open-nomad/website/source/docs/jobops/logs.html.md

layout	page_title	sidebar_current	description
docs	Operating a Job: Accessing Logs	docs-jobops-logs	Learn how to operate a Nomad Job.

Accessing Logs

Accessing applications logs is critical when debugging issues, performance problems or even for verifying the application is starting correctly. To make this as simple as possible, Nomad provides both a CLI tool and an API for accessing application logs and data files.

To see this in action we can just run the example job which created using nomad init:

$ nomad init
Example job file written to example.nomad

This job will start a redis instance in a Docker container. We can run it now:

$ nomad run example.nomad
==> Monitoring evaluation "7a3b78c0"
    Evaluation triggered by job "example"
    Allocation "c3c58508" created: node "b5320e2d", group "cache"
    Evaluation status changed: "pending" -> "complete"
==> Evaluation "7a3b78c0" finished with status "complete"

We can grab the allocation ID from above and use the nomad fs command to access the applications logs. Logs are stored under the following directory structure: alloc/logs/<task-name>.<stdout/stderr>.<index>. Nomad has built in log rotation, documented in the Jobspec. The index is a monotonically increasing number starting at zero and incremented each time the log is rotated.

Thus to access the stdout we can issue the below command:

$ nomad fs c3c58508 alloc/logs/redis.stdout.0
                 _._
            _.-``__ ''-._
       _.-``    `.  `_.  ''-._           Redis 3.2.1 (00000000/0) 64 bit
   .-`` .-```.  ```\/    _.,_ ''-._
  (    '      ,       .-`  | `,    )     Running in standalone mode
  |`-._`-...-` __...-.``-._|'` _.-'|     Port: 6379
  |    `-._   `._    /     _.-'    |     PID: 1
   `-._    `-._  `-./  _.-'    _.-'
  |`-._`-._    `-.__.-'    _.-'_.-'|
  |    `-._`-._        _.-'_.-'    |           http://redis.io
   `-._    `-._`-.__.-'_.-'    _.-'
  |`-._`-._    `-.__.-'    _.-'_.-'|
  |    `-._`-._        _.-'_.-'    |
   `-._    `-._`-.__.-'_.-'    _.-'
       `-._    `-.__.-'    _.-'
           `-._        _.-'
               `-.__.-'

 1:M 28 Jun 19:49:30.504 # WARNING: The TCP backlog setting of 511 cannot be enforced because /proc/sys/net/core/somaxconn is set to the lower value of 128.
 1:M 28 Jun 19:49:30.505 # Server started, Redis version 3.2.1
 1:M 28 Jun 19:49:30.505 # WARNING overcommit_memory is set to 0! Background save may fail under low memory condition. To fix this issue add 'vm.overcommit_memory = 1' to /etc/sysctl.conf and then reboot or run the command 'sysctl vm.overcommit_memory=1' for this to take effect.
 1:M 28 Jun 19:49:30.505 # WARNING you have Transparent Huge Pages (THP) support enabled in your kernel. This will create latency and memory usage issues with Redis. To fix this issue run the command 'echo never > /sys/kernel/mm/transparent_hugepage/enabled' as root, and add it to your /etc/rc.local in order to retain the setting after a reboot. Redis must be restarted after THP is disabled.
 1:M 28 Jun 19:49:30.505 * The server is now ready to accept connections on port 6379

Replacing stdout for stderr would display the respective stderr output.

While this works well for quickly accessing logs, we recommend running a log-shipper for long term storage of logs. In many cases this will not be needed and the above will suffice but for use cases in which log retention is needed Nomad can accomodate.

Since we place application logs inside the alloc/ directory, all tasks within the same task group have access to each others logs. Thus we can have a task group as follows:

group "my-group" {
    task "log-producer" {...}
    task "log-shipper" {...}
}

In the above example, the log-producer task is the application that should be run and will be producing the logs we would like to ship and the log-shipper reads these logs from the alloc/logs/ directory and ships them to a long term storage such as S3.