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πŸ€ Ratlog 1.0.0 - Application Logging for Rats, Humans and Machines

Ratlog is a specification of an application logging format.

The format is language independent and libraries for producing and parsing logs are available in different programming languages.

While Ratlog specifies a text-based output format, it also provides useful semantics for logging.

Even if not one is bound to use another representation for logs, we invite to make use of Ratlog's semantics of message, tags and *fields.



The most basic Ratlog line is an empty line. Even if no output is produced, the system should know that something happened:

All log lines should have a message describing the event:

System started

Tags allow to scope logs in context or categorize them:

[warn] Disk space running low

Tags can be chained and their order can carry meaning:

[fs|warn|critical] Disk space running low

Additional information can be added to logs using fields:

File not found | path: /tmp/notfound.txt

Putting everything together, a log line consists of tags, message and fields:

[http|request|error] File not found | code: 404 | method: GET | route: /admin

Logs should be easy to parse by programs and CLI tools such as grep and cut :

  • Filter by tag:
cat ./logs.rat | grep '\[|\|warn'
  • Filter by field:
cat ./logs.rat | grep '| code: 404'
  • Count tag:
cat ./logs.rat | grep -c '\[|\|warn'
  • Ignore fields:
cat ./logs.rat | cut -d'|' -f1
  • Ignore tags:
cat ./logs.rat | cut -d] -f2-


  • Create a log format independent from any programming language or library.
  • Logs should be readable by humans without external tooling. JSON is not readable.
  • Logs should be parsable by machines.
  • Logging should never fail. There are no invalid logs.
  • Provide semantics useful for the majority of application logging. More specific semantics can be built on top of Ratlog using tags and fields.
  • Every log has a message, even if it is empty.
  • Log levels are arbitrary and restricting. Use tags as flexible alternative instead.
  • A log event is always a single line of text.
  • Applications don't need to worry about logging timestamps.
  • Applications don't need to know their own name.
  • Utilize supervisor tools such as docker and SystemD: Don't duplicate information they already collect, such as service names and timestamps.


A Ratlog line consists of the segments tags, message and fields, in this order.


  • Text should be UTF-8 encoded. If that is not possible, the reason and encoding must be clearly stated.


  • A log is always a single line ending with a Unix line separator \n.
  • Line breaks in the log data must be escaped as \\n.


  • The tags segment must start at the beginning of the line.
  • The tags segment starts with [ and ends with ].
  • Leave no space before and after [.
  • Leave no space before ].
  • Leave a space after ].
  • Tags are separated by | without spaces.
  • Inside the tags segment ] can be escaped as \] and | as \|.
  • An empty tag value is still a valid tag.
  • The tags segment can be omitted. If no tags exist no [] is printed. [] represents a single empty tag.
  • If there is no matching closing token, there is no tags segment and the opening [ and all following characters are treated as part of the message segment.
  • Tag values are not unique. They can be repeated.
  • The sorting of tags may be user-defined and may be relevant.
  • In terms of data structures tags, should be thought of as an ordered list.


  • The message segment always exists, even if it is completely empty. It is never omitted.
  • In the message segment [ can be escaped as \[ and | as \|.


  • The fields segment may be omitted if no field exists.
  • Each field starts with |.
  • Leave a space around | in front of each field.
  • Field key and value are separated by : followed by a space.
  • Space before : is treated as part of the field key.
  • Inside a field | can be escaped as \| and : as \:.
  • A completely empty field key or value is still valid.
  • If a field's value is empty, the separator : may be omitted.
  • Field keys must be unique.
  • If any field is invalid, the fields segment is ignored and all potential field characters, even those containing valid fields, are treated as part of the message segment instead.
  • The sorting of fields is not relevant and may be ignored. It is recommended to sort fields alphabetically.
  • In terms of data structures fields should be thought of as a hashmap.


  • All additional spaces not mentioned above are treated as data.

Library and Tooling Development

We always welcome new developers to contribute new tooling to the Ratlog ecosystem. If you would like to create or extend a logging library in a programming language of your choice, you can use the JSON test suite to verify your logger or parser is producing or consuming logs correctly:

  • The test suite contains a JSON object with the attributes "meta", "generic" and "parsing".
  • The attributes "generic" and "parsing" contain arrays of test cases.
  • "parsing" is only for validating parsers.
  • Each test case is an object with "log" and "data" attributes.
  • "log" is a string containing a log line.
  • "data" is an object with "message", "fields" and "tags" attributes. All but "message" are optional.
  • "message" is a string containing the logged message.
  • "fields" is an object with arbitrary attributes mapping strings to strings.
  • "tags" is an array of arbitrary strings.

When writing a logging library only use generic. Use "data" as input and verify that the produced line matches "log". Since all parameters here are well defined and all values are represented as strings, make sure your logging library is also handling other input passed to it, which - depending of the strictness of the type system of the language you are working with - might have all sorts of shapes. Keep in mind - logging should never crash a program.

When writing a parser "log" should be parsed and the fields specified in "data" should be retrieved from it. Parser should also be able to handle the test cases listed under "parsing".


Are there not enough logging libraries already?

There are enough libraries. They all have different goals and functionality. Some are simple, some are quite sophisticated. But hardly any two of them produce matching formats - or formatting is not one of their concerns at all and they leave it up to the user.

The closest to a common format on which a few libraries agree on is JSON. But JSON doesn't know about logging. Each developer has to come up with their own JSON format. And JSON is not made for humans. You need additional tooling to query and read logs in a sane way.

Why doesn't Ratlog come with timestamps?

Timestamps are a fundamental part of event-based data such as application logs.

However in the context applications are embedded in a supervisor that is collecting the logs most likely already adds timestamps anyways. Therefore there is no need to duplicate that information.

If you have a Docker setup and you run docker logs -t myapp, you get logs displayed with timestamps:

2018-03-29T11:10:29.116Z [file-import|warning] file not found | code: 404 | path: /tmp/notfound.txt

Similarly when using SystemD journalctl -u myapp includes timestamps in its output:

Apr 13 22:15:34 myhost myapp[1234]: [file-import|warning] file not found | code: 404 | path: /tmp/notfound.txt

Running a standalone application, timestamps can be added easily by piping the output through ts:

$ ./myapp | ts
Apr 14 12:03:38 [file-import|warning] file not found | code: 404 | path: /tmp/notfound.txt

And in case you really need the application to log timestamps directly, you can simply add a timestamp field.

What about Common Log Format?

There are some standardized logging formats for specific use cases such as the Common Log Format for server logs, which is very useful if you are building an HTTP server.

If you have a more specific output format to use for your domain, you can provide more meaningful context - which is great. Ratlog tries to be a foundation for generic application logging.

If you build a web server, have a look at the Common Log Format.

What about logfmt?

logfmt describes logs as key-value pairs. It is simpler and (arguably) easier to read than JSON. It is a great start for adding some structure to log output. The key-value semantics of logfmt are similar to fields in Ratlog, but Ratlog additionally gives you the semantics of message and tags, which I would argue are generally useful for logging. If you rather only have field semantics, logfmt is a great and even simpler alternative to Ratlog.

How do I add Ratlog to my library/module/package?

Libraries shouldn't produce logs. They should return machine readable information about their state to the application code making use of the library. The application should be responsible for deciding on what and how to log the state.

Should I use Ratlog for my CLI tool?

Certain CLI tools produce generic output similar to application logs. In those scenarios Ratlog might be a great way of displaying consistent, readable, parsable output.

Many tools, however, produce very specific output. For example, it wouldn't be helpful if git would try to display its output as Ratlog.

Mostly the output tools write to stdout is too specific and not log-like. But tools sometimes like to provide additional information about their state via stderr. That might be a good use case of Ratlog.

How do I collect metrics from Ratlog logs?

Logs are for humans. Metrics, events and other output are better collected separately because there are tools better suited for that task (such as Prometheus for metrics). Metrics should be handled different to text based logs. Metrics can be aggregated and have very different performance characteristics and usage goals.

How do I log errors?

Errors should not be logged. Other tools are better suited at handling them (such as Sentry). Tools specialized on collecting errors can provide many useful features such as grouping, silencing or linking stack-traces to source code. Those features are not in the scope of logging.

How do I query logs across many services?

Ratlog is just text. You can collect logs from many services in a central place like you are already used to with tools such as fluentd, Elastic, CloudWatch or OK Log.

Why are there no nested fields or lists inside fields?

Ratlog tries to be as simple and generic as possible. If you need to store more complex data, custom formats can be built on top of Ratlog. It is also possible to store data in a format such as JSON inside a field. However, we recommend keeping fields simple. It makes the logs easier to read an query. Don't log more than absolutely necessary and especially don't write sensitive user data inside logs.

Why use [] and | as separators?

They conflict little with natural language or other popular formats such as JSON or XML/HTML, while at the same time being common enough that you should be able to locate them on your keyboard.