Synapse Configuration Language

The Synapse configuration language is designed to support a processing model where messages come into Synapse, are processed via some number of mediators and then delivered to an endpoint somewhere. It is currently direction agnostic, but directionality can easily be added as a selection mechanism for mediators (see below for details).

Overall Structure

A Synapse configuration looks like the following at the top level:

   <registry provider="string">...</registry>?
   <localEntry key="string">...</localEntry>?
   <sequence name="string">...</sequence>?
   <endpoint name="string">...</endpoint>?
   <proxy name="string" ...>...</proxy>?


The <registry> element is used to define the remote registry which are referenced from within the configuration. The registry provider specifies the implementation class for the registry used, and optionally a number of configuration parameters may be specified to configure the registry.

 <registry provider="string"/>
   <parameter name="string">text | xml</parameter>*

Registry entries loaded from a remote registry are cached within Synapase as dictated by the registry, and reloaded after the cache periods expires. Hence it is possible to define configuration elements such as (dynamic) sequences and endpoints, as well as resources such as XSLT's or XSDs off the registry, and update the configuration as these change dynamically over time.

Local Entry

The <localEntry> element is used to declare registry entries that are local to the Synapse instance, as shown below

  <localEntry key="string" [src="url"]>text | xml</localEntry>

These entries are top level entries which are set globally for the entire system. Values of these entries can be retrieved via the extension XPath function "synapse:get-property(prop-name)".

An entry can be static text specified as inline text or static XML specified as an inline XML fragment or specified as a URL (using the src attribute). These local entries can overide any existing entries with the same keys of the remote registry.


A <sequence> element is used to define a sequence of mediators that can be invoked later by name as a sequence of mediators.

 <sequence name="string" [onError="string"] [key="string"]>

If the configuration defines a sequence named "main" then it is considered as the main mediation sequence of Synapse. If such a sequence is not defined locally, and a registry has been specified, the registry is looked up for a key named "main" to find the main mediator sequence. Synapse also supports the specification of mediators directly within the <definitions> tag, and if any mediators are present, will constitute the main sequence. In the absence of a main sequence, the Synapse runtime will create a default main sequence that consists of an implicit send mediator.

Synapse considers a sequence named "fault", or in its absence a registry entry with a key "fault" as its general fault handler sequence. If Synapse encounters an erroneous situation, it executes the defined error handling sequence for the current context - which may be specified as the 'onError' sequence for a sequence mediator. If a fault sequence is not specified or cannot be found through the registry, Synapse will create a defualt fault sequence that will perform a log of the message at the log level 'full'.

If an optional error handler sequence name is specified on any sequence through the attribute 'onError', an exception on this sequence will invoke the sequence specified by this key.

A Dynamic Sequence may be defined by specifying a key reference to a registry entry. As the remote registry entry changes, the sequence will dynamically be updated accordingly.


An <endpoint> element defines a destination for an outgoing message. An endpoint may be specified as an address endpoint, WSDL based endpoint, a load balanced endpoint or a fail-over endpoint as follows:

<endpoint [name="string"] [key="string"]>
address-endpoint | wsdl-endpoint | load-balanced-endpoint | fail-over-endpoint </endpoint> 

All above endpoint types can have a name attribute. Such named endpoints can be reffered by other endpoints, which only contain the key attribute. For example if there is an endpoint named as "foo", following endpoint can be used in any place, where "foo" has to be used.

<endpoint key="foo"/>

Address Endpoint

Address endpoint is an endpoint defined by specifying the EPR and other attributes of the endpoint directly in the configuration. uri attribute of the address element contains the EPR of the target endpoint. Message format for the endpoint and the method to optimize attachments can be specified in the format and optimize attributes respectively. Reliable messaging and security policies for the endpoint can be specified in the policy attribute of the enableRM and enableSec elements respectively. WS-Addressing can be engaged for the messaging going to the endpoint by the enableAddressing element. suspendDurationOnFailure attribute specifies the time duration in seconds to suspend this endpoint, if it is detected as failed. If this attribute is not specified, endpoint will never be recovered after a failure.

Address endpoints can timeout the responses if they failed to arrive Synapse in predefined time duration. This is done by the timeout elements. duration element inside the timeout element contains duration for the timeout in seconds. action element specifies the action to be performed once a timeout occurs. Allowed actions are discard and fault. If discard is specified, responses arriving after the timeout will be discarded without further processing. If fault is specified as the action, fault sequence applicable for the endpoint will be activated as soon as the timeout occurs and responses will not be processed after that.

<address uri="endpoint-address" format="soap|soap11|soap12|pox" [optimize="mtom|swa"]>
  <enableRM [policy="key"]/>?
  <enableSec [policy="key"]/>?

WSDL Endpoint

WSDL endpoint is an endpoint based on a WSDL document. It can extract the target EPR from a given WSDL. Currently it only supports WSDL 1.1. WSDL document can be specifed either as an uri or as inline with the configuration. uri attribute can be set to specify the WSDL as an uri. WSDL can be specified inline as a child element of the wsdl element. Service and port name containing the target EPR has to be specified in service and port attributes respectively. enableRM, enableSec, enableAddressing, suspendDurationOnFailure and timeout elements are same as in the Address endpoint.

<wsdl [uri="wsdl-uri"] service="qname" port/endpoint="qname">
  <enableRM [policy="key"]/>?
  <enableSec [policy="key"]/>?

Load balanced Endpoint

Load balance endpoint distributes the messages (load) arriving at it among the set of listed endpoints by evaluating the load balancing policy and other parameters. policy attribute of the load balance element specifies the load balance policy (algorithm) to be used for selecting the target endpoint. Currently only the roundRobin policy is supported. failover attribute determines if the next endpoint should be selected once the currently selected endpoint has failed. Default is true. The set of endpoints among which the load is distributed can be listed under the loadBalance element. Those endpoints can belong to any endpoint type mentioned in this document. For example, failover endpoints can be listed inside the load balance endpoint to load balance between failover groups.

The optional session element makes the endpoint a session affinity based load balancing endpoint. If it is specified, sessions are bound to endpoints in the first message and all successive messages for those sessions are directed to their associated endpoints. http sessions are supported currently, which identifies sessions based on http cookies. failover attribute mentioned above is not applicable for session affinity based endpoints and it is always set to false. If it is required to have failover behavior in session affinity based load balance endpoints, list failover endpoints as the target endpoints.

<session type="http"/>?
<loadBalance [policy="roundRobin"] [failover="true|false"]>
  <endpoint .../>+

Failover Endpoint

Failover endpoints send messages to the listed endpoints with the following failover behavior. At the start, first listed endpoint is selected as the primary and all other endpoints are treated as backups. Incoming messages are always sent only to the primary endpoint. If the primary endpoint fails, next active endpoint is selected as the primary and failed endpoint is marked as incative. Thus it sends messages successfully as long as there is at least one active endpoint among the listed endpoints.

  <endpoint .../>+

Proxy service

A <proxy> element is used to define a Synapse Proxy service.

 <proxy name="string" [transports="(http |https |jms )+|all"]>
   <target [inSequence="name"] [outSequence="name"] [faultSequence="name"] [endpoint="name"]>
   <publishWSDL key="string" uri="string">
      <description>...</description> | <definitions>...</definitions>
   <enableSec/>?                                   // These two tags will removed after the recognition of the Security and RM can be done by looking at policy
   <policy key="string">...</policy>?       // optional service level policies
                                                            // (e.g. WS-Security and/or WS-RM policies)
   <parameter name="string">                // optional service parameters
      string | xml                                      // (e.g. transport.jms.ConnectionFactory)

A proxy service is created and exposed on the specified transports through the underlying Axis2 instance, exposing service EPR's as per the standard Axis2 conventions - based on the service name. (Note: that currently Axis2 does not allow custom URI's to be set for services on some transports.) The Proxy service could be exposed over all enabled Axis2 transports such as http, https, JMS etc. or on a subset of these. Each service could define the target for received messages as a named sequence or a direct endpoint. Target inSequence or endpoint is required for the proxy configuration. Any supplied policies would apply as service level policies, and any properties could be passed into the proxy services' AxisService instance (e.g. the JMS destination etc). If the proxy service should enable WS-Reliable Messaging or Security, the appropriate modules could be engaged.

A Dynamic Proxy may be defined by specifying a proxy with the key as its definition. As the remote registry entry changes, the proxy will dynamically be updated accordingly.


A mediator token refers to any of the following tokens:

In addition to the above, Synapse will be able to load mediators via the J2SE Service Provider model. Mediator extensions must implement the MediatorFactory interface, similarly to the configuration extensions mentioned previously.

Core Mediators


The send token represents a <send> element. The <send> element is used to send messages out of Synapse to some endpoint, and stop further mediation of the message. The send mediator also copies any message context properties from the current message context to the reply message received on the execution of the send operation. This allows the reply messages to be correlated to the original messages in a flexible manner. Messages may be correlated by WS-A MessageID, or even simple custom text labels. Also see the property mediator.

In the simplest case, the place to send the message to is implicit in the message (via a property of the message itself)- that is indicated by the following:


If the message is to be sent to one or more endpoints, then the following is used:

   (endpointref | endpoint)+

where the endpointref token refers to the following:

 <endpoint key="name"/>

and the endpoint token refers to an anonymous endpoint defined.


The drop token refers to a <drop> element which is used to drop a message:


Once the <drop> mediator executes, further processing of the current message stops.


The log token refers to a <log> element which may be used to log messages being mediated:

 <log [level="string"] [separator="string"]>
   <property name="string" (value="literal" | expression="xpath")/>*

The optional level attribute selects a pre-defined subset of properties to be logged.


  • simple = To, From, WSAction, SOAPAction, ReplyTo, MessageID and any properties
  • headers = All SOAP header blocks and any properties
  • full = all attributes included in log level 'simple' and the SOAP envelope and any properties
  • custom = Only properties specified to the Log mediator

A separator if defined will be used to seperate the attributes being logged. The default separator is the ',' comma.


 <property name="string" [action=set|remove] (value="literal" | expression="xpath") [scope=transport|axis2]/>

The property token refers to a <property> element which is a mediator that has no direct impact on the message but rather on the message context flowing through Synapse. The properties which does not specify the action thus set on the message context applies only to the current message and can be later retrieved through the synapse:get-property(prop-name) extension function. If a scope is specified for a property, the property could be set as a transport header property or an (underlying) Axis2 message context property. Using the property element with action specified as "remove" you can remove the message context properties if available.

There are some well-defined properties that you can get/set:

  • RESPONSE - 'true' means the message is a response message
  • ERROR_MESSAGE - this is set to any error message

There are also some Axis2 and module properties that are useful to set (with scope="axis2"):

  • Sandesha2RMSpecVersion - can be '1.0' or '1.1'
  • Sandesha2SequenceKey - can be an identifier specifying an Sandesha internal sequence key, and
  • Sandesha2LastMessage - 'true' will make this the last message and terminate the sequence


 <sequence key="name"/>

A sequence ref token refers to a <sequence> element which is used to invoke a named sequence of mediators.


 <validate [source="xpath"]>
   <property name="validation-feature-id" value="true|false"/>*
   <schema key="string"/>+

The <validate> mediator validates the result of the evaluation of the source xpath expression, against the schema specified. If the source attribute is not specified, the validation is performed against the first child of the SOAP body of the current message. If the validation fails, the on-fail sequence of mediators is executed. Properties could be used to turn on/off some of the underlying features of the validator (See

Message Transformations


 <makefault [version="soap11|soap12"]>
   <code (value="literal" | expression="xpath")/>
   <reason (value="literal" | expression="xpath")>

The <makefault> mediator transforms the current message into a fault message, but does NOT send it. The <send> mediator needs to be invoked to send a fault message created this way. The fault message "to" header is set to the "faultTo" of the original message if such a header existed on the original message.


 <xslt key="string" [source="xpath"]>
   <property name="string" (value="literal" | expression="xpath")/>*

The <xslt> mediator applies the specified XSLT transformation to the given element. If the source element is not specified, it defaults to the first child of the soap body. Optionally parameters (XSLT) could be passed into the transformations through the <property> elements.


 <header name="qname" (value="literal" | expression="xpath") [action="set"]/>
 <header name="qname" action="remove"/>

The <header> mediator sets or removes a specified header from the current soap message. Currently the set header only supports simple valued headers. In the future we may extend this to have XML structured headers by embedding the XML content within the element itself. The optional action attribute specifies whether the mediator should set or remove the header. If omitted, it defaults to a set-header.



 <filter (source="xpath" regex="string") | xpath="xpath">

The <filter> mediator either test the given xpath expression as a boolean expression, or match the evaluation result of a source xpath expression against the given regular expression. If the test succeeds, the filter mediator will execute the enclosed mediators in sequence.


 <switch source="xpath">
   <case regex="string">

The <switch> mediator will evaluate the given source xpath expression into its string value, and match it against the given regular expressions. If the specified cases does not match and a default case exists, it will be executed.

In / Out

<in>  mediator+

The In and Out mediators will execute the child mediators over the current message if the message matches the direction of the mediator. Hence all incoming messages would pass through the "<in>" mediators and vice versa.

WS-Reliable Messaging


 <RMSequence (correlation="xpath" [last-message="xpath"]) | single="true" [version="1.0|1.1"]/>

The <RMSequence> mediator can be used to create a sequence of messages to communicate via WS-Reliable Messaging with an WS-RM enabled endpoint (<enableRM>). The simple use case of this mediator is to specify a single="true" property, because this means that only one message is involved in the same sequence. However if multiple messages should be sent in the same sequence, the correlation property should be used with a XPath expression that selects an unique element value from the incoming message. With the result of the XPath expression, Synapse can group messages together that belong to the same sequence. To close the sequence neatly, for the last message of the sequence also an XPath expression should be specified. With the version attribute the WS-RM specification version to be used can be specified, 1.0 or 1.1.

Extension mediator

Class Mediator

 <class name="class-name">
   <property name="string" (value="literal" | expression="xpath")/>*

The class mediator creates an instance of the specified class and sets it as a mediator. The class must implement the org.apache.synapse.api.Mediator interface. If any properties are specified, the corresponding setter methods are invoked on the class. However, Synapse currently supports only String properties.

Extensibility of Synapse

The Synapse configuration language could be easily extended, with configuration extensions as well as mediation extensions. The Spring mediator is such an example.

Spring Configuration

A Spring configuration could be created as a localEntry or remote registry entry providing a URL or a key reference to a Registry. The configuration is then created on first use or as necessary (as per registry lookup semantics) by the mediators which reference this configuration.

 <localEntry key="string"/>
 <localEntry key="string" src="url"/>

The name attribute specifies a unique name for the configuration, and the src, key or inlined XML references to the Spring configuration

Spring mediator

 <spring:spring bean="exampleBean1" key="string"/>

The <spring> element creates an instance of a mediator, which is managed by Spring. This Spring bean must implement the Mediator interface for it to act as a Mediator. The key will reference the Spring ApplicationContext/Configuration used for the bean

Scripting language mediators

Synapse supports Mediators implemented in a variety of scripting languages such as JavaScript, Python or Ruby. There are two ways of defining script mediators, either with the script program statements stored in a separate file which is referenced via the local or remote registry entry, or with the script program statements embedded in-line within the Synapse configuration. A script mediator using a script off the registry (local or remote) is defined as follows:

 <script key="string" language="string" [function="script-function-name"]/>

The property key is the registry key to load the script. The language attribute specifies the scripting language of the script code (e.g. "js" for Javascript, "rb" for ruby, "groovy" for Groovy, "py" for Python..). The function is an optional attribute defining the name of the script function to invoke, if not specified it defaults to a function named 'mediate'. The function is passed a single parameter - which is the Synapse MessageContext. The function may return a boolean, if it does not, then true is assumed, and the script mediator returns this value. An inline script mediator has the script source embedded in the configuration as follows:

 <script language="string">...script source code...<script/>

The execution context environment of the script has access to the Synapse MessageContext predefined in a script variable named 'mc' . An example of an inline mediator using JavaScript/E4X which returns false if the SOAP message body contains an element named 'symbol' which has a value of 'IBM' would be:

 <script language="js">mc.getPayloadXML()..symbol != "IBM";<script/>

Synapse uses the Apache Bean Scripting Framework for the scripting language support, any script language supported by BSF may be used to implement a Synapse Mediator.

Implementing a Mediator with a script language can have advantages over using the built in Synapse Mediator types or implementing a custom Java class Mediator. Script Mediators have all the flexibility of a class Mediator with access to the Synapse MessageContext and SynapseEnvironment APIs, and the ease of use and dynamic nature of scripting languages allows rapid development and prototyping of custom mediators. An additional benefit of some scripting languages is that they have very simple and elegant XML manipulation capabilities, for example JavaScript E4X or Ruby REXML, so this makes them well suited for use in the Synapse mediation environment. For both types of script mediator definition the MessageContext passed into the script has additional methods over the standard Synapse MessageContext to enable working with the XML in a way natural to the scripting language. For example when using JavaScript getPayloadXML and setPayloadXML, E4X XML objects, and when using Ruby, REXML documents.