subscriptions

subscriptions

One producing session, three Observer clients, three different sinks. None of them ever issue a command.

Before ARCP

Most teams sidecar the agent with a tee: agent emits to stdout, a shipper tails the log, a second tail re-parses for metrics, a third process writes to SQLite for replay. Three pipelines diverge over time, none know about each other, and adding a fourth consumer means another sidecar.

With ARCP

let client: Client = todo!();              // observer client
let sub_id = subscribe(&client, target, Some(&["metric"])).await?;
// for await env in client.events() {
//     if let Some(inner) = unwrap_event(&env) { sink.handle(inner).await }
// }

Three observers. One transport each. Filters declared inline. The producing session never knows they exist.

ARCP primitives

• Subscriptions, filters, Observer role — RFC §13, §5.
• since.after_message_id backfill + the synthetic subscription.backfill_complete marker — §13.3.
• Standard metrics + trace spans — §17.
• Stream-kind filtering for kind: thought redaction — §11.4.

File tour

• main.rs — boots three clients in parallel via tokio::join!.
• sinks.rs — StdoutSink, SqliteSink (uses arcp::store::eventlog), OtlpSink (metric / trace.span exporter).

Variations

• Replace SQLite with ClickHouse for fleet-wide replay.
• Tee stdout into Slack via a min_priority: critical filter.
• A fourth subscriber on kind: thought only, gated by stricter access control.