Service callout with dynamic host name - apigee

Question

I have a proxy in Apigee that uses a service callout to another proxy in the same environment. I would like to set the URL host for the callout to match the host of the initial request.
For example, if a request is made in the dev environment to:
https://example-dev.apigee.com/awesome-proxy
I need to make a call to:
https://example-dev.apigee.com/support-proxy
In a test environment the first call is to:
https://example-test.apigee.com/awesome-proxy
The support call needs to go to:
https://example-test.apigee.com/support-proxy
Here is how I would like to define the service callout policy:
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<ServiceCallout async="false" continueOnError="false" enabled="true" name="serviceCallout">
<DisplayName>serviceCallout</DisplayName>
<FaultRules/>
<Properties/>
<Request clearPayload="true" variable="example.request">
<IgnoreUnresolvedVariables>false</IgnoreUnresolvedVariables>
</Request>
<Response>example.response</Response>
<HTTPTargetConnection>
<Properties/>
<URL>{client.host}/support-proxy</URL>
</HTTPTargetConnection>
</ServiceCallout>
This will not save and complains about no protocol. The help indicates that this must be hard coded:
<HTTPTargetConnection>/<URL> element
The URL to the service being called. While the hostname portion of URL must be hard-coded, you can supply the remainder of the URL dynamically with a variable.
I found a variable to define the URL of a service callout:
servicecallout.{policy-name}.target.url
I attempted to use an assign message policy to dynamically set the variable, as follows:
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<AssignMessage async="false" continueOnError="false" enabled="true" name="assignCalloutURL">
<DisplayName>assignCalloutURL</DisplayName>
<FaultRules/>
<Properties/>
<AssignVariable>
<Name>servicecallout.serviceCallout.target.url</Name>
<Value>{client.host}</Value>
<Ref/>
</AssignVariable>
<IgnoreUnresolvedVariables>true</IgnoreUnresolvedVariables>
<AssignTo createNew="false" transport="http" type="request"/>
</AssignMessage>
This sets the URL to the literal text {client.host}
I have used the assign message policy in a similar fashion for other purposes and it actually resolves the variable listed. I'm not sure what's happening here.

Answer 1

client.host is not the correct variable to use, it returns an IP address 192.168....
I tried a few other variables:
proxy.url returns a strange host, it looks like internal Apigee machine names with a port. The proxy.url host times out when hit directly.
I ended up using virtualhost.aliases and proxy.pathsuffix. Here is the full JavaScript that solved it:
var base = context.getVariable("proxy.basepath");
var aliases = context.getVariable("virtualhost.aliases");
var url = "https://" + aliases[0] + base + "/support-proxy";
context.setVariable("servicecallout.serviceCallout.target.url", url);

Answer 2

Modifying target URLs gets a little goofy when dealing with the linter that ensures URLs have valid values.
This will not save and complains about no protocol.
This is because you're missing the scheme (https:// or http://) before the URL (client.host doesn't include scheme).
This sets the URL to the literal text {client.host}
That bit's not working properly because you need to use the ref tag to retrieve the existing variable:
<AssignVariable>
<Name>servicecallout.serviceCallout.target.url</Name>
<Ref>client.host</Ref>
</AssignVariable>
Now, that might work for the service callout, but it's not probably not going to work for setting the target URL.
I've ended up creating JavaScript policies to handle target.url, since the AssignMessage has been problematic for me:
var scheme = context.getVariable("client.scheme");
var host = context.getVariable("client.host");
var pathsuffix = context.getVariable("proxy.pathsuffix");
var newUrl = scheme + host + pathsuffix;
context.setVariable("target.url", newUrl);

Answer 3

Take look at this - it worked for me.