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<h2>Chapter 7: Diagnostic Context</h2>
<div class="author">
Authors: Ceki Gülcü, Sébastien Pennec
</div>
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<p>Copyright © 2000-2006, QOS.ch</p>
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<p>
One of the design goals of logback is to audit and debug complex distributed applications.
Most real-world distributed systems need to deal with multiple clients simultaneously.
In a typical multithreaded implementation of such a system, different threads will handle
different clients. A possible but discouraged approach to differentiate the logging output of
one client from another consists of instantiating a new and separate logger for each client.
This technique promotes the proliferation of loggers and considerably increases
their management overhead.
</p>
<p>
A lighter technique consists of uniquely stamping each
log request servicing a given client. Neil Harrison described this method in the book
<em>Patterns for Logging Diagnostic Messages</em> in
Pattern Languages of Program Design 3, edited by R. Martin, D. Riehle,
and F. Buschmann (Addison-Wesley, 1997). Logback offers a variant of this technique:
Mapped Diagnostic Contexts (MDC).
</p>
<p>
To uniquely stamp each request, the user puts contextual information into the
<code><a href="../xref/ch/qos/logback/classic/MDC.html">MDC</a></code>,
the abbreviation of Mapped Diagnostic Context.
The public interface of the MDC class is shown below.
</p>
<div class="source"><pre>package ch.qos.logback.classic;
public class MDC {
//Put a context value as identified by <em>key</em>
//into the current thread's context map.
<b>public static void put(String key, String val);</b>
//Get the context identified by the <code>key</code> parameter.
<b>public static String get(String key);</b>
//Remove the the context identified by the <code>key</code> parameter.
<b>public static void remove(String key);</b>
//Clear all entries in the MDC.
<b>public static void clear();</b>
//Returns the keys in the MDC as a Set. The returned value can be null.
<b>public static Set<String> getKeys();</b>
}</pre></div>
<p>
The <code>MDC</code> class contains only static methods.
It lets the developer place information in a <em>diagnostic context</em> that can be
subsequently retrieved by certain logback components. The
<code>MDC</code> manages contextual information on a per thread basis.
Typically, while starting to service a new client request, the developer will
insert pertinent contextual information, such as the client id, client's IP
address, request parameters etc. into the <code>MDC</code>. Logback components,
if appropriately configured, will automatically include this information
in each log entry.
</p>
<p>
The next application named
<code><a href="../xref/chapter7/SimpleMDC.html">SimpleMDC</a></code>
demonstrates this basic principle.
</p>
<em>Example 7.1: Basic MDC usage (<a href="../xref/chapter7/SimpleMDC.html">
logback-examples/src/main/java/chapter7/SimpleMDC.java)</a></em>
<div class="source"><pre>package chapter7;
import org.slf4j.LoggerFactory;
import ch.qos.logback.classic.Logger;
import ch.qos.logback.classic.MDC;
import ch.qos.logback.classic.PatternLayout;
import ch.qos.logback.core.ConsoleAppender;
public class SimpleMDC {
static public void main(String[] args) throws Exception {
// You can put values in the MDC at any time. We first put the
// first name
<b>MDC.put("first", "Dorothy");</b>
// Configure logback
PatternLayout layout = new PatternLayout();
layout.setPattern("%X{first} %X{last} - %m%n");
layout.start();
ConsoleAppender appender = new ConsoleAppender();
appender.setLayout(layout);
appender.start();
Logger root = (Logger)LoggerFactory.getLogger("root");
root.addAppender(appender);
// get a logger
Logger logger = (Logger)LoggerFactory.getLogger(SimpleMDC.class);
// We now put the last name
<b>MDC.put("last", "Parker");</b>
// The most beautiful two words in the English language according
// to Dorothy Parker:
logger.info("Check enclosed.");
logger.debug("The most beautiful two words in English.");
MDC.put("first", "Richard");
MDC.put("last", "Nixon");
logger.info("I am not a crook.");
logger.info("Attributed to the former US president. 17 Nov 1973.");
}
}</pre></div>
<p>
The main method starts by associating the value <em>Dorothy</em> with
the key <em>first</em> in the <code>MDC</code>. You can place as many
value/key associations in the <code>MDC</code> as you wish.
Multiple insertions with the same key will overwrite older values.
The code then proceeds to configure logback.
Note the usage of the <em>%X</em> specifier within the
<code>PatternLayout</code> conversion pattern. The <em>%X</em>
conversion specifier is employed twice, once for the key <em>first</em>
and once for the key <em>last</em>. After configuring the root logger,
the code associates the value <em>Parker</em> with the key <em>last</em>.
It then invokes the logger twice with different messages.
The code finishes by setting the <code>MDC</code> to different values
and issuing several logging requests. Running SimpleMDC yields:
</p>
<div class="source"><pre>Dorothy Parker - Check enclosed.
Dorothy Parker - The most beautiful two words in English.
Richard Nixon - I am not a crook.
Richard Nixon - Attributed to the former US president. 17 Nov 1973.</pre></div>
<p>
The <code>SimpleMDC</code> application illustrates how logback layouts,
if configured appropriately, automatically output <code>MDC</code> information.
Moreover, the information placed into the <code>MDC</code> can be used by
multiple logger invocations.
</p>
<p>
Mapped Diagnostic Contexts shine brightest within client server architectures.
Typically, multiple clients will be served by multiple threads on the server.
Although the methods in the <code>MDC</code> class are static,
the diagnostic context is managed on a per thread basis, allowing each server
thread to bear a distinct <code>MDC</code> stamp. <code>MDC</code> operations
such as <code>put()</code> and <code>get()</code> affect the <code>MDC</code>
of the <em>current</em> thread only. The <code>MDC</code> in other threads remain
unaffected. Given that <code>MDC</code> information is managed on a
per thread basis, each thread will have its own copy of the <code>MDC</code>.
Thus, there is no need for the developer to worry about thread-safety or
synchronization when programming with the <code>MDC</code> because
it safely and transparently handles these issues.
</p>
<p>
The next example is somewhat more advanced.
It shows how the <code>MDC</code> can be used in a client-server setting.
The server-side implements the <code>NumberCruncher</code> interface shown in
Example 7.2 below. <code>The NumberCruncher</code> interface contains a single
method named <code>factor()</code>. Using RMI technology, client invokes the
<code>factor()</code> method of the server application to retrieve the distinct
factors of an integer.
</p>
<em>Example 7.2: The service interface (<a href="../xref/chapter7/NumberCruncher.html">
logback-examples/src/main/java/chapter7/NumberCruncher.java)</a></em>
<div class="source"><pre>package chapter7;
import java.rmi.Remote;
import java.rmi.RemoteException;
/**
* NumberCruncher factors positive integers.
*/
public interface NumberCruncher extends Remote {
/**
* Factor a positive integer <code>number</code> and return its
* <em>distinct</em> factor's as an integer array.
* */
int[] factor(int number) throws RemoteException;
}</pre></div>
<p>
The <code>NumberCruncherServer</code> application, listed in Example 7.3 below,
implements the <code>NumberCruncher</code> interface. Its main method exports
an RMI Registry on the local host that accepts requests on a well-known port.
</p>
<em>Example 7.3: The server side (<a href="../xref/chapter7/NumberCruncherServer.html">
logback-examples/src/main/java/chapter7/NumberCruncherServer.java)</a></em>
<div class="source"><pre>package chapter7;
import java.rmi.RemoteException;
import java.rmi.registry.LocateRegistry;
import java.rmi.registry.Registry;
import java.rmi.server.UnicastRemoteObject;
import java.util.Vector;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import ch.qos.logback.classic.LoggerContext;
import ch.qos.logback.classic.MDC;
import ch.qos.logback.classic.joran.JoranConfigurator;
import ch.qos.logback.core.joran.spi.JoranException;
/**
* A simple NumberCruncher implementation that logs its progress when
* factoring numbers. The purpose of the whole exercise is to show the
* use of mapped diagnostic contexts in order to distinguish the log
* output from different client requests.
* */
public class NumberCruncherServer extends UnicastRemoteObject
implements NumberCruncher {
private static final long serialVersionUID = 1L;
static Logger logger = LoggerFactory.getLogger(NumberCruncherServer.class);
public NumberCruncherServer() throws RemoteException {
}
public int[] factor(int number) throws RemoteException {
// The client's host is an important source of information.
try {
<b>MDC.put("client", NumberCruncherServer.getClientHost());</b>
} catch (java.rmi.server.ServerNotActiveException e) {
logger.warn("Caught unexpected ServerNotActiveException.", e);
}
// The information contained within the request is another source
// of distinctive information. It might reveal the users name,
// date of request, request ID etc. In servlet type environments,
// useful information is contained in the HttpRequest or in the
// HttpSession.
<b>MDC.put("number", String.valueOf(number));</b>
logger.info("Beginning to factor.");
if (number <= 0) {
throw new IllegalArgumentException(number +
" is not a positive integer.");
} else if (number == 1) {
return new int[] { 1 };
}
Vector<Integer> factors = new Vector<Integer>();
int n = number;
for (int i = 2; (i <= n) && ((i * i) <= number); i++) {
// It is bad practice to place log requests within tight loops.
// It is done here to show interleaved log output from
// different requests.
logger.debug("Trying " + i + " as a factor.");
if ((n % i) == 0) {
logger.info("Found factor " + i);
factors.addElement(new Integer(i));
do {
n /= i;
} while ((n % i) == 0);
}
// Placing artificial delays in tight-loops will also lead to
// sub-optimal resuts. :-)
delay(100);
}
if (n != 1) {
logger.info("Found factor " + n);
factors.addElement(new Integer(n));
}
int len = factors.size();
int[] result = new int[len];
for (int i = 0; i < len; i++) {
result[i] = ((Integer) factors.elementAt(i)).intValue();
}
<b>// clean up
MDC.remove("client");
MDC.remove("number");</b>
return result;
}
static void usage(String msg) {
System.err.println(msg);
System.err.println("Usage: java chapter7.NumberCruncherServer configFile\n" +
" where configFile is a logback configuration file.");
System.exit(1);
}
public static void delay(int millis) {
try {
Thread.sleep(millis);
} catch (InterruptedException e) {
}
}
public static void main(String[] args) {
if (args.length != 1) {
usage("Wrong number of arguments.");
}
String configFile = args[0];
if (configFile.endsWith(".xml")) {
try {
LoggerContext lc = (LoggerContext) LoggerFactory.getILoggerFactory();
JoranConfigurator configurator = new JoranConfigurator();
configurator.setContext(lc);
lc.shutdownAndReset();
configurator.doConfigure(args[0]);
} catch (JoranException je) {
je.printStackTrace();
}
}
NumberCruncherServer ncs;
try {
ncs = new NumberCruncherServer();
logger.info("Creating registry.");
Registry registry = LocateRegistry.createRegistry(Registry.REGISTRY_PORT);
registry.rebind("Factor", ncs);
logger.info("NumberCruncherServer bound and ready.");
} catch (Exception e) {
logger.error("Could not bind NumberCruncherServer.", e);
return;
}
}
}</pre></div>
<p>
The implementation of the <code>factor(int number)</code> method is
of particular relevance. It starts by putting the client's hostname into the
<code>MDC</code> under the key <em>client</em>. The number to factor,
as requested by the client, is put into the <code>MDC</code> under the key
<em>number</em>. After computing the distinct factors of the integer
parameter, the result is returned to the client. Before returning the
result however, the values for the <em>client</em> and <em>number</em> are
cleared by calling the <code>MDC.remove()</code> method. Normally,
a <code>put()</code> operation should be balanced by the corresponding
<code>remove()</code> operation. Otherwise, the <code>MDC</code> will
contain stale values for certain keys. We would recommend that whenever
possible <code>remove()</code> operations be performed within finally blocks,
ensuring their invocation regardless of the execution path of the code.
</p>
<p>
After these theoretical explanations, we are ready to run the number
cruncher example. Start the server with the following command:
</p>
<div class="source"><pre>java chapter7.NumberCruncherServer src/main/java/chapter7/mdc1.xml</pre></div>
<p>
The <em>mdc1.xml</em> configuration file is listed below:
</p>
<em>Example 7.4: Configuration file (logback-examples/src/main/java/chapter7/mdc1.xml)</em>
<div class="source"><pre><configuration>
<appender name="CONSOLE"
class="ch.qos.logback.core.ConsoleAppender">
<layout class="ch.qos.logback.classic.PatternLayout">
<Pattern>%-4r [%thread] %-5level <b>C:%X{client} N:%X{number}</b> - %msg%n</Pattern>
</layout>
</appender>
<root>
<level value ="debug"/>
<appender-ref ref="CONSOLE"/>
</root>
</configuration></pre></div>
<p>
Note the use of the <em>%X</em> conversion specifier within the
<span class="option">Pattern</span> option.
</p>
<p>
The following command starts an instance of <code>NumberCruncherClient</code>
application:
</p>
<div class="source"><pre>java chapter7.NumberCruncherClient <em>hostname</em></pre></div>
<p>
where <em>hostname</em> is the host where the
<code>NumberCruncherServer</code> is running
</p>
<p>
Executing multiple instances of the client and requesting the server to factor
the numbers 129 from the first client and shortly thereafter
the number 71 from the second client, the server outputs the following:
</p>
<div class="source"><pre>
<b>70984 [RMI TCP Connection(4)-192.168.1.6] INFO C:orion N:129 - Beginning to factor.</b>
70984 [RMI TCP Connection(4)-192.168.1.6] DEBUG C:orion N:129 - Trying 2 as a factor.
71093 [RMI TCP Connection(4)-192.168.1.6] DEBUG C:orion N:129 - Trying 3 as a factor.
71093 [RMI TCP Connection(4)-192.168.1.6] INFO C:orion N:129 - Found factor 3
71187 [RMI TCP Connection(4)-192.168.1.6] DEBUG C:orion N:129 - Trying 4 as a factor.
71297 [RMI TCP Connection(4)-192.168.1.6] DEBUG C:orion N:129 - Trying 5 as a factor.
71390 [RMI TCP Connection(4)-192.168.1.6] DEBUG C:orion N:129 - Trying 6 as a factor.
<b>71453 [RMI TCP Connection(5)-192.168.1.6] INFO C:orion N:71 - Beginning to factor.</b>
71453 [RMI TCP Connection(5)-192.168.1.6] DEBUG C:orion N:71 - Trying 2 as a factor.
71484 [RMI TCP Connection(4)-192.168.1.6] DEBUG C:orion N:129 - Trying 7 as a factor.
71547 [RMI TCP Connection(5)-192.168.1.6] DEBUG C:orion N:71 - Trying 3 as a factor.
71593 [RMI TCP Connection(4)-192.168.1.6] DEBUG C:orion N:129 - Trying 8 as a factor.
71656 [RMI TCP Connection(5)-192.168.1.6] DEBUG C:orion N:71 - Trying 4 as a factor.
71687 [RMI TCP Connection(4)-192.168.1.6] DEBUG C:orion N:129 - Trying 9 as a factor.
71750 [RMI TCP Connection(5)-192.168.1.6] DEBUG C:orion N:71 - Trying 5 as a factor.
71797 [RMI TCP Connection(4)-192.168.1.6] DEBUG C:orion N:129 - Trying 10 as a factor.
71859 [RMI TCP Connection(5)-192.168.1.6] DEBUG C:orion N:71 - Trying 6 as a factor.
71890 [RMI TCP Connection(4)-192.168.1.6] DEBUG C:orion N:129 - Trying 11 as a factor.
71953 [RMI TCP Connection(5)-192.168.1.6] DEBUG C:orion N:71 - Trying 7 as a factor.
72000 [RMI TCP Connection(4)-192.168.1.6] INFO C:orion N:129 - Found factor 43
72062 [RMI TCP Connection(5)-192.168.1.6] DEBUG C:orion N:71 - Trying 8 as a factor.
72156 [RMI TCP Connection(5)-192.168.1.6] INFO C:orion N:71 - Found factor 71</pre></div>
<p>
The clients were run from a machine called <em>orion</em> as can be seen in
the above output. Even if the server processes the requests of clients
near-simultaneously in separate threads, the logging output pertaining
to each client request can be distinguished by studying the output of the
<code>MDC</code>. Note for example the stamp associated with <em>number</em>,
i.e. the number to factor.
</p>
<p>
The attentive reader might have observed that the thread name could
also have been used to distinguish each request. The thread name can cause
confusion if the server side technology recycles threads. In that case,
it may be hard to determine the boundaries of each request, that is,
when a given thread finishes servicing a request and when it begins servicing the next.
Because the <code>MDC</code> is under the control of the application developer,
<code>MDC</code> stamps do not suffer from this problem.
</p>
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