Tuesday, November 19, 2013

Don't swallow Hibernate exceptions!

Every once in a while, I see a code pattern that makes me go "hmmm". Once instance of this is code that catches a Hibernate exception within a transaction and swallows it before doing some compensating action. Normally, the usual pattern is that if Hibernate throws an exception, you let it propagate past the transaction boundary, rolling back the transaction, before handling the error. You violate the established practice at your peril.

What peril? Well, Hibernate's reference manual says the following (section 13.2.3 on Exception Handling):

If the Session throws an exception, including any SQLException, immediately rollback the database transaction, call Session.close() and discard the Session instance. Certain methods of Session will not leave the session in a consistent state. No exception thrown by Hibernate can be treated as recoverable.

I have emphasized the relevant text in bold. If you're catching the exception to do some other action within the same transaction, there is a chance you're corrupting data. If you're lucky, at some point the transaction got marked rollback-only and Hibernate will not let you commit the mischief. But the Hibernate folks are saying that all bets are off if you swallow that exception.

If you think about it, this sort of pattern makes no sense in a transactional setting. When you demarcate an operation as being transactional, you are saying "this operation either succeeds entirely, or it fails entirely as if it never happened". Atomicity is a guarantee of a transaction. So if Hibernate throws an exception it means the operation has failed, and you're mucking with the meaning of a transaction if you attempt a compensating action. If you want to log an error, retry or otherwise record a failure, you are free to do it outside of that transaction.

Wednesday, June 26, 2013

Dynamically changing a log4j configuration

For most of my career, I've used log4j as my logging back end. Whether during development, testing or production, I've encountered situations where I need to change the log level of a running web app on Tomcat. Restarting the application is often not feasible: either the situation I'm trying to diagnose would be wiped out, or other users would be impacted.  Here's an easy tweak, if you're using Spring, to dynamically change a log4j configuration. With this addition to your web.xml, log4j will pick up any changes you make to your log4j.properties file within 5 seconds:
The log4jRefreshInterval parameter is the key. Without that, the log4j configuration won't reload. While this solution is documented separately elsewhere, I wanted to call attention to this tweak because it really deserves wider knowledge.
There are, of course, other possible approaches such as writing your own custom JMX MBean, a custom servlet or using JRebel. These have their own advantages and disadvantages, but I feel the Spring Log4jConfigListener approach is the easiest and most straighforward for a developer.

Thursday, March 28, 2013

Is your language strongly, weakly, statically or dynamically typed?

Would you describe a programming language to be strongly typed, weakly typed, statically typed or dynamically typed? People often use the terms "strongly typed" and "statically typed" interchangeably, and likewise "weakly typed" and "dynamically typed" are often used to mean the same thing. I think this imprecision of terminology is unfortunate. That is, the definition of strong v.s. weak typing is so vague that I would claim it's possible to see them as completely orthogonal concepts.

Wednesday, February 27, 2013

Yammer/Chatter as an alternative to your daily scrum

The daily standup meeting, also known as the daily scrum, did not work very well for me. One day, we looked at each other and asked "does anyone actually want to do this?" Nobody did, so that was the end of the daily scrum. The intention is admirable: you want team members to get a status update and know what's going on with the rest of the team. But the daily scrum seems the wrong way to accomplish that. There must be better ways.

Tuesday, January 15, 2013

Don’t forget to index your Oracle foreign keys

This is another note-to-myself blog post. Like PostgreSQL, Oracle does not create indexes on foreign key columns. This can lead to both poor performance and deadlocks. This is because when the parent table’s column where the foreign key points to is updated Oracle needs to verify the child table’s FK constraints. In the absence of an index, this means a full table scan and a table lock. Normally foreign keys point to primary keys in the parent table, and you rarely need to change a primary key, so you’d think this should not happen often. But deleting rows from the parent table also has the potential of violating FK constraints. So operations that delete rows in the parent table can result in deadlocks in some cases.
Anyway, here’s the Oracle script to identify unindexed foreign keys:

Friday, January 4, 2013

JmsTemplate is not evil

A while back, I watched a presentation on JMS messaging where the presenter (Mark Richards) declared that Spring’s JmsTemplate is “evil,” and he had the benchmarks to prove it. Since Mark was kind enough to provide the source code (based on ActiveMQ), I decided to dig a little deeper to determine the cause. It turns out that the apparent poor performance of JmsTemplate was not due to anything intrinsic to this class, and you can indeed get message sending performance with it comparable to using the standard JMS API. It’s not evil. Really.

Read on if you want the details.