Skip to main content

Carrying over Log4j context in threadpool

We use MDC feature of slf4j/log4j to automatically log the details about user/client that is making the request so we can trace the things happening in the thread. 

    public static void setRequestId(String requestId) {
        if (requestId != null) {
            MDC.put("MDC_REQUEST_ID", requestId);
        } else {
            MDC.put("MDC_REQUEST_ID", "");

    public static void setUserInContext(long userId) {
        MDC.put("MDC_USER_ID", String.valueOf(userId));

and the log4j appender users

    <appender name="RFA_ROOT" class="org.apache.log4j.rolling.RollingFileAppender">
        <rollingPolicy class="org.apache.log4j.rolling.TimeBasedRollingPolicy">
          <param name="FileNamePattern" value="${catalina.base}/logs/kp.log.%d{yyyy-MM-dd-HH}"/>
          <param name="ActiveFileName" value="${catalina.base}/logs/kp.log"/>
        <layout class="org.apache.log4j.PatternLayout">
            <param name="ConversionPattern" value="%p %d{ISO8601} %t D-%X{MDC_DOMAIN} U-%X{MDC_USER_ID} S-%X{MDC_SYNC_CID} R-%X{MDC_REQUEST_ID} %c - %m%n"/>

This will automatically print User and request Id but when you want to get the user requests funnel through some threadpool to avoid thundering herd problem then the log4j context is not carried over and this breaks traceability. To solve this problem the easy solution is to just create your own Callable and use that in thread pool.

public abstract class MyCallable implements Callable {
    private Map contextMap = AppLogger.getCopyOfContextMap();
    public T call() throws Exception {
        try {
            return internalCall();
        } finally {

    protected abstract T internalCall() throws Exception;


        MyCallable task = new MyCallable() {
            public String internalCall() throws Exception {


and use

This way your context would be carried over to background threads.


Popular posts from this blog

RabbitMQ java clients for beginners

Here is a sample of a consumer and producer example for RabbitMQ. The steps are
Download ErlangDownload Rabbit MQ ServerDownload Rabbit MQ Java client jarsCompile and run the below two class and you are done.
This sample create a Durable Exchange, Queue and a Message. You will have to start the consumer first before you start the for the first time.

For more information on AMQP, Exchanges, Queues, read this excellent tutorial
import com.rabbitmq.client.Connection; import com.rabbitmq.client.Channel; import com.rabbitmq.client.*; public class RabbitMQProducer { public static void main(String []args) throws Exception { ConnectionFactory factory = new ConnectionFactory(); factory.setUsername("guest"); factory.setPassword("guest"); factory.setVirtualHost("/"); factory.setHost(""); factory.setPort(5672); Conne…

Spring query timeout or transaction timeout

If you are using spring to manage transactions then you can specify default transaction timeout using

    <bean id="transactionManager"
        <property name="dataSource" ref="dataSource" />
        <property name="defaultTimeout" value="30" /> <!--30 sec--->             

or you can override the timeout in the annotation

    @Transactional(readOnly = false, timeout=30)

or if you are doing it programatic transactions then you can do

DataSourceTransactionManager transactionManager = new DataSourceTransactionManager(dataSource);

 or you can override the timeout for one particular transaction

TransactionTemplate transactionTemplate = new TransactionTemplate();

Python adding pid file

I have a thumbnail generator that launches multiple processes and the correct way to shut it down is to send kill -HUP to the parent process. To automate I had to write a pid file from python, it was a piece of cake
def writePidFile(): pid = str(os.getpid()) f = open('', 'w') f.write(pid) f.close()