using #[handler] + Dep<T> DI
• examples/macro-handlers - standalone #[handler] + #[dead_letter_handler] with builder
• examples/jaeb-demo - full demo with tracing + metrics exporter
• examples/summer-jaeb-demo - summer-rs plugin + #[event_listener]
• examples/observability-stack - Grafana + Prometheus + Loki + Tempo demo
• examples/jaeb-visualizer - TUI visualizer for event bus activity

Run an example:

cargo run -p axum-integration

Feature Flags

Flag	Default	Description
macros	off	Re-exports #[handler] and #[dead_letter_handler]
metrics	off	Enables Prometheus-compatible instrumentation via metrics
trace	off	Enables tracing spans and events for dispatch diagnostics
test-utils	off	Exposes TestBus helpers for integration tests

When metrics is enabled, JAEB records:

• eventbus.publish (counter, labels: event)
• eventbus.handler.duration (histogram, labels: event, handler)
• eventbus.handler.error (counter, labels: event, listener)
• eventbus.dead_letter (counter, labels: event, handler) — fires when a dead letter is created

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 (sync listeners only)
• name

use jaeb::{DeadLetter, EventBus, HandlerResult};
use summer::{App, AppBuilder, async_trait};
use summer::extractor::Component;
use summer::plugin::{MutableComponentRegistry, Plugin};
use summer_jaeb::{SummerJaeb, event_listener};

#[derive(Debug)]
struct OrderPlacedEvent {
    order_id: u32,
}

/// A dummy database pool registered as a summer Component via a plugin.
#[derive(Clone, Debug)]
struct DbPool;

impl DbPool {
    fn log_order(&self, order_id: u32) {
        println!("DbPool: persisted order {order_id}");
    }
}

struct DbPoolPlugin;

#[async_trait]
impl Plugin for DbPoolPlugin {
    async fn build(&self, app: &mut AppBuilder) {
        app.add_component(DbPool);
    }
    fn name(&self) -> &str { "DbPoolPlugin" }
}

/// Async listener — `DbPool` is injected automatically from summer's DI container.
#[event_listener(retries = 2, retry_strategy = "fixed", retry_base_ms = 500, dead_letter = true)]
async fn on_order_placed(event: &OrderPlacedEvent, Component(db): Component<DbPool>) -> HandlerResult {
    db.log_order(event.order_id);
    Ok(())
}

/// Sync dead-letter listener — auto-detected from the `DeadLetter` event type.
#[event_listener(name = "dead_letter")]
fn on_dead_letter(event: &DeadLetter) -> HandlerResult {
    eprintln!("dead letter: event={}, attempts={}", event.event_name, event.attempts);
    Ok(())
}

#[tokio::main]
async fn main() {
    App::new()
        .add_plugin(DbPoolPlugin)
        .add_plugin(SummerJaeb::new().with_dependency("DbPoolPlugin"))
        .run()
        .await;
}

All #[event_listener] functions are auto-discovered via inventory and subscribed during plugin startup — no manual registration needed.

Standalone Macros

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

#[handler] generates a struct named <FunctionName>Handler that implements HandlerDescriptor. Register it via EventBusBuilder::handler. Policy attributes are supported:

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

#[dead_letter_handler] generates a struct that implements DeadLetterDescriptor. The function must be synchronous and accept &DeadLetter. Register it via EventBusBuilder::dead_letter.

Dep<T> parameters are supported for both macros and are resolved from EventBusBuilder::deps(...) at build time. Supported forms:

• Dep(name): Dep<T>
• name: Dep<T>

use std::time::Duration;
use jaeb::{DeadLetter, EventBus, HandlerResult, dead_letter_handler, handler};

#[derive(Debug)]
struct Payment {
    id: u32,
}

#[handler(retries = 2, retry_strategy = "fixed", retry_base_ms = 50, dead_letter = true, name = "payment-processor")]
async fn process_payment(event: &Payment) -> HandlerResult {
    println!("processing payment {}", event.id);
    Ok(())
}

#[dead_letter_handler]
fn on_dead_letter(event: &DeadLetter) -> HandlerResult {
    println!(
        "dead-letter: event={}, handler={:?}, attempts={}, error={}",
        event.event_name, event.handler_name, event.attempts, event.error
    );
    Ok(())
}

#[tokio::main]
async fn main() -> Result<(), jaeb::EventBusError> {
    let bus = EventBus::builder()
        
        .handler(process_payment)
        .dead_letter(on_dead_letter)
        .build()
        .await?;
    bus.publish(Payment { id: 7 }).await?;
    tokio::time::sleep(Duration::from_millis(300)).await;
    bus.shutdown().await
}

Dep<T> example:

use std::sync::Arc;
use jaeb::{Dep, Deps, EventBus, HandlerResult, handler};

struct AuditLog;

struct Payment;

#[handler]
async fn process_with_dep(_event: &Payment, Dep(log): Dep<Arc<AuditLog>>) -> HandlerResult {
    let _ = log;
    Ok(())
}

#[handler]
fn process_with_wrapper(_event: &Payment, log: Dep<Arc<AuditLog>>) -> HandlerResult {
    let _inner: Arc<AuditLog> = log.0;
    Ok(())
}

#[tokio::main]
async fn main() -> Result<(), jaeb::EventBusError> {
    let log = Arc::new(AuditLog);
    let bus = EventBus::builder()
        .handler(process_with_dep)
        .handler(process_with_wrapper)
        .deps(Deps::new().insert(log))
        .build()
        .await?;
    bus.shutdown().await
}

Notes

• JAEB requires a running Tokio runtime.
• Events published through JAEB must be Send + Sync + 'static.
• 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.