Last Updated: 30 April 2015

exceptions logging logs

Table of Contents

Logs are a stream of time-stamped events aggregated from the output streams of all of your app’s running processes, system components, and backing services. Heroku’s Logplex routes log streams from all of these diverse sources into a single channel, providing the foundation for truly comprehensive logging.

Types of logs

Heroku aggregates three categories of logs for your app:

  • App logs - Output from your application. This will include logs generated from within your application, application server and libraries. (Filter: --source app)
  • System logs - Messages about actions taken by the Heroku platform infrastructure on behalf of your app, such as: restarting a crashed process, sleeping or waking a web dyno, or serving an error page due to a problem in your app. (Filter: --source heroku)
  • API logs - Messages about administrative actions taken by you and other developers working on your app, such as: deploying new code, scaling the process formation, or toggling maintenance mode. (Filter: --source heroku --ps api)

Log history limits

Logplex is designed for collating and routing log messages, not for storage. It keeps the last 1,500 lines of consolidated logs. If you’d like to persist more than 1,500 lines for long-term storage, search, alerting, filtering and other processing, you can drain your logs to a drain service provided by one of the many logging Add-on Providers. You can also implement your own log drains and get full control over what happens to your logs.

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.

In Ruby, you could use something like:

puts "Hello, logs!"

In Java:

System.err.println("Hello, logs!");
System.out.println("Hello, logs!");

The same holds true for all other languages supported by Heroku.

To take advantage of the real-time 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. These might require extra configuration. For example, when using the Ruby on Rails TaggedLogger by ActiveSupport, you should add the following into your app’s configuration to get stdout logging:

config.logger = Logger.new(STDOUT)

Log retrieval

Log format

Each log line is formatted as follows:

timestamp source[dyno]: message
  • 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.
  • Source - All of your app’s dynos (web dynos, background workers, cron) have the source, app. All of Heroku’s system components (HTTP router, dyno manager) have the source, heroku.
  • Dyno - The name of the dyno or component that wrote the log line. For example, worker #3 appears as worker.3, and the Heroku HTTP router appears as router.
  • Message - The content of the log line. Lines generated by dynos that exceed 10000 bytes are split into 10000 byte chunks without extra trailing newlines. Each chunk is submitted as a separate log line.

View logs

To fetch your logs, use the heroku logs command.

$ heroku logs
2010-09-16T15:13:46.677020+00:00 app[web.1]: Processing PostController#list (for 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="" 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. You can specify the number of log lines to retrieve (up to a maximum of 1,500 lines) by using the --num (or -n) option.

$ heroku logs -n 200

Real-time tail

Similar to tail -f, real-time tail displays recent logs and leaves the session open for real-time logs to stream in. 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.

You can tail your logs using --tail (or -t).

$ heroku logs --tail

When you are done, press Ctrl+C to return to the prompt.


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="" 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="" 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 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, can be used.

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

$ heroku logs --source app --tail

Log message ordering

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