I have been asked many times about how robust is SAP XI. That translates into two questions: 1) how many messages can XI handle? and 2) how big a message XI can handle?
The classical answer to this question is it depends. Here is a list of points you need to take into consideration to answer this question.
By design, SAP XI processes its messages based on open standards. All messages are in XML format. If the source and target systems are different, most probably the message requires mapping so that the target system can understand the message coming from the source system. Depending on the systems involved, this mapping could range from very simple (without mapping) to very complicated (calling external systems for mapping information). Therefore a same purchase order can take drastically different time to process if their target systems are different. So when we talk about message mapping requirement, we have to do apple to apple, but not apple to orange comparison.
Another area we need to look into is whether there is an adapter involved. An adapter can be viewed as a translator to translate a message into or from XIs XML format. Many of these adapters are Java Connectivity Architecture (JCA) based and is running on SAPs J2EE Engine as part of Web AS. The adapter itself requires CPU power and time to process messages.
Third area we need to look at is the message size. XI can handle a message with multi-line data or a single line data. The larger of a message, the more CPU power and time it needs to process it.
With above points in mind, we recommend using SAPs XI QuickSizer to do hardware estimation for each individual customers needs. There is no generic answer to customer X processes 400 messages per hour on a Windows based server, can customer Y using Unix based server process 100 messages per hour.
If you are still curious to know other customers example, here is one to share: more than one million messages per day on a SAP XI 2.0 system at one of our customers.
If you are from SAP R/3 world, you know we can have multiple application servers of an R/3 system when number of MM or SD users increases over the time. SAP XI is based on the same Web AS (formerly known as R/3 Basis) technology. With the increase of message numbers, we can add more application servers. The robustness of this approach has been proved over the past decades.
Then how big can an individual message be? There is a straight answer in Sizing SAP Exchange Infrastructure 3.0 documentation: The size of the largest message thus depends mainly on the size of the available main memory. On a normal 32Bit operating system, there is an upper boundary of approximately 1.5 to 2 GByte per process, limiting the respective largest message size.