Zel RPC Framework

A type-safe RPC framework built on Iroh! Out of the box support for methods, subscriptions (server to client stream), notifications (client to server stream), and raw bidirectional streams.

Quick Start

# Cargo.toml
[dependencies]
zel_core = "0.3.2"
# Note: No need to add zel_types or zel_macros - everything is re-exported!

// Import everything with the prelude
use zel_core::prelude::*;

// Define your service
#[zel_service(name = "math")]
trait Math {
    #[method(name = "add")]
    async fn add(&self, a: i32, b: i32) -> Result<i32, ResourceError>;
}

// Implement it
#[derive(Clone)]
struct MathService;

#[async_trait]
impl MathServer for MathService {
    async fn add(&self, ctx: RequestContext, a: i32, b: i32)
        -> Result<i32, ResourceError>
    {
        Ok(a + b)
    }
}

Table of Contents

• Architecture Overview
• Iroh Integration
  • Connection Lifecycle
  • Stream Types
• RequestContext & Extensions
  • Three-Tier Extension System
  • Extension Lifecycle
  • Connection Hooks
  • Request Middleware
• Service Definition
  • Methods
  • Subscriptions
  • Notifications
  • Raw Streams
• Examples

---

Architecture Overview

Zel RPC provides four types of endpoints:

1. Methods (#[method]) - Request/response RPC calls
2. Subscriptions (#[subscription]) - Server-to-client streaming
3. Notifications (#[notification]) - Client-to-server streaming
4. Raw Streams (#[stream]) - Bidirectional custom protocols (BYOP)

All endpoints receive a RequestContext providing access to:

• The underlying Iroh connection
• Three-tier extension system (server/connection/request)
• Remote peer information

---

Iroh Integration

Connection Lifecycle

sequenceDiagram
    participant C as Client
    participant S as Server<br/>(ALPN: myapp/1)
    participant SVC as Service: users
    participant RES as Resource: create

    Note over C,S: Connection Setup
    C->>S: connect(peer_id, "myapp/1")
    S->>S: accept connection

    Note over C,RES: Request Flow
    C->>S: open_bi() stream
    S->>S: accept_bi() → spawn handler
    C->>S: Request { service: "users", resource: "create", body: {...} }
    S->>SVC: Route to service "users"
    SVC->>RES: Route to resource "create"
    RES->>RES: Handle with RequestContext
    RES->>S: Response
    S->>C: Response

    Note over C,S: Each request gets own stream

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---

RequestContext & Extensions

Zel uses a three-tier extension system to share context across different scopes:

Server Extensions (Shared)

• Database connection pools, configuration, shared caches, metrics collectors

Connection Extensions (Isolated)

• User sessions, authentication state, per-peer metrics

Request Extensions (Unique)

• Distributed trace IDs, request timing, per-call context

📚 For complete documentation, see doc_more/EXTENSIONS.MD

💡 For working examples, see examples/context_extensions_demo.rs

---

Service Definition

Methods

Request/response RPC - handler receives RequestContext as first parameter

Subscriptions

Server-to-client streaming - handler receives RequestContext and typed sink

Notifications

Client-to-server streaming - handler receives RequestContext, receive stream, and ack sender

Raw Streams

Custom bidirectional protocols - handler receives RequestContext, SendStream, RecvStream

See examples/macro_service_example.rs and examples/notification_example.rs for complete service definitions

---

P2P Resilience

Zel provides a few resilience tools out of the box:

• Per-Peer Circuit Breakers: Isolate sketchy peers (RpcServerBuilder::with_circuit_breaker(CircuitBreakerConfig::builder().failure_threshold(0.5).build())).
• Automatic Retries: Exponential backoff/jitter on client (RpcClient::builder().with_retry_config(RetryConfig::builder().max_attempts(3).build())).
• Error Classification: Infra errors (network/timeout) trip breakers; app errors (validation) don't.
• Monitoring: RequestContext::remote_id(), connection_rtt(), connection_stats().

Example:

let server = RpcServerBuilder::new(b"app/1", endpoint)
    .with_circuit_breaker(CircuitBreakerConfig::builder()
        .failure_threshold(0.5)
        .consecutive_failures(5)
        .build())
    .build();

See doc_more/P2P-RESILIENCE.md for complete details.

Advanced Iroh Integration

Beyond transport, leverage:

• IrohBundle::watch_addr(): Subscribe to EndpointAddr changes (NAT rebinding via netwatch).
• IrohBundle::notify_network_change(): Trigger addr refresh.
• IrohBundle::wait_online() / is_online(): Ensure node ready before RPCs.

P2P Quickstart Extension:

let bundle = IrohBundle::builder(None).await?.finish().await;
bundle.wait_online().await;  // Wait for holepunching/relays

Examples

See zel_core/examples/README.md for full list/table.

Quick Start: macro_service_example.rs