5f1033263b
Add redirects from /docs/ -> /guides/ and update old redirects to point to the new location.
126 lines
4.4 KiB
Markdown
126 lines
4.4 KiB
Markdown
---
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layout: "guides"
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page_title: "Accessing Logs - Operating a Job"
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sidebar_current: "guides-operating-a-job-accessing-logs"
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description: |-
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Nomad provides a top-level mechanism for viewing application logs and data
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files via the command line interface. This section discusses the nomad alloc
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logs command and API interface.
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---
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# Accessing Logs
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Viewing application logs is critical for debugging issues, examining performance
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problems, or even just verifying the application started correctly. To make this
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as simple as possible, Nomad provides:
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- Job specification for [log rotation](/docs/job-specification/logs.html)
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- CLI command for [log viewing](/docs/commands/alloc/logs.html)
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- API for programatic [log access](/api/client.html#stream-logs)
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This section will utilize the job named "docs" from the [previous
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sections](/guides/operating-a-job/submitting-jobs.html), but these operations
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and command largely apply to all jobs in Nomad.
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As a reminder, here is the output of the run command from the previous example:
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```text
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$ nomad job run docs.nomad
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==> Monitoring evaluation "42d788a3"
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Evaluation triggered by job "docs"
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Allocation "04d9627d" created: node "a1f934c9", group "example"
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Allocation "e7b8d4f5" created: node "012ea79b", group "example"
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Allocation "5cbf23a1" modified: node "1e1aa1e0", group "example"
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Evaluation status changed: "pending" -> "complete"
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==> Evaluation "42d788a3" finished with status "complete"
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```
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The provided allocation ID (which is also available via the `nomad status`
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command) is required to access the application's logs. To access the logs of our
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application, we issue the following command:
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```shell
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$ nomad alloc logs 04d9627d
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```
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The output will look something like this:
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```text
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<timestamp> 10.1.1.196:5678 10.1.1.196:33407 "GET / HTTP/1.1" 200 12 "curl/7.35.0" 21.809µs
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<timestamp> 10.1.1.196:5678 10.1.1.196:33408 "GET / HTTP/1.1" 200 12 "curl/7.35.0" 20.241µs
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<timestamp> 10.1.1.196:5678 10.1.1.196:33409 "GET / HTTP/1.1" 200 12 "curl/7.35.0" 13.629µs
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```
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By default, this will return the logs of the task. If more than one task is
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defined in the job file, the name of the task is a required argument:
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```shell
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$ nomad alloc logs 04d9627d server
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```
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The logs command supports both displaying the logs as well as following logs,
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blocking for more output, similar to `tail -f`. To follow the logs, use the
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appropriately named `-f` flag:
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```shell
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$ nomad alloc logs -f 04d9627d
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```
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This will stream logs to our console.
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If you wish to see only the tail of a log, use the `-tail` and `-n` flags:
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```shell
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$ nomad alloc logs -tail -n 25 04d9627d
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```
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This will show the last 25 lines. If you omit the `-n` flag, `-tail` will
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default to 10 lines.
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By default, only the logs on stdout are displayed. To show the log output from
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stderr, use the `-stderr` flag:
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```shell
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$ nomad alloc logs -stderr 04d9627d
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```
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## Log Shipper Pattern
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While the logs command works well for quickly accessing application logs, it
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generally does not scale to large systems or systems that produce a lot of log
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output, especially for the long-term storage of logs. Nomad's retention of log
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files is best effort, so chatty applications should use a better log retention
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strategy.
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Since applications log to the `alloc/` directory, all tasks within the same task
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group have access to each others logs. Thus it is possible to have a task group
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as follows:
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```hcl
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group "my-group" {
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task "server" {
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# ...
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# Setting the server task as the leader of the task group allows us to
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# signal the log shipper task to gracefully shutdown when the server exits.
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leader = true
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}
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task "log-shipper" {
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# ...
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}
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}
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```
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In the above example, the `server` task is the application that should be run
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and will be producing the logs. The `log-shipper` reads those logs from the
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`alloc/logs/` directory and sends them to a longer-term storage solution such as
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Amazon S3 or an internal log aggregation system.
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When using the log shipper pattern, especially for batch jobs, the main task
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should be marked as the [leader task](/docs/job-specification/task.html#leader).
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By marking the main task as a leader, when the task completes all other tasks
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within the group will be gracefully shutdown. This allows the log shipper to
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finish sending any logs and then exiting itself. The log shipper should set a
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high enough [`kill_timeout`](/docs/job-specification/task.html#kill_timeout)
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such that it can ship any remaining logs before exiting.
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