Apache Camel Interview Questions.

In this post we will look at Apache Camel Interview questions. Examples are provided with explanation.

What is Apache Camel ?

In an enterprise, a number of systems of different types exist. Some of these may be legacy systems while some may be new. These systems often interact with each other,and need to be integrated. This interaction or integration is not easy as the implementations of the systems, their message formats may differ. One way to achieve this is to implement code which bridges these differences. However this will be point to point integration. If tomorrow again if there is change in a system the other might also have to be changed which is not good. Instead of this point to point integration which causes tight coupling we can implement an additional layer to mediate the differences between the systems. This results in loose coupling and not affect much our existing systems. Apache Camel is a rule-based routing and mediation engine that provides a Java object- based implementation of the Enterprise Integration Patterns using an API (or declarative Java Domain Specific Language) to configure routing and mediation rules.

What are routes in Apache Camel?

The core functionality of Apache Camel is its routing engine. It allocates messages based on the related routes. A route contains flow and integration logic. It is implemented using EIPs and a specific DSL.

What are DSLs and which DSLs have you used?

Routes in a variety of domain-specific languages (DSL).The most popular ones are Java DSL - A Java based DSL using the fluent builder style. Spring XML - A XML based DSL in Spring XML files
Code example for Camel Java DSL are present here along with source code and explanation

What is an ESB? Have you deployed camel on any ESB?

ESB stands for Enterprise Service Bus. It can be defined as a tool designed to help implement an application using SOA principles Not for all projects projects is the use of ESB an optimum solution ESB should be used when projects involve integrating a number of Endpoints like Webservices, JMS, FTP etc. Have deployed JBoss Fuse ESB for Apache Camel Deployement.
Code example for Apache Camel Deployment on JBoss Fuse are available here
JBoss Fuse Interview Questions

What are EIPs in Apache Camel?

EIPs stand for Enterprise Integration Pattern. These are Design patterns for the use of enterprise application integration and message-oriented middleware in the form of a pattern. Various EIPs are used in Apache Camel. Some of them are- Splitter Pattern- Split the data on the basis of some token and then process it. Content Based Router-The Content-Based Router inspects the content of a message and routes it to another channel based on the content of the message. Using such a router enables the message producer to send messages to a single channel and leave it to the Content-Based Router to inspect messages and route them to the proper destination. This alleviates the sending application from this task and avoids coupling the message producer to specific destination channels. Message Filter-A Message Filter is a special form of a Content-Based Router. It examines the message content and passes the message to another channel if the message content matches certain criteria. Otherwise, it discards the message. Recipient List-A Content-Based Router allows us to route a message to the correct system based on message content. This process is transparent to the original sender in the sense that the originator simply sends the message to a channel, where the router picks it up and takes care of everything. Wire Tap-Wire Tap allows you to route messages to a separate location while they are being forwarded to the ultimate destination.
Apache Camel EIPs with code are explained here
Apache Camel Routing Slip Pattern
Apache Camel Dynamic Router Pattern

What is an exchange in Apache camel?

The message to be routed in Camel route is present in the Exchange. It is the message holder. Apache camel uses Message Exchange Patterns(MEP). Apache camel exchange can hold any kind of message. It supports a variety of formats like xml, JSON etc.

What are endpoints in apache camel?

Camel supports the Message Endpoint pattern using the Endpoint interface. Endpoints are usually created by a Component and Endpoints are usually referred to in the DSL via their URIs.

What are various components in apache camel? Which ones have you used?

Apache camel provides us with a number of components. These components make interacting create endpoints with which a system can interact with other external systems. For example using an ActiveMQ component we expose an ActiveMQ endpoint for interaction with external system. There are more than 100 components provided by Apache Camel. Some of them are FTP,JMX, Webservices, HTTP. Apache camel also allows users to create custom components.
Here are examples of FileComponent, JMSComponent, CXF Component.

Have you made database calls using Apache Camel?

Yes. Have integrated Apache Camel MySQL Database using SQL Queries
Apache Camel Tutorial- Integrate with MySQL DB using SQL query

Have you used Apache Camel with Spring?

Yes. Have integrated Apache Camel with Spring. It helps in utilizing features like Spring Dependency injection, Datasource and Transaction management.
Apache Camel Java DSL + Spring Integration Example

Have you exposed a SOAP webservice endpoint using Apache Camel?

Yes. Using Apache CXF exposed a webservice to be consumed. Used Contract first approach to generate the classes from wsdl.
Apache Camel + Apache CXF SOAP Webservices

Have you exposed a REST webservice endpoint using Apache Camel?

Yes. Using Apache CXF exposed a REST Endpoint. This can be done using either JAX-RS or CXFRS
Apache Camel + JAX-RS REST Webservice
Apache Camel + CXFRS REST Webservice

How did you integrate Apache Camel with Database?

Using Apache Camel SQL component.
Apache Camel + SQL component(MySQL DB)

How did you execute JUnit test cases for Apache camel?

Using CamelSpringTestSupport - Apache Camel Unit Testing

How are exception handled using Apache Camel?

Exception can be handled using the <try> <catch> block, <OnException> block or the <errorHandler> block.
The errorHandler is used to handle any uncaught Exception that gets thrown during the routing and processing of a message. Conversely, onException is used to handle specific Exception types when they are thrown. Understanding Apache Camel Exception Handling Using Simple Example

What is Redelivery policy in Apache Camel?

A redelivery policy defines rules when Camel Error Handler perform redelivery attempts. For example you can setup rules that state how many times to try redelivery, and the delay in between attempts, and so forth. Understanding Apache Camel Redelivery policy using simple example

What is CamelContext?

The CamelContext represents a single Camel routing rulebase. We use the CamelContext in a similar way to the Spring ApplicationContext. public interface CamelContext extends SuspendableService, RuntimeConfiguration. Interface used to represent the context used to configure routes and the policies to use during message exchanges between endpoints.

What is RouterContext?

It is now possible to define routes outside <camelContext/> which you do in a new <routeContext/> tag. The routes defined in <routeContext/> can be reused by multiple <camelContext/>. However its only the definition which is reused. At runtime each CamelContext will create its own instance of the route based on the definition.

Have you used Quartz with Apache Camel?

Yes. Apache camel was used to trigger certain jobs everyday at a particular time of the day.
Apache Camel + Spring + Quartz Example

Have you used Apache Camel with Spring Boot?

Have integrated Apache Camel with Spring Boot. Made use of Apache Camel Spring Boot starter dependency. Apache Camel + Spring Boot

What is Apache Camel Idempotent Consumer pattern?

In Apache Camel we use the Idempotent Consumer pattern to filter out duplicate messages. Consider a scenario where we have to process files only once. If there are any duplicates they should be skipped. Using Apache Camel we can use Idempotent Consumer directly within the component so it will skip files that are processed once. This feature is be enabled by setting the idempotent=true option. In order to achieve this Apache Camel keeps track of the consumed files using a message id which is stored in the repository called Idempotent Repository. Apache Camel provides the following types of IdempotentRepository.
Apache Camel Tutorial - Idempotent Consumer using MemoryIdempotentRepository and FileIdempotentRepository

See Also
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Enterprise Service Bus- ESB Interview Questions.
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