Apache Camel Interview Questions and Answers

Apache Camel is a powerful open source integration framework based on known Enterprise Integration Patterns with powerful bean integration.

1)Does Camel work on IBMs JDK

Answer)Yes, It has been tested Camel with IBM’s JDK on the AIX and Linux platforms. There are a few things to look out for though.

EXCEPTION USING CAMEL-HTTP

BUILDING CAMEL-SPRING COMPONENT

RUBY SCRIPTING SUPPORT

2)How does Camel compare to Mule?

Answer)The main differences are as follows:

Camel uses a Java Domain Specific Language in addition to Spring XML for configuring the routing rules and providing Enterprise Integration Patterns

Camel’s API is smaller & cleaner (IMHO) and is closely aligned with the APIs of JBI, CXF and JMS; based around message exchanges (with in and optional out messages) which more closely maps to REST, WS, WSDL & JBI than the UMO model Mule is based on

Camel allows the underlying transport details to be easily exposed (e.g. the JmsExchange, JbiExchange, HttpExchange objects expose all the underlying transport information & behaviour if its required). See How does the Camel API compare to

Camel supports an implicit Type Converter in the core API to make it simpler to connect components together requiring different types of payload & headers

Camel uses the Apache 2 License rather than Mule’s more restrictive commercial license

3)How does Camel compare to servicemix?

Answer)Camel is smart routing and mediation engine which implements the Enterprise Integration Patterns and is designed to be used either inside an ESB like ServiceMix, in a Message Broker like ActiveMQ or in a smart endpoint or web services framework like CXF. ServiceMix is an ESB, a JBI container and an integration platform. So they both address different needs though they are both designed to work great together.

Camel can be deployed as a component within ServiceMix to provide EIP routing and mediation between existing JBI components together with communicating with any of the other Camel. Components along with defining new JBI components on the NMR. So Camel is similar to the ServiceMix EIP component.

To work with Camel and ServiceMix you take your Camel Spring configuration and turn it into a JBI Service Unit using the maven plugin or archetype. For more details see ServiceMix Camel plugin.

So you could start out using Camel routing inside your application via Java or Spring; then later on if you choose to you could wrap up your routing and mediation rules as a JBI deployment unit and drop it into your ServiceMix ESB. This provides a nice agile approach to integration; start small & simple on an endpoint then as and when you need to migrate your integration components into your ESB for more centralised management, governance and operational monitoring etc.

4)How does Camel compare to ServiceMix EIP?

Answer)ServiceMix EIP was the ancestor though they both do similar things.

The main difference with ServiceMix EIP is its integrated into the existing ServiceMix XBean XML configuration whereas Camel has more Enterprise Integration Patterns and can be used outside of JBI (e.g. just with pure JMS or MINA). Also Camel supports a Java DSL or XML configuration.

CONVERTING FROM SERVICEMIX EIP TO CAMEL

5)How does Camel compare to Synapse?

We are Camel developers so take what you read here with a pinch of salt. If you want to read a less biased comparison try reading this review which has a slight Synapse bias since the author mostly uses Synapse :smile:

However we think the main differences are:

the Camel community is way more active according to the nabble statistics (Synapse is inside the Apache Web Services bar) or by comparing Camel and Synapse on markmail.

Camel is the default routing engine included in Apache ActiveMQ for Message Orientated middleware with EIP and Apache ServiceMix the ESB based around OSGi and JBI at Apache - both of which are very popular too.

Camel is designed from the ground up around Enterprise Integration Patterns — having an EIP pattern language implemented in Java and Spring XML.

Camel is designed to work with pretty much all kinds of transport as well as working with any Data Format. When we first looked at Synapse it was based around Axis 2 and WS-* though apparently thats no longer the case.

6)What is Camel

Answer)Apache Camel is a versatile open-source integration framework based on known Enterprise Integration Patterns.

Camel empowers you to define routing and mediation rules in a variety of domain-specific languages, including a Java-based Fluent API, Spring or Blueprint XML Configuration files. This means you get smart completion of routing rules in your IDE, whether in a Java or XML editor.

Apache Camel uses URIs to work directly with any kind of Transport or messaging model such as HTTP, ActiveMQ, JMS, JBI, SCA, MINA or CXF, as well as pluggable Components and Data Format options. Apache Camel is a small library with minimal dependencies for easy embedding in any Java application. Apache Camel lets you work with the same API regardless which kind of Transport is used — so learn the API once and you can interact with all the Components provided out-of-box.

Apache Camel provides support for Bean Binding and seamless integration with popular frameworks such as CDI, Spring and Blueprint. Camel also has extensive support for unit testing your routes.

The following projects can leverage Apache Camel as a routing and mediation engine:

Apache ServiceMix — a popular distributed open source ESB and JBI container

Apache ActiveMQ — a mature, widely used open source message broker

Apache CXF — a smart web services suite (JAX-WS and JAX-RS)

Apache Karaf — a small OSGi based runtime in which applications can be deployed

Apache MINA — a high-performance NIO-driven networking framework

7)How do I specify which method to use when using beans in routes?

Answer)However if you have overloaded methods you need to specify which of those overloaded method you want to use by specifying parameter type qualifiers.

8)How can I get the remote connection IP address from the camel-cxf consumer ?

Answer)From Camel 2.6.0, you can access the CXF Message by using the key of CamelCxfMessage from message header, and you can get the ServletRequest instance from the CXF message, then you can get the remote connection IP easily.

Here is the code snippet:

// check the remote IP from the CXF Message

org.apache.cxf.message.Message cxfMessage = exchange.getIn().getHeader(CxfConstants.CAMEL_CXF_MESSAGE, org.apache.cxf.message.Message.class);

ServletRequest request = (ServletRequest) cxfMessage.get("HTTP.REQUEST");

String remoteAddress = request.getRemoteAddr();

8)How can I stop a route from a route?

Answer)The CamelContext provides API for managing routes at runtime. It has a stopRoute(id) and startRoute(id) methods.

Stopping a route during routing an existing message is a bit tricky. The reason for that is Camel will Graceful Shutdown the route you are stopping. And if you do that while a message is being routed the Graceful Shutdown will try to wait until that message has been processed.

The best practice for stopping a route from a route, is to either:

signal to another thread to stop the route

spin off a new thread to stop the route

Using another thread to stop the route is also what is normally used when stopping Camel itself, or for example when an application in a server is stopped etc. Its too tricky and hard to stop a route using the same thread that currently is processing a message from the route. This is not advised to do, and can cause unforeseen side effects.

9)How does Camel work?

Answer)Camel uses a Java based Routing Domain Specific Language (DSL) or an XML Configuration to configure routing and mediation rules which are added to a CamelContext to implement the various Enterprise Integration Patterns.

At a high level Camel consists of a CamelContext which contains a collection of Component instances. A Component is essentially a factory of Endpoint instances. You can explicitly configure Component instances in Java code or an IoC container like Spring or Guice, or they can be auto-discovered using URIs.

An Endpoint acts rather like a URI or URL in a web application or a Destination in a JMS system; you can communicate with an endpoint; either sending messages to it or consuming messages from it. You can then create a Producer or Consumer on an Endpoint to exchange messages with it.

The DSL makes heavy use of pluggable Languages to create an Expression or Predicate to make a truly powerful DSL which is extensible to the most suitable language depending on your needs. Many of the Languages are also supported as Annotation Based Expression Language.

10)How does Camel work with ActiveMQ?

Answer)You can use Camel to do smart routing and implement the Enterprise Integration Patterns inside:

the ActiveMQ message broker

the ActiveMQ JMS client

So Camel can route messages to and from Mail, File, FTP, JPA, XMPP other JMS providers and any of the other Camel Components as well as implementating all of the Enterprise Integration Patterns such as Content Based Router or Message Translator.

11)How does Camel work with ServiceMix?

Answer)You can use Camel to do smart routing and implement the Enterprise Integration Patterns inside of the JBI container, routing between existing JBI components together with communicating with any of the other Camel Components.

To do this you take your Camel Spring configuration and turn it into a JBI Service Unit using the maven plugin or archetype.

12)How can I get the remote connection IP address from the camel-cxf consumer ?

Here is the code snippet:

// check the remote IP from the CXF Message

org.apache.cxf.message.Message cxfMessage = exchange.getIn().getHeader(CxfConstants.CAMEL_CXF_MESSAGE, org.apache.cxf.message.Message.class);

ServletRequest request = (ServletRequest) cxfMessage.get("HTTP.REQUEST");

String remoteAddress = request.getRemoteAddr();

13)How does Camel look up beans and endpoints?

Answer)There are many times using Camel that a name is used for a bean such as using the Bean endpoint or using the Bean Language to create a Expression or Predicate or referring to any Component or Endpoint.

Camel uses the Registry to resolve names when looking up beans or components or endpoints. Typically this will be Spring; though you can use Camel without Spring in which case it will use the JNDI registry implementation.

Lots of test cases in the camel-core module don’t use Spring (as camel-core explicitly doesn’t depend on spring) - though test cases in camel-spring do.

So you can just define beans, components or endpoints in your Registry implementation then you can refer to them by name in the Endpoint URIs or Bean endpoints or Bean Language expressions.

13)How do I change the logging?

Answer)We use commons-logging to log information in the broker client and the broker itself so you can fully configure the logging levels desired, whether to log to files or the console, as well as the underlying logging implementation (Log4J, Java SE logger, etc.) you wish to use. For Log4J, full instructions are in its manual, but in a nutshell:

Add log4j.jar to your classpath

Create a log4j.properties file specifying desired logging configuration (The Camel distribution has example log4j.properties files you can use — see for example in the /examples/camel-example-as2/src/main/resources folder.)

Place the log4j.properties file in the folder where the compiled .class files are located (typically the classes folder) — this will place the properties file on the classpath, where it needs to be at runtime.

14)How do I configure endpoints?

Answer)There are a few different approaches to configuring components and endpoints.

USING JAVA CODE

You can explicitly configure a Component using Java code as shown in this example

Or you can explicitly get hold of an Endpoint and configure it using Java code as shown in the Mock endpoint examples.

SomeEndpoint endpoint = camelContext.getEndpoint("someURI", SomeEndpoint.class);

endpoint.setSomething("aValue");

14)How do I configure password options on Camel endpoints without the value being encoded?

Answer)When you configure Camel endpoints using URIs then the parameter values gets url encoded by default.

This can be a problem when you want to configure passwords as is.

To do that you can tell Camel to use the raw value, by enclosing the value with RAW(value).

15)How do I configure the default maximum cache size for ProducerCache or ProducerTemplate?

Answer)This applies to ConsumerCache and ConsumerTemplate as well.

You can configure the default maximum cache size by setting the Exchange.MAXIMUM_CACHE_POOL_SIZE property on CamelContext.

getCamelContext().getProperties().put(Exchange.MAXIMUM_CACHE_POOL_SIZE, "50");

And in Spring XML its done as:

</properties>

...

</camelContext>

The default maximum cache size is 1000.

At runtime you can see the ProducerCache in JMX as they are listed in the services category.

15)How do I configure the maximum endpoint cache size for CamelContext?

Answer)CamelContext will by default cache the last 1000 used endpoints (based on a LRUCache).

CONFIGURING CACHE SIZE

Available as of Camel 2.8

You can configure the default maximum cache size by setting the Exchange.MAXIMUM_ENDPOINT_CACHE_SIZE property on CamelContext.

getCamelContext().getProperties().put(Exchange.MAXIMUM_ENDPOINT_CACHE_SIZE, "500");

You need to configure this before CamelContext is started.

And in Spring XML its done as:

</properties>

...

</camelContext>

At runtime you can see the EndpointRegistry in JMX as they are listed in the services category.

16)How do I debug my route?

Answer)If you’ve created a route and its not doing what you think it is you could try using one of these features from version 1.4 onwards:

Tracer to trace in commons-logging / log4j each step that Camel takes

Debugger to let you set breakpoints at points in the route and examine historic message exchanges

Debug from your unit test if you use the Camel camel-test component

16)How do I handle failures when consuming for example from a FTP server?

Answer)When you do a route such as:

from("ftp://foo@somesever.com?password=secret").to("bean:logic?method=doSomething");

And there is a failure with connecting to the remote FTP server. The existing Error handling in Camel is based on when a message is being routed. In this case the error occurs before a message has been initiated and routed. So how can I control the error handling?

The FTP component have a few options (maximumReconnectAttempts, reconnectDelay to control number of retries and delay in between.

But you can also plugin your own implementation and determine what to do using the pollStrategy option which has more documentation Polling Consumer. Notice that the option pollStrategy applies for all consumers which is a ScheduledPollConsumer consumer. The page lists those.

16)How do I retry failed messages forever?

Answer)If you want to keep the bad message in the original queue, then you are also blocking the messages that has arrived on the queue after the bad message.

By default Camel will retry consuming a message up til 6 times before its moved to the default dead letter queue.

If you configure the Dead Letter Channel to use maximumRedeliveries = -1 then Camel will retry forever.

When you consume a message you can check the in message header org.apache.camel.redeliveryCount that contains the number of times it has been redlivered.

Or org.apache.camel.Redelivered that contains a boolean if its redelivered or if its the first time the message is processed.

16)How should I invoke my POJOs or Spring Services?

Answer)The various options are described in detail in Bean Integration, in particular the Bean Binding describes how we invoke a bean inside a route.

See the POJO Consuming for examples using either the @Consume annotation or using the routing DSL:

from("jms:someQueue").bean(MyBean.class, "someMethod");

17)How should I package applications using Camel and ActiveMQ?

Answer)So you may wish to use Camel’s Enterprise Integration Patterns inside the ActiveMQ Broker. In which case the stand alone broker is already packaged to work with Camel out of the box; just add your EIP routing rules to ActiveMQ’s XML Configuration like the example routing rule which ships with ActiveMQ 5.x or later. If you want to include some Java routing rules, then just add your jar to somewhere inside ActiveMQ’s lib directory.

If you wish to use ActiveMQ and/or Camel in a standalone application, we recommend you just create a normal Spring application; then add the necessary jars and customise the Spring XML and you’re good to go.

18)How to define a static Camel converter method in Scala?

Answer)When you use Scala object you can define the static method for others to use. Scala will create a class which implements the singleton pattern for that class object.

If the object name is A, you can find the singleton class name with A$. Using javap to recompile the class A and A$, you will find A has bunch of static method, and A$ doesn’t have any of them. If you specify the converter class package name in META-INF/service/org/apache/camel/TypeConverter, Camel will load the class A and A$ at the same time. As the A$ construction method is not supposed to be invoked, Camel will complain that he cannot load the converter method which you are supposed to use because he can’t create an instance of A$.

To avoid this kind of error, we need to specify the full class name of A in the TypeConverter to let Camel load the converter directly.

19)How to send the same message to multiple endpoints?

Answer)When you need to send the same message to multiple endpoints then you should use Multicast.

In the sample below we consume messages from the activemq queue foo and want to send the same message to both seda:foo and seda:bar. Sending the same message requires that we use Multicast. This is done by adding the multicast() before the to type:

from("activemq:queue:foo").multicast().to("seda:foo", "seda:bar");

Pipeline is default in Camel

If you have a route such as:

from("activemq:queue:foo").to("seda:foo", "seda:bar");

It is by default a pipeline in Camel (that is the opposite to Multicast). In the above example using pipes and filters then the result from seda:foo is sent to seda:bar, ie. its not the same message sent to multiple destinations, but a sent through a chain (the pipes and the filters).

20)Why does FTP component not download any files?

Answer)The FTP component has many options. So make sure you have configured it properly. Also a common issue is that you have to use either active or passive mode. So you may have to set passiveMode=true on the endpoint configuration.

21)Why does useOriginalMessage with error handler not work as expected?

Answer)If you use the useOriginalMessage option from the Camel Error Handler then it matters if you use this with EIPs such as:

Recipient List

Splitter

Multicast

Then the option shareUnitOfWork on these EIPs influence the message in use by the useOriginalMessage option.

22)Why is my message body empty?

Answer)In Camel the message body can be of any types. Some types are safely readable multiple times, and therefore do not 'suffer' from becoming 'empty'. So when you message body suddenly is empty, then that is often related to using a message type that is no re-readable; in other words, the message body can only be read once. On subsequent reads the body is now empty. This happens with types that are streaming based, such as java.util.InputStream, etc.

A number of Camel components supports and use streaming types out of the box. For example the HTTP related components, CXF, etc.

Camel offers a functionality Stream caching; that caches the stream, so it can be re-readable. By enabling this cache, the message body would no longer be empty.

23)Why use multiple CamelContext?

Answer)In general, you don’t tend to want multiple camel contexts in your application, if you’re running Camel as a standalone Java instance. However, if you’re deploying Camel routes as OSGi bundles, or WARs in an application server, then you can end up having multiple routes being deployed, each in it’s own, isolated camel context, in the same JVM. This makes sense: you want each Camel application to be deployable in isolation, in it’s own Application Context, and not affected by the other Camel applications.

If you want the endpoints or producers in different camel contexts to communicate with another, there are a number of solutions. You can use the ServiceMix NMR, or you can use JMS, or you can use Camel’s VM transport.

24)How do I invoke Camel routes from JBI?

Answer)When you use the JBI endpoint as follows:

from("jbi:endpoint:http://foo.bar.org/MyService/MyEndpoint")

you automatically expose the endpoint to the NMR bus where service qname is:

{http://foo.bar.org}MyService

and endpoint name is MyEndpoint.

Then if you send a message via the JBI NMR to this JBI endpoint then it will be sent to the above Camel route.

Sending works in the same way. You use:

to("jbi:endpoint:http://foo.bar.org/MyService/MyEndpoint")

to send messages to JBI endpoint deployed to the bus.

I noticed that people are used to somehow 'declaring' endpoints in SMX. In Camel it is enough to simply start a flow from a jbi endpoint and Camel will create it automatically.

24)How Do I Make My JMS Endpoint Transactional?

Answer)I have a JMS route like this:

from("activemq:Some.Queue")

.bean(MyProcessor.class);

24)How do the direct, event, seda and vm endpoints compare?

Answer)VM and SEDA endpoints are basically the same; they both offer asychronous in memory SEDA queues; they differ in visibility — endpoints are visible inside the same JVM or within the same CamelContext respectively.

Direct uses no threading; it directly invokes the consumer when sending.

Spring Event adds a listener to Spring’s application events; so the consumer is invoked the same thread as Spring notifies events. Event differs in that the payload should be a Spring ApplicationEvent object whereas Direct, SEDA and VM can use any payload.

24)How do the Timer and Quartz endpoints compare?

Answer)Timer is a simple, non persistence timer using the JDK’s in built timer mechanism.

Quartz uses the Quartz library which uses a database to store timer events and supports distributed timers and cron notation.

24)Why does my JMS route only consume one message at once?

Answer)The default JMS endpoint configuration defines concurrentConsumers to be 1 so only 1 message is processed concurrently at any point in time. To change this to make things more concurrent, just configure this value; either at the JMS component level or endpoint level.

E.g.

from("activemq:SomeQueue?concurrentConsumers=25").

bean(SomeCode.class);

25) Why do Camel throw so many NoClassDefFoundException on startup?

Answer)Camel uses a runtime strategy to discover features while it starts up. This is used to register components, languages, type converters, etc.

If you are using the uber .jar (the big camel.jar) with all the Camel components in a single .jar filen, the this problem can typically occur. Especially the type converters is know to cause NoClassDefFoundException in the log during startup. The reasons is that some of these type converters rely on 3rd. party .jar files.

To remedy this either add the missing .jars to the classpath, or stop using the big .jar and use the fine grained jars.

Apache Camel Interview Questions and Answers

You may also be interested in