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Judul : Log4j Tips : Role Mdc Or Mapped Dignostic Context To Distinguish Logging Per Customer Or Request
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Log4j Tips : Role Mdc Or Mapped Dignostic Context To Distinguish Logging Per Customer Or Request

The MDC or Mapped Diagnostic Context is a concept or characteristic of Log4j logging library which tin live on used to grouping related log messages together. For example, past times using MDC you lot tin postage a unique identification String similar clientId or orderId on each log message in addition to therefore past times using grep ascendance inwards Linux, you lot tin extract all log messages for a item customer or club to empathise precisely what happened to a item order. This is peculiarly really useful inwards multi-threaded, concurrent Java applications where multiple threads are simultaneously processing multiple orders from multiple clients.  In such applications, searching for relevant log messages inwards a big log file where log messages for multiple orders or clients are overlapping is a big task.

The MDC concept is non a novel concept in addition to available since log4j 1.2 version. Since most of existent Blue Planet systems e.g. high-frequency trading application procedure thousands of club simultaneously where dissimilar thread runs the same slice of code but amongst dissimilar data, each execution is unique, but without a unique identification it's hard to honour all log messages related to processing a item order. The MDC or Mapped Diagnostic Context characteristic of Log4j solves that work past times stamping unique identification string on relevant log messages.

Once you lot seat this information on MDC at the foremost of your processing cycle, all log messages effect amongst the processing of that club volition live on stamped amongst same OrderId, but processing of dissimilar club volition live on having dissimilar orderId, this way you lot tin runway consummate processing of a item order. You tin fifty-fifty honour if an club made to your arrangement or not.

Why role Mapped Diagnostic Context or MDC inwards Log4j?

If you lot bring worked inwards a multithreaded high majority transaction processing system, therefore you lot bring faced a situation, where you lot demand to describe processing of a item order, without MDC, it's nightmare. You demand to include Order specific information inwards every unmarried log messages you lot print.

With MDC or ThreadContext from Log4j2 onwards, you lot only demand to add together that information 1 time, at the foremost of your processing, afterward that it's available inwards every log messages.

Btw, this number is non only for multi-threaded heart in addition to someone Java application, you lot volition confront similar issues on an company or spider web applications, where multiple customer requests are processed simultaneously past times dissimilar threads. Log messages from processing ii dissimilar orders may interleave together. MDC allows you lot to postage each log messages amongst a unique identification number e.g. orderId, which is unique for every order, sessionId, which is unique for every session or username, which is unique for every user.

In short, Mapped Diagnostic Context volition care you lot inwards debugging, troubleshooting, in addition to fifty-fifty inwards auditing. It's likewise 1 of the logging best practices to employ on production systems.

Log4j likewise provides a similar utility called NDC, known as Nested Diagnostic Context, both of which are replaced by ThreadContext class in Log4j 2.

The ThreadContext class provides a Map and a Set to supervene upon MDC in addition to NDC. The Thread Context Map allows whatever number of items to live on added in addition to live on identified using key/value pairs, while ThreadContextStack allows 1 or to a greater extent than items to live on pushed on the Stack in addition to therefore live on identified past times their club inwards the Stack or past times the information itself. 

Since key/value pairs are to a greater extent than flexible, the Thread Context Map is recommended when information items may live on added during the processing of the asking or when in that location are to a greater extent than than 1 or ii items. Btw, if you lot an experienced Java developer, I advise you lot reading a skillful majority e.g. Java Performance Definitive Guide By Scott Oaks to larn to a greater extent than close how logging impacts performance. It's really of import for a seasoned developer to know what he is doing. 

How to role MDC to log customer specific information inwards log4j

You tin seat whatever information into Mapped Diagnostic Context or MDC past times calling the put() method. MDC is a static class i.e. a shape amongst only static methods, which way you lot tin lead telephone telephone them without creating whatever event of MDC.

Remember, this information is stored every bit thread local variable, which may campaign a retention leak inwards a spider web application if used incorrectly (see). If you lot are using MDC to log customer or club specific information e.g. orderId inwards a spider web application using a filter than you lot tin likewise withdraw it in 1 trial done.

try{   MDC.put("tradeId", trade.getId()); }finally{   MDC.remove.remove("tradeId"); }

By the way from log4j2 onwards, MDC in addition to NDC (Nested Diagnostic Context) are merged into a shape called ThreadContext So, instead of using MDC, you lot demand to role ThreadContext shape to seat unique context data.

try{    ThreadContext.put("tradeId", trade.getId()); }finally {    ThreadContext.clear(); }

Once available inwards MDC, this information tin live on printed inwards the log file using PatternLayout. You tin role %X{tradeId) to impress tradeId inwards log messages, hollo upwardly tradeId for dissimilar trades volition live on different, which allows you lot to describe logging messages for private trades. You tin role next Patterns to include MDC contents inwards log files:

• Use %X past times itself to include the total contents of the Map.
• Use %X{key} to include the specified key.
• Use %x to include the total contents of the Stack

Remember MDC is managed on a per thread basis in addition to every kid thread automatically inherits a re-create of the mapped diagnostic context from its parent. This is achieved past times using InheritableThreadLocal class, which is a subclass of the ThreadLocal class.

This shape extends ThreadLocal to furnish inheritance of values from bring upwardly thread to kid thread: when a kid thread is created, the kid receives initial values for all inheritable thread-local variables for which the bring upwardly has values.

Here is an example, where both bring upwardly in addition to kid thread is using the same value from MDC or ThreadContextMap:

Normally the child's values volition live on identical to the parent's; however, the child's value tin live on made an arbitrary portion of the parent's past times overriding the childValue() method


That's all guys, if you lot bring been using Log4j 1.x or Log4j2 in addition to non using MDC, you lot are missing a really of import feature, peculiarly if you lot ofttimes demand to know what happened to a item order, asking or user. I bring constitute putting information similar OrderId, Username, RquestId, or something which tin distinguish each asking adds tremendous value during troubleshooting in addition to debugging. It's likewise easier to describe orders in addition to respond back upwardly queries. If you lot bring a unique id to distinguish each message, therefore e'er include them into MDC to distinguish logging information from customer or club basis.

Further Learning
Design Patterns Library
Clean Code: Writing Code for Humans
SOLID Principles of Object Oriented Design

Other Java Logging Tutorials in addition to Best Practices you lot may like

• 5 Java Performance tuning books for experienced Programmers (list)
• Why role Log4j logging vs System.out.println inwards Java? (answer)
• Why role SLF4j over log4j for logging inwards Java? (answer)
• How to enable SSL debugging log inwards Java Virtual Machine? (tips)
• How to configure Log4j without XML or Properties file inwards Java? (tutorial)
• Difference betwixt Functional in addition to Non-Functional Requirement (article)
• 10 Essential JVM Options for Real Blue Planet Java Applications (tips)

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