How does modern day ESB compare to SCA ( in terms of routing / aggregating services ) ?
This is what i think [ from past experience ].
Lets say we have two services S1 and S2, and a composite service S3 which is composed of S1 and S2. Assume S1 and S2 have two different endpoints and protocols.
How to access using ESB? ( WSO2 / Apache Synapse )
- We can access S1 first and S2 second ( creating a pipe )
- We can access S3 directly to provide output. (protocol conversion / transformation / orchestration ) being done here.
How to access using SCA? ( Apache Tuscany )
- Lookup S1 first and S2 next ( using SCA API )
- Create a composite S3 out of S1 and S2 and invoke S3 ( protocol conversion / transformation / orchestration ) being done here.
Are people out there using both SCA composition and ESB Orchestration together?
SCA is not an alternative to ESB It's just a way to build Service Oriented Architecture.
As david.a says you could use an oriented SCA ESB like Websphere ESB, or Oracle SOA-suite which is an SCA oriented integration solution.
You could learn more about SCA here : http://www.davidchappell.com/writing/Introducing_SCA.pdf
In Oracle and IBM usage, SCA composition is a way to combined different kind of implementation, It could be use to build a solution with a mediation part (ESB) in front of another element (BPEL, java bean etc...).
From a Service Oriented Architecture view, SCA help you to combined your ESB with your upper layer (BPEL, BR etc...)