Access application logs for troubleshooting

Viewing application logs is critical for debugging issues, examining performance problems, or even verifying the application started correctly. To make this as simple as possible, Nomad provides:

• Job specification for log rotation
• CLI command for log viewing
• API for programmatic log access

This section will use the job named "docs" from the previous sections, but these operations and command largely apply to all jobs in Nomad.

As a reminder, here is the output of the run command from the previous example:

$ nomad job run docs.nomad.hcl
==> Monitoring evaluation "42d788a3"
    Evaluation triggered by job "docs"
    Allocation "04d9627d" created: node "a1f934c9", group "example"
    Allocation "e7b8d4f5" created: node "012ea79b", group "example"
    Allocation "5cbf23a1" modified: node "1e1aa1e0", group "example"
    Evaluation status changed: "pending" -> "complete"
==> Evaluation "42d788a3" finished with status "complete"

The provided allocation ID (which is also available via the nomad status command) is required to access the application's logs. To access the logs of our application, issue the following command:

$ nomad alloc logs 04d9627d

The output will look something like this:

<timestamp> 10.1.1.196:5678 10.1.1.196:33407 "GET / HTTP/1.1" 200 12 "curl/7.35.0" 21.809µs
<timestamp> 10.1.1.196:5678 10.1.1.196:33408 "GET / HTTP/1.1" 200 12 "curl/7.35.0" 20.241µs
<timestamp> 10.1.1.196:5678 10.1.1.196:33409 "GET / HTTP/1.1" 200 12 "curl/7.35.0" 13.629µs

By default, this will return the logs of the task. If more than one task is defined in the job file, the name of the task is a required argument:

$ nomad alloc logs 04d9627d server

The logs command supports both displaying the logs as well as following logs, blocking for more output, similar to tail -f. To follow the logs, use the appropriately named -f flag:

$ nomad alloc logs -f 04d9627d

This will stream logs to your console.

If you are only interested in the "tail" of the log, use the -tail and -n flags:

$ nomad alloc logs -tail -n 25 04d9627d

This will show the last 25 lines. If you omit the -n flag, -tail will default to 10 lines.

By default, only the logs on stdout are displayed. To show the log output from stderr, use the -stderr flag:

$ nomad alloc logs -stderr 04d9627d

Consider the "log shipper" pattern

While the logs command works well for quickly accessing application logs, it generally does not scale to large systems or systems that produce a lot of log output, especially for the long-term storage of logs. Nomad's retention of log files is best effort, so chatty applications should use a better log retention strategy.

Since applications log to the alloc/ directory, all tasks within the same task group have access to each others logs. Thus it is possible to have a task group as follows:

group "my-group" {
  task "server" {
    # ...

    # Setting the server task as the leader of the task group allows Nomad to
    # signal the log shipper task to gracefully shutdown when the server exits.
    leader = true
  }

  task "log-shipper" {
    # ...
  }
}

In the above example, the server task is the application that should be run and will be producing the logs. The log-shipper reads those logs from the alloc/logs/ directory and sends them to a longer-term storage solution such as Amazon S3 or an internal log aggregation system.

When using the log shipper pattern, especially for batch jobs, the main task should be marked as the leader task. By marking the main task as a leader, when the task completes all other tasks within the group will be gracefully shutdown. This allows the log shipper to finish sending any logs and then exiting itself. The log shipper should set a high enough kill_timeout such that it can ship any remaining logs before exiting.