This documentation is for WSO2 ESB version 4.8.0. View documentation for the latest release.

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The main role of an Enterprise Service Bus (ESB) is to act as the backbone of an organization’s service-oriented architecture. It is the spine through which all the systems and applications within the enterprise (and external applications that integrate with the enterprise) communicate with each other. As such, an ESB often has to deal with many wire level protocols, messaging standards, and remote APIs. But applications and networks can be full of errors. Applications crash. Network routers and links get into states where they cannot pass messages through with the expected efficiency. These error conditions are very likely to cause a fault or trigger a runtime exception in the ESB.

Using fault sequences

WSO2 ESB provides fault sequences for dealing with errors. A fault sequence is a collection of mediators just like any other sequence, and it can be associated with another sequence or a proxy service. When the sequence or the proxy service encounters an error during mediation or while forwarding a message, the message that triggered the error is delegated to the specified fault sequence. Using the available mediators it is possible to log the erroneous message, forward it to a special error-tracking service, and send a SOAP fault back to the client indicating the error or even send an email to the system admin.

It is not mandatory to associate each sequence and proxy service with a fault sequence. In situations where a fault sequence is not specified explicitly, a default fault sequence will be used to handle errors. Sample 4: Introduction to Error Handling shows how to specify a fault sequence with a regular mediation sequence.

Whenever an error occurs in WSO2 ESB, the mediation engine attempts to provide as much information as possible on the error to the user by initializing the following properties on the erroneous message:


Within the fault sequence, you can access these property values using the get-property XPath function. Sample 4 uses the log mediator as follows to log the actual error message:  
<log level="custom">
  <property name="text" value="An unexpected error occured"/>
    <property name="message" expression="get-property('ERROR_MESSAGE')"/>      

Note how the ERROR_MESSAGE property is being used to get the error message text. If you want to customize the error message that is sent back to the client, you can use the makefault mediator as demonstrated in Sample 5: Creating SOAP Fault Messages and Changing the Direction of a Message.

Error codes

This section describes error codes and their meanings.

Transport error codes

Error Code Detail
101000Receiver input/output error sending
101001Receiver input/output error receiving
101500Sender input/output error sending
101501Sender input/output error receiving
101503Connection failed
101504Connection timed out (no input was detected on this connection over the maximum period of inactivity)
101505Connection closed
101506NHTTP protocol violation

Connection canceled

101508Request to establish a new connection timed out
101509Send abort
101510Response processing failed

If the HTTP PassThrough transport is used, and a connection level error occurs, the error code is calculated using the following equation:

There is a state machine in the transport sender side, where the protocol state changes according to the phase of the message.

Following are the possible protocol states and the description for each:

Protocol StateDescription
REQUEST_READY (0)Connection is at the initial stage ready to send a request
REQUEST_HEAD(1)Sending the request headers through the connection
REQUEST_BODY(2)Sending the request body
REQUEST_DONE(3)Request is completely sent
RESPONSE_HEAD(4)The connection is reading the response headers
RESPONSE_BODY(5)The connection is reading the response body
RESPONSE_DONE(6)The response is completed
CLOSING(7)The connection is closing
CLOSED(8)The connection is closed

Since there are several possible protocol states in which a request can time out, you can calculate the error code accordingly using the values in the table above.

For example, in a scenario where you send a request and the request is completely sent to the backend, but a timeout happens before the response headers are received, the error code is calculated as follows:

In this scenario, the base error code is CONNECTION_TIMEOUT(101504) and the protocol state is REQUEST_DONE(3).


Error code = 101504 + 3 = 101507

Endpoint failures

This section describes the error codes for endpoint failures. For more information on handling endpoint errors, see Endpoint Error Handling.

General errors

Error Code Detail
303000Load Balance endpoint is not ready to connect
303000Recipient List Endpoint is not ready
303000Failover endpoint is not ready to connect
303001Address Endpoint is not ready to connect
303002WSDL Address is not ready to connect

Failure on endpoint in the session

Error Code Detail
309001Session aware load balance endpoint, No ready child endpoints
309002Session aware load balance endpoint, Invalid reference
309003Session aware load balance endpoint, Failed session

Non-fatal warnings

Error Code Detail
303100A failover occurred in a Load balance endpoint
304100A failover occurred in a Failover endpoint

Referring real endpoint is null

Error Code Detail
305100Indirect endpoint not ready

Callout operation failed

Error Code Detail
401000 Callout operation failed (from the callout mediator)

Custom error codes

Error Code Detail

Endpoint Custom Error - This error is triggered when the endpoint is prefixed by <property name="FORCE_ERROR_ON_SOAP_FAULT" value="true"/>, which enhances the failover logic by marking an endpoint as suspended when the response is a SOAP fault.

For information on best practices for handling errors in WSO2 ESB, see WSO2 ESB by Example - Best practices for error handling on the WSO2 ESB.

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