Cap jobs running at once

Say exports are heavy, and you want each customer workspace to run at most 3 at once. The 4th should wait or be refused, and a crashed export mustn’t keep its slot forever.

1. Create the limit

curl -X POST https://api.limitry.com/v1/limits \
  -H "Authorization: Bearer $LIMITRY_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"name":"Exports at once","subjectKind":"workspace","action":"export",
       "amount":3,"window":"concurrent"}'

2. Take a slot for the job

hold takes a slot before the job starts and gives it back when the job ends, whether it succeeds or throws:

import { LimitExceeded } from "@limitry/sdk";

try {
  await limits.hold(
    {
      subject: { kind: "workspace", id: workspaceId },
      action: "export",
      ttlSeconds: 600,
    },
    async (held) => {
      for (const chunk of chunks) {
        await exportChunk(chunk);
        await held.extend(); // still running: keep the slot another 600 s
      }
    },
  );
} catch (err) {
  if (err instanceof LimitExceeded) {
    // All 3 slots are taken. Try again in err.retryAfterSeconds.
    return requeue(job, err.retryAfterSeconds);
  }
  throw err;
}

3. If the worker dies

A slot is a reservation, and it lasts ttlSeconds. A job that’s still running extends it as it goes. A worker that crashes stops extending, and the slot frees itself within ttlSeconds. Pick a ttlSeconds comfortably longer than the gap between your extend calls.

Combine with a daily limit

A concurrency limit doesn’t count use. To also cap exports per day, add a second limit on the same action with the window day. The hold above then has to fit both.