Logging

Heroku aggregates three categories of logs for your app:

1. App logs - Output from your application.

   This will include logs generated from

   1. within your application,
   2. application server and
   3. libraries.
   (Filter: --source app)
2. System logs -

   Messages about actions taken by the Heroku platform infrastructure on behalf of your app, such as:

   1. restarting a crashed process,
   2. sleeping or waking a web dyno, or
   3. serving an error page due to a problem in your app.
   (Filter: --source heroku)
3. API logs -

   Messages about administrative actions taken by you and other developers working on your app, such as:

   1. deploying new code,
   2. scaling the process formation, or
   3. toggling maintenance mode.
   (Filter: --source heroku --ps api)

   [~/rack/rack-rock-paper-scissors(master)]$ heroku logs --source heroku --ps api
   2013-10-23T21:33:41.105090+00:00 heroku[api]: Deploy 5ec1351 by chuchu.chachi.leon@gmail.com
   2013-10-23T21:33:41.154690+00:00 heroku[api]: Release v7 created by chuchu.chachi.leon@gmail.com
   

Logplex is designed for collating and routing log messages, not for storage. It keeps the last 1,500 lines of consolidated logs.

Heroku recommends using a separate service for long-term log storage; see Syslog drains for more information.

Writing to your log

Anything written to standard out (stdout) or standard error (stderr) is captured into your logs. This means that you can log from anywhere in your application code with a simple output statement:

puts "Hello, logs!"

To take advantage of the realtime logging, you may need to disable any log buffering your application may be carrying out. For example, in Ruby add this to your config.ru:

$stdout.sync = true

Some frameworks send log output somewhere other than stdout by default.

To fetch your logs

$ heroku logs
2010-09-16T15:13:46.677020+00:00 app[web.1]: Processing PostController#list (for 208.39.138.12 at 2010-09-16 15:13:46) [GET]
2010-09-16T15:13:46.677023+00:00 app[web.1]: Rendering template within layouts/application
2010-09-16T15:13:46.677902+00:00 app[web.1]: Rendering post/list
2010-09-16T15:13:46.678990+00:00 app[web.1]: Rendered includes/_header (0.1ms)
2010-09-16T15:13:46.698234+00:00 app[web.1]: Completed in 74ms (View: 31, DB: 40) | 200 OK [http://myapp.heroku.com/]
2010-09-16T15:13:46.723498+00:00 heroku[router]: at=info method=GET path=/posts host=myapp.herokuapp.com fwd="204.204.204.204" dyno=web.1 connect=1ms service=18ms status=200 bytes=975
2010-09-16T15:13:47.893472+00:00 app[worker.1]: 2 jobs processed at 16.6761 j/s, 0 failed ...

In this example, the output includes log lines from one of the app’s web dynos, the Heroku HTTP router, and one of the app’s workers.

The logs command retrieves 100 log lines by default.

Log message ordering

When retrieving logs, you may notice that the logs are not always in order, especially when multiple components are involved.

This is likely an artifact of distributed computing.

Logs originate from many sources (router nodes, dynos, etc) and are assembled into a single log stream by logplex.

It is up to the logplex user to sort the logs and provide the ordering required by their application, if any

Log history limits

You can fetch up to 1500 lines using the -num (or -n) option:

$ heroku logs -n 200

Heroku only stores the last 1500 lines of log history. If you’d like to persist more than 1500 lines, use a logging add-on or create your own syslog drain^52.1.

Log format

Each line is formatted as follows:

1. timestamp source[dyno]: message
2. Timestamp - The date and time recorded at the time the log line was produced by the dyno or component. The timestamp is in the format specified by RFC5424, and includes microsecond precision.
3. Source -
   1. All of your app’s dynos (web dynos, background workers, cron) have a source of app.
   2. All of Heroku’s system components (HTTP router, dyno manager) have a source of heroku.
4. Dyno - The name of the dyno or component that wrote this log line. For example, worker #3 appears as worker.3, and the Heroku HTTP router appears as router.
5. Message - The content of the log line. Dynos can generate messages up to approximately 1024 bytes in length and longer messages will be truncated.

Realtime tail

1. Similar to tail -f, realtime tail displays recent logs and leaves the session open for realtime logs to stream in.
2. By viewing a live stream of logs from your app, you can gain insight into the behavior of your live application and debug current problems.
3. You may tail your logs using --tail (or -t).

   $ heroku logs --tail
   

   When you are done, press Ctrl-C to close the session.

Filtering

If you only want to fetch logs with a certain source, a certain dyno, or both, you can use the --source (or -s) and --ps (or -p) filtering arguments:

$ heroku logs --ps router
2012-02-07T09:43:06.123456+00:00 heroku[router]: at=info method=GET path=/stylesheets/dev-center/library.css host=devcenter.heroku.com fwd="204.204.204.204" dyno=web.5 connect=1ms service=18ms status=200 bytes=13
2012-02-07T09:43:06.123456+00:00 heroku[router]: at=info method=GET path=/articles/bundler host=devcenter.heroku.com fwd="204.204.204.204" dyno=web.6 connect=1ms service=18ms status=200 bytes=20375

$ heroku logs --source app
2012-02-07T09:45:47.123456+00:00 app[web.1]: Rendered shared/_search.html.erb (1.0ms)
2012-02-07T09:45:47.123456+00:00 app[web.1]: Completed 200 OK in 83ms (Views: 48.7ms | ActiveRecord: 32.2ms)
2012-02-07T09:45:47.123456+00:00 app[worker.1]: [Worker(host:465cf64e-61c8-46d3-b480-362bfd4ecff9 pid:1)] 1 jobs processed at 23.0330 j/s, 0 failed ...
2012-02-07T09:46:01.123456+00:00 app[web.6]: Started GET "/articles/buildpacks" for 4.1.81.209 at 2012-02-07 09:46:01 +0000

$ heroku logs --source app --ps worker
2012-02-07T09:47:59.123456+00:00 app[worker.1]: [Worker(host:260cf64e-61c8-46d3-b480-362bfd4ecff9 pid:1)] Article#record_view_without_delay completed after 0.0221
2012-02-07T09:47:59.123456+00:00 app[worker.1]: [Worker(host:260cf64e-61c8-46d3-b480-362bfd4ecff9 pid:1)] 5 jobs processed at 31.6842 j/s, 0 failed ...

When filtering by dyno, either the base name, --ps web, or the full name, --ps web.1, may be used.

You can also combine the filtering switches with --tail to get a realtime stream of filtered output.

$ heroku logs --source app --tail