LogAnnotation

LogAnnotation

Some data processing platforms have troubles with a large amount of log fields.
One possible way to limit the number of fields your application logs is listening annotations and using them every time you log a value.

Having a list of annotations in front of your eyes, you can easily make sure that the same names are not assigned to different values (which can lead to an invalid json and corrupt your structured logs).

Another problem can occur if the same field is logged with different types (e.g. as a string and as a number) . LogAnnotation statically types logging values. This helps also to store and retrieves values from data structures like Map[LogAnnotation[_], Any] (useful for zio-users who stores log context on FiberRefs).

Exapmle

case class MyId(id: String)

object LogKey {
  val count: LogAnnotation[Int] = LogAnnotation.make[Int]("count")
  val myId: LogAnnotation[MyId] = Loggable[String].contramap((_: MyId).id).logAnnotation("id")
}

val maybeId = Some(MyId("123"))

log.info("Hello", LogKey.count -> 5, LogKey.myId -> maybeId)
// log.error("this won't compile", LogKey.count -> "5")

will produce:

{
  "message": "Hello",
  "count": 5,
  "id": "123"
}