How To Debug Working Docker Containers

July 13, 2024

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Picture by Editor | Midjourney & Canva

Containers can generally behave unexpectedly as a consequence of configuration points, utility bugs, or useful resource constraints. On this tutorial, we’ll go over the totally different strategies to debug working containers by taking the instance of a Postgres container.

Stipulations

To comply with alongside to this tutorial:

You must have Docker put in in your improvement atmosphere. Get Docker in case you haven’t already.
You ought to be snug with how Docker works and fundamental instructions to tug pictures, begin, cease, and handle containers.

Pull and Begin a PostgreSQL Container

First, let’s pull the most recent PostgreSQL picture from DockerHub and begin a container. Right here, we pull postgres:16 utilizing the docker pull command:

$ docker pull postgres:16

As soon as the pull is full, you can begin the postgres container with the next docker run command. Notice that the POSTGRES_PASSWORD is the required atmosphere variable it is advisable begin the container:

$ docker run --name my_postgres -e POSTGRES_PASSWORD=my_postgres_password -d postgres

This command begins a brand new container named my_postgres with PostgreSQL working inside it. You possibly can confirm it by working the docker ps command:

Which outputs:

CONTAINER ID   IMAGE         COMMAND                  CREATED          STATUS         PORTS      NAMES
5cb6fabbbc8b   postgres:16   "docker-entrypoint.s…"   18 seconds in the past   Up 9 seconds   5432/tcp   my_postgres

1. Examine the Container

You should use the docker examine command to retrieve detailed details about a container. This may be helpful for checking the container’s configuration, community settings, and state:

$ docker examine my_postgres

This command outputs a JSON object with all the small print in regards to the container. You should use instruments like jq to parse and extract particular info of curiosity from this output.

Truncated output of docker examine my_postgres

2. View Container Logs

The docker logs command fetches the logs from a working container. That is helpful for troubleshooting points associated to the applying working contained in the container.

$ docker logs my_postgres

Truncated output of docker logs my_postgres

3. Execute Instructions Contained in the Container

Generally it’s useful to get into the container and run a bunch of diagnostic instructions. You are able to do this with the docker exec command that means that you can run instructions inside a working container. That is helpful for inspecting the container’s filesystem, checking atmosphere variables, or the like.

Right here’s how one can begin an interactive shell session contained in the working container:

$ docker exec -it my_postgres /bin/bash

This command opens an interactive bash shell contained in the my_postgres container. So you’ll be able to run instructions from inside the container.

4. Test Working Processes

The docker prime command exhibits the working processes inside a container. This may help you establish if any processes are consuming extra assets than anticipated or if there are any surprising processes working:

UID                 PID                 PPID                C                   STIME               TTY                 TIME                CMD
ollama              8116                8096                0                   09:41               ?                   00:00:00            postgres
ollama              8196                8116                0                   09:41               ?                   00:00:00            postgres: checkpointer
ollama              8197                8116                0                   09:41               ?                   00:00:00            postgres: background author
ollama              8199                8116                0                   09:41               ?                   00:00:00            postgres: walwriter
ollama              8200                8116                0                   09:41               ?                   00:00:00            postgres: autovacuum launcher
ollama              8201                8116                0                   09:41               ?                   00:00:00            postgres: logical replication launcher

5. Connect to the Container

The docker connect command means that you can connect your terminal to a working container’s important course of. This may be helpful for debugging interactive processes:

$ docker connect my_postgres

Notice that attaching to a container is totally different from docker exec because it connects you to the container’s important course of and streams its normal output and normal error to your terminal.

6. View Useful resource Utilization

With the docker stats command, you will get real-time statistics on container useful resource utilization together with CPU, reminiscence, and community. Which is useful if you wish to analyze efficiency points:

$ docker stats my_postgres

Here is a pattern output:

CONTAINER ID   NAME          CPU %     MEM USAGE / LIMIT     MEM %     NET I/O       BLOCK I/O         PIDS
5cb6fabbbc8b   my_postgres   0.03%     34.63MiB / 3.617GiB   0.94%     10.6kB / 0B   38.4MB / 53.1MB   6



CONTAINER ID   NAME          CPU %     MEM USAGE / LIMIT     MEM %     NET I/O       BLOCK I/O         PIDS
5cb6fabbbc8b   my_postgres   0.03%     34.63MiB / 3.617GiB   0.94%     10.6kB / 0B   38.4MB / 53.1MB   6



CONTAINER ID   NAME          CPU %     MEM USAGE / LIMIT     MEM %     NET I/O       BLOCK I/O         PIDS
5cb6fabbbc8b   my_postgres   0.03%     34.63MiB / 3.617GiB   0.94%     10.6kB / 0B   38.4MB / 53.1MB   6


...

Debugging Docker containers includes inspecting container config, viewing logs, executing instructions contained in the container, and monitoring useful resource utilization. With these methods, you’ll be able to successfully troubleshoot and resolve points together with your containerized purposes. Joyful debugging!

Further Sources

Try the next assets in case you’d wish to discover additional:

Bala Priya C is a developer and technical author from India. She likes working on the intersection of math, programming, knowledge science, and content material creation. Her areas of curiosity and experience embody DevOps, knowledge science, and pure language processing. She enjoys studying, writing, coding, and occasional! Presently, she’s engaged on studying and sharing her information with the developer group by authoring tutorials, how-to guides, opinion items, and extra. Bala additionally creates participating useful resource overviews and coding tutorials.

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