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 of them know about each other, and adding a fourth consumer means another sidecar.

With ARCP

$client = elided();                      // observer client
$client->open(...);                      // subscriptions: true, nothing else
$sub = $client->subscribe(
    ['session_id' => [$target], 'types' => ['metric']],
    static fn (Envelope $env) => $sink->handle($env),
);

Three observers. One transport each. Filters declared inline. The agent 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.php — boots three observer attachments.
• sinks/StdoutSink.php — log summarizer.
• sinks/SqliteSink.php — Arcp\Store\eventlog schema.
• sinks/OtlpSink.php — metric and trace.span → OTLP.

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.