Restart Policies

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Why Containers Stop

1. Normal completion
   The primary process finishes successfully (exit code 0), for example, a script completes its task.
2. Failure
   The process crashes or throws an error, exiting with a non-zero code (e.g., bad input).
3. Manual intervention
   Running docker container stop sends a SIGTERM, then a SIGKILL after a timeout:
   • If the process traps SIGTERM and exits cleanly, it may return code 0.
   • If it’s killed with SIGKILL, it usually exits with a non-zero code.

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Docker Restart Policies

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Quick Reference

• no: No auto-restart.
• on-failure: Auto-restart only on errors (non-zero exit).
• always: Auto-restart on any exit.
• unless-stopped: Auto-restart on any exit, but not after a manual stop.

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Examples

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1. Default (no)

docker run --name test-no --restart=no ubuntu \
  expr 3 + 5
# Exits with 0; Docker does not restart.

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2. On Failure

docker run --name test-fail --restart=on-failure:3 ubuntu \
  expr three + 5
# Exits with 1; Docker retries up to 3 times, then stops.

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3. Always

docker run --name test-always --restart=always ubuntu \
  sleep 5

• After sleep 5 finishes (exit 0), Docker restarts immediately.
• docker stop test-always prevents restart only until the next Docker daemon reboot.

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4. Unless-Stopped

docker run --name test-unless --restart=unless-stopped ubuntu \
  sleep 5

• Behaves like always on crashes or normal exit.
• Honors manual docker stop even if the daemon restarts later.

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Live Restore of Containers

1. Edit or create /etc/docker/daemon.json:
   {
     "live-restore": true
   }
   
2. Restart the Docker service:
   sudo systemctl restart docker
   # or
   sudo systemctl reload docker
   

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Verifying Live Restore

docker run --name web httpd
sudo systemctl stop docker
# Containers stop when daemon stops.
sudo systemctl start docker
docker ps
# web is not running

docker run --name web httpd
sudo systemctl stop docker
# web stays running.
sudo systemctl start docker
docker ps
# web is still running

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Links and References

• Docker Restart Policy Documentation
• Docker Daemon Configuration
• Docker Live Restore Deep Dive