appexample/app:1.4.2
Runs the image example/app:1.4.2.
Limits memory to 512m. The container is killed if it exceeds this.
Docker Compose check · low
Services without a restart policy
Without a restart policy Docker uses "no": a container that crashes, or that was running when the host or Docker daemon restarted, stays stopped until someone starts it. The explainer flags services with neither restart nor deploy.restart_policy as low. For long-running services, restart: unless-stopped is a common default.
$ explain docker-compose.yml ! app has no restart policy line 2 fix: see below $
What the free Docker Compose explainer reports on the risky example below.
Why it matters
Crashes happen: an out-of-memory kill, an unhandled error, a dependency that was briefly unavailable. Without a policy, one such event leaves the service down until it is noticed.
After a host reboot, Docker brings back containers whose policy says to, and leaves the others stopped.
The policy is also part of what the file documents: a reader can see which services are meant to stay up.
Risky and safer docker-compose.yml
services:
app:
image: example/app:1.4.2
mem_limit: 512m
services:
app:
image: example/app:1.4.2
restart: unless-stopped
mem_limit: 512m
How to fix it
- Add restart: unless-stopped to long-running services: they come back after crashes and daemon restarts, but stay stopped if you stopped them by hand.
- Use restart: on-failure for jobs that should retry on errors but not run again after a clean exit.
- For one-off tasks such as migrations, set restart: "no" explicitly so the choice is visible; the explainer accepts that as a policy.
When it's acceptable
- One-off or batch containers that should run once and exit.
- Stacks managed by something else that restarts them, such as a process supervisor or an orchestrator.
Check your own docker-compose.yml
The risky example is loaded below. Paste your own file to check it for this and every other Docker Compose rule. It runs in your browser; nothing is uploaded.
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Explanation
1 service0 networks0 volumes1 low
Top level
1 service defined: app.
Questions
What is the default restart policy in docker-compose?
"no": Docker does not restart the container automatically, whether it crashed or the host rebooted.
What is the difference between always and unless-stopped?
Both restart the container after it exits. With always, a container you stopped manually starts again when the Docker daemon restarts; with unless-stopped it stays stopped.
Does deploy.restart_policy count as a restart policy?
Yes. The explainer accepts either restart or deploy.restart_policy on the service.