Converting asynchronous backend service to synchronous request

[#1]

Hi All,

We have an asynchronous service which needs to be serviced as synchronous API to a front-end application. The service responds with a 201 immediately with a requestId and eventually with actual response on a callbackUrl with the same requestId in response body. The callbackUrl is provided as part of request and be another Apigee endpoint of course. We have a need to make this service synchronous using Apigee. API will make the initial call and

- return an error code if anything other than 201 is received

- if 201 is received with requestId, request will be blocked until a response is received in callbackUrl for that responseId OR a timeout is hit. Delayed callback response will need to be ignored.

I have seen several references in the forum to make service callout asynchronously and later wait for responses. However, I am having hard time finding an API example to achieve what we need.

Thanks in advance, Ashish

[sidd-harth]

HI @Ashish Pokharel, maybe we can use a Service Callout(SC) Policy. A SC policy with Response tag is an synchronous call. If you omit he Response tag, the SC will fire a call & proceed forward without any response.

In your case use the Response tag & in SC also set the timeout in milliseconds.

<ServiceCalloutasync="false"continueOnError="false"enabled="true"name="SC-1">
	<DisplayName>SC</DisplayName>
	<Response>calloutResponse</Response>
	<Timeout>60000</Timeout>
	<HTTPTargetConnection>
		<URL>http://example.com</URL>
	</HTTPTargetConnection>
</ServiceCallout>

After SC policy, use calloutResponse.status.code flow variable to check the status code & raise an error if required.

[#3]

@Siddharth Barahalikar thanks but that may not be what we need. I am ok to wait for intermediate response synchronously from backend. If that is 201, I will get a requestId. Now, until the backend responds asynchronously (meaning it will be a different POST call from backend to a callbackUrl we can set from Apigee) I have to block original request and wait for a response for that particular requestId so I can attach the final response from backend tonoriginal request. So, some questions are

1. What callbackUrl will I set to get response

2. How will I block original request and wait for callback respose as response is a different POST from backend

3. How do I respond to the right requestId only

Thanks, Ashish

[sidd-harth]

Maybe we need to use Nodejs/Javascript. Have a look at,

https://community.apigee.com/articles/2340/asynchronous-http-requests-in-an-api-proxy.html

https://docs.apigee.com/api-platform/reference/javascript-object-model#makingjavascriptcalloutswithh...

[#5]

IDK, I am not able to see a convincing solution here. To me, these articles read how make a call asynchronously to more or less a synchronous backend so calls could be parallelized and later synchronized. That't not really what we need. We are looking for implementation of this facade described back in 2012

https://www.slideshare.net/apigee/essential-api-facade-patterns-synchronous-to-asynchronous-conversi...

I actually guessed this was a piece of cake but is becoming tricky 🙂

[#6]

Will do. Thanks! I noticed a slideshare presentation by Santanu Dey which explains this facade pattern but has no code reference 😞