Updating Doker Containers Without Downtime

Marc Gallofré
Aug 31, 2020 2 min read Docker, Swarm, docker-compose

Updating a running container may be a critical task if not done properly, as it can cause undesired side effects or stop production processes. Thus, with this guide I want to show you how to conduct a conteiner update process in an easy and safe way.

I assume you are using a container deployment and managment tool such as Swarm or Kubernates, otherwise I strongly recommend you to get familar with such kind of tools and use it on your applications. For the rest of this guide, I will use Swarm as a example.

Lets imagine that we have the following service definition in a docker-compose.yml file:

services:
  cassandra1:
    <<: *cassandra-base
    environment:
      CASSANDRA_CLUSTER_NAME: cassandra-cluster
      CASSANDRA_PASSWORD: cassandra
      CASSANDRA_BROADCAST_ADDRESS: tasks.cassandra1
      CASSANDRA_LISTEN_ADDRESS: tasks.cassandra1
      CASSANDRA_SEEDS: "tasks.cassandra1,tasks.cassandra2,tasks.cassandra3" 
      CASSANDRA_PASSWORD_SEEDER: "yes"
    ports:
      - target: 7000
        published: 7000
      - target: 9042
        published: 9042

This service creates an Apache Cassandra node which is connected in a cluster to other 2 nodes, making a cluster of 3 nodes. It also opens the ports 7000 and 9042.

Now, we run docker stack deploy -c docker-compose.yml example_project and our cluster of 3 nodes is succesfuly running and working. So far, so god!

However, after a while we realise that we have forgoten to open the port 7199 from where we can connect with nodetols tool to manage our cluster and get some statistics. So what can we do now?

It’s easy, (1) update the docker-compose.yml as we wish, e.g., with the port 7199:

services:
  cassandra1:
    <<: *cassandra-base
    environment:
      CASSANDRA_CLUSTER_NAME: cassandra-cluster
      CASSANDRA_PASSWORD: cassandra
      CASSANDRA_BROADCAST_ADDRESS: tasks.cassandra1
      CASSANDRA_LISTEN_ADDRESS: tasks.cassandra1
      CASSANDRA_SEEDS: "tasks.cassandra1,tasks.cassandra2,tasks.cassandra3" 
      CASSANDRA_PASSWORD_SEEDER: "yes"
    ports:
      - target: 7000
        published: 7000
      - target: 9042
        published: 9042
      - target: 7199
        published: 7199

(2) Run again the command:

docker stack deploy -c docker-compose.yml example_project

In that way the Docker Swarm will create a new version of our service and stop old version after that.

Finally, if everything has worked fine, we should be able to do something like this nodetool tablestats -h <CASSANDRA_NODE_IP>.

Docker Swarm docker-compose

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