Identifying RabbitMQ consumers

One of the challenges my team was facing was to accurately identify the consumers of a RabbitMQ queue. Let's say if you have 5 consumers and you want to kill one of them through the dashboard, you need to identify the consumer.

Each consumer of RabbitMQ has a tag value that can be used to uniquely identify it. By default, RabbitMQ assigns some random number as the consumer tag, but just by looking at this random string tag there is no way to identify the actual consumer. 

To resolve this, you need to create a ConsumerTagStrategy and associate it with the MessageListenerContainer. Code snippet given below: 

Caching Simplified - Magic of Spring Annotations

Spring Boot makes is super simple to add caching abilities to your application. With just a few lines of code and some annotation magic, you have a complete caching solution available with you.

Spring provides wrappers for many cache implementations - Google Guava, ehcache, MongoDB, Redis, etc. At the simplest level, you can also use a ConcurrentHashMap as the cache manager.

Given below are three code snippets that shows how simple it is to enable caching for static data in a Spring Boot application - Welcome to the power of Spring !

Step 1:  [CachingConfiguration.java]  Configure a ConcurrentHashMap cache manager

Step 2:  [SpringBootCachingApplication.java] Annotate your Spring Boot application with @EnableCaching.

Magic behind the scenes: The @EnableCaching annotation triggers a post processor that inspects every Spring bean for the presence of caching annotations  on public methods. If such an annotation is found, a proxy is automatically created to intercept the method call  and handle the caching behavior accordingly.

Step 3: [StaticDataService.java] Annotate your methods that return the static data with  @Cacheable annotation. You can pass the cache name in the annotation - e.g.  @Cacheable("SomeStaticData")


 

That's it! You have configured caching. In the next blog-post, we will see how we can evict items from the cache when necessary.

Circular Dependencies in Spring Boot

Recently one of my team member was struggling with a strange issue in Spring Boot. The application was running fine on his local machine, but when deployed to the cloud it started giving errors - throwing BeanCurrentlyInCreationException.

 The core issue was identified as Circular Dependencies in Spring. An excellent article on how to identify such dependencies and resolve them is here - http://www.baeldung.com/circular-dependencies-in-spring

It is recommended best to avoid circular dependencies, but in worst case if you must have them, then there are multiple options such as:
1. Using the @Lazy annotation on one of the dependency.
2. Not use Constructor injection, but use a Setter injection
3. Use @PostConstruct to set the Bean

Database connection reconnection strategy

In any database connection pool, there is a risk of stale connections due to network outages or database server restarts. How do we refresh the connection pool without restarting the client application?

The standard technique used is to plug-in a validation query; that gets fired every time a connection is requested from the pool. This validation query is typically a default test query that does not result in any IO / disk access.  Examples of validation queries for different databases are given below:

• Oracle - select 1 from dual
• SQL Server - select 1 (tested on SQL-Server 9.0, 10.5 [2008])
• Postgresql - select 1
• hsqldb - select 1 from INFORMATION_SCHEMA.SYSTEM_USERS
• derby - values 1
• H2 - select 1
• DB2 - select 1 from sysibm.sysdummy1
• Mysql - select 1

We were using Spring Boot that uses the Tomcat JDBC Connection Pool by default. We tried setting all the parameters required for the validation check as given here, but in vain. 

Finally we decided to go with HikariCP connection pool as suggested here. 

First, we added the dependency in our maven pom as shown below. 

Next we added the following properties in our application.properties file. We will pick up these properties to create our HikariCP connection. pool.
Finally we wrote a @Configuration bean to wire up the HirakiCP datasource as below:

Spring beans annotations, scope and loading sequence

Spring beans can have different scopes as listed here . By default, the scope is singleton and the bean is eagerly initialized on start-up. If you want to lazy load a bean, then you have to decorate it with the @Lazy annotation.

By default, a bean has singleton scope and does not have the lazy-init property set to true. Hence by default all beans are eagerly created by the application context. This will affect the time taken for your Spring Boot application to start-up. 

The @Lazy annotation can be used on both @Component and @Bean. It is also important to understand how objects are constructed in Spring. The whole Spring framework works on the fundamental design pattern of 'Inversion of Control'or 'Dependency Injection'. Using Spring, you just declare components and Spring wires them up and manages the dependency between them.

When we mark a class with a @Component annotation, Spring will automatically create an instance of this class (make a bean) by scanning the classpath. This bean can be assessed anywhere in your application using the @Autowired annotation on fields, constructors, or methods in a component.

But what if you don't want Spring to auto-create the bean and want to control how the bean is constructed. To do this, you write a method that constructs your bean as you want and annotate that method with the @Bean tag. Please note that the @Bean annotation decorates a method and not a class. Thus a method marked with the @Bean annotation is a bean producer.

But how will Spring know where you have created @Bean annotated methods? Scanning all the classes will take a lot of time. Hence you annotate these classes which have @Bean methods with two annotations - @Configuration and @ComponentScan.
@Configuration annotation marks a class as a source of the bean definitions.
@ComponentScan annotation is used to declare the packages where Spring should scan for annotated components.

An excellent cheat sheet on Spring Annotations is available here.

Ruminating on Builder design pattern in Spring Security

While configuring Spring Security, you can use the properties file to configure the filters and other classes, but another popular option is declaring the configuration using code - using the Builder design pattern.

Understanding the way to use the builder is a bit tricky. Lets look at the below code and try to dissect it.

The 'http' object represents the org.springframework.security.config.annotation.web.builders.HttpSecurity object and is similar to Spring Security's XML element.
The .authorizeRequests() method returns the ExpressionInterceptUrlRegistry method that can be used to add HTTP URL matching patterns. After this, you can add additional information as required. 

A good blog-post explaining more such options is given here - https://www.javabullets.com/securing-urls-using-spring-security/

Spring Sleuth Tips and Tricks

Spring Sleuth coupled with Zipkin in an excellent tool to measure the performance bottlenecks in your distributed microservices architecture.

Spring Sleuth automatically creates HTTP and Messaging interceptors when calls are made to other microservices over REST or RabbitMQ. But what if you want to instrument the time taken to execute a method or time take for a third-party service to respond?

Initially we tried with the @NewSpan annotation on methods, but soon realized that this annotation DOES NOT work in the same class. A detailed discussion thread on this limitation is available here - https://github.com/spring-cloud/spring-cloud-sleuth/issues/617.

The core reason is that the proxies that Spring creates do not work for the method calls in the same object. Sleuth creates a proxy of the object so if we are calling a proxied method from the proxy itself then the method will be called directly without going through the proxy logic, hence no span will be created. Hence we need to manually add the span as follows and it would show up in the beautiful Zipkin UI.

Another great blog post around these techniques is here - http://www.baeldung.com/spring-cloud-sleuth-single-application

Using Spring MVC to create REST style services

We have been always using Apache CXF or Axis 2 to build web services, but recently I was pretty impressed with the simplicity with which we can create REST style services using Spring MVC. OOTB, Spring MVC integrates with Jackson (for JSON serialization) and JAXB (for XML serialization).

We just have to annotate our Spring Controller class with @RequestMapping and @ResponseBody attributes and let Spring handle the rest. The following links would provide more info on the same.

http://www.javacodegeeks.com/2013/04/spring-mvc-easy-rest-based-json-services-with-responsebody.html

http://xpadro.blogspot.in/2013/02/create-and-test-rest-services-with.html

Using Spring MVC with Ext-JS MVC
http://justinrodenbostel.wordpress.com/2013/03/15/restful-spring-mvc-and-extjs-episode-1-the-spring-stuff/
http://justinrodenbostel.wordpress.com/2013/04/02/restful-spring-mvc-and-extjs-episode-2-the-extjs-stuff/