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Friday, August 30, 2013

Spring AMQP and RabbitMQ High Availability (HA)

Recently, I finished a project to use RabbitMQ as the message broker.  It is pretty easy to set up a RabbitMQ cluster with High Availability (HA).  The RabbitMQ has the pretty good documentation on how to set up the cluster.  I also document it here on how to set up a 2-node cluster on a single Mac machine.  In this post, I would like to show how we can leverage spring-amqp to connect to RabbitMQ cluster.  I have a rabbit cluster running with 2 nodes: localhost:5673 and localhost:5672.

It is pretty easy to use spring-amqp.  I used maven to manage the dependencies:

jackson-jaxrs-json-provider is used to serialize java object to json, and deserialize json back to java object.

When creating ConnectionFactory, the addresses should be used instead of the host and port:

The addresses are the comma separated host:port pairs which consist of the cluster.

For producer, we use rabbitTemplate to send messages:

For consumer, a MessageListenerContainer is created to consume message asynchronously:

The MessageHandler code is as follows:

This class can be called anything you like, but the method must be called handleMessaege and with the correct signature (here it is Employee to match producer).  If you want to change the method name, you have to call:


The source code can be download from github.

Tuesday, June 11, 2013

Skip cobertura coverage check when skip unit test

After I introduced embedded mysql for unit testing to my project, the build is taking longer now.  For my local development, sometimes I want to disable the unit test:

      mvn clean package -DskipTests

Note that you can even skip the compiling of test class by using

     mvn clean package -Dmaven.test.skip

However, if you use cobertura for code coverage check, you would get the following error:

[ERROR] Project failed check. Total branch coverage rate of 0.0% is below 85.0%
Project failed check. Total line coverage rate of 0.0% is below 85.0%

So you need to skip cobertura as well.  if you google "skip cobertura", most answers suggested using haltOnFailure. However, cobertura already provides the skip functionality:

     mvn clean package -DskipTests -Dcobertura.skip

With -Dcobertura.skip, cobertura check will be skipped:

[INFO] --- cobertura-maven-plugin:2.5.2:instrument (default) @ tag-service ---
[INFO] Skipping cobertura execution

Monday, June 10, 2013

Using embedded mysql database for unit test with maven and spring

I used H2 in memory database to unit test my DAO.  However, my production database is MySQL.
Generally, H2 is compatible with MySQL.  However, H2 doesn't support ENUM type yet, and my schema uses ENUM for a column definition.  Inspired by this post and another, I successfully used embedded MySQL for unit test.  Here is what I did:

1.  Add mysql-connector-mxj dependency:

2. Create and

3. Create spring bean datasource:
4. Run unit test with Spring:
A couple things are worth mentioning:

1. The test uses Spring annotation @DirtiesContext(classMode = ClassMode.AFTER_EACH_TEST_METHOD) which reload the context after every test method.  It means every test method starts a new mysql instance.  You may group test methods to 2 different categories, one changes data, and another not.  For those methods don't change data, only need to create one instance of mysql to speed up testing.

2. If you see the following error message on Linux server:
/lib64/ version `GLIBC_2.7' not found (required by /tmp/test_db_2420035739165052/bin/mysqld)

You need to either downgrade mysql-connector-mxj to 5.0.11 or upgrade your linux OS.  I got the above error message on CentOS 5.4 with 5.0.12, but no problem with 5.0.11. However, 5.0.11 isn't in any public maven repo, but you can download it from here and install it to your local repo, or upload to your company's repo. Using the following command to check CentOS version:

cat /etc/redhat-release

3. The code is tested on Mac and CentOS only, and can be downloaded from github

Monday, June 3, 2013

Example for enabling CORS support in Spring Rest Api 3.2

My last year's post "Enable CORS support in REST services with Spring 3.1" seems causing some confusion.  I decided to create an example to show how to enable CORS with Spring rest api.  The CorsFilter is same as before:

Below are 2 endpoints from

The update method adds the header Access-Control-Allow-Origin with "*", but delete method doesn't.  Therefore, the update method is enabled with CORS, but delete isn't.  If delete endpoint is called, the following error will be shown in Chrome:

"cannot load http://localhost:8080/rest/employee/1. Origin is not allowed by Access-Control-Allow-Origin."

However, the delete method is still invoked on the server side since the pre-flight request (OPTIONS)
allows DELETE method to be called.

The entire project can be downloaded from github.  Following README to test it.

My intention was to disable/enable CORS support in each individual method by setting "Access-Control-Allow-Origin", but it seems not working as expected:  Although the browser returns correct info, the method call is still invoked on the server side even Access-Control-Allow-Origin is not set.  If you are allowed to enable all endpoints with CORS support, the code can be simplified as below:

The only difference is that addHeader("Access-Control-Allow-Origin") is moved out the if check. And then the update method can be simplified as:

The code can be downloaded from github too.

Wednesday, May 15, 2013

Add an existing project to a newly created git repository

I recently created a project on my local machine using maven archetype.  See my previous post on how to generate a new project from installed maven catalog.  I then got created a repository for this project, for example:

How can I associate my existing project with the newly created git repo?  There are 2 ways to do it:

First option:

1. clone the git repo:
git clone

It will clone the repo to test1 folder.

2. copy your existing project to test1 folder

cp -r /path/to/project /path/to/test1

3.  commit and push all files

git add .
git commit -m "first commit"
git push -u origin master

Second option (more git style)

1. go to your project folder

cd /path/to/project

2. run the following commands:

git init
git add .
git commit -m "first commit"
git remote add origin

The above commands associate your project to git remote repo.

3. run the following commands to to merge the conflict:

git fetch
git merge origin/master
 --run it if conflicts exist
git mergetool
git commit

git push -u origin master

Tuesday, April 9, 2013

Riak: configure memory_backend

Riak supports several backends:
1. bitcask
2. memory
3. leveldb
4. multi

My recent project required me to use memory backend.  I have a test server with 24GB memory.  I tried to use 4GB as the max_memory.  It seems quite easy at beginning, and I just followed the doc from  I just replace riak_kv_bitcask_backend with riak_kv_memory_backend:

{riak_kv, [
  {storage_backend, riak_kv_memory_backend},
Then I want to see how much data I can load with default configuration.  I loaded a little over 11 million records, then riak crashed.  Then I set max_memory to 4GB to add the following right under {eleveldb ... configuration:

{memory_backend, [
            {max_memory, 4096}, %% 4GB in megabytes
Note that the following is copied from riak's doc.  Then interesting, riak crashed with same number of records (11M+).  I then changed it to 8192, but riak still crashed with the same number of records.  I spent a couple days, and couldn't figure out the reason.  I went to #riak IRC and asked the following question, and those folks were very responsive and solved my problem right away:

[10:30] <zhentao> hi folks, how to configure the max_memory for memory_backend?
[10:30] <@alexmoore> Hi zhentao
[10:30] <zhentao> Hi
[10:30] <zhentao> I added the following to config file:
[10:30] <zhentao> {memory_backend, [              {max_memory, 8192} %% 8GB in megabytes             ]},
[10:31] <zhentao> but it didn't work
[10:31] <@alexmoore> In regards to your earlier question, if you don't specify a max_memory, or a TTL, the memory backend will continue to grow until it runs out of memory.
[10:31] <@alexmoore> Let me look at your config here
[10:31] <zhentao> it seems like that
[10:32] <zhentao> the node crashed after I loaded some data
[10:32] <zhentao> then I specify the max_memory to 8gb, and it still crashed with same amount of data
[10:32] <@alexmoore> How much RAM does the machine have?
[10:33] <sully_> Cluster health seems like it's degrading again.  We're starting to see the same errors as before.
[10:34] <zhentao> 24 GB
[10:34] <sully_> We are planning to add 3 more nodes.
[10:34] <@evanmcc> zhentao: that's 8GB per vnode
[10:34] <@rzezeski> sully_ ok, can you tar.gz the latest log files again, that might allow me to find the cause before it gets rotated out by the logger
[10:34] <@jcaprice> zhentao: did you restart the node after adding the memory constraint?
[10:35] <sully_> Getting you the latest logs.
[10:35] <zhentao> yes, I restarted it
[10:35] <@evanmcc> zhentao: how many nodes, and what is your ring size?
[10:36] <@alexmoore> zhentao, how many physical machines do you have in your cluster, and what is your ring size?
[10:36] <zhentao> it is a test server and just one machine
[10:36] <@jcaprice> ring size?
[10:37] <zhentao> i am new to riak, and where to find th ring size?
[10:37] <@evanmcc> if you didn't set it, it's 64
[10:37] <@alexmoore> In your vm.args file
[10:37] <@evanmcc> so you're limiting memory to 8GB * 64
[10:38] <zhentao> let me check
[10:38] <@evanmcc> alexmoore: it's in app.config under riak_core
[10:38] <@evanmcc> ring_creation_size
[10:38] <@alexmoore> Whoops, make that the app.config
[10:39] <zhentao> it is 64
[10:39] <zhentao> {ring_creation_size, 64},
[10:39] <@evanmcc> so you want to change max memory to 128
[10:39] <@evanmcc> if you want to limit it at 8GB
[10:40] <@jcaprice> zhentao: max_memory limits the amount of memory used per vnode, not for the node itself
[10:40] <zhentao> @jcaprice, so what number I should use for max_memory for my test server?
[10:41] <@evanmcc> 128, like I said above
[10:41] <zhentao> 128 mb?
[10:41] <@evanmcc> yes
[10:41] <@jcaprice> as evanmcc said, you'll want 8192 / 64
[10:41] <zhentao> thx, let me try it
So in  summary, {max_memory, 4096} is for each vnode, not for each machine.  Since I wanted to limit the memory usage for one machine to 4GB, I should use the following:
{max_memory, 64}
The reason is that the ring_creattion_size is 64 which means there are 64 vnodes on my single test server. 
64MB * 64 = 4096MB
After I changed max_memory to 64, riak is happy, and it didn't crash when I tried to load 20MM records.
Some interesting things I noticed:
1. since Riak use LRU for memory_backend, the old records are evicted if max_memory can't hold all records.
2. With the key as "70f21766-1cde-38d4-5920-a380003723b3", and value as "1365720608095#TQB:1.4:2:1", 
4GB memory with 64 vnodes can  hold about 2.5MM records
4GB memory with 4 vnodes can hold about 7MM records
Seems the number of vnodes/machine has big overhead


Thursday, February 14, 2013

eclipse JUNO hangs on start up

I use Eclipse JUNO every day.  However, I couldn't start it today after I restart my Mac.  Mac updated with Microsoft 2011 so I have to reboot Mac machine.  The strange thing happened: Eclipse got stuck.  I manually killed the start up process several times, but it didn't work.  Then I googled it, and found the solution from this post, and here is what I did:

rm $WORKSPACE_DIR/.metadata/.plugins/org.eclipse.e4.workbench/workbench.xmi
Then I can start eclipse, and happy coding.