1.can i use two different databases(DB2, ORacle) in single compute mode?

ans : No u cannot use 2 different databases in single compute node.
         but u can use two different data sources of same type.

2.default http port in iib ?

ans : 4415

3. i have message flow 

MQinput -> compute->compute -> MQoutput

scenario: in first compute node i have done some DB update and in second
 compute node some simple traformation and send a message to ouput queue
 the transaction is incomplete unfortunatly, so what vl happen with DB updation which
 is done in first compute node.

ans: if u set Transaction property as Commit then it vl update in the DB if it is automatic then it vl lost.

4.Scenario,

MQinput -> compute -> mQoutput

in message there are some elements like <root> <Queue1>a</Queue1>.. etc

i want to send a message to Queue a and so on.. how do u do that.

Ans: OutputLocalEnvironment.Destination.MQ.DestinationData[1].queueName = InputRoot.root.Queue1;

5. Difference between Environment and local environment?

ans : Environment Single instance of it is maintained throughout the message flow.

      Environment is the logical message tree which is used to the values and its scope throughout the message       flow.

      LocalEnvironment is also the logical message tree which is used to store the values throuout the message       flow
      and its reference can be used to create destinations which describes on where the message should be             landed.

      LocalEnvironment  tree can be used to populate the destination list where as Environment cant used for         the same.