scala-steward / blindsight

Blindsight is a Scala logging API that allows for fluent logging, semantic logging, and context aware logging.

Blindsight

Suffering in silence, you check the logs for fresh telemetry.

You think: That can't be right.

-- Blindsight, Peter Watts

Blindsight is "observability through logging" where observability is defined as baked in high cardinality structured data with field types. The name is taken from Peter Watts' excellent first contact novel, Blindsight.

Blindsight is a logging library written in Scala that wraps SLF4J to add useful features that solve several outstanding problems with logging:

• Rendering structured logs in multiple formats through a format-independent AST and DSL.
• Managing complex relationships and schema through JSON-LD.
• Expressing domain specific objects as arguments through type classes.
• Resolving operation-specific loggers through logger resolvers.
• Building up complex logging statements through fluent logging.
• Enforcing user supplied type constraints through semantic logging.
• Minimal-overhead tracing and causality tracking through flow logging.
• Providing thread-safe context to logs through context aware logging.
• Time-based and targeted diagnostic logging through conditional logging.
• Hooks into logging entries through entry transformation
• Application accessible debug and trace logs through event buffers
• Method or line based logging overrides at runtime through scripting.
• Easier "printf debugging" through macro based inspections.

See the documentation for more details.

Example

You can check out a "starter project" at https://github.com/tersesystems/blindsight-starter.

There's an example application at https://github.com/tersesystems/play-blindsight that integrates with Honeycomb Tracing using the flow logger:

Dependencies

The only hard dependency is the SLF4J API, but the DSL functionality is only implemented for Logback with logstash-logback-encoder.

Blindsight is a pure SLF4J wrapper: it delegates all logging through to the SLF4J API and does not configure or manage the SLF4J implementation at all.

Versions are published for Scala 2.11, 2.12, 2.13, and 3.0.0.

Install

See Setup for how to install Blindsight.

You can check out a "starter project" at https://github.com/tersesystems/blindsight-starter.

Because Blindsight uses a very recent version of Logstash that depends on Jackson 2.11.0, you may need to update your dependencies for the jackson-scala-module if you're using Play or Akka.

libraryDependencies += "com.fasterxml.jackson.module" %% "jackson-module-scala" % "2.11.0"

Usage

The easiest way to use Blindsight is to import the base package and the DSL:

import com.tersesystems.blindsight._
import com.tersesystems.blindsight.DSL._

To use a Blindsight Logger:

val logger = LoggerFactory.getLogger
logger.info("I am an SLF4J-like logger")

or in block form for diagnostic logging:

logger.debug { debug => debug("I am an SLF4J-like logger") }

Structured DSL:

import com.tersesystems.blindsight._
import com.tersesystems.blindsight.DSL._

logger.info("Logs with argument {}", bobj("array" -> Seq("one", "two", "three")))

Statement Interpolation:

val dayOfWeek = "Monday"
val temp = 72 

// macro expands this to:
// Statement("It is {} and the temperature is {} degrees.", Arguments(dayOfWeek, temp))
val statement: Statement = st"It is ${dayOfWeek} and the temperature is ${temp} degrees."

logger.info(statement)

Marker/Argument Type Classes:

case class Lotto(
  id: Long,
  winningNumbers: List[Int],
  winners: List[Winner],
  drawDate: Option[java.util.Date]
) {
  lazy val asBObject: BObject = "lotto" ->
      ("lotto-id"          -> id) ~
        ("winning-numbers" -> winningNumbers) ~
        ("draw-date"       -> drawDate.map(_.toString)) ~
        ("winners"         -> winners.map(w => w.asBObject))
}

object Lotto {
  implicit val toArgument: ToArgument[Lotto] = ToArgument { lotto => Argument(lotto.asBObject) }
}

val winners =
  List(Winner(23, List(2, 45, 34, 23, 3, 5)), Winner(54, List(52, 3, 12, 11, 18, 22)))
val lotto = Lotto(5, List(2, 45, 34, 23, 7, 5, 3), winners, None)

logger.info("message {}", lotto) // auto-converted to structured output

JSON-LD:

implicit val nodeObjectToArgument: ToArgument[NodeObject] = ToArgument[NodeObject] { nodeObject =>
  Argument(BlindsightASTMapping.toBObject(nodeObject))
}

implicit val nodeObjectToMarkers: ToMarkers[NodeObject] = ToMarkers { nodeObject =>
  Markers(BlindsightASTMapping.toBObject(nodeObject))
}

implicit val nodeObjectToStatement: ToStatement[NodeObject] = ...

class Foo extends LDContext { // LDContext contains all the type safe bindings
  def sayHello(): Unit = {
    val willPerson = NodeObject(
      `@type`    -> "Person",
      `@id`      -> willId,
      givenName  -> "Will",
      familyName -> "Sargent",
      parent     -> parentId,
      occupation -> Occupation(
        estimatedSalary = MonetaryAmount(Currency.getInstance("USD"), 1),
        name = "Code Monkey"
      )
    )

    logger.info("as an argument {}", willPerson) // as an argument
    logger.info(Markers(willPerson), "as a marker") // as a marker
    
    logger.semantic[NodeObject].info(willPerson) // or as a statement
  }
}

Fluent logging:

logger.fluent.info
  .message("The Magic Words are")
  .argument(Arguments("Squeamish", "Ossifrage"))
  .logWithPlaceholders()

Semantic logging:

// log only user events
logger.semantic[UserEvent].info(userEvent)

// Works well with refinement types
import eu.timepit.refined.api.Refined
import eu.timepit.refined.string._
import eu.timepit.refined._
logger.semantic[String Refined Url].info(refineMV(Url)("https://tersesystems.com"))

Flow logging:

import com.tersesystems.blindsight.flow._

implicit def flowBehavior[B]: FlowBehavior[B] = new SimpleFlowBehavior

val arg1: Int = 1
val arg2: Int = 2
val result:Int = logger.flow.trace(arg1 + arg2)

Conditional logging:

logger.withCondition(booleanCondition).info("Only logs when condition is true")

logger.info.when(booleanCondition) { info => info("when true") }

Context aware logging:

import DSL._

// Add key/value pairs with DSL and return a logger
val markerLogger = logger.withMarker(bobj("userId" -> userId))

// log with generated logger
markerLogger.info("Logging with user id added as a context marker!")

// can retrieve state markers
val contextMarkers: Markers = markerLogger.markers

Entry Transformation:

val logger = LoggerFactory.getLogger
               .withEntryTransform(e => e.copy(message = e.message + " IN BED"))

logger.info("You will discover your hidden talents")

Event Buffer:

val queueBuffer = EventBuffer(1)
val logger      = LoggerFactory.getLogger.withEventBuffer(queueBuffer)

logger.info("Hello world")

val event = queueBuffer.head

Scripting:

val scriptHandle = new ScriptHandle {
  override def isInvalid: Boolean = false // on file modification, etc
  override val script: String =
    """import strings as s from 'std.tf';
      |alias s.ends_with? as ends_with?;
      |
      |library blindsight {
      |  function evaluate: (long level, string enc, long line, string file) ->
      |    if (ends_with?(enc, "specialMethodName")) then true
      |    else false;
      |}
      |""".stripMargin
  override def report(e: Throwable): Unit = e.printStackTrace()
}
val scriptManager = new ScriptManager(scriptHandle) 
val location = new ScriptAwareLocation(scriptManager)

def specialMethodName = {
  // inside the specialMethodName method here :-)
  logger.debug.when(location.here) { log => 
    log("script allows selective logging by method or by line")
  }
}

Inspections:

import com.tersesystems.blindsight.inspection.InspectionMacros._

decorateIfs(dif => logger.debug(s"${dif.code} = ${dif.result}")) {
  if (System.currentTimeMillis() % 17 == 0) {
    println("branch 1")
  } else if (System.getProperty("derp") == null) {
    println("branch 2")
  } else {
    println("else branch")
  }
}

Benchmarks

Benchmarks are available here.

License

Blindsight is released under the "Apache 2" license. See LICENSE for specifics and copyright declaration.