8.7. Routing Slip

Overview

The routing slip pattern, shown in Figure 8.9, “Routing Slip Pattern”, enables you to route a message consecutively through a series of processing steps, where the sequence of steps is not known at design time and can vary for each message. The list of endpoints through which the message should pass is stored in a header field (the slip), which Apache Camel reads at run time to construct a pipeline on the fly.

Figure 8.9. Routing Slip Pattern

Routing Slip Pattern

The slip header

The routing slip appears in a user-defined header, where the header value is a comma-separated list of endpoint URIs. For example, a routing slip that specifies a sequence of security tasks—decrypting, authenticating, and de-duplicating a message—might look like the following: 
cxf:bean:decrypt,cxf:bean:authenticate,cxf:bean:dedup

The current endpoint property

From Camel 2.5 the Routing Slip will set a property (Exchange.SLIP_ENDPOINT) on the exchange which contains the current endpoint as it advanced though the slip. This enables you to find out how far the exchange has progressed through the slip.
The Routing Slip will compute the slip beforehand which means, the slip is only computed once. If you need to compute the slip on-the-fly then use the Dynamic Router pattern instead.

Java DSL example

The following route takes messages from the direct:a endpoint and reads a routing slip from the aRoutingSlipHeader header: 
from("direct:b").routingSlip("aRoutingSlipHeader");
You can specify the header name either as a string literal or as an expression.
You can also customize the URI delimiter using the two-argument form of routingSlip(). The following example defines a route that uses the aRoutingSlipHeader header key for the routing slip and uses the # character as the URI delimiter: 
from("direct:c").routingSlip("aRoutingSlipHeader", "#");

XML configuration example

The following example shows how to configure the same route in XML: 
<camelContext id="buildRoutingSlip" xmlns="http://camel.apache.org/schema/spring">
  <route>
    <from uri="direct:c"/>
    <routingSlip uriDelimiter="#">
      <headerName>aRoutingSlipHeader</headerName>
    </routingSlip>
  </route>
</camelContext>

Ignore invalid endpoints

The Routing Slip now supports ignoreInvalidEndpoints, which the Recipient List pattern also supports. You can use it to skip endpoints that are invalid. For example: 
    from("direct:a").routingSlip("myHeader").ignoreInvalidEndpoints();
In Spring XML, this feature is enabled by setting the ignoreInvalidEndpoints attribute on the <routingSlip> tag: 
   <route>
       <from uri="direct:a"/>
       <routingSlip ignoreInvalidEndpoints="true">
         <headerName>myHeader</headerName>
       </routingSlip>
   </route>
Consider the case where myHeader contains the two endpoints, direct:foo,xxx:bar. The first endpoint is valid and works. The second is invalid and, therefore, ignored. Apache Camel logs at INFO level whenever an invalid endpoint is encountered.

Options

The routingSlip DSL command supports the following options:
Name Default Value Description
uriDelimiter , Delimiter used if the Expression returned multiple endpoints.
ignoreInvalidEndpoints false If an endpoint uri could not be resolved, should it be ignored. Otherwise Camel will thrown an exception stating the endpoint uri is not valid.
cacheSize 0 Camel 2.13.1/2.12.4: Allows to configure the cache size for the ProducerCache which caches producers for reuse in the routing slip. Will by default use the default cache size which is 0. Setting the value to -1 allows to turn off the cache all together.