Heroku Logs Tutorial

Introduction

Heroku's logging feature is not the same as traditional Linux servers so I'd like to write a post talking about this feature in more detail.

Basic concepts and workflow

First, I'd like you to have a basic understanding of how Heroku logging process works.

Heroku’s Logplex router is responsible for collating and distributing the log entries generated by your app and other components of the Heroku platform

As you can see, if you use Heroku CLI to check the log, then can see logs from many different sources.

Heroku logs command

Heroku CLI has command to help you quickly check the Heroku logs.

USAGE
  $ heroku logs

OPTIONS
  -a, --app=app        (required) app to run command against

  -d, --dyno=dyno      only show output from this dyno type (such as "web" or
                       "worker")

  -n, --num=num        number of lines to display

  -r, --remote=remote  git remote of app to use

  -s, --source=source  only show output from this source (such as "app" or
                       "heroku")

  -t, --tail           continually stream logs

  --force-colors       force use of colors (even on non-tty output)

Since the log come from different sourcee, when using heroku logs command, you can add some options to filter the logs.

# display logs for web
$ heroku logs -d web -a heroku-app

# display logs for redis plugin
$ heroku logs -d heroku-redis -a heroku-app

# display logs come from app
$ heroku logs --source app

You can also add -t to see realtime log just like this.

$ heroku logs -a heroku-app -t

Heroku logs look like this

DATATIME app[heroku-redis]: blah blah blah
DATATIME app[web.1]: blah blah blah
DATATIME heroku[router] blah blah blah

So the log format is something like this {datetime} {source}[{dyno}]: blah blah blah, so -d and -s can help you filter logs in good way.

Error log with Sentry

Since Heroku logs do not to do store job (it works like streaming service), so here I'd like to give you a way to do error logging.

Heroku has plugin called Sentry to help you do error logging, it would catch error, save the error trackback data, and send you email notification. The free plan is good to go for most projects.

1. You need to add the add-on for your Heroku app (you can do that in web page or terminal command heroku addons:create sentry)

2. After you add the add-on, a new config variable SENTRY_DSN would be adeed, just check using heroku config

3. Then you need to add Sentry code to your project. (you can get the code on sentry homepage)

For, example, if you want to use Sentry with Django project.

1. pip install sentry-sdk

Add code below to your Django settings file

import sentry_sdk
import os

from sentry_sdk.integrations.django import DjangoIntegration

sentry_sdk.init(
    dsn=os.environ.get('SENTRY_DSN'),
    integrations=[DjangoIntegration()]
)

Sentry would help you aggregate the error logging and traceback data, it would also send you email error report. So you might not need to Heroku log to troubleshoot some error.

Logging Persistence

Since Heroku log work like stream service and it would not save it for you (you can only get the latest 1500 lines), so if you want to save the log to somewhere, there are some ways to do that.

1. Use some add-on, I would recommend logentries, which also have free plan for you to get started.

2. Another way is to implement custom log drains, please check Heroku doc for more details.

Conclusion

In this Heorku tutorial, I talked about how to use heroku logs and how to use Sentry to record error log for Heroku app.

If you have any question, please feel free to contact us.