Writing to Application Logs as an Add-on Partner
Last updated 13 July 2020
Add-on Partners can improve the development experience of their Add-on by inserting data into the app’s log stream. This article explains how to connect your service to an app’s log stream via Logplex.
If you’d like to read application logs as an add-on, see this documentation instead.
To gain access to the app’s Logplex endpoint, which will give you the capability to write to an app’s log stream, you will first need to store its URL that is submitted in the Add-on provision request. The URL will be available in the initial provision request and in the App Info API, once you have added the
"log_input" value to your manifest’s
requires field. More details of the provision request can be found in the Add-on Partner API Spec. If you have not previously stored the
log_input_url from the provision request, you can use the App Info API to query for the endpoint URL.
Data must be delivered to Logplex via HTTP. Using keep-alive connections and dense payloads, you can efficiently deliver all of your customer’s logs to Logplex via HTTP. Here is cURL example demonstrating a simple HTTP request:
$ URL="https://token:firstname.lastname@example.org/logs" $ curl $URL \ -d "62 <190>1 2013-03-27T20:02:24+00:00 hostname t.01234567-89ab-cdef-0123-456789abcdef procid - - foo62 <190>1 2013-03-27T20:02:24+00:00 hostname t.123 procid - - bar" \ -X "POST" \ -H "Content-Length: 130" \ -H "Content-Type: application/logplex-1" \ -A 'MyAddonName (https://elements.heroku.com/addons/my-addon-name; Ruby/2.1.2)'
Please make sure that you set a
User-Agent header that includes a string that identifies your add-on uniquely.
Note that the URL in the example above is purely illustrative. This URL can vary from app to app and will always have different credentials. You’ll need to store these URLs for each installed add-on that requires writing logs or fetching them from the App Info API as needed.
log_input_url field for a given app is subject to change, for instance if credentials must be rotated. If your HTTP request gets a
4xx response code, please see if the
log_input_url for the app has changed using the App Info API; if so, update your record for the app and retry.
Checkout all of the API details in the Logplex API Docs.
The headers of the request should contain
Content-Type. The value of
Content-Length should be the integer value of the byte length of the body. The value of
Content-Type should be
The body of the HTTP request should contain length delimited syslog packets. Syslog packets are defined in RFC5424. The following line summarizes the RFC protocol:
<prival>version time hostname appname procid msgid structured-data msg
You can use
<190> (local7.info) for the prival and
1 for the version. The time field should be set to the time at which the logline was created in rfc3339 format. The hostname should be set to the name of your service (e.g. postgresql). The appname field is reserved for the app’s Logplex token. The Logplex token is required to write to an app’s log stream. The procid is not used by logplex but is a great way to identify the process which is emitting the logs. Similarly, the msgid, and structured-data is not used by logplex and the value
- should be used. Finally, the msg is the section of the packet where the log message can be stored.
You should format your log messages in a way that is optimized for both human readability and machine parsability. With that in mind, log data should:
- Consist of a single message
- Use key-value pairs of the format
- Use a
sourcekey-value pair in log lines for distinguishing machines or environments (example:
- Show hierarchy with dots, from least to most specific, as in
- Units must immediately follow the number and must only include
These are log events that would benefit from statistical aggregation by a log consumer:
Periodically, your service may inject pre-aggregated metrics into a user’s logstream. The event could include the total number of database tables, active connections, cache usage:
sample#tables=30 sample#active-connections=3 sample#cache-usage=72.94 sample#cache-keys=1073002
Our experience logging for Postgres suggests that once a minute is a reasonable frequency for reporting aggregate metrics. Any higher frequency is potentially too noisy or expensive for storage/analysis. Be mindful of this when choosing to periodically log metrics from your add-on.
A reference implementation is provided at heroku/lpxc. This implementation highlights batching and keep-alive connections.