Last updated September 09, 2024

A Heroku app’s logs come from the output streams of all of its running processes, system components, and backing services. Heroku’s Logplex routes log streams from all sources into a single channel, providing a foundation for comprehensive logging.

Runtime Logs

Heroku aggregates the following categories of logs for a deployed app:

• App logs - Logging output from the application itself, including logs generated by your app’s code and dependencies. (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 app --dyno api)
• Add-on logs - Messages from add-on services. See the add-on’s Dev Center article for details. (Filter varies by add-on)

Build Logs

The logs generated while building and deploying your app are separate from the app’s runtime logs. Logs for both successful and unsuccessful builds are available from your app’s Activity tab in the Heroku Dashboard:

Click View build log for any build event in the Activity Feed to see its logs:

Log History Limits

Logplex is designed for collating and routing log messages, not for storage. It retains the most recent 1,500 lines of your consolidated logs, which expire after 1 week.

For more production-ready persistence of logs, add one of the Heroku platform’s available logging add-ons to your app. Most of these add-ons offer a free plan to get started.

Alternatively, implement your own log drains for full control over what happens to your logs.

Writing to Your Log

Your logs capture anything your app writes to standard out (stdout) or standard error (stderr). 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 real-time logging, disable any log buffering your application performs. For example, in Ruby, add the following code to your config.ru file:

$stdout.sync = true

Some frameworks send log output somewhere other than stdout by default and require extra configuration. For example, when using the Ruby on Rails TaggedLogger by ActiveSupport, add the following into your app’s configuration to get stdout logging:

config.logger = Logger.new(STDOUT)

See Writing Best Practices For Application Logs for more info.

View Logs

You can view logs with the Heroku CLI, the dashboard, your logging add-on, or in your log drain.

View Logs with the Heroku CLI

To fetch your app’s most recent logs, use the heroku logs command:

$ 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://example-app-1234567890ab.heroku.com/]
2010-09-16T15:13:46.723498+00:00 heroku[router]: at=info method=GET path="/posts" host=example-app-1234567890ab.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. See Heroku’s Log Format and Message Ordering for more info.

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’re done, press Ctrl+C to return to the prompt.

A real-time tail session automatically terminates after one hour of inactivity.

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 --dyno (or -d) filtering arguments:

$ heroku logs --dyno 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 --dyno 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, you can use either the base name (like --dyno web) or the full name ( like --dyno web.1).

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

$ heroku logs --source app --tail

View Logs with the Heroku Dashboard

You can view your logs on the web by logging into your Heroku dashboard. Navigate to the app you want to see, for example, https://dashboard.heroku.com/apps/<app-name>. On this page, select “more” tosee a drop-down menu:

From this menu, select View logs. See Heroku’s Log Format and Message Ordering for more info.

View Logs with a Logging Add-on

It’s recommended to add one of the Heroku platform’s available logging add-ons to your app. Most of these add-ons offer a free plan to get started. See your add-on’s documentation for instructions on retrieving logs.

Alternatively, you can implement a custom log drain.

Heroku’s Log Format and Message Ordering

​Heroku’s Logplex router collates and distributes the log entries generated by your app and other components of the platform.

Logs aren’t in chronological order. Logs originate from many sources and Logplex assembles them into a single log stream. Logplex itself uses a distributed architecture to ensure high availability, so log messages can also be out of order as they collect across multiple Logplex nodes.

The output format of the heroku logs command is as follows:

timestamp source[dyno]: message

• Timestamp - The date and time recorded at the time the dyno or component produced the log line. 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. Logplex splits any lines generated by dynos that exceed 10,000 bytes into 10,000-byte chunks without extra trailing newlines. It submits each chunk is submitted as a separate log line.