21 Nov WS-ReliableMessaging (WS-RM) between IBM and SAP back-end
More than 10 years ago, SAP started service enabling its applications. SAP implements a large number of web services as documented on SAP Enterprise Services Workplace.
SAP distinguishes 3 types of services:
- B2B: for exchanging data with business partners
- A2A: one-way, reliable services for application-to-application
- A2X: two-way request/respone for interaction with front-end application and use in composite applications
The A2X and A2A services are directly supported by SAP back-end systems. Their implementation is done in SAP’s proprietary programming language ABAP, leveraging the SAP Proxy Framework.
The A2X request/response web services are standard SOAP web services. The asynchronous services communicate typically via SAP’s integration solution, Process Integration or SAP PI. This reliable communication between the A2A web services and SAP PI is based on a SAP specific protocol (XMB, based on SOAP with Attachments and a number of custom SAP header fields).
But more interesting is the support of SAP’s A2A web services for WS-ReliableMessaging. WS-RM is one of those many WS-* standards. WS-RM is a protocol for reliably delivering messages over SOAP. SAP supports versions 1.0 and 1.1 of WS-RM.
We recently connected WebSphere ESB 7.5 with SAP using WS-RM. The communication from SAP over WS-RM to the IBM ESB was rather trivial. WS-RM was well supported. One attention point was the configuration of a WS-Addressing action for each operation.
Going van WebSphere ESB to SAP was a bit trickier. Messages from WESB over WS-RM 1.1 to SAP always failed with the error message “Only the anonymous value for wsrm:AcksTo is supported”. The WS-RM 1.1 uses the URI http://docs.oasis-open.org/ws-rx/wsrm/200608/anonymous.
When switching back from WS-RM 1.1 to 1.0, we finally got the communication from WESB to SAP working. The anonymous URI is http://w3.org/2005/08/addressing/anonymous is then used.
It seems that SAP has an error (bug) in its implementation of WS-RM 1.1, only accepting the URI http://w3.org/2005/08/addressing/anonymous to specify “anonymous”.
SAP is also coming up with an alternative for WS-RM, called “SAP-RM” or “plain SOAP“. It simply boils down to a HTTP POST with filtering of duplicate messages. This mechanism has been available in SAP’s integration solution PI but is now also being supported in modern SAP back-end applications. We only tested this with soapUI, not yet from IBM WESB.