Run Docker containers with high availability

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We’ll start with a small Node.js script, index.js, that prints hello world and exits after three seconds:

cat > index.js <<'EOF'
console.log('hello world');

setTimeout(() => process.exit(), 3000);
EOF

Create a Dockerfile that copies the script into the image and runs it:

cat > Dockerfile <<'EOF'
FROM mhart/alpine-node
COPY index.js .
CMD node index.js
EOF

Build the image with the name helloworld:

docker build -t helloworld .

Run a container named test1:

docker run --name test1 helloworld

The script exits after three seconds, so the container stops. Check its status with docker ps, filtering for test1:

docker ps -a --filter name=test1

The container is exited and is no longer running. To restart a container automatically whenever its main process exits, use the --restart=always option:

docker run --name test2 --restart=always helloworld

Inspect test2 to confirm that it is still running:

docker ps --filter name=test2

Docker restarts the container after the script exits, so it remains available.

Docker Compose provides another way to configure containers for high availability. Create a docker-compose.yml file with the helloworld service and set its restart policy to always:

cat > docker-compose.yml <<'EOF'
version: "3"

services:
  helloworld:
    image: helloworld
    restart: always
EOF

Start three instances of the service with Docker Compose:

docker-compose up --scale helloworld=3

Docker Compose creates three containers for the helloworld service. Because the service uses restart: always, each container restarts whenever the script exits, so all three instances remain available.