Apache Camel Interview Questions and Answers

Ques 1. 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 according to the requirements. 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 (message transformation, rules to integrate, validations etc). 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.

Ques 2. 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 (Enterprise Integration Patterns) and a specific DSL (Domain Specific Language).

Ques 3. What are DSLs and which DSLs have you used?

Routes in a variety of domain-specific languages (DSL). The most popular ones are: 

Ques 4. 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.

Ques 5. 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.

Ques 6. 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.

Ques 7. 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. It also provides different connector components to interact with Cloud services (AWS/Azure etc).

Ques 8. How to make database calls using Apache Camel?

You can call database using apache camel component.

Ques 9. How to expose a REST webservice endpoint using Apache Camel?

You can expose the REST webservice API endpoint using Apache camel  (JAX-RS or CXFRS).

Ques 10. How did you execute JUnit test cases for Apache camel?

Using CamelSpringTestSupport.

Ques 11. 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.

Ques 12. 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.

Ques 13. How to use Spring Boot with Apache Camel?

You can use Spring boot with Apache Camel.

Ques 14. 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: 

Ques 15. Have you used Apache Camel with Spring?

Ques 16. 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. 

Ques 17. How to use Quartz Scheduler in Apache Camel?

You can use quartz scheduler in apache camel.

Ques 18. What is Idempotent Consumer pattern in Apache Camel?

In Apache Camel we use the Idempotent Consumer pattern to filter out duplicate messages. Consider a scenario where we have to process messages 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 messages 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 messages using a message id which is stored in the repository called Idempotent Repository. Apache Camel provides the following types of IdempotentRepository.

Ques 19. How to connect with AWS service from Apache Camel?

Yes we can connect AWS services from Apache Camel using connector components.

Ques 20. How to connect with Azure from Apache Camel?

Yes. We can connect with azure services using Apache camel connector components.