EJB Good Practices

EJBs abstract your middleware or business logic layer. They are transactional in nature so when you hit your persistence layer (mostly through JPA), the transaction is already there for your database session. As a result, all DB operations are going to complete or none of them, i.e. EJB operation is atomic. Let's cover some of the good practices:

Don't create EJB methods for CRUD operations

Imagine creating operations in your EJB for creating, fetching, updating or deleting your entity. It's not going to serve the purpose; quite clearly, CRUD operations are not your business logic!

In fact, CRUD operations will be part of your more sophisticated business operations. Let's take that you want to transfer x amount from a bank account A to another account B. There should be just a single method which reads appropriate records from DB, modifies them and performs the update.

Also, creating a CRUD operation gives the impression that EJB is created for each entity. We should create EJB for a group of related problems like manage accounts, policy manager etc. You can abstract your CRUD operations in Data Access Layer though!

Minimise Injecting EJBs in the Presentation layer

Imagine yourself working in the presentation layer (Servlets, web services, ..) and having to deal with multiple EJBs to delegate the call. You are going to struggle to find appropriate EJB and then a method for delegating the incoming calls.  Especially when someone else is taking care of business layer!

This is going to defeat the separation of concern principle which is important to manage your complex distributed system. So what's the solution to deal with this - Bundle related EJBs in a single (super) EJB and inject this super EJB. 

But make sure that, in doing so, you are not putting un-related EJBs together just for the sake of minimizing the number of EJBs.  Each EJB (including super) should adhere to the single responsibility principle. 

Reusing EJB methods

Suppose you have quite complex use cases which resulted in a big EJB class definition. So, the obvious question would be, how do you achieve reusability with EJB methods?

Just like a normal Java class, you can create helper EJBs with reusable methods. Multiple EJBs can use the services provided by this helper EJB. And to make these helper utilities more clear, you can put this in a utility or helper package inside your main EJB package. 

References

http://palkonyves.blogspot.in/2013/03/how-to-design-clean-business-tier-with.html

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would love to hear your suggestions/feedback about this post.

The Cost of Concurrency

Concurrency is not free!

Modern libraries provide a wonderful abstraction for programmers, so doing certain task concurrently or asynchronously is quite trivial. It is as simple as instantiating an object and calling few methods on it, and you are done! These libraries are abstracted in such a way that they don't even remind to programmers that you are going to deal with threads. And this is where the lazy programmer can take things for granted.

You need to process 100 task, create 50 threads.
     Collection<Task> task = fetchTasks();   //from somewhere
     int numberOfThreads = 50;
    obj.executeConcurrently(tasks, numberOfThreads);
In object oriented world, all it takes is a method call. 

To understand the cost of concurrency, let's take a step back and ask yourself how is it implemented? It is implemented through locks. Locks provide mutual exclusion and ensure that the visibility of change occurs in an ordered manner. 

Locks are expensive because they require arbitration when contended. This arbitration is achieved by a context switch at the OS level which will suspend threads waiting for lock until it is released. Context switch might cause performance penalty as OS might decide to do some other housekeeping job and so will lose the cached instruction and data. This is even more evident in multicore CPUs where each core has its own cache.  In the worst case, this might cause latency equivalent to that of an I/O operation. 

Another aspect of concurrency is managing the lifecycle of threads. OS does the dirty job of creating threads and managing them on behalf of your platform (or runtime environment). There are certain limits on the number of threads which can be created at the system level. So definitely, proper thoughts should be given on how many threads are required to accomplish a job.

Don't blindly decide to execute task concurrently!

Concurrency or Thread Model of Java

Thread Model in Java is built around shared memory and Locking!

Concurrency basically means two or more tasks happen in parallel and they compete to access a resource.  In the object-oriented world, the resource would be an object which could abstract a database, file, socket, network connection etc. In the concurrent environment, multiple threads try to get hold of the same resource. Locks are used to ensuring consistency in case there is a possibility of concurrent execution of a piece of code. Let's cover these aspects briefly:

Concurrent Execution is about

1. Mutual Exclusion or Mutex
2. Memory Consistency or Visibility of change

Mutual Exclusion

Mutual exclusion ensures that at a given point of time only one thread can modify a resource.  If you write an algorithm which guarantees that a given resource can be modified by (only) one thread then mutual exclusion is not required.  

Visibility of Change

This is about propagating changes to all threads. If a thread modifies the value of a resource and then (right after that) another thread wants to read the same then the thread model should ensure that read thread/task gets the updated value. 

The most costly operation in a concurrent environment contends write access. Write access to a resource, by multiple threads, requires expensive and complex coordination. Both read as well as write requires that all changes are made visible to other threads. 

Locks

Locks provide mutual exclusion and ensure that visibility of change is guaranteed (Java implements locks using the synchronized keyword which can be applied on a code block or method).

Read about the cost of locks, here. 

Related Post: http://geekrai.blogspot.in/2014/02/atomic-operations-in-java.html

Case Study: http://martinfowler.com/articles/lmax.html
References: Java Memory Model

Preventing logging overhead in Log4J

Have you seen something like below in your application, and ever wondered about the overhead (due to the if condition). This post covers ways to get rid of the overhead with Log4j 2.x and Java 8.

if(log.isDebugEnabled()){
   log.debug("Person="+ person);
}

Above style is quite common in log4j-1.x; though it adds few extra lines it improves the performance.

Below log calls toString method on the person even if it's not going to get logged.
log.debug(person);  //big NO; avoid this !

So how do we remove the overhead of if check

The if condition is an overhead and it's going to appear multiple places in method/class. Also, if you don't do logging judiciously then it can be spread all over.

log4j 2.x
log4j 2.x is out there after a long gap and this particular issue has been addressed. Inspired by SLF4J it has added parameterized logging.

log.debug("{}"+person); //will not call .toString method on person
log.debug("{}"+person.toString());   //this is still going to be an issue

So log4j 2.x parameterized logging mechanism will not call implicit (toString) method on the person, but if you call it explicitly then it will make the call. So log4j 2 has partially addressed the problem.

log4j 2.4 + Java 8 lambda

log4j 2.4 supports lambda based mechanism which solves the problem completely. So it doesn't call the method (implicit or explicit) at all if the statement being logged is at the level less specific than the current level.

log.debug(() -> person.toString());   //all good 
log.debug("{}"+ this::expensiveOperation());   //all good

Final Note:
Be mindful of logging overhead when you do code review!

References
https://logging.apache.org/log4j/2.0/manual/api.html
http://marxsoftware.blogspot.pt/2015/10/log4j2-non-logging-performance.html
http://www.jroller.com/bantunes/entry/java_lazy_logging_features_ways
http://www.infoq.com/news/2014/07/apache-log4j2
http://www.grobmeier.de/log4j-2-performance-close-to-insane-20072013.html

Extracting root cause from Stack Trace

Don't tell me the problem; show me the logs first!

Whether you are a fresh graduate, an experienced programmer, QA engineer, production engineer or even a product manager - a good understanding of stack trace is vital to crack critical issues. Your ability to find the real culprit from a lengthy stack trace will be instrumental in resolving a problem. This is even more important if you work on a distributed system where you use many libraries so stack trace is not well structured. Let's start with a simple scenario-

Scenario 1: Simple

This is the most trivial case, where the exception gets thrown by a method of your project and during the call duration, it doesn't go out of your code base. This is the most trivial scenario which you will encounter but very important to understand how the stack trace gets printed.

Shown below is Eclipse screenshot of MyController.java which two classes. Right click and run the program. 

Let's Decode Above stack trace:

• RuntimeException is shown in line number 29, in method MyService.four()
• Method MyService.four() gets called by MyService.three() in line number 25
• Method MyService.three() gets called by MyController.two() in line 11
• Method MyController.two() gets called by MyController.one() in line 6
• Method MyController.one() gets called by MyController.main() in line 17

First frame of stack trace holds all important information required to know the root cause. 

Be mindful of the very important line number 

Scenario 2: Chain Of Exception

Let's modify above code a bit by catching exception at the origin and then throwing a brand new Exception. 

Let's Decode Above stack trace:

This stack trace has a caused by section. It has only one caused by but in real applications you can have multiple caused by sections. The last caused by will have the root cause of the exception.

Caused by: java.lang.RuntimeException: here, comes the exception!

at MyService.four(MyController.java:30)

... 4 more

But, if you are using external jars or libraries, finding the root cause could be bit tricky as the real reason might be nested deep inside. In such case you should look for Class.method name which belongs to your application. Also, you should look the complete stack trace carefully as the real root cause could lie in any part of stack trace. 

References:
http://www.nurkiewicz.com/2011/09/logging-exceptions-root-cause-first.html
http://stackoverflow.com/questions/12688068/how-to-read-and-understand-the-java-stack-trace
http://stackoverflow.com/questions/3988788/what-is-a-stack-trace-and-how-can-i-use-it-to-debug-my-application-errors