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Our First Logs

Let's take a look at Docker Logging

Tasks:

Task 1: Start a Traefik Proxy to log

Now that Docker is setup, it's time to get our hands dirty. In this section, you are going to run a reverse proxy called Traefik container (a high-performance webserver, load-balancer, and proxy) on your system and get hands-on with the docker logs command.

  1. To get started, create a new directory traefik and change to the traefikdirectory

    mkdir traefik
cd traefik

  2. Using your favorite editor create a new file named traefik.tomland add the below configuration to the newly created file.

    ################################################################
# API and dashboard configuration
################################################################
123[api]
################################################################
# Docker configuration backend
################################################################
[docker]
domain = "docker.local"
watch = true
################################################################
# Enable Access Logs
################################################################
[accessLog]

  3. Start the Traefik proxy. Ensure the traefik.toml is in your current working directory.

docker run -d -p 8080:8080 -p 80:80 --name traefik \
-v $PWD/traefik.toml:/etc/traefik/traefik.toml  \
-v /var/run/docker.sock:/var/run/docker.sock traefik

  1. Ensure the Traefik container is running by running the ls command with -l showing last container

     docker container ps -l
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS                     PORTS       NAMES
0b2062765e41        traefik             "/traefik"          8 minutes ago       Exited (1) 8 minutes ago                  Traefik

    Uh oh, what happend? We can actually use the docker logs command on stopped containers to troubleshoot. Great, let's do it.

  2. Check the logs of the Traefik container to see why the container didn't start

     docker container logs traefik
 2019/04/25 15:11:51 Error reading TOML config file /etc/traefik/traefik.toml : 
 Near line 3 (last key parsed ''): bare keys cannot         contain '['

  3. OK, remove the Traefik container

     docker container rm -f traefik

    This is the forceful way to remove it. With great power comes great responsability. You are warned!

  4. Fix the traefik.toml configuration file line 4 removing 123 in front of the [API] block

    ################################################################
# API and dashboard configuration
################################################################
[api]
################################################################
# Docker configuration backend
################################################################
[docker]
domain = "docker.local"
watch = true
################################################################
# Enable Access Logs
################################################################
[accessLog]

  5. Start Traefik with the fixed configuration file. Ensure the traefik.toml is in your current working directory.

    docker run -d -p 8080:8080 -p 80:80 --name traefik \
-v $PWD/traefik.toml:/etc/traefik/traefik.toml  \
-v /var/run/docker.sock:/var/run/docker.sock traefik

  6. Ensure the Traefik container is running

     docker container ps -l
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS                         NAMES
e72a26a2b752        traefik             "/traefik"          5 seconds ago       Up 3 seconds        0.0.0.0:80->80/tcp,0.0.0.0:8080->8080/tcp   traefik

  7. Test the Traefik container with curl or open a browser tab and navigate to: https://0.0.0.0

     curl 0.0.0.0

    Go ahead and send a few curl/refresh request to the Traefik container.

  8. Check the logs

     docker container logs traefik

    What do we see different? We should now see each curl/refresh we sent to the Traefik container

    We should see a 404 error about no backends configured.

Now, we will connect a whoami container to the Traefik proxy. This whoami container will register itself automatically with the proxy. The Traefik proxy routes traffic from 0.0.0.0from the Traefik proxy to our new whoami application.

Traefik watches the Docker daemon for new containers that join and start on thee server. When a new container starts it automatically registers it with Traefik

  1. Start the whoami container

    docker run -d --name whoami emilevauge/whoami

  2. curl the whoami container using the Virtual Hostname test.docker.localconfigured in Traefik

     curl --header 'Host: whoami.docker.local' 'http://localhost:80/'

Response

``` 
Hostname: 299f3c36eb18
IP: 127.0.0.1
IP: 172.17.0.5
GET / HTTP/1.1
Host: test.docker.local
User-Agent: curl/7.47.0
Accept: */*
Accept-Encoding: gzip
X-Forwarded-For: 172.17.0.1
X-Forwarded-Host: test.docker.local
X-Forwarded-Port: 80
X-Forwarded-Proto: http
X-Forwarded-Server: 10744bcc8a7d
X-Real-Ip: 172.17.0.1
```

  1. Finally, run the docker container logs command on the proxy to ensure everything is now working as expected

    docker container logs traefik

    We now see the hostname which is queried and a HTTP 200 success code

    172.17.0.1 - - [25/Apr/2019:15:37:33 +0000] "GET / HTTP/1.1" 200 326 "-" "curl/7.47.0" 5 "Host-whomai-docker-local-0" "http://172.17.0.5:80" 1ms
172.17.0.1 - - [25/Apr/2019:15:37:36 +0000] "GET / HTTP/1.1" 200 326 "-" "curl/7.47.0" 6 "Host-whoami-docker-local-0" "http://172.17.0.5:80" 1ms
172.17.0.1 - - [25/Apr/2019:15:37:37 +0000] "GET / HTTP/1.1" 200 326 "-" "curl/7.47.0" 7 "Host-whoami-docker-local-0" "http://172.17.0.5:80" 0ms

Task 2: Understanding the Docker Logs Command

The docker container logs command is a powerful command and is used for troubleshooting, analyzing, and general information gathering. The command is useful to Developers to debug new applications as well as Operations for information gathering.

  1. Great! Let's now take a look at the docker container logs help to better understand how we can best use the command

     docker container logs --help

Usage:  docker logs [OPTIONS] CONTAINER

Fetch the logs of a container

Options:
    --details        Show extra details provided to logs
-f, --follow         Follow log output
    --since string   Show logs since timestamp (e.g. 2013-01-02T13:23:37) or relative (e.g. 42m for 42 minutes)
    --tail string    Number of lines to show from the end of the logs (default "all")
-t, --timestamps     Show timestamps
    --until string   Show logs before a timestamp (e.g. 2013-01-02T13:23:37) or relative (e.g. 42m for 42 minutes)

  2. Add timestamps to our logging output. This will help with narrowing down when events occured if no timestamp is created in the log message.

     docker container logs -t traefik

  3. Tail the last n number of lines in the log file

    This is extremely helpful when logs become very big. If you were to run a docker logs command on a large log it could over run your terminal winder

     docker container logs --tail 5 traefik

  4. Follow the log for real-time updates.

     docker container logs -t -f traefik

    Curl the whoami container a couple times to see the log update. Remember the IP address of the node you are on by looking at the command prompt root@ip_address.

    Switch to a different worker node and run the below command.

    curl --header 'Host: whoami.docker.local' 'http://<ip_address_host>:80/'

  5. Switch back to the original Worker and Restart the Traefik container

    docker container restart traefik

  6. Check the logs again. What do you notice?

     docker container logs traefik

    The logs still persist inside the container from our previous tests.

  7. Stop and remove the Traefik container

     docker container rm -f traefik

  8. Start Traefik again

    docker run -d -p 8080:8080 -p 80:80 --name traefik \
-v $PWD/traefik.toml:/etc/traefik/traefik.toml  \
-v /var/run/docker.sock:/var/run/docker.sock traefik

  9. Check the logs. What do you notice?

     docker container logs traefik

    This time we removed the container and started a new container. It is important to notice now the logs didn't persist. This is why it is important we persist logs outside of the containers.

  10. Cleanup running containers

    docker container rm -f whoami traefik

Task 3: docker-compose and logging

We have now seen how logging works in a single container. Now, we want to see what logs look like when multiple containers are running in a compose file. In this example we will use the docker voting application. This stack contains 3 different containers running with one docker-compose file.

  1. In the Setupsection we cloned the Repo. If you haven't done so please do it now

     git clone https://github.com/vegasbrianc/docker-compose-demo.git

  2. Start the Compose demo with docker-compose.

     cd docker-compose-demo

 docker-compose up -d

  3. Once the voting application stack has started. Check the logs of the voting app stack.

     docker-compose logs

    What we notice is that Docker color codes the log based on container names. Since we have 3 different containers this makes it easier when viewing from a terminal window.

  4. Expanding the command to capture certain containers

     docker-compose logs reverse-proxy

  5. Combing everything we learned follow and timestamp the logs

     docker-compose logs -f -t reverse-proxy

Cleanup

  1. Time to stop and remove the running containers

     docker-compose rm -fs

Recap

What did we learn in this section?

  • Running docker container logs on stopped containers
  • Troubleshooting containers
  • The docker container logscommand is actually quite powerful and can be combined with other tools like the Linux grep command or others
  • Logs don't persist in containers once the container is removed
  • docker-compose logs works similarly to docker container logs but displays all containers in the compose stack

Next Steps, Docker Swarm & Logs

For the next step in the workshop, head over to Docker Swarm & Logging