Hi,
I suspect you could use the Microgateway mode of the REST service to solve this.
Could you explain your flow in a bit more detail …
How is the mainframe publishing events to Solace? Also via HTTP? If so I guess the publisher includes a correlation id as a HTTP header? The HTTP endpoint, that messages are pushed to - it will also attach the correlation id in a header in the HTTP response?
Microgateway mode will preserve all HTTP headers in the request and response flow, it maps HTTP headers into the message’s user properties
It may also work if the mainframe is publishing using JMS, SMF or similar but will require some more thought to make sure messages are published in a way that the Solace REST publisher (RDP) can generate the appropriate HTTTP request.
Have a look at the documentation here bearing in mind that the transport layer under the covers is Solace messaging and therefore publisher and subscribers could be using other (non HTTP) protocols:
You may also find this thread and blog post useful: