How to use backend and string to give me base-url in backend-service policy?

Question

How to use backend and string to give me base-url in backend-service policy?

Martin Kallukalam 540

I have an mtls backend defined: mtlsbackend https://middleware.contoso.com

I want to use in inbound policy an expression which will evaluate to

https://middleware.contoso.com/service1 using a policy such as

<set-backend-service base-url="base URL of the backend service" />

How do I use a variable expression using backend and string (service1) to set base-url?

PS: I do not want to define my backend as https://middleware.contoso.com/service1 because I will have 100s of service which will use the backend. Hence not scalable to define a backend for every fqdn/service.
Instead I want to define backend as fqdn and then use expression to give me base-url

Sina Salam 26,661 Reputation points Volunteer Moderator

2024-06-15T20:43:25.71+00:00
Hello Martin Kallukalam,

Welcome to the Microsoft Q&A and thank you for posting your questions here.

Problem

I understand that you would like to dynamically construct the base-url in an inbound policy in Azure API Management by using a variable expression that combines a predefined backend URL with a specific service path to avoid the need to define separate backend services for each unique service endpoint.

Solution

According to the information provided you can dynamically set the base-url in an Azure API Management (APIM) policy. This following are the best practices to implement it:

Firstly

Define a backend service in APIM with the base URL https://middleware.contoso.com. This backend is named mtlsbackend. So, in your APIM instance, define a backend service with the following details:

Name: mtlsbackend

Base URL: https://middleware.contoso.com

Secondly

Use policy expressions within the inbound policy to dynamically build the full URL by concatenating the base URL with a specific service path. This means you will use variables to construct the Full URL like the example in this xml snippet below:

<inbound> <base />  <set-variable name="baseUrl" value="backend('mtlsbackend').url" />  <set-variable name="servicePath" value="/service1" />  <set-backend-service base-url="{{var.baseUrl}}{{var.servicePath}}" /> </inbound>

What we have done in the inbound policy above, we set variables for the base URL, and the service path, then concatenate these variables to form the full URL.

References

For more reading and step by steps information about the above configurations, kindly us the source links listed below:

Source: Policy Expressions in Azure API Management. Accessed, 6/15/2024.

Source: Set Variable in Azure API Management. Accessed, 6/15/2024.

Source: Policies in Azure API Management. Accessed, 6/15/2024.

Source: Send request to a service. Accessed, 6/15/2024.

Accept Answer

I hope this is helpful! Do not hesitate to let me know if you have any other questions.

** Please don't forget to close up the thread here by upvoting and accept it as an answer if it is helpful ** so that others in the community facing similar issues can easily find the solution.

Best Regards,

Sina Salam
Martin Kallukalam 540 Reputation points

2024-06-16T03:24:46.07+00:00

Thank you for understanding the problem description . However the solution you described doesnt work. You are suggesting using {{ }} syntax which doesnt work in APIM.
You are also suggesting to use the context backend , but that doesn't work either. Because there is no builtin backend context.
So overall the solution you suggested doesnt work plus it is syntactically wrong.
Martin Kallukalam 540 Reputation points

2024-06-16T03:45:10.92+00:00

@Sina Salam
The {{ }} form of variable expansion doesn't seem to work.
Also the builtin context backend is not a valid context when I try to save it
See the screen shots.
It doesn't like the {{var}} syntax but it allows me to save even though there is an error.
But if I add the <set-backend-service base-url="{{var.baseUrl}}{{var.servicePath}}" />

the editor wouldnt even allow me to save it and shows the error in red exclamation.
Sina Salam 26,661 Reputation points Volunteer Moderator

2024-06-16T15:08:18.8833333+00:00
Hi @Martin Kallukalam

Thank you for your feedback and sorry for the inconvenience if that confused you.

To resolve the issues, you're encountering, here is explanation:

The {{ }} form of variable expansion doesn't seem to work.

In Azure API Management policies, variable expansion should be done using the @(variable) syntax rather than {{variable}}.

Also the builtin context backend is not a valid context when I try to save it See the screen shots.

The correct way to reference the backend URL in policy expressions is by using the context variables and policy expressions properly. The backend('mtlsbackend').url is not valid. Instead, you should retrieve and use context and policy variables directly.

It doesn't like the {{var}} syntax but it allows me to save even though there is an error. But if I add the <set-backend-service base-url="{{var.baseUrl}}{{var.servicePath}}" />

As mentioned above, use the @(variable) syntax to correctly reference variables.

the editor wouldnt even allow me to save it and shows the error in red exclamation.

This is because the policy syntax is incorrect. Correcting the syntax as per the above points should resolve this.

This is the corrected XML snippet for the inbound policy:

<inbound>  <set-variable name="baseUrl" value="https://middleware.contoso.com" />  <set-variable name="servicePath" value="/service1" />  <set-backend-service base-url="@{context.Variables['baseUrl'] + context.Variables['servicePath']}" /> </inbound>

At this point you should be able to dynamically set the base URL in your APIM policy without encountering syntax errors or validation issues.

Success,

Sina
Martin Kallukalam 540 Reputation points

2024-06-16T20:22:24.8233333+00:00

You suggested a solution first, which is syntactically wrong and doesn't work because there is no builtin context called backend.
Then you suggest a solution which obviously doesn't address my original question.
My original ask was how to use the backend to form the base-url.
Sina Salam 26,661 Reputation points Volunteer Moderator

2024-06-17T12:42:10.9733333+00:00
Hi,

These are the three steps solution I have provided here:

Define service path dynamically as needed.

Retrieve the backend URL using the backend('backendName').url.

Concatenate and set the full URL using the correct syntax.

And I repost to give more understanding to use the @() syntax to concatenate variables and expressions properly.

backend('mtlsbackend').url is the correct method to get the URL of the backend configured in APIM.

@{} is the correct syntax for expression evaluation in APIM.

If there is other issue, you should let's know. There are other experts that can fix your issue better.

Rg,
Martin Kallukalam 540 Reputation points

2024-06-17T14:30:29.0933333+00:00

Pls see reply from a peer of your own (below).
The step2 you describe , there is no such builtin context called "backend" available in APIM to get the backend URL.
Sina Salam 26,661 Reputation points Volunteer Moderator

2024-06-17T14:45:40.5133333+00:00

Thank you, I'm not arguing with you. The answer given you was tested before posted it.

And for your comment:

The step2 you describe , there is no such builtin context called "backend" available in APIM to get the backend URL.

I will advice you to learn more and use best practices, not just what work for you.

In Azure API Management, the backend context does exist and it allows you to retrieve information about the backend service configured in the APIM service instance. The backend('identifier').url expression is used to get the URL of a backend service with the specified identifier.

Success.

1 answer

Your answer

Martin Kallukalam 540 Reputation points

2024-06-16T03:24:46.07+00:00

Thank you for understanding the problem description . However the solution you described doesnt work. You are suggesting using {{ }} syntax which doesnt work in APIM.
You are also suggesting to use the context backend , but that doesn't work either. Because there is no builtin backend context.
So overall the solution you suggested doesnt work plus it is syntactically wrong.
Martin Kallukalam 540 Reputation points

2024-06-16T03:45:10.92+00:00

@Sina Salam
The {{ }} form of variable expansion doesn't seem to work.
Also the builtin context backend is not a valid context when I try to save it
See the screen shots.
It doesn't like the {{var}} syntax but it allows me to save even though there is an error.
But if I add the <set-backend-service base-url="{{var.baseUrl}}{{var.servicePath}}" />

the editor wouldnt even allow me to save it and shows the error in red exclamation.
Sina Salam 26,661 Reputation points Volunteer Moderator

2024-06-16T15:08:18.8833333+00:00

Hi @Martin Kallukalam

Thank you for your feedback and sorry for the inconvenience if that confused you.

To resolve the issues, you're encountering, here is explanation:

The {{ }} form of variable expansion doesn't seem to work.

In Azure API Management policies, variable expansion should be done using the @(variable) syntax rather than {{variable}}.

Also the builtin context backend is not a valid context when I try to save it See the screen shots.

The correct way to reference the backend URL in policy expressions is by using the context variables and policy expressions properly. The backend('mtlsbackend').url is not valid. Instead, you should retrieve and use context and policy variables directly.

It doesn't like the {{var}} syntax but it allows me to save even though there is an error. But if I add the <set-backend-service base-url="{{var.baseUrl}}{{var.servicePath}}" />

As mentioned above, use the @(variable) syntax to correctly reference variables.

the editor wouldnt even allow me to save it and shows the error in red exclamation.

This is because the policy syntax is incorrect. Correcting the syntax as per the above points should resolve this.

This is the corrected XML snippet for the inbound policy:

<inbound>  <set-variable name="baseUrl" value="https://middleware.contoso.com" />  <set-variable name="servicePath" value="/service1" />  <set-backend-service base-url="@{context.Variables['baseUrl'] + context.Variables['servicePath']}" /> </inbound>

At this point you should be able to dynamically set the base URL in your APIM policy without encountering syntax errors or validation issues.

Success,

Sina
Martin Kallukalam 540 Reputation points

2024-06-16T20:22:24.8233333+00:00

You suggested a solution first, which is syntactically wrong and doesn't work because there is no builtin context called backend.
Then you suggest a solution which obviously doesn't address my original question.
My original ask was how to use the backend to form the base-url.
Sina Salam 26,661 Reputation points Volunteer Moderator

2024-06-17T12:42:10.9733333+00:00

Hi,

These are the three steps solution I have provided here:

Define service path dynamically as needed.

Retrieve the backend URL using the backend('backendName').url.

Concatenate and set the full URL using the correct syntax.

And I repost to give more understanding to use the @() syntax to concatenate variables and expressions properly.

backend('mtlsbackend').url is the correct method to get the URL of the backend configured in APIM.

@{} is the correct syntax for expression evaluation in APIM.

If there is other issue, you should let's know. There are other experts that can fix your issue better.

Rg,
Martin Kallukalam 540 Reputation points

2024-06-17T14:30:29.0933333+00:00

Pls see reply from a peer of your own (below).
The step2 you describe , there is no such builtin context called "backend" available in APIM to get the backend URL.
Sina Salam 26,661 Reputation points Volunteer Moderator

2024-06-17T14:45:40.5133333+00:00

Thank you, I'm not arguing with you. The answer given you was tested before posted it.

And for your comment:

The step2 you describe , there is no such builtin context called "backend" available in APIM to get the backend URL.

I will advice you to learn more and use best practices, not just what work for you.

In Azure API Management, the backend context does exist and it allows you to retrieve information about the backend service configured in the APIM service instance. The backend('identifier').url expression is used to get the URL of a backend service with the specified identifier.

Success.

Answer 1

JananiRamesh-MSFT 29,436 Moderator

@Martin Kallukalam Thanks for reaching out. From the description i understand that you would need to set the base URL of the backend in the set-backend-service policy from the backend section and append it with a string dynamically using an expression in an inbound policy correct me if i am wrong here User's image If it is the above case, then unfortunately we don't have a way to retrieve the backend information currently please refer the supported policy expressions: https://learn.microsoft.com/en-us/azure/api-management/api-management-policy-expressions#ContextVariables.

Feel free to leave your feedback at aka.ms/apimwish.

do let me know incase of further queries, I would be happy to assist you.

Martin Kallukalam 540 Reputation points

2024-06-17T18:18:28.24+00:00

@JananiRamesh-MSFT

Your comment says there is no way to retrieve backend information , which I agree with . And it is reflected in the documentation as well.
However your peer Sina Salam says otherwise. that backend('identifier').url is a valid expression.
It would be great if yall agree upon a common answer.
Jason Williams 90 Reputation points

2024-07-02T22:23:09.27+00:00

I have a similar use case - I have a backend which has https://host/path and I've realised that I have a single use case where I need another path and so ideally would like to have a single API policy that retrieves the existing backend and then replaces the path component. I was attempting to use the set-backend-service policy and then looked to use a policy expression to retrieve that URL and modify it, then using the set-backend-service policy again to set the new URL. However I wasn't able to find a context attribute in the inbound policy section that contained the backend url. Unfortunately context.Request.Url.Host was null. From what I can see in the trace it looks like the set-backend-service isn't happening until the backend section (which isn't useful).

So just to confirm - there is no way to retrieve a backend in a policy expression? (except for a wildly complex side-car request to the APIM management APIs)
Martin Kal 20 Reputation points

2024-07-02T23:03:25.8466667+00:00
I am using the below workaround until the time APIM team comes up with a context attribute to retrieve the backend url from the "backend"

<inbound> <base /> <set-backend-service backend-id="putbackendidhere" /> <rewrite-uri template="/hello" copy-unmatched-params="true" /> </inbound>

This will take the backend url and append /hello
So, if my backend is defined as https://www.foo.com/foobar
the request forwarded to backend will be https://www.foo.com/foobar/hello

Share via

How to use backend and string to give me base-url in backend-service policy?

Problem

Solution

Firstly

Secondly

References

Accept Answer

1 answer

Your answer