The document presents an introduction to Mapped Diagnostic Context (MDC) logging in Scala, emphasizing etiquette during sessions and providing guidance on utilizing MDC for enhanced logging in multi-threaded applications. It discusses the importance of contextualizing log statements for effective debugging and demonstrates how to configure dependencies and propagate MDC across threads. The session includes practical advice on maintaining logging context despite thread changes and highlights the necessity of a custom execution context for multi-thread MDC logging.

1. Presented By:
Prateek Gupta(Software Consultant)
Introduction to
MDC Logging in Scala

2. Lack of etiquette and manners is a huge turn off.
KnolX Etiquettes
Punctuality
Respect Knolx session timings, you
are requested not to join sessions
after a 5 minutes threshold post
the session start time.
Feedback
Make sure to submit a constructive
feedback for all sessions as it is
very helpful for the presenter.
Avoid Disturbance
Avoid leaving your window
unmuted after asking a question.
Mute
Please keep your window on mute.

3. Agenda
02 What is MDC ?
03 Dependency and Configuration
04 MDC Across a Single Thread
05 MDC Across Multiple Threads
06 Demo
01 Problem with multi-threaded Logging

4. Problem with multi-threaded Logging
● Imagine you have a multi-user web application. It gets a HTTP request, calls a few functions and serves
a response.
● But this might not be enough for useful debugging.

5. ● So we need a way to put the log messages into context, like this:
● With this we can search and filter our logs for the request we're interested in.

6. What is MDC ?
● The Mapped Diagnostic Context, in short MDC, allows to put context to all log statements for a
single thread.
● We can also propagate MDC across multiple threads or futures.
● For that purpose, we have to create our own ExecutionContext where we can copy the MDC to any
new thread that is being used.
● This provide us multi-thread MDC logging.

7. Dependency and Configuration
● Add the following dependency in your build.sbt.
libraryDependencies += "ch.qos.logback" % "logback-classic" % "1.2.3"
● In logback.xml, add the following pattern.
<pattern>%date (%level) [%thread] [%X{contextID}]: %message%n</pattern>

8. MDC Across a Single Thread
● It is very easy to provide context to all the log statements across a single thread.
MDC.put(“contextId”, UUID.randomUUID().toString)
● This puts a context value as identified by the key in the current thread’s context map.
● With this, everything that is in the current thread will use the same MDC.

9. MDC Across Multiple Threads
● MDC works fine if the thread doesn’t change.
● So if we use a new Thread or a Future, we are losing our precious MDC!
● To get the MDC across that new thread, we'll need to provide our own ExecutionContext.

10. MdcExecutionContext

11. Demo

12. Thank You !