Chapter 1. Building Blocks for Route Definitions

To use the Domain Specific Language (DSL), you extend the RouteBuilder class and override its configure() method (where you define your routing rules).

You can define as many RouteBuilder classes as necessary. Each class is instantiated once and is registered with the CamelContext object. Normally, the lifecycle of each RouteBuilder object is managed automatically by the container in which you deploy the router.

As a router developer, your core task is to implement one or more RouteBuilder classes. There are two alternative RouteBuilder classes that you can inherit from:

The RouteBuilder class defines methods used to initiate your routing rules (for example, from(), intercept(), and exception()).

Example 1.1, “Implementation of a RouteBuilder Class” shows a minimal RouteBuilder implementation. The configure() method body contains a routing rule; each rule is a single Java statement.

import org.apache.camel.builder.RouteBuilder;

public class MyRouteBuilder extends RouteBuilder {

public void configure() {
  // Define routing rules here:
  from("file:src/data?noop=true").to("file:target/messages");

  // More rules can be included, in you like.
  // ...
}
}

The form of the rule from(URL1).to(URL2) instructs the router to read files from the directory src/data and send them to the directory target/messages. The option ?noop=true instructs the router to retain (not delete) the source files in the src/data directory.

A Domain Specific Language (DSL) is a mini-language designed for a special purpose. A DSL does not have to be logically complete but needs enough expressive power to describe problems adequately in the chosen domain. Typically, a DSL does not require a dedicated parser, interpreter, or compiler. A DSL can piggyback on top of an existing object-oriented host language, provided DSL constructs map cleanly to constructs in the host language API.

Consider the following sequence of commands in a hypothetical DSL:

command01;
command02;
command03;

You can map these commands to Java method invocations, as follows:

command01().command02().command03()

You can even map blocks to Java method invocations. For example:

command01().startBlock().command02().command03().endBlock()

The DSL syntax is implicitly defined by the data types of the host language API. For example, the return type of a Java method determines which methods you can legally invoke next (equivalent to the next command in the DSL).

Apache Camel defines a router DSL for defining routing rules. You can use this DSL to define rules in the body of a RouteBuilder.configure() implementation. Figure 1.1, “Local Routing Rules” shows an overview of the basic syntax for defining local routing rules.

Figure 1.1. Local Routing Rules

A local rule always starts with a from("EndpointURL") method, which specifies the source of messages (consumer endpoint) for the routing rule. You can then add an arbitrarily long chain of processors to the rule (for example, filter()). You typically finish off the rule with a to("EndpointURL") method, which specifies the target (producer endpoint) for the messages that pass through the rule. However, it is not always necessary to end a rule with to(). There are alternative ways of specifying the message target in a rule.

A local rule always starts by defining a consumer endpoint, using from("EndpointURL"), and typically (but not always) ends by defining a producer endpoint, using to("EndpointURL"). The endpoint URLs, EndpointURL, can use any of the components configured at deploy time. For example, you could use a file endpoint, file:MyMessageDirectory, an Apache CXF endpoint, cxf:MyServiceName, or an Apache ActiveMQ endpoint, activemq:queue:MyQName. For a complete list of component types, see Apache Camel Component Reference.

An exchange object consists of a message, augmented by metadata. Exchanges are of central importance in Apache Camel, because the exchange is the standard form in which messages are propagated through routing rules. The main constituents of an exchange are, as follows:

Using an Exchange object makes it easy to generalize message processing to different message exchange patterns. For example, an asynchronous protocol might define an MEP that consists of a single message that flows from the consumer endpoint to the producer endpoint (an InOnly MEP). An RPC protocol, on the other hand, might define an MEP that consists of a request message and a reply message (an InOut MEP). Currently, Apache Camel supports the following MEPs:

Where these message exchange patterns are represented by constants in the enumeration type, org.apache.camel.ExchangePattern.

Sometimes it is useful to have a single exchange that encapsulates multiple exchange instances. For this purpose, you can use a grouped exchange. A grouped exchange is essentially an exchange instance that contains a java.util.List of Exchange objects stored in the Exchange.GROUPED_EXCHANGE exchange property. For an example of how to use grouped exchanges, see Section 8.5, “Aggregator”.

A processor is a node in a route that can access and modify the stream of exchanges passing through the route. Processors can take expression or predicate arguments, that modify their behavior. For example, the rule shown in Figure 1.1, “Local Routing Rules” includes a filter() processor that takes an xpath() predicate as its argument.

Expressions (evaluating to strings or other data types) and predicates (evaluating to true or false) occur frequently as arguments to the built-in processor types. For example, the following filter rule propagates In messages, only if the foo header is equal to the value bar:

from("seda:a").filter(header("foo").isEqualTo("bar")).to("seda:b");

Where the filter is qualified by the predicate, header("foo").isEqualTo("bar"). To construct more sophisticated predicates and expressions, based on the message content, you can use one of the expression and predicate languages (see Part II, “Routing Expression and Predicate Languages”).

The router schema — which defines the XML DSL — belongs to the following XML schema namespace:

http://camel.apache.org/schema/spring

The location of the router schema is normally specified to be http://camel.apache.org/schema/spring/camel-spring.xsd, which references the latest version of the schema on the Apache Web site. For example, the root beans element of an Apache Camel Spring file is normally configured as shown in Example 1.2, “Specifying the Router Schema Location”.

<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:camel="http://camel.apache.org/schema/spring"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:schemaLocation="
       http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.0.xsd
       http://camel.apache.org/schema/spring http://camel.apache.org/schema/spring/camel-spring.xsd">

  <camelContext id="camel" xmlns="http://camel.apache.org/schema/spring">
    <!-- Define your routing rules here -->
  </camelContext>
</beans>

At run time, Apache Camel does not download the router schema from schema location specified in the Spring file. Instead, Apache Camel automatically picks up a copy of the schema from the root directory of the camel-spring JAR file. This ensures that the version of the schema used to parse the Spring file always matches the current runtime version. This is important, because the latest version of the schema posted up on the Apache Web site might not match the version of the runtime you are currently using.

Generally, it is recommended that you edit your Spring files using a full-feature XML editor. An XML editor’s auto-completion features make it much easier to author XML that complies with the router schema and the editor can warn you instantly, if the XML is badly-formed.

XML editors generally do rely on downloading the schema from the location that you specify in the xsi:schemaLocation attribute. In order to be sure you are using the correct schema version whilst editing, it is usually a good idea to select a specific version of the camel-spring.xsd file. For example, to edit a Spring file for the 2.3 version of Apache Camel, you could modify the beans element as follows:

<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:camel="http://camel.apache.org/schema/spring"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:schemaLocation="
       http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.0.xsd
       http://camel.apache.org/schema/spring http://camel.apache.org/schema/spring/camel-spring-2.3.0.xsd">
...

Change back to the default, camel-spring.xsd, when you are finished editing. To see which schema versions are currently available for download, navigate to the Web page, http://camel.apache.org/schema/spring.

Apache Camel endpoints are the sources and sinks of messages in a route. An endpoint is a very general sort of building block: the only requirement it must satisfy is that it acts either as a source of messages (a consumer endpoint) or as a sink of messages (a producer endpoint). Hence, there are a great variety of different endpoint types supported in Apache Camel, ranging from protocol supporting endpoints, such as HTTP, to simple timer endpoints, such as Quartz, that generate dummy messages at regular time intervals. One of the major strengths of Apache Camel is that it is relatively easy to add a custom component that implements a new endpoint type.

Endpoints are identified by endpoint URIs, which have the following general form:

scheme:contextPath[?queryOptions]

The URI scheme identifies a protocol, such as http, and the contextPath provides URI details that are interpreted by the protocol. In addition, most schemes allow you to define query options, queryOptions, which are specified in the following format:

?option01=value01&option02=value02&...

For example, the following HTTP URI can be used to connect to the Google search engine page:

http://www.google.com

The following File URI can be used to read all of the files appearing under the C:\temp\src\data directory:

file://C:/temp/src/data

Not every scheme represents a protocol. Sometimes a scheme just provides access to a useful utility, such as a timer. For example, the following Timer endpoint URI generates an exchange every second (=1000 milliseconds). You could use this to schedule activity in a route.

timer://tickTock?period=1000

Sometimes endpoint URIs can become quite long due to all the accompanying configuration information supplied. In JBoss Fuse 6.2 onwards, there are two approaches you can take to make your working with lengthy URIs more manageable.

Many of the Apache Camel components have options whose value is a time period (for example, for specifying timeout values and so on). By default, such time period options are normally specified as a pure number, which is interpreted as a millisecond time period. But Apache Camel also supports a more readable syntax for time periods, which enables you to express the period in hours, minutes, and seconds. Formally, the human-readable time period is a string that conforms to the following syntax:

[NHour(h|hour)][NMin(m|minute)][NSec(s|second)]

Where each term in square brackets, [], is optional and the notation, (A|B), indicates that A and B are alternatives.

For example, you can configure timer endpoint with a 45 minute period as follows:

from("timer:foo?period=45m")
  .to("log:foo");

You can also use arbitrary combinations of the hour, minute, and second units, as follows:

from("timer:foo?period=1h15m")
  .to("log:foo");
from("timer:bar?period=2h30s")
  .to("log:bar");
from("timer:bar?period=3h45m58s")
  .to("log:bar");

By default, the option values that you specify in a URI are automatically URI-encoded. In some cases this is undesirable beahavior. For example, when setting a password option, it is preferable to transmit the raw character string without URI encoding.

It is possible to switch off URI encoding by specifying an option value with the syntax, RAW(RawValue). For example,

from("SourceURI")
 .to("ftp:joe@myftpserver.com?password=RAW(se+re?t&23)&binary=true")

In this example, the password value is transmitted as the literal value, se+re?t&23.

Some endpoint URI options get mapped to Java enum constants. For example, the level option of the Log component, which can take the enum values, INFO, WARN, ERROR, and so on. This type conversion is case-insensitive, so any of the following alternatives could be used to set the logging level of a Log producer endpoint:

<to uri="log:foo?level=info"/>
<to uri="log:foo?level=INfo"/>
<to uri="log:foo?level=InFo"/>

From Camel 2.17, the resource based components such as XSLT, Velocity can load the resource file from the Registry by using ref: as prefix.

For example, ifmyvelocityscriptbean and mysimplescriptbean are the IDs of two beans in the registry, you can use the contents of these beans as follows:

Velocity endpoint:
------------------
from("velocity:ref:myvelocityscriptbean").<rest_of_route>.

Language endpoint (for invoking a scripting language):
-----------------------------------------------------
from("direct:start")
  .to("language:simple:ref:mysimplescriptbean")
 Where Camel implicitly converts the bean to a String.

Each URI scheme maps to an Apache Camel component, where an Apache Camel component is essentially an endpoint factory. In other words, to use a particular type of endpoint, you must deploy the corresponding Apache Camel component in your runtime container. For example, to use JMS endpoints, you would deploy the JMS component in your container.

Apache Camel provides a large variety of different components that enable you to integrate your application with various transport protocols and third-party products. For example, some of the more commonly used components are: File, JMS, CXF (Web services), HTTP, Jetty, Direct, and Mock. For the full list of supported components, see the Apache Camel component documentation.

Most of the Apache Camel components are packaged separately to the Camel core. If you use Maven to build your application, you can easily add a component (and its third-party dependencies) to your application simply by adding a dependency on the relevant component artifact. For example, to include the HTTP component, you would add the following Maven dependency to your project POM file:

<!-- Maven POM File -->
  <properties>
    <camel-version>{camelFullVersion}</camel-version>
    ...
  </properties>

  <dependencies>
    ...
    <dependency>
      <groupId>org.apache.camel</groupId>
      <artifactId>camel-http</artifactId>
      <version>${camel-version}</version>
    </dependency>
    ...
  </dependencies>

The following components are built-in to the Camel core (in the camel-core artifact), so they are always available:

A consumer endpoint is an endpoint that appears at the start of a route (that is, in a from() DSL command). In other words, the consumer endpoint is responsible for initiating processing in a route: it creates a new exchange instance (typically, based on some message that it has received or obtained), and provides a thread to process the exchange in the rest of the route.

For example, the following JMS consumer endpoint pulls messages off the payments queue and processes them in the route:

from("jms:queue:payments")
  .process(SomeProcessor)
  .to("TargetURI");

Or equivalently, in Spring XML:

<camelContext id="CamelContextID"
              xmlns="http://camel.apache.org/schema/spring">
  <route>
    <from uri="jms:queue:payments"/>
    <process ref="someProcessorId"/>
    <to uri="TargetURI"/>
  </route>
</camelContext>

Some components are consumer only — that is, they can only be used to define consumer endpoints. For example, the Quartz component is used exclusively to define consumer endpoints. The following Quartz endpoint generates an event every second (1000 milliseconds):

from("quartz://secondTimer?trigger.repeatInterval=1000")
  .process(SomeProcessor)
  .to("TargetURI");

If you like, you can specify the endpoint URI as a formatted string, using the fromF() Java DSL command. For example, to substitute the username and password into the URI for an FTP endpoint, you could write the route in Java, as follows:

fromF("ftp:%s@fusesource.com?password=%s", username, password)
  .process(SomeProcessor)
  .to("TargetURI");

Where the first occurrence of %s is replaced by the value of the username string and the second occurrence of %s is replaced by the password string. This string formatting mechanism is implemented by String.format() and is similar to the formatting provided by the C printf() function. For details, see java.util.Formatter.

A producer endpoint is an endpoint that appears in the middle or at the end of a route (for example, in a to() DSL command). In other words, the producer endpoint receives an existing exchange object and sends the contents of the exchange to the specified endpoint.

For example, the following JMS producer endpoint pushes the contents of the current exchange onto the specified JMS queue:

from("SourceURI")
  .process(SomeProcessor)
  .to("jms:queue:orderForms");

Or equivalently in Spring XML:

<camelContext id="CamelContextID" xmlns="http://camel.apache.org/schema/spring">
  <route>
    <from uri="SourceURI"/>
    <process ref="someProcessorId"/>
    <to uri="jms:queue:orderForms"/>
  </route>
</camelContext>

Some components are producer only — that is, they can only be used to define producer endpoints. For example, the HTTP endpoint is used exclusively to define producer endpoints.

from("SourceURI")
  .process(SomeProcessor)
  .to("http://www.google.com/search?hl=en&q=camel+router");

If you like, you can specify the endpoint URI as a formatted string, using the toF() Java DSL command. For example, to substitute a custom Google query into the HTTP URI, you could write the route in Java, as follows:

from("SourceURI")
  .process(SomeProcessor)
  .toF("http://www.google.com/search?hl=en&q=%s", myGoogleQuery);

Where the occurrence of %s is replaced by your custom query string, myGoogleQuery. For details, see java.util.Formatter.

To enable the router to do something more interesting than simply connecting a consumer endpoint to a producer endpoint, you can add processors to your route. A processor is a command you can insert into a routing rule to perform arbitrary processing of messages that flow through the rule. Apache Camel provides a wide variety of different processors, as shown in Table 1.1, “Apache Camel Processors”.

To get some idea of how to use processors in a route, see the following examples:

The choice() processor is a conditional statement that is used to route incoming messages to alternative producer endpoints. Each alternative producer endpoint is preceded by a when() method, which takes a predicate argument. If the predicate is true, the following target is selected, otherwise processing proceeds to the next when() method in the rule. For example, the following choice() processor directs incoming messages to either Target1, Target2, or Target3, depending on the values of Predicate1 and Predicate2:

from("SourceURL")
    .choice()
        .when(Predicate1).to("Target1")
        .when(Predicate2).to("Target2")
        .otherwise().to("Target3");

Or equivalently in Spring XML:

<camelContext id="buildSimpleRouteWithChoice" xmlns="http://camel.apache.org/schema/spring">
  <route>
    <from uri="SourceURL"/>
    <choice>
      <when>
        <!-- First predicate -->
        <simple>header.foo = 'bar'</simple>
        <to uri="Target1"/>
      </when>
      <when>
        <!-- Second predicate -->
        <simple>header.foo = 'manchu'</simple>
        <to uri="Target2"/>
      </when>
      <otherwise>
        <to uri="Target3"/>
      </otherwise>
    </choice>
  </route>
</camelContext>

In the Java DSL, there is a special case where you might need to use the endChoice() command. Some of the standard Apache Camel processors enable you to specify extra parameters using special sub-clauses, effectively opening an extra level of nesting which is usually terminated by the end() command. For example, you could specify a load balancer clause as loadBalance().roundRobin().to("mock:foo").to("mock:bar").end(), which load balances messages between the mock:foo and mock:bar endpoints. If the load balancer clause is embedded in a choice condition, however, it is necessary to terminate the clause using the endChoice() command, as follows:

from("direct:start")
    .choice()
        .when(bodyAs(String.class).contains("Camel"))
            .loadBalance().roundRobin().to("mock:foo").to("mock:bar").endChoice()
        .otherwise()
            .to("mock:result");

The filter() processor can be used to prevent uninteresting messages from reaching the producer endpoint. It takes a single predicate argument: if the predicate is true, the message exchange is allowed through to the producer; if the predicate is false, the message exchange is blocked. For example, the following filter blocks a message exchange, unless the incoming message contains a header, foo, with value equal to bar:

from("SourceURL").filter(header("foo").isEqualTo("bar")).to("TargetURL");

Or equivalently in Spring XML:

<camelContext id="filterRoute" xmlns="http://camel.apache.org/schema/spring">
  <route>
    <from uri="SourceURL"/>
    <filter>
      <simple>header.foo = 'bar'</simple>
      <to uri="TargetURL"/>
    </filter>
  </route>
</camelContext>

The throttle() processor ensures that a producer endpoint does not get overloaded. The throttler works by limiting the number of messages that can pass through per second. If the incoming messages exceed the specified rate, the throttler accumulates excess messages in a buffer and transmits them more slowly to the producer endpoint. For example, to limit the rate of throughput to 100 messages per second, you can define the following rule:

from("SourceURL").throttle(100).to("TargetURL");

Or equivalently in Spring XML:

<camelContext id="throttleRoute" xmlns="http://camel.apache.org/schema/spring">
  <route>
    <from uri="SourceURL"/>
    <throttle maximumRequestsPerPeriod="100" timePeriodMillis="1000">
      <to uri="TargetURL"/>
    </throttle>
  </route>
</camelContext>

If none of the standard processors described here provide the functionality you need, you can always define your own custom processor. To create a custom processor, define a class that implements the org.apache.camel.Processor interface and overrides the process() method. The following custom processor, MyProcessor, removes the header named foo from incoming messages:

public class MyProcessor implements org.apache.camel.Processor {
public void process(org.apache.camel.Exchange exchange) {
  inMessage = exchange.getIn();
  if (inMessage != null) {
      inMessage.removeHeader("foo");
  }
}
};

To insert the custom processor into a router rule, invoke the process() method, which provides a generic mechanism for inserting processors into rules. For example, the following rule invokes the processor defined in Example 1.3, “Implementing a Custom Processor Class”:

org.apache.camel.Processor myProc = new MyProcessor();

from("SourceURL").process(myProc).to("TargetURL");