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· 5 min read
Charlie Drage

In this blog post we will learn how take a multi-container Compose application and use it within Podman Desktop.

Objectives

  • Start the Compose YAML through podman compose up.
  • View the guestbook web application.
  • Confirm the web application is being synchronized and running correctly with the database.
  • Use Podman Desktop to view, inspect and access the terminal of the Redis cluster.

What is Compose

Compose is a specification for defining and running multi-container Docker applications. With pose, you use a YAML file to configure your application’s services, networks, and volumes. This allows you to capture in a single file the entire configuration necessary to run a set of interconnected containers as an application. For example, if you have an application that requires a web server, a database, and a caching service, you can define these components and their relationships in your Compose file.

To use the Compose YAML, you can use a specification implementation such as podman compose and docker compose.

Before we begin

If you do not have Compose installed, let's go through the onboarding process to install the Compose implementation binary:

  1. Get to Resources under Settings > Resources.
  2. Click Setup under Compose (it will appear if it has not been installed yet).
  3. Go through the onboarding process.

Onboarding process

Confirm that you are able to run podman compose:

podman compose
Run compose workloads via an external provider such as docker-compose or podman-compose

Description:
This command is a thin wrapper around an external compose provider such as docker-compose or podman-compose. This means that podman compose is executing another tool that implements the compose functionality but sets up the environment in a way to let the compose provider communicate transparently with the local Podman socket. The specified options as well the command and argument are passed directly to the compose provider.
...

Download and run the example application

Our example application is located at github.com/redhat-developer/podman-desktop-demo.

We will use git clone so we can build the Go binary web application:

git clone https://github.com/redhat-developer/podman-desktop-demo
cd podman-desktop-demo/guestbook-compose

Run podman compose up -d to start the application:

podman compose up -d
>>>> Executing external compose provider "/usr/local/bin/docker-compose". Please refer to the documentation for details. <<<<

[+] Running 3/3
✔ Container redis-replica Started 0.0s
✔ Container web Started 0.0s
✔ Container redis-leader Started 0.0s

Viewing the guestbook application

Within Podman Desktop, you can now see that all three containers are up and operational.

Click the "Open Browser" button to view the web application:

Open browser

Within the Guestbook web application, you can:

  • "Sign" the guestbook, which will write to the Redis leader and synchronize to the replicas.
  • "Read" from the guestbook, which will read from the pool of Redis replicas. This allows for readability even if the Redis leader is unavailable.
  • /env: View the container's environment variables.
  • /info: View information about the Redis cluster.

Guestbook application

Viewing and modifying the database

Using Podman Desktop, you can also access the container's terminal directly from the GUI and modify the database.

Click "Open Terminal" to access the redis-leader terminal:

Open terminal

Modify the database as if you are doing database administration:

  1. Run redis-cli within the container to access the Redis database.
  2. Type LPUSH guestbook "Hello World!" and you will see your web application update in real-time.
  3. Type DEL guestbook and you will see that your database drops the guestbook key and clears the database.

Redis CLI

Changes will reflect in real-time on the guestbook.

You can further modify the database and see the changes propagate to the Redis replicas.

For example, view the logs of the redis-replica, and you will notice that there are periodic database synchronizations as well as reads to the database:

Redis replica logs

How does it work?

A quick overview of how the architecture works in this multi-container scenario:

  • Within the Guestbook application, it looks for a database with the names redis-leader and redis-replica on port 6379.
  • Because it is a Compose application, the containers are connected on the same network. This means that a neighboring container can be network-accessible simply by its container name.

There is a set of environment variables that the web application can modify in the Compose application:

  • REDIS_LEADER: The default is redis-leader.
  • REDIS_REPLICAS: The default is redis-replica. Can be comma-separated, such as redis-replica-1,redis-replica-2.
  • REDIS_PORT: The default is 6379.
  • SERVER_PORT: The default is 8080.

Scaling more replicas

Want to scale more replicas? This can be achieved by adding an environment variable to your compose.yaml and duplicating your redis-replica entry.

Modify your compose.yaml as follows:

services:
redis-leader:
container_name: redis-leader
image: redis:latest
ports:
- '6379'

redis-replica:
container_name: redis-replica
image: redis:latest
ports:
- '6379'
command: redis-server --replicaof redis-leader 6379

redis-replica-2:
container_name: redis-replica-2
image: redis:latest
ports:
- '6379'
command: redis-server --replicaof redis-leader 6379

web:
container_name: web
build: ./web
environment:
- REDIS_REPLICAS=redis-replica1,redis-replica2
ports:
- '8080:8080'

Run podman compose up -d again to ensure the new container has been added and the new environment variable has propagated:

podman compose up -d
>>>> Executing external compose provider "/usr/local/bin/docker-compose". Please refer to the documentation for details. <<<<

[+] Running 4/4
✔ Container redis-replica-2 Started 0.0s
✔ Container redis-leader Running 0.0s
✔ Container web Started 0.0s
✔ Container redis-replica Running 0.0s