[Haskell-cafe] ANN: katip - a new structured logging framework for Haskell

Ozgun Ataman ozgun.ataman at soostone.com
Mon Mar 14 18:20:15 UTC 2016

Fellow Haskellers,

We're delighted to release a new logging framework today called "katip",
centered around the idea of making it easy to include contextual data in
individual log items in the form of structured fields. Katip has become,
rather quickly, our go-to logging library at Soostone, included in
everything from simple ad-hoc scripts to production systems under load.

You can find katip on both Hackage[1] and Github[2]. The core library has
also been added to Stackage nightly, though the ElasticSearch backend is
waiting on a dependency conflict on aeson. We hope you find it useful and
look forward to any contributions or feedback!

[1] http://hackage.haskell.org/package/katip
[2] https://github.com/Soostone/katip

As a little summary, here's the features checklist from the README:

* *Structured:* Logs are structured, meaning they can be individually
  tagged with key value data (JSON Objects). This helps you add
  critical details to log messages before you need them so that when
  you do, they are available. Katip exposes a typeclass for log
  payloads so that you can use rich, domain-specific Haskell types to
  add context that will be automatically merged in with existing log

* *Easy to Integration:* Katip was designed to be easily integrated
  into existing monads. By using typeclasses for logging facilities,
  individual subsystems and even libraries can easily add their own
  namespacing and context without having any knowledge of their
  logging environment.

* *Practical Use:* Katip comes with a set of convenience facilities
  built-in, so it can be used without much headache even in small

    * A `Handle` backend for logging to files in simple settings.

    * A `AnyLogPayload` key-value type that makes it easy to log
      structured columns on the fly without having to define new data

    * A `Monadic` interface where logging namespace can be obtained
      from the monad context.

    * Multiple variants of the fundamental logging functions for
      optionally including fields and line-number information.

* *Extensible:* Can be easily extended (even at runtime) to output to
  multiple backends at once (known as scribes). See
  `katip-elasticsearch` as an example. Backends for other forms of
  storage are trivial to write, including both hosted database systems
  and SaaS logging providers.

* *Debug-Friendly:* Critical details for monitoring production systems
  such as host, PID, thread id, module and line location are
  automatically captured. User-specified attributes such as
  environment (e.g. Production, Test, Dev) and system name are also

* *Configurable:* Can be adjusted on a per-scribe basis both with
  verbosity and severity.

    * *Verbosity* dictates how much of the log structure should
      actually get logged. In code you can capture highly detailed
      metadata and decide how much of that gets emitted to each backend.

    * *Severity* AKA "log level" is specified with each message and
      individual scribes can decide whether or not to record that
      severity. It is even possible to at runtime swap out and replace
      loggers, allowing for swapping in verbose debug logging at runtime
      if you want.

* *Battle-Tested:* Katip has been integrated into several production
  systems since 2015 and has logged hundreds of millions of messages
  to files and ElasticSearch.
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