Module context 

Optional typed per-delivery context exposing native JetStream metadata.

A handler reads native JetStream delivery metadata - the stream and consumer names, the stream and consumer sequence numbers, the server-side redelivery count, and the pending count - by declaring JetStreamContext as its per-delivery context and reading fields with the compile-time keys. The runtime builds the context once per delivery via BuildContext; resolving a key is a direct field read, with no hashing, boxing, or downcasting.

This is purely additive: the default context is () (no fields), so existing handlers are unaffected. Opting in means changing the handler’s context type to JetStreamContext.

These fields are genuinely native: they come from the JetStream $JS.ACK reply subject, not from the payload or the message Headers, so they are not reachable any other way. Core (non-JetStream) NATS deliveries carry no such metadata - their only native datum is the reply inbox, already surfaced as the reply-to header - so Core handlers should keep the default () context. A handler bound to JetStreamContext still works on a Core subscription; every key just reads None there.

Examples

use ruststream::IncomingMessage;
use ruststream::runtime::{Context, HandlerResult};
use ruststream_nats::context::{JetStreamContext, keys};

async fn handle<M: IncomingMessage>(
    _msg: &M,
    ctx: &mut Context<'_, JetStreamContext>,
) -> HandlerResult {
    // `None` on a core delivery; the stream sequence on a JetStream one.
    if let Some(seq) = ctx.context(keys::STREAM_SEQUENCE) {
        println!("stream sequence {seq}");
    }
    // The server-side delivery count distinguishes a first delivery from a redelivery.
    if ctx.context(keys::DELIVERED).is_some_and(|n| n > 1) {
        println!("redelivery");
    }
    HandlerResult::Ack
}

Modules

keys

Compile-time Field keys, one per native JetStream metadatum.

Structs

JetStreamContext

Native JetStream per-delivery metadata, built once per delivery from the broker message.