In one of our projects we had a scenario of integrating 2 systems SFDC and Oracle E-Biz. The requirement was to synchronize both systems. If a record gets inserted in SFDC same needs to be inserted in Oracle E-Biz using fusion middleware and vice versa. In this particular integration scenario which is usually the case with most systems the primary id of the record in both systems are not the same. So the problem was how to identify the records in the case an update happens. One of the options is to have one more column in both systems to store the other systems primary key which requires both systems to be affected. The other alternative which seems to be a better option is to handle it in the middleware layer. For this ESB has this cross referencing feature.
ESB dynamic cross-referencing feature assigns a common or global key to data objects and maintains mappings between application-specific keys and the common key in a dynamic cross-referencing table. There is import/export utility provided with Cross-Reference which helped us with initial data loads. The utility is provided by xrefimport and xrefexport (bat/sh)files respectively. Xreftool(bat/sh) is the file which helps you in creating tables ,adding columns, deleting and listing values.
To know more on the implementation refer the Oracle® Enterprise Service Bus Developer's Guide
Or access the link @ ESB Cross-Reference Feature