This section explains, through an example scenario, how the Dead Letter Channel EIP can be implemented using the ESB profile of WSO2 EI. The following topics are covered:
Introduction to Dead Letter Channel
The Dead Letter Channel (DLC) EIP outlines how messaging systems can handle messages that cannot be delivered to the recipient. Due to system/network failures or failures at the recipient's end, messages sometimes do not get delivered to the target. In such cases, the messaging system can deliver the message to a DLC. Different mechanisms implemented in the DLC take care of delivering the dead message to the target. One method is periodically retrying to send the message to the recipient over a defined period of time. Persistence of the dead message is another option, which ensures that the dead messages are delivered to the receivers once the system is rebooted, even if the messaging system fails.
For more information, go to Dead Letter Channel.
Figure 1: Dead Letter Channel EIP
This example takes a proxy service called
StockQuoteProxy, which fronts a service by the name
SimpleStockQuoteService. As long as the
SimpleStockQuoteService is running, the clients calling
StockQuoteProxy service get responses. But if the
SimpleStockQuoteService is down or a failure occurs while trying to send the message to the
faultSequence of the
StockQuoteProxy will get invoked, and the message will be forwarded to the dead letter channel.
The diagram below depicts how to simulate the example scenario using the ESB profile.
Figure 2: Example Scenario of the Dead Letter Channel EIP
Before digging into implementation details, let's take a look at the relationship between the example scenario and the Dead Letter Channel EIP by comparing their core components. We use three constructs of the ESB profile to implement the Dead Letter Channel EIP.
|Dead Letter Channel EIP (Figure 1)||Dead Letter Channel Example Scenario (Figure 2)|
|Sender||Stock Quote Client|
|Intended Recipient||Stock Quote Service Instance|
|Dead Letter Channel||Store mediator, Message stores, Message processors|
|Intended Receiver||Simple Stock Quote Service|
The diagram below depicts the DLC architecture in the ESB profile.
Figure 3: DLC architecture in the ESB profile
The store mediator stores the dead message in the specified message store. The message processor retrieves stored messages from its associated message store and tries to resend those messages to the target receiver. The message store and message processor combination acts as the dead letter channel.
The ESB configuration
Given below is the ESB configuration of this sample. Log in to the Management Console of the ESB profile, click Main, and then click Source View in the Service Bus menu to view this.
How the implementation works
Let's investigate the elements of the configuration in detail.
faultSequence - The fault sequence. The proxy service
StockQuoteProxyruns in the event of a fault or error.
store - The store mediator loads the message store defined in line 53.
messageStore - Defines the message store to use, which we created using the ESB profile Management Console in step 2 above.
messageProcessor - The
messageProcessordefines the processing algorithm, the period of time to retry sending messages in case of a failure, and the maximum number of retry attempts.
Simulating the sample scenario
Now, let's try out the sample scenario explained above.
Setting up the environment
Executing the sample
Use SoapUI or ESB profile's Try It tool to send the following request to the
StockQuoteProxy service. For information on the
Stock Quote Client and its operation modes, go to Stock Quote Client in the WSO2 EI Documentation.
Analyzing the output
- A message similar to the one below appears in the simple Axis2 server.
- Stop the Axis2 server, and resend the same request. Note that the message sending fails, and the message processor tries to resend the message every few seconds.
- Restart the Axis2 server, and note that the message will be delivered to
SimpleStockQuoteServiceonce the server is running.
Execute the following command for Stock Quote Client:
ant stockquote -Dtrpurl=y
Generating quote for : IBM
Stock Quote Client output:
Standard :: Stock price = $146.31993212382798
Stop the Axis2 server, and resend the same request. View the message count beomes 1 in the Stores (Management Console)
Restart the Axis2 server, and note that the message will be delivered to SimpleStockQuoteService once the server is running.