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OpenZeppelin Relayer Pub/Sub Queue Backend Example

This example runs the relayer with QUEUE_BACKEND=pubsub in two modes:

  • Local (default): the GCP Pub/Sub emulator with automatic topic + subscription provisioning at startup — no real GCP project required. See Getting Started.
  • Real GCP: your own project's topics/subscriptions with Application Default Credentials, via docker-compose.gcp.yaml. See Running against real GCP.

Both modes use Redis for the repository, distributed locks, and deferred-job scheduling.

Getting Started

Prerequisites

Step 1: Clone the repository

git clone https://github.com/OpenZeppelin/openzeppelin-relayer
cd openzeppelin-relayer

Step 2: Create a signer

cargo run --example create_key -- \
  --password <DEFINE_YOUR_PASSWORD> \
  --output-dir examples/gcp-pubsub-queue-storage/config/keys \
  --filename local-signer.json

Create .env:

cp examples/gcp-pubsub-queue-storage/.env.example examples/gcp-pubsub-queue-storage/.env

Then set:

  • KEYSTORE_PASSPHRASE
  • API_KEY
  • WEBHOOK_SIGNING_KEY

Step 3: Configure notification webhook

Edit examples/gcp-pubsub-queue-storage/config/config.json and set notifications[0].url to your webhook endpoint (for example from Webhook.site).

Step 4: Start services

docker compose -f examples/gcp-pubsub-queue-storage/docker-compose.yaml up

Services started:

  • relayer — QUEUE_BACKEND=pubsub, PUBSUB_PROJECT_ID=test-project, PUBSUB_EMULATOR_HOST=pubsub-emulator:8085, DISTRIBUTED_MODE=false
  • redis — repository, distributed locks, and the deferred-job scheduled sets
  • pubsub-emulator — gcloud beta emulators pubsub start
  • pubsub-init — creates all 8 topics + 8 subscriptions, then exits

pubsub-init runs to completion before the relayer starts, so all required resources exist when the backend probes them at startup.

Step 5: Verify relayer

curl -X GET http://localhost:8080/api/v1/relayers \
  -H "Content-Type: application/json" \
  -H "AUTHORIZATION: Bearer YOUR_API_KEY"

Step 6: Submit a transaction

Submit a transaction via the API and watch the logs: you'll see the per-subscription pubsub workers pick it up (publish → pull → handler) and the status-check re-checks run until the transaction reaches a final state.

Topics + subscriptions created by this example

For the default prefix relayer, pubsub-init creates these 8 topic/subscription pairs:

Topic Subscription
relayer-transaction-request relayer-transaction-request-sub
relayer-transaction-submission relayer-transaction-submission-sub
relayer-status-check relayer-status-check-sub
relayer-status-check-evm relayer-status-check-evm-sub
relayer-status-check-stellar relayer-status-check-stellar-sub
relayer-notification relayer-notification-sub
relayer-token-swap-request relayer-token-swap-request-sub
relayer-relayer-health-check relayer-relayer-health-check-sub

No dead-letter topics are required (see below).

How scheduling, retries, and exhaustion work

Pub/Sub has no native delayed delivery, so deferred jobs and retry backoff are held in a Redis sorted set and published when due (the apalis store-and-run-when-due pattern). The topic therefore only ever carries already-due jobs.

  • A failed bounded job is re-enqueued with an incremented retry_attempt; once its budget (max_retries) is exhausted it stops retrying, and the transaction's terminal state in the repository is the durable record — the relayer does not publish to a dead-letter topic (matching the Redis/SQS backends). Handler panics and lease timeouts count as failed attempts too, so a consistently-failing bounded job stops at max_retries rather than looping.
  • Status-check queues are unbounded: they re-run (with increasing backoff) until the transaction reaches a final state.

A handler that runs longer than a subscription's default ack deadline is not double-processed — the worker extends the lease to 600s (Pub/Sub's max) before processing and bounds the handler to it. If that extension can't be secured (after a few retries), the message is released for redelivery rather than run under a too-short lease, so it is never executed concurrently. Ctrl-C triggers a graceful shutdown that drains in-flight work without acking incomplete jobs.

Backlog depth under the emulator

Backlog depth is read from Cloud Monitoring, which the emulator does not serve, so depth counts are reported as unavailable locally (not 0) — the health endpoint's reachability still works. Against real GCP, depth populates from subscription/num_undelivered_messages.

Running against real GCP

The same backend runs against a real GCP project using docker-compose.gcp.yaml (the emulator + auto-provisioning are replaced by your real topics/subscriptions

  • ADC). Complete the signer, .env, and webhook setup from Getting Started (Steps 1–3), then:

Step A: Provision topics + subscriptions

The relayer never creates resources — it probes them at startup and fails fast if any are missing. Create them with the idempotent helper (or your own Terraform):

PUBSUB_PROJECT_ID=my-project ./examples/gcp-pubsub-queue-storage/scripts/provision-gcp.sh

It uses an authenticated gcloud (run gcloud auth login first) and prints the IAM bindings to apply — unlike init-pubsub.sh, which targets the emulator.

Step B: Provide credentials (ADC)

The relayer's principal needs roles/pubsub.publisher + roles/pubsub.subscriber (or roles/pubsub.editor) and roles/monitoring.viewer (for the backlog-depth read). Provide Application Default Credentials either way:

  • Service-account key (default): place the key JSON at examples/gcp-pubsub-queue-storage/config/keys/gcp-sa-key.json (gitignored, mounted read-only). The relayer fails to start if it is missing.

  • Your gcloud ADC: set GCP_CREDENTIALS_FILE in .env to its path:

    GCP_CREDENTIALS_FILE="$HOME/.config/gcloud/application_default_credentials.json"

In production (GKE/Cloud Run), prefer Workload Identity or the GCE metadata server over a key file.

Step C: Set the project and start

Set PUBSUB_PROJECT_ID in .env, then:

docker compose -f examples/gcp-pubsub-queue-storage/docker-compose.gcp.yaml up

Against real GCP the startup log shows depth_read=true: the backend reads backlog depth from Cloud Monitoring (subscription/num_undelivered_messages, ~1–3 min sampling lag), whereas under the emulator that read is unavailable. The depth is exposed per queue as the queue_depth Prometheus gauge (labels backend, queue_type) — metrics are off by default in this compose, so set METRICS_ENABLED=true and publish :8081 to scrape it. /api/v1/ready reports aggregate queue health; /api/v1/health is a plain liveness probe.

See the configuration docs for production provisioning details.