Log4j Tips : Last 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 stamp a unique identification String similar clientId or orderId on each log message in addition to and thence past times using grep ascendence inward Linux, you lot tin extract all log messages for a item customer or guild to sympathize just what happened to a item order. This is peculiarly real useful inward multi-threaded, concurrent Java applications where multiple threads are simultaneously processing multiple orders from multiple clients.  In such applications, searching for relevant log messages inward 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 dry reason systems e.g. high-frequency trading application procedure thousands of guild simultaneously where unlike thread runs the same slice of code but alongside unlike data, each execution is unique, but without a unique identification it's hard to notice 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 position this information on MDC at the showtime of your processing cycle, all log messages effect alongside the processing of that guild volition live on stamped alongside same OrderId, but processing of unlike guild volition live on having unlike orderId, this way you lot tin runway consummate processing of a item order. You tin fifty-fifty notice if an guild made to your organization or not.

Why role Mapped Diagnostic Context or MDC inward Log4j?

If you lot accept worked inward a multithreaded high majority transaction processing system, in addition to thence you lot accept faced a situation, where you lot demand to delineate processing of a item order, without MDC, it's nightmare. You demand to include Order specific information inward every unmarried log messages you lot print.

With MDC or ThreadContext from Log4j2 onwards, you lot merely demand to add together that information 1 time, at the showtime of your processing, later that it's available inward every log messages.

Btw, this number is non merely for multi-threaded heart in addition to someone Java application, you lot volition aspect upward similar issues on an corporation or spider web applications, where multiple customer requests are processed simultaneously past times unlike threads. Log messages from processing 2 unlike orders may interleave together. MDC allows you lot to postage stamp each log messages alongside 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 aid you lot inward debugging, troubleshooting, in addition to fifty-fifty inward auditing. It's equally good 1 of the logging best practices to employ on production systems.

Log4j equally good 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 supersede MDC in addition to NDC. The Thread Context Map allows whatsoever 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 and thence live on identified past times their guild inward 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 at that spot are to a greater extent than than 1 or 2 items. Btw, if you lot an experienced Java developer, I advise you lot reading a expert majority e.g. Java Performance Definitive Guide By Scott Oaks to larn to a greater extent than near how logging impacts performance. It's real of import for a seasoned developer to know what he is doing. 

How to role MDC to log customer specific information inward log4j

You tin position whatsoever information into Mapped Diagnostic Context or MDC past times calling the put() method. MDC is a static class i.e. a course of written report alongside merely static methods, which agency you lot tin straight telephone telephone them without creating whatsoever event of MDC.

Remember, this information is stored equally thread local variable, which may drive a retentiveness leak inward a spider web application if used incorrectly (see). If you lot are using MDC to log customer or guild specific information e.g. orderId inward a spider web application using a filter than you lot tin equally good take away it 1 time 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 course of written report called ThreadContext So, instead of using MDC, you lot demand to role ThreadContext course of written report to position unique context data.

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

Once available inward MDC, this information tin live on printed inward the log file using PatternLayout. You tin role %X{tradeId) to impress tradeId inward log messages, recall tradeId for unlike trades volition live on different, which allows you lot to delineate logging messages for private trades. You tin role next Patterns to include MDC contents inward 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 course of written report extends ThreadLocal to supply inheritance of values from nurture thread to kid thread: when a kid thread is created, the kid receives initial values for all inheritable thread-local variables for which the nurture has values.

Here is an example, where both nurture 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 business office of the parent's past times overriding the childValue() method


That's all guys, if you lot accept been using Log4j 1.x or Log4j2 in addition to non using MDC, you lot are missing a real of import feature, peculiarly if you lot oftentimes demand to know what happened to a item order, asking or user. I accept establish putting information similar OrderId, Username, RquestId, or something which tin distinguish each asking adds tremendous value during troubleshooting in addition to debugging. It's equally good easier to delineate orders in addition to reply back upward queries. If you lot accept a unique id to distinguish each message, in addition to thence e'er include them into MDC to distinguish logging information from customer or guild basis.

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

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