In this session, we'll learn about MDC Logging in Scala. Also, we'll see how we can use it across a single or multiple threads.

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

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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 !