Recently I have been reading about some of the advanced policies available in APIM and the one that caught my eye was the “Wait” policy found here https://docs.microsoft.com/en-us/azure/api-management/api-management-advanced-policies. This policy will execute child policies in parallel and will wait until either one or all of the child polices have completed before continuing.
I was pondering on a good use case for this policy and the first one came to mind was a scatter-gather pattern shown below. I could easily create this pattern without having to provision any other required Azure services.
The Scatter-Gather pattern is created by exposing a REST endpoint through APIM which will accept a JSON payload. When a request comes into APIM, a custom policy will forward this request onto the service endpoints defined in the policy and wait until they have all responded before continuing on with the rest of the statements in the parent policy. Once all the destination endpoints have responded, the return status code on each service is then checked for success or an error. If an error does occur, then the error is returned immediately otherwise the message from each service is combined into a composite JSON message which is returned back to the caller as the response message.
For the scenario, I will be sending a price request for a product to 3 suppliers simultaneously. In Azure APIM, I created a method which will accept a JSON price request message as shown below.
Then I added an Inbound processing policy to that method which is broken down into two parts. Below is the first part of the policy where it sends the inbound request to multiple endpoints in parallel using the <send-request> tag. Named values are used for the service endpoint address defined in the <set-url>{{serviceendpoint_xxx}}</set-url> tags. The response from each service call is inserted into the variable response-variable-name=”response_xxx” which is used in the second part of this policy.
<set-variable name="requestPayload" value="@(context.Request.Body.As<string>(true))" />
<wait for="all">
<send-request mode="copy" response-variable-name="response_1" timeout="120" ignore-error="false">
<set-url>{{serviceendpoint_1}}</set-url>
<set-method>POST</set-method>
<set-body>@(context.Variables.GetValueOrDefault<string>("requestPayload"))</set-body>
</send-request>
<send-request mode="copy" response-variable-name="response_2" timeout="120" ignore-error="false">
<set-url>{{serviceendpoint_2}}</set-url>
<set-method>POST</set-method>
<set-body>@(context.Variables.GetValueOrDefault<string>("requestPayload"))</set-body>
</send-request>
<send-request mode="copy" response-variable-name="response_3" timeout="120" ignore-error="false">
<set-url>{{serviceendpoint_3}}</set-url>
<set-method>POST</set-method>
<set-body>@(context.Variables.GetValueOrDefault<string>("requestPayload"))</set-body>
</send-request>
</wait>
<choose>
<when condition="@((int)((IResponse)context.Variables["response_1"]).StatusCode != 200)">
<return-response response-variable-name="response_1" />
</when>
<when condition="@((int)((IResponse)context.Variables["response_2"]).StatusCode != 200)">
<return-response response-variable-name="response_2" />
</when>
<when condition="@((int)((IResponse)context.Variables["response_3"]).StatusCode != 200)">
<return-response response-variable-name="response_3" />
</when>
<otherwise>
<return-response>
<set-status code="200" reason="OK" />
<set-header name="Content-Type" exists-action="override">
<value>application/json</value>
</set-header>
<set-body>@(new JObject(
new JProperty("service_1",((IResponse)context.Variables["response_1"]).Body.As<JObject>()),
new JProperty("service_2",((IResponse)context.Variables["response_2"]).Body.As<JObject>()),
new JProperty("service_3",((IResponse)context.Variables["response_3"]).Body.As<JObject>())
).ToString())</set-body>
</return-response>
</otherwise>
</choose>
<policies>
<inbound>
<set-variable name="requestPayload" value="@(context.Request.Body.As<string>(true))" />
<wait for="all">
<send-request mode="copy" response-variable-name="response_1" timeout="120" ignore-error="false">
<set-url>{{serviceendpoint_1}}</set-url>
<set-method>POST</set-method>
<set-body>@(context.Variables.GetValueOrDefault<string>("requestPayload"))</set-body>
</send-request>
<send-request mode="copy" response-variable-name="response_2" timeout="120" ignore-error="false">
<set-url>{{serviceendpoint_2}}</set-url>
<set-method>POST</set-method>
<set-body>@(context.Variables.GetValueOrDefault<string>("requestPayload"))</set-body>
</send-request>
<send-request mode="copy" response-variable-name="response_3" timeout="120" ignore-error="false">
<set-url>{{serviceendpoint_3}}</set-url>
<set-method>POST</set-method>
<set-body>@(context.Variables.GetValueOrDefault<string>("requestPayload"))</set-body>
</send-request>
</wait>
<choose>
<when condition="@((int)((IResponse)context.Variables["response_1"]).StatusCode != 200)">
<return-response response-variable-name="response_1" />
</when>
<when condition="@((int)((IResponse)context.Variables["response_2"]).StatusCode != 200)">
<return-response response-variable-name="response_2" />
</when>
<when condition="@((int)((IResponse)context.Variables["response_3"]).StatusCode != 200)">
<return-response response-variable-name="response_3" />
</when>
<otherwise>
<return-response>
<set-status code="200" reason="OK" />
<set-header name="Content-Type" exists-action="override">
<value>application/json</value>
</set-header>
<set-body>@(new JObject(
new JProperty("service_1",((IResponse)context.Variables["response_1"]).Body.As<JObject>()),
new JProperty("service_2",((IResponse)context.Variables["response_2"]).Body.As<JObject>()),
new JProperty("service_3",((IResponse)context.Variables["response_3"]).Body.As<JObject>())
).ToString())</set-body>
</return-response>
</otherwise>
</choose>
</inbound>
<backend>
<base />
</backend>
<outbound>
<base />
</outbound>
<on-error>
<base />
</on-error>
</policies>
Thanks for the article. I was looking to implement such a policy but was not getting much details on how to do this. This article helped me do a similar implementation.