synchronizing a database insert and select behind a web service - mule

Question

I'm struggling to figure out how to solve this problem in mule using the studio and thought that perhaps reaching out to the good users of SO may be of some help.
I have a simple webservice that takes a request from a client. this request will preform an insert into a database table, effectively using this database as a message queue. A separate process periodically polls this table, performs additional processing on the message, and then writes results to an output table. the database insert and subsequent select will be linked by a a correlationId that I can pass along to ensure I get the result for the message that was sent. Unfortunately, the software this will integrate with requires this pattern to work correctly.
Here's the workflow that is needed:
HttpRequest -> insert record into a table -> wait(or poll/retry/etc?) until a record is written to another table by a separate process(with the same correlationId) -> return data from this other table back to the httpRequest
here's a sample flow that is as close as i've been able to get with this. Oddly enough, this flow does actually return a payload, however it seems to always be "1". i can't quite see how to make this flow retry the database query until a row exists and then return the resulting row.
How should i be synchronizing 2 database calls? is this possible within mule perhaps with a different combination of components?
Thanks.
<flow name="mainFlow">
<http:listener config-ref="HTTP_Listener_Configuration" path="hello" doc:name="HTTP"/>
<cxf:jaxws-service doc:name="CXF" configuration-ref="CXF_Configuration" serviceClass="kansas.MuleTestServiceImpl"/>
<db:insert config-ref="Oracle_Configuration" doc:name="Database">
<db:parameterized-query><![CDATA[insert into tblRequest (id, correlationId) values(#[payload], #[message.correlationId])]]></db:parameterized-query>
</db:insert>
<until-successful objectStore-ref="MyObjectStore" maxRetries="5" millisBetweenRetries="2000" doc:name="Until Successful" > <!-- failureExpression="???" -->
<db:select config-ref="Oracle_Configuration" doc:name="Database">
<db:parameterized-query><![CDATA[select correlationId,msgResponse from tblResponse where correlationId = #[message.correlationId]]]></db:parameterized-query>
</db:select>
</until-successful>
<logger level="INFO" doc:name="Logger" message="#[payload]"/> <!-- why is payload always = 1? -->
</flow>

Answer 1

Mule is great tool but it makes your life too easy. Sometime so easy that you forget simple things.
In your case you forgot that payload is one object which is result of last component. Think about flow as rails with just one cart. Whatever you load on last station is delivered to the next one. And then process repeats. What was originally delivered to the station does not matter. Matters what you load.
In your case first database component has original payload from CXF and stores something in the database. It returns result of the INSERT statement which is 1 - one row is inserted. So our payload keeps deliver new cargo - 1.
But you need original payload from CXF. Where it is? It is gone - we have only one flow, one pair of trails, one cart.
What to do in this situation? Keep required information not in the cart but somewhere else. For example in flow variables. Store original payload in some variable and then restore it when it required again. Like this
<flow name="mainFlow">
<http:listener config-ref="HTTP_Listener_Configuration" path="hello" doc:name="HTTP"/>
<cxf:jaxws-service doc:name="CXF" configuration-ref="CXF_Configuration" serviceClass="kansas.MuleTestServiceImpl"/>
<set-variable variableName="storedPaylod" value="#[payload]" doc:name="Store original payload"/>
<db:insert config-ref="Oracle_Configuration" doc:name="Database">
<db:parameterized-query><![CDATA[insert into tblRequest (id, correlationId) values(#[payload], #[message.correlationId])]]></db:parameterized-query>
</db:insert>
<set-payload value="#[flowVars.storedPaylod]" doc:name="Restore Payload"/>
<until-successful objectStore-ref="MyObjectStore" maxRetries="5" millisBetweenRetries="2000" doc:name="Until Successful" > <!-- failureExpression="???" -->
<db:select config-ref="Oracle_Configuration" doc:name="Database">
<db:parameterized-query><![CDATA[select correlationId,msgResponse from tblResponse where correlationId = #[message.correlationId]]]></db:parameterized-query>
</db:select>
</until-successful>
<logger level="INFO" doc:name="Logger" message="#[payload]"/> <!-- why is payload always = 1? -->
</flow>
Good idea will be to check that first database component really returns 1 - record is inserted. Do this, produce alerts on the error, and then restore original payload and continue your flow.

Answer 2

The best solution to avoid killing the actual value of your payload after the database insert is to make use of the Message Enricher processor.
try this code below:
<flow name="mainFlow">
<http:listener config-ref="HTTP_Listener_Configuration" path="hello" doc:name="HTTP"/>
<cxf:jaxws-service configuration-ref="CXF_Configuration" serviceClass="kansas.MuleTestServiceImpl" doc:name="CXF"/>
<enricher source="#[payload]" target="#[flowVars.insertResponse]" doc:name="Message Enricher">
<db:insert config-ref="Oracle_Configuration" doc:name="Database">
<db:parameterized-query><![CDATA[insert into tblRequest (id, correlationId) values(#[payload], #[message.correlationId])]]></db:parameterized-query>
</db:insert>
</enricher>
<flow-ref name="dbSelectSubFlow" doc:name="dbSelectSubFlow"/>
<logger message="#[payload]" level="INFO" doc:name="Logger"/>
</flow>
<sub-flow name="dbSelectSubFlow">
<until-successful objectStore-ref="MyObjectStore" maxRetries="5" millisBetweenRetries="2000" doc:name="Until Successful">
<db:select config-ref="Oracle_Configuration" doc:name="Database">
<db:parameterized-query><![CDATA[select correlationId,msgResponse from tblResponse where correlationId = #[message.correlationId]]]></db:parameterized-query>
</db:select>
</until-successful>
</sub-flow>