Fetch APIM Backends in <set-variable>

Question

Fetch APIM Backends in <set-variable>

Nagashree Balasundaram 741

Is it possible to read the backend url defined under Backends using the id into a variable using the set-variable policy?

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MuthuKumaranMurugaachari-MSFT 22,451 Moderator

@Nagashree Balasundaram Thank you for reaching out to Microsoft Q&A. Based on your statement, you are setting up a backend URL using a policy like "<set-backend-service backend-id="test-Backend" />" and would like to assign it to a variable using a set-variable policy.

set-variable policy can assign a value either a string literal or using a policy expression (https://learn.microsoft.com/en-us/azure/api-management/api-management-advanced-policies#set-variable). The policy expression has access only to context variable (docs: https://learn.microsoft.com/en-us/azure/api-management/api-management-policy-expressions#ContextVariables share the full info about the fields) and hence we cannot directly read backend URL specified using backend id.

I tested with the sample policy below and I was able to retrieve the backend URL in the outbound processing using Context.Request.Url. Note, set-backend-service policy is evaluated during backend processing and hence you cannot perform the same in inbound processing to get new backend URL as it is not evaluated yet.

I hope this helps in answering your question. Feel free to reach out if any questions.

Please 'Accept as answer' and ‘Upvote’ if it helped so that it can help others in the community looking for help on similar topics.

Nagashree Balasundaram 741 Reputation points

2022-07-07T15:42:07.89+00:00

Thank you @MuthuKumaranMurugaachari-MSFT for your response. I am looking at reading the backend host value from the backend id in the inbound flow.
For eg: if you have a backend defined as test-backend with a value https://www.test.net
I want to set the backend url as {{test-backend}}/flow. I want to append the /flow to the backend host without having to hardcode it in the policy. Is there some option to do this?

I can think of name-value option, where I can define the hostname and append it. But its a little difficult for maintenance as you need to remember backend host is defined in named-value and not under the backends configuration.
MuthuKumaranMurugaachari-MSFT 22,451 Reputation points Moderator

2022-07-07T16:12:33.01+00:00

@Nagashree Balasundaram Thank you for clarifying it. You can use rewrite-uri policy like below to append /flow to the backend URL and validate if this resolves your problem.

<set-variable name="varBackendAppend" value="flow" />
<rewrite-uri template="@{return "/" + context.Variables.GetValueOrDefault<string>("varBackendAppend");}" copy-unmatched-params="true" />

Refer to docs: https://learn.microsoft.com/en-us/azure/api-management/api-management-transformation-policies#RewriteURL for more info on this policy.

Thanks,
Muthu
Please 'Accept as answer' and ‘Upvote’ if it helped so that it can help others in the community looking for help on similar topics.
MuthuKumaranMurugaachari-MSFT 22,451 Reputation points Moderator

2022-07-12T00:04:37.807+00:00

@Nagashree Balasundaram Following up on my previous comment to check if you are able to resolve the issue with rewrite-uri policy as mentioned above. If so, please confirm and accept as an answer.

Thanks,
Muthu
Please 'Accept as answer' and ‘Upvote’ if it helped so that it can help others in the community looking for help on similar topics.
Nagashree Balasundaram 741 Reputation points

2022-07-12T00:25:07.647+00:00

@MuthuKumaranMurugaachari-MSFT , I was able to use the rewrite-url policy as mentioned in your response to append the required string. Thank you.

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Nagashree Balasundaram 741 Reputation points

2022-07-07T15:42:07.89+00:00

Thank you @MuthuKumaranMurugaachari-MSFT for your response. I am looking at reading the backend host value from the backend id in the inbound flow.
For eg: if you have a backend defined as test-backend with a value https://www.test.net
I want to set the backend url as {{test-backend}}/flow. I want to append the /flow to the backend host without having to hardcode it in the policy. Is there some option to do this?

I can think of name-value option, where I can define the hostname and append it. But its a little difficult for maintenance as you need to remember backend host is defined in named-value and not under the backends configuration.
MuthuKumaranMurugaachari-MSFT 22,451 Reputation points Moderator

2022-07-07T16:12:33.01+00:00

@Nagashree Balasundaram Thank you for clarifying it. You can use rewrite-uri policy like below to append /flow to the backend URL and validate if this resolves your problem.

<set-variable name="varBackendAppend" value="flow" />
<rewrite-uri template="@{return "/" + context.Variables.GetValueOrDefault<string>("varBackendAppend");}" copy-unmatched-params="true" />

Refer to docs: https://learn.microsoft.com/en-us/azure/api-management/api-management-transformation-policies#RewriteURL for more info on this policy.

Thanks,
Muthu
Please 'Accept as answer' and ‘Upvote’ if it helped so that it can help others in the community looking for help on similar topics.
MuthuKumaranMurugaachari-MSFT 22,451 Reputation points Moderator

2022-07-12T00:04:37.807+00:00

@Nagashree Balasundaram Following up on my previous comment to check if you are able to resolve the issue with rewrite-uri policy as mentioned above. If so, please confirm and accept as an answer.

Thanks,
Muthu
Please 'Accept as answer' and ‘Upvote’ if it helped so that it can help others in the community looking for help on similar topics.
Nagashree Balasundaram 741 Reputation points

2022-07-12T00:25:07.647+00:00

@MuthuKumaranMurugaachari-MSFT , I was able to use the rewrite-url policy as mentioned in your response to append the required string. Thank you.

Answer 1

Sujithreddy Komma 581 Microsoft Employee

Hi NagaShree,

i doubt you have straight forward way to do that

There is rest api to get the based on the ID. try the below

https://learn.microsoft.com/en-us/rest/api/apimanagement/current-ga/backend/get#code-try-0

you might need to use a send request policy to this and also the token and fetch it

Thanks,
Sujith.

Nagashree Balasundaram 741 Reputation points

2022-07-07T15:43:38.067+00:00

Thanks @Sujithreddy Komma . OK that's a little too complicated to be added in an API that processes normal traffic. I might have to look at other options to define the backend host name and fetch it from named values may be. I wanted to avoid that, because that's not the standard place where you have the backends and may get missed when you have make changes to the backend host name.

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Fetch APIM Backends in <set-variable>

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