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Apache Camel for Micro­service Architectures

I've been using microservice architectures before I knew they were called so. I used to work with pipeline applications made up of isolated modules that interact with each other through queues. Since then a number of (ex)ThoughtWorks gurus talked about microservices. First Fred George, then James Lewis and finally Martin Fowler blogged about microservices making it the next buzzword so every company wants to have few microservices. Nowadays there are #hashtags, endorsements, likes, trainings, even 2 day conference about it. The more I read and listen about microservice architectures, the more I realize how Apache Camel (and the accompanying projects around it) fits perfectly to this style of applications. In this post we will see how Apache Camel framework can help us create microservice style applications in Java without much hussle.

Microservices Characteristics
There is nothing new in microservices. Many applications have already been designed and implemented as such for a long time. Microservices is just a new term that describes a style of software systems that have certain characteristics and follow certain principles. It is an architectural style where an application or software system is composed of individual standalone services communicating using lightweight protocols in event based manner. The same way as TDD helps us to create decoupled single responsibility classes, microservices principles guide us to create simple applications at system level. Here we will not discuss the principles and characteristics of such architectures or argue whether it is a way of implementing SOA in practice or a totally new approach to application design, but rather look at the most common practices used for implementing microservices and how Apache Camel can helps us accomplish that in practice. There is not definitive list (yet) but if you read around or watch the videos posted above, you will notice that the following are quite common practices for creating microservices:

1. Small in size. The very fundamental principle of micro services says that each application is small in size and it only does one thing and does it well. It is debatable what is small or large, the number varies from 10 LOC to 1000 but form me I like the idea that it should be small enough to fit in your head. There are people with big heads, so even that is debatable ;) but I think as long as an application does one thing and does it well so that it is not considered a nanoservices, that is a good size.
Camel applications are inherently small in size. A camel context with couple of routes with error handling and helper beans is approximately 100 LOC. Thanks to Camel DSLs and URIs for abstracting endpoints, receiving an event either through HTTP or JMS, unmarshaling it, persisting and sending a response back is around 50 LOC. That is small enough to be tested end-to-end, rewritten and even thrown away without feel any remorse.

2. Having transaction boundaries. An application consisting of multiple microservices forms an eventually consistent system of systems where the state of the whole system is not known at any given time. This on its own creates a barrier for understanding and adopting microservices with teams who are not used to work with this kind of distributed applications. Even though the state of the whole system is not fixed, it is important to have transaction boundaries that define where a message currently belongs. 
Ensuring transactional behaviour across heteregenous systems is not an easy task, but Camel has great transactional capabilities. Camel has endpoints that can participate in transactions, transacted routes and error handlers, idempotent consumers and compensating actions, all of which help developers easily create services with transactional behavior.

3. Self monitoring. This is one of my favorite areas with microservices. Services should expose information that describes the state of various resources it depends on and the service itself. These are statistics such as average, min, max time to process a message, number of successful and failed messages, being able to track a message and so forth.
This is something you get OOTB with Camel without any effort. Each Camel application gathers JMX statistics by default for the whole application, individual routes, endpoints, etc. It will tell you how many messages have completed successfully, how many failed, where they failed, etc. This is not read only API, JMX allows also updating and tuning the application at run time, so based on these statistics, using the same API you can tune the application. Also the information can be accessed with tools such as jConsole, VisualVM, Hyperic HQ, exposed over HTTP using Jolokia or feed into a great web UI called hawtio.
If the functionality that is available OOTB doesn't fit your custom requirements, there multiple extension points such as the nagios, jmx, amazon cloudwatch and the new metrics components, or use Event Notifiers for custom events.
Logging in messaging applications is another challenge, but Camel's MDC logging combined with Throughput logger makes it easy to track individual messages or get aggregated statistics as part of the logging output.

5. Designed for failure - Each of the microservices can be down or unresponsive for some time but that should not bring the whole system down. Thus microservices should be fault tolerant and be able to recover when that is possible.
Camel has lots of helpful tools and patterns to cope with these scenarios too. Dead Letter Channel can make sure messages are not lost in case of failure, the retry policy can retry to send a message couple of times for certain error conditions using custom backoff method and collision avoidance. Patterns such as Load balancer which supports Circuit breaker, Failover and other policies, Throttler to make sure certain endpoints do not get overload, Detour, Sampler, are all needed in various failure scenarios. So why not use them rather than reinventing the wheel in each service.

6. Highly Configurable - It should be easy to configure the same application for high availability, scale it for reliability or throughput, or said another way: have different degrees of freedom through configuration.
When creating a Camel application using the DSLs, all we do is to define the message flow and configure various endpoints and other characteristics of the application. So Camel applications are highly configurable by design. When all the various options are externalized using properties component, it is possible to configure an application for different expectations and redeploy without touching the actual source code at all. Camel is so configurable that you can change an endpoint with another (for example replace HTTP endpoint with JMS) without changing the application code which we will cover next.

7. With smart endpoints.  Micro services favour RESTish protocols and lightweight messaging rather than Web Services.
Camel favors anything. It has HTTP support as no other framework. It has components for Asynchronous Http, GAE URL fetch service, Apache HTTP Client, Jetty, Netty, Servlet, Restlet, CXF and multiple data formats for serializing/deserializing messages. In addition the recent addition of Rest DSL makes REST a first class citizen in the Camel world and simply creating such services a lot. As for the queuing support, OOTB there are connectors for JMS, ActiveMQ, ZeroMQ, Amazon SQS, Amazon SNS, AMQP, Kestrel, Kafka, Stomp, you name it.

8. Testable. There is no common view on this characteristic. Some favor no testing at all and relying on business metrics. Some cannot afford to have bad business metrics at all. I like TDD and for me having the ability to test my business POJOs in isolation from the actual message flow, then test the flow separately by mocking some of the external endpoints is invaluable. Camel testing support can intercept and mock endpoints, simulate events, verify expectations with ease. Having a well tested microservice for the expected behavior is the only guarantee to have the whole system to work as expected.

9. Provisioned individually. The most important characteristics of microservices is that they run in isolation from other services most commonly as standalone Java applications. Camel can be embedded in Spring, OSGI or web containers. Camel can also run as a standalone Java application with embedded Jetty endpoints easily. But managing multiple processes, all running in isolation without a centralized tool is a hard job. This is what Fabric8 is made for. Fabric8 is developed by the same guys who developed Camel and supported by Red Hat JBoss. It is a poly Java application provisioning and management tool that can deploy and manage a variety of Java containers and standalone processes. To find out more about Fabric8, here is nice post by Christian Posta.

10. Language neutral. Having small and independently deployed applications allow developers to choose thebest suited language for the given task. Camel has XML, Java, Scala, Groovy and few other DSLs with similar syntax and capabilities .But if you don't want to you use Camel at all for a specific micro service, you can still use Fabric8 do deploy and manage applications written in other languages and run them as native processes.

In summary: Microservices are not strictly defined and that's the beauty. It is a lightweight style of implementing SOA that works. So is Apache Camel. It is not a full featured ESB, but it can be as part of JBoss Fuse. It is not a strictly defined specification driven project, but a lightweight tool that works and developers love it.

Referecences
1. Micro-Service Architecture by Fred George (video)
2. Micro-Services - Java, the UNIX way by James Lewis
3. Microservices by Martin Fowler
4. ┬ÁServices by Peter Kriens
5. Micro Services the easy way with Fabric8 by James Strachan (with video)
6. Fabric8 by Red Hat
7. Meet Fabric8: An open­source integration platform by Christian Posta
8. Micro Services the easy way with Fabric8 by James Strachan

Clustered Idempotent Consumer Pattern with Infinispan

I've created a small project that shows how to use JBoss Infinispan with Apache Camel and the Idempotent Consumer Pattern to guarantee a message will not be processed twice in a clustered environment.
Imagine you have an application that has to scale out easily by deploying it on multiple containers. But the application has to process each unique request only once across the cluster.
The solution is simple: use Idempotent Consumer Pattern in Camel with a repository that can scale out easily. This is where Infinispan comes into play. Infinispan is extremely scalable, highly available key/value store and data grid. If you use InfinispanIdempotentRepository with an idempotent consumer, it will create an in-memory cache to store the requests, and the moment you start another instance of the application, the cache instances will sync and the idempotent consumers in all applications will not process existing requests any longer.
With this project (idempotent consumer demo5) you can start as many containers as you want, each container will start a rest endpoint on a new port starting from 8080 (http://localhost:8080/idempotent/KEY), and if you perform a GET request with a key, the subsequent requests with the same key to any other container will be rejected. Behind the scene Infinispan is replicating all the processed keys across the cluster of Camel applications and ensuring consistency.
The core of the application is the following route definition that finds a new free port number for each instance of the application:
Simple, isn't it.

Why bother contributing a Camel component?

Camel has a staggering number of connectors and that is thanks to you - the community. May be you also know a cool library that still doesn't have a Camel connector and wonder whether you should create a connector and contribute it to Apache Camel? Hopefully this article will give you the answer why you should do so.

The learning experience - There are great books about Apache Camel (mine is not bad either), but nothing can teach you better than banging your head while trying to write your first component. Understanding how Camel components works equals to understanding half of the project (the other half is the framework lifecycle and EIPs which are similar to components). So if you want to learn Camel and do more than than hello world, try writing a component.

A sense of accomplishment - the moment your code is pushed to Apache repository by a committer it becomes available to everyone and it is owned by the Apache foundation. But your name remains in the commit history forever, and no matter how much the code has changed, you can always claim: "Oh yeah, I've created that Camel component".

Becoming a Camel contributor - you get the right to add your name to the Camel team list. It is one of the most popular open source projects in the Java world and being part of something great is ... great. And who knows, may be this small contribution will spark a new passion and you will follow up with many other contributions and become a Camel committer in a short time.

Sharing can only bring you good - may be you are luckier than the average developer and you work for a company that let's you hack whatever you want for a day every fortnights. Or may be you are even luckier and you get paid for contributing to open source projects as my colleagues at Red Hat do (here is a list with Camel jobs at Red Hat for those interested). If you are neither of those groups, you can still work on something that is interesting to you and contribute to open source projects. All you need is couple of hours in a train or Starbucks.

Why a component? Writing a component is the easiest way to contribute something to Camel. It doesn't require deep Camel or EIP knowledge. You need good understanding of Camel components, good understand of the library you want to create connector for, and a mindset open for sharing.

This article is only the appetizer, if you liked it, the next article will explain how to create a Camel component.

10 Good Reasons for using Apache Camel to create Microservices

Microservices is an architectural style for distributed applications that is taking over the developer community in high pace. There are tens of blog posts on the net every day, conferences and even books on the topic already.
To contribute to all the buzz around it I've written a post where I gave 10 good reasons why Apache Camel and Fabric8 are great tools for creating microservices applications. If you are interested in microservices topic, the article is published on jaxenter.com.

Circuit Breaker Pattern in Apache Camel

Camel is very often used in distributed environments for accessing remote resources. Remote services may fail for various reasons and periods. For services that are temporarily unavailable and recoverable after short period of time, a retry strategy may help. But some services can fail or hang for longer period of time making the calling application unresponsive and slow. A good strategy to prevent from cascading failures and exhaustion of critical resources is the Circuit Breaker pattern described by Michael Nygard in the Release It! book.
Circuit Breaker is a stateful pattern that wraps the failure-prone resource and monitors for errors. Initially the Circuit Breaker is in closed state and passes all calls to the wrapped resource. When the failures reaches a certain threshold, the circuit moves to open state where it returns error to the caller without actually calling the wrapped resource. This prevents from overloading the already failing resource. While at this state, we need a mechanism to detect whether the failures are over and start calling the protected resource. This is where the third state called half-open comes into play. This state is reached after a certain time following the last failure. At this state, the calls are passed through to the protected resource, but the result of the call is important. If the call is successful, it is assumed that the protected resource has recovered and the circuit is moved into closed state, and if the call fails, the timeout is reset, and the circuit is moved back to open state where all calls are rejected. Here is the state diagram of Circuit Breaker from Martin Fowler's post:

How Circuit Breaker is implemented in Camel?

Circuit Breaker is available in the latest 2.14 version of Camel as a Load balancer policy. Camel Load Balancer already has policies for Round Robin, Random, Failover, etc. and now also CircuiBreaker policy.
Here is an example load balancer that uses Circuit Breaker policy with threshold of 2 errors and halfOpenAfter timeout of 1 second. Notice also that this policy applies only to errors caused by MyCustomException
And here is the same example using Spring XML DSL: