Why do we need listener for Oracle EBS application server?
The ICM starts up a Service Manager on each node that is enabled for concurrent processing, by instructing the node's Applications Listener (which is dedicated to Concurrent Processing) to spawn a process running the Service Manager executable (FNDSM). 

The Applications Listener must be configured to source the Oracle E-Business Suite environment file before FNDSM is spawned.

Following startup, the Service Manager acts as an agent of the ICM to start and stop concurrent managers on that node, according to their defined work shifts.

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