rust-pubsub

Proof of concept: asynchronous service communication via Google Cloud Pub/Sub from Rust.

Two Axum / Tokio services exchange messages through Pub/Sub using the gcloud-pubsub crate:

1. api-service — accepts loan applications over HTTP, publishes them to Pub/Sub, and lets users poll for results.
2. risk-processor — subscribes to applications, makes a random approve/reject decision, and publishes the result back.

Architecture

                        ┌──────────────────┐
  POST /applications ──>│   api-service    │──> loan-applications topic
                        │  (HTTP + state)  │
  GET /applications/:id │                  │<── loan-decisions topic
          <─────────────└──────────────────┘

                        ┌──────────────────┐
  loan-applications ──> │ risk-processor   │──> loan-decisions topic
          topic         │  (subscriber)    │
                        └──────────────────┘

Quick Start

-> % ./scripts/start.sh

Building and starting containers...
[+] Building 1.3s (31/31) FINISHED
(...)
Tailing logs (Ctrl+C to stop watching, containers keep running)...
Waiting for API service to become ready...

API service is ready at http://localhost:3000

-> % ./scripts/submit-application.sh u-7712 15000 PLN
{
  "application_id": "loan-24198"
}

-> % ./scripts/check-status.sh loan-24198
{
  "application_id": "loan-24198",
  "status": "APPROVED",
  "interest_rate": 6.73,
  "max_term_months": 22
}

% ./scripts/logs.sh

rust-pubsub-risk-processor-1  | Connected to Pub/Sub.
rust-pubsub-risk-processor-1  | Listening for loan applications on 'loan-applications-sub'...
rust-pubsub-risk-processor-1  | Processing application loan-24198 for user u-7712 (15000 PLN)
rust-pubsub-risk-processor-1  | Decision for loan-24198: Approved
rust-pubsub-api-service-1     | Connected to Pub/Sub.
rust-pubsub-api-service-1     | Listening on 0.0.0.0:3000
rust-pubsub-api-service-1     | Background listener subscribed to 'loan-decisions-sub'
rust-pubsub-api-service-1     | Submitting application loan-24198 for user u-7712 (15000 PLN)
rust-pubsub-api-service-1     | Received decision for loan-24198: Approved

Running

Everything runs in Docker via the official Pub/Sub emulator:

./scripts/start.sh

Or directly:

docker compose up --build

This starts the emulator, creates topics and subscriptions, and launches both services. The API listens on port 3000.

API

Submit an application

./scripts/submit-application.sh u-7712 15000 PLN

POST /applications  {"user_id": "u-7712", "amount": 15000, "currency": "PLN"}

Check application status

./scripts/check-status.sh loan-12345

GET /applications/loan-12345

The response starts as "status": "PENDING" and changes to "APPROVED" or "REJECTED" once the risk processor publishes its decision (typically 1-2 seconds).

Limitations

Error handling is not production-ready. Both services use ? to propagate errors, which means the process exits on the first failure (e.g. a failed publish or ack). A real system would need structured error types, retries for transient failures, and dead-letter handling for poison messages.

Stopping

./scripts/stop.sh