How do I send a reply in Microgateway mode to a dynamic topic?

Hey @andrew_j_roberts . Great question! Looking through the docs (https://docs.solace.com/RESTMessagingPrtl/Solace-REST-Message-Encoding.htm#_Toc426703633), I do see a way to specify the "reply-to", but unfortunately, it's only for REST Messaging mode, not Gateway mode:

I guess it's because in Gateway mode, the broker is handling everything automatically, sets up the funny-looking #P2P/... reply-to topic automatically. So if your Replier application absolutely needs to send the reply message to a constructed, defined topic hierarchy, I think you either have to:

a) double publish
b) use a Processor-type application (1-in-1-out), which is essentially just offloading that responsibility to another app

May I ask though: what use case or functional capability it that you are trying to achieve? Perhaps there's another way..?

Ok, so you're hoping that you could build a response topic that contains "ok" or "error" or something in the topic, eh? And then filter based on that..?

But (if I read you correctly) what you're really trying to do is filter on the response somehow. Obviously, if you could use Solace hierarchical topics directly, you'd embed something in the topic, but what other options are there? Another option which might work for an existing REST flow is took look at the HTTP response code. The backend REST service will/should pass along a response code, right? 200 OK, or 40x for something bad? HTTP codes are passed through the broker as User Properties on the message. And while we can't do topic filtering on that, you could use a Selector to look for certain error codes? This might actually be one of the good uses of a Topic Endpoint in Solace, because it performs its Selector on ingress, rather than a Queue which stores all messages, and performs Selectors on egress based on a client.

... goes and tries ...

So yes, you can definitely do this! I made a Topic Endpoint "te1", and then used a Solace client (SdkPerf in my case) to connect a client to that TE and specify a subscription (as you had above: #P2P/v:*/#rest-*/POST/> for all POST responses) (maybe use the specific topic rather than > to be more selective) as well as a Selector JMS_Solace_HTTP_status_code>299.

BTW, I tested this using SdkPerf C and a Topic Endpoint called te-rest-errors:

./sdkperf_c -cip=localhost -sdl=te-rest-errors -stl="#P2P/v:*/#rest-*/>" -ssl="JMS_Solace_HTTP_status_code>299" -md

And then (after having configured the VPN for MicroGateway and RDP to a backend service -- SEMP in my case) I got this on the command line when issuing a bad request:

^^^^^^^^^^^^^^^^^^ Start Message ^^^^^^^^^^^^^^^^^^^^^^^^^^^
Destination:                            Topic '#P2P/v:cd6c1f921db8/#rest-06c7bdc3dedfb2c5/GET/SEMP/v2/config/msgVpns'
ApplicationMessageId:                   ID:Solace-c6e9493cdae8b1ed
CorrelationId:                          ID:Solace-c6e9493cdae8b1ed
HTTP Content Type:                      application/json
Class Of Service:                       COS_1
DeliveryMode:                           NON-PERSISTENT
Message Id:                             3
Reply Message
User Property Map:
  Key 'JMS_Solace_HTTP_field_Date' (STRING) Tue, 27 Apr 2021 06:41:26 GMT
  Key 'JMS_Solace_HTTP_field_Server' (STRING) Solace_VMR/9.2.0.14
  Key 'JMS_Solace_HTTP_field_Cache-Control' (STRING) no-cache
  Key 'JMS_Solace_HTTP_field_WWW-Authenticate' (STRING) Basic realm="Unauthorized"
  Key 'JMS_Solace_HTTP_field_Access-Control-Allow-Credentials' (STRING) true
  Key 'JMS_Solace_HTTP_field_Access-Control-Allow-Headers' (STRING) Authorization, Content-Type, X-Requested-With
  Key 'JMS_Solace_HTTP_field_Access-Control-Allow-Methods' (STRING) GET, POST, PUT, PATCH, DELETE, OPTIONS
  Key 'JMS_Solace_HTTP_field_Strict-Transport-Security' (STRING) max-age=31536000
  Key 'JMS_Solace_HTTP_status_code' (UINT16) 401
  Key 'JMS_Solace_HTTP_reason_phrase' (STRING) Unauthorized
Binary Attachment String:               len=318
<SNIP>

So if you really wanted to, you could Selector on ANY of those User Properties that get populated by the MicroGateway!

Nice. I like this.

Hey @Frankee787 ..! I somehow missed this post you made back in December. Apologies for that. Unfortunately, (currently) there's no easy way to get the response code from an RDP that is configured in Messaging mode. The REST response goes back to the REST Consumer ends there.

HOWEVER! A couple months ago I was helping a colleague with a similar ask, and thought it would be possible to "glue together" two Message VPNs (easy if running the software broker yourself), where you have one VPN configured as Messaging and one configured as Gateway. And then the Messaging VPN RDP simply POSTs to the incoming REST port of the Gateway VPN so the REST request has to "go through there", which means that the REST response comes back as a message to pass back to the RDP in the Messaging VPN.

Here's a diagram I made for my colleague. "SF" in this case was SalesForce, trying to push into that:

I started writing some code for a "REST Correlator" app that connected to the Gateway VPN and listened to all the requests and responses as messages, and would (should?) publish a message when something goes wrong. https://github.com/aaron-613/solace-rest-response-correlator

Not sure if this would help, but thought I'd reply here. I'm sure the performance isn't spectacular, because you have two RDPs now, but it does work.