Skip to main content

Pitfalls of defensive programming

We programmers some times add too much defensive code in order to protect ourselves from the caller not asserting preconditions before making the call. So for e.g. if we have to save a file in some directory, we would first go and check if the directory exists and if it exists then create the file. Now NFS is not designed to work at cloud scale and we saw lots of calls just stuck in file.exists call in threaddumps. The solution was simple, some of these directories could be created at tomcat startup or app node installer can create them. Also some code can assume that directory exists and if if gets a FileNotFoundExcpetion then create it and retry the operation. Removing these defensive coding practices reduced a lot of unnecessary stat calls on filers and improved performance. This is just an example but similar pattern can be observed in other areas of the code and fixed. Defensive programming is good but too much of it is bad and can be improved by making some assumptions or providing better documentation of the api.


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()