Katip

Represents a heirarchy of namespaces going from general to specific. For instance: ["processname", "subsystem"]. Note that single-segment namespaces can be created using IsString/OverloadedStrings, so "foo" will result in Namespace ["foo"].

Application environment, like prod, devel, testing.

Verbosity controls the amount of information (columns) a Scribe emits during logging.

The convention is: - V0 implies no additional payload information is included in message. - V3 implies the maximum amount of payload information. - Anything in between is left to the discretion of the developer.

Katip requires JSON objects to be logged as context. This typeclass provides a default instance which uses ToJSON and produces an empty object if toJSON results in any type other than object. If you have a type you want to log that produces an Array or Number for example, you'll want to write an explicit instance here. You can trivially add a ToObject instance for something with a ToJSON instance like:

instance ToObject Foo

Payload objects need instances of this class. LogItem makes it so that you can have very verbose items getting logged with lots of extra fields but under normal circumstances, if your scribe is configured for a lower verbosity level, it will only log a selection of those keys. Furthermore, each Scribe can be configured with a different Verbosity level. You could even use registerScribe, unregisterScribe, and clearScribes to at runtime swap out your existing scribes for more verbose debugging scribes if you wanted to.

When defining payloadKeys, don't redundantly declare the same keys for higher levels of verbosity. Each level of verbosity automatically and recursively contains all keys from the level before it.

This has everything each log message will contain.

Field selector by verbosity within JSON payload.

Construct a simple log from any JSON item.

Reasonable defaults for a scribe. Buffer size of 4096.

A concrete monad you can use to run logging actions. Use this if you prefer an explicit monad transformer stack and adding layers as opposed to implementing Katip for your monad.

Execute KatipT on a log env.

Create a reasonable default InitLogEnv. Uses an AutoUpdate which updates the timer every 1ms. If you need even more timestamp precision at the cost of performance, consider setting _logEnvTimer with getCurrentTime.

Add a scribe to the list. All future log calls will go to this scribe in addition to the others. Writes will be buffered per the ScribeSettings to prevent slow scribes from slowing down logging. Writes will be dropped if the buffer fills.

Remove a scribe from the environment. This does not finalize the scribe. This mainly only makes sense to use with something like MonadReader's local function to temporarily disavow a single logger for a block of code.

Unregister all scribes. Note that this is not for closing or finalizing scribes, use closeScribes for that. This mainly only makes sense to use with something like MonadReader's local function to temporarily disavow any loggers for a block of code.

Call this at the end of your program. This is a blocking call that stop writing to a scribe's queue, waits for the queue to empty, finalizes each scribe in the log environment and then removes it. Finalizers are all run even if one of them throws, but the exception will be re-thrown at the end.

Finalize a scribe. The scribe is removed from the environment, its finalizer is called so that it can never be written to again and all pending writes are flushed. Note that this will throw any exceptions yoru finalizer will throw, and that LogEnv is immutable, so it will not be removed in that case.

Log message with Builder underneath; use <> to concat in O(1).

Pack any string-like thing into a LogStr. This will automatically work on String, ByteString, Text and any of the lazy variants.

Shorthand for logStr

Convert any showable type into a LogStr.

Logs to a file handle such as stdout, stderr, or a file. Contexts and other information will be flattened out into bracketed fields. For example:

[2016-05-11 21:01:15][MyApp][Info][myhost.example.com][PID 1724][ThreadId 1154][main:Helpers.Logging Helpers/Logging.hs:32:7] Started
[2016-05-11 21:01:15][MyApp.confrabulation][Debug][myhost.example.com][PID 1724][ThreadId 1154][confrab_factor:42.0][main:Helpers.Logging Helpers/Logging.hs:41:9] Confrabulating widgets, with extra namespace and context
[2016-05-11 21:01:15][MyApp][Info][myhost.example.com][PID 1724][ThreadId 1154][main:Helpers.Logging Helpers/Logging.hs:43:7] Namespace and context are back to normal

Returns the newly-created Scribe. The finalizer flushes the handle. Handle mode is set to LineBuffering automatically.

Logs to a file handle such as stdout, stderr, or a file. Takes a custom ItemFormatter that can be used to format Item as needed.

Returns the newly-created Scribe. The finalizer flushes the handle. Handle mode is set to LineBuffering automatically.

A specialization of mkHandleScribe that takes a FilePath instead of a Handle. It is responsible for opening the file in AppendMode and will close the file handle on closeScribe/closeScribes. Does not do log coloring. Sets handle to LineBuffering mode.

A custom ItemFormatter for logging Items. Takes a Bool indicating whether to colorize the output, Verbosity of output, and an Item to format.

See bracketFormat and jsonFormat for examples.

A traditional bracketed log format. Contexts and other information will be flattened out into bracketed fields. For example:

[2016-05-11 21:01:15][MyApp][Info][myhost.example.com][PID 1724][ThreadId 1154][main:Helpers.Logging Helpers/Logging.hs:32:7] Started
[2016-05-11 21:01:15][MyApp.confrabulation][Debug][myhost.example.com][PID 1724][ThreadId 1154][confrab_factor:42.0][main:Helpers.Logging Helpers/Logging.hs:41:9] Confrabulating widgets, with extra namespace and context
[2016-05-11 21:01:15][MyApp][Info][myhost.example.com][PID 1724][ThreadId 1154][main:Helpers.Logging Helpers/Logging.hs:43:7] Namespace and context are back to normal

Logs items as JSON. This can be useful in circumstances where you already have infrastructure that is expecting JSON to be logged to a standard stream or file. For example:

{"at":"2018-10-02T21:50:30.4523848Z","env":"production","ns":["MyApp"],"data":{},"app":["MyApp"],"msg":"Started","pid":"10456","loc":{"loc_col":9,"loc_pkg":"main","loc_mod":"Helpers.Logging","loc_fn":"Helpers\\Logging.hs","loc_ln":44},"host":"myhost.example.com","sev":"Info","thread":"ThreadId 139"}
{"at":"2018-10-02T21:50:30.4523848Z","env":"production","ns":["MyApp","confrabulation"],"data":{"confrab_factor":42},"app":["MyApp"],"msg":"Confrabulating widgets, with extra namespace and context","pid":"10456","loc":{"loc_col":11,"loc_pkg":"main","loc_mod":"Helpers.Logging","loc_fn":"Helpers\\Logging.hs","loc_ln":53},"host":"myhost.example.com","sev":"Debug","thread":"ThreadId 139"}
{"at":"2018-10-02T21:50:30.4523848Z","env":"production","ns":["MyApp"],"data":{},"app":["MyApp"],"msg":"Namespace and context are back to normal","pid":"10456","loc":{"loc_col":9,"loc_pkg":"main","loc_mod":"Helpers.Logging","loc_fn":"Helpers\\Logging.hs","loc_ln":55},"host":"myhost.example.com","sev":"Info","thread":"ThreadId 139"}

Scribes are handlers of incoming items. Each registered scribe knows how to push a log item somewhere.

Guidelines for writing your own Scribe

Scribes should always take a Severity and Verbosity.

Severity is used to exclude log messages that are lower than the provided Severity. For instance, if the user passes InfoS, DebugS items should be ignored. Katip provides the permitItem utility for this. The user or the scribe may use permitAND and permitOR to further customize this filtering, even dynamically if they wish to.

Verbosity is used to select keys from the log item's payload. Each LogItem instance describes what keys should be retained for each Verbosity level. Use the payloadObject utility for extracting the keys that should be written.

Scribes provide a finalizer IO action (scribeFinalizer) that is meant to synchronously flush any remaining writes and clean up any resources acquired when the scribe was created. Internally, katip keeps a buffer for each scribe's writes. When closeScribe or closeScribes is called, that buffer stops accepting new log messages and after the last item in its buffer is sent to liPush, calls the finalizer. Thus, when the finalizer returns, katip can assume that all resources are cleaned up and all log messages are durably written.

While katip internally buffers messages per ScribeSettings, it sends them one at a time to the scribe. Depending on the scribe itself, it may make sense for that scribe to keep its own internal buffer to batch-send logs if writing items one at a time is not efficient. The scribe implementer must be sure that on finalization, all writes are committed synchronously.

Signature of a function passed to Scribe constructor and mkScribe* functions that decides which messages to be logged. Typically filters based on Severity, but can be combined with other, custom logic with permitAND and permitOR

AND together 2 permit functions

OR together 2 permit functions

Should this item be logged given the user's maximum severity? Most new scribes will use this as a base for their PermitFunc

Constrain payload based on verbosity. Backends should use this to automatically bubble higher verbosity levels to lower ones.

Convert log item to its JSON representation while trimming its payload based on the desired verbosity. Backends that push JSON messages should use this to obtain their payload.

Provides a simple transformer that defines a KatipContext instance for a fixed namespace and context. Just like KatipT, you should use this if you prefer an explicit transformer stack and don't want to (or cannot) define KatipContext for your monad . This is the slightly more powerful version of KatipT in that it provides KatipContext instead of just Katip. For instance:

  threadWithLogging = do
    le <- getLogEnv
    ctx <- getKatipContext
    ns <- getKatipNamespace
    forkIO $ runKatipContextT le ctx ns $ do
      $(logTM) InfoS "Look, I can log in IO and retain context!"
      doOtherStuff