apigee overwriting the target.url with a custom variable - variable-assignment

Question

I have been trying to overwrite the target.url with a variable using the Assign Message Policy. Per other solutions, I have put this in the "Target EndPoint" Section. The issue is, unless I hard-code the root section of the URL, the substitution fails. I have tried all the commented VALUE stmts below with and then started adding the "REF"stmts to attempt to solve the issue - to no avail. You can see I have tried cutting the target into various snippets using Extract policies, but cannot get a solution that works.
Thanks for help.
For the purposes of the code snippet below
entireURL = "http://my.root.url/thestuff/morestuff"
AppServerURL = "my.root.url/thestuff/morestuff"
AppServerRoot = "my.root.url"
AppServerSfx = "thestuff/morestuff"
codee from Assign Message Policy
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<AssignMessage async="false" continueOnError="false" enabled="true" name="Post-to-named-serverL">
<DisplayName>Post to named server</DisplayName>
<FaultRules/>
<Properties/>
<AssignVariable>
<Name>target.url</Name>
<Value>http://${AppServerRoot}/{AppServerSfx}</Value>
<Ref/>
<!--
<Value>http://my.root.url/{AppServerSfx}</Value> works but I need the root changed
<Value>http://{AppServerRoot}/{AppServerSfx}</Value>
<Value>http://${AppServerRoot}/{AppServerSfx}</Value>
<Value>http://{AppServerURL}</Value>
<Value>http://${AppServerURL}</Value>
<Value>entireURL</Value>
<Value>{entireURL}</Value> -- this was my first try
<Value>${entireURL}</Value>
<Ref>entireURL</Ref>
<Ref>{entireURL}</Ref>
<Ref>${entireURL}</Ref>
-->
</AssignVariable>
<IgnoreUnresolvedVariables>false</IgnoreUnresolvedVariables>
<AssignTo createNew="false" transport="http" type="request"/>
</AssignMessage>

Answer 1

You are correctly putting the target.url manipulation in the Target Request flow.
Using AssignMessage/AssignVariable can be limiting. The Value element doesn't allow you to do any variable substitutions.
The following worked for me:
<Ref>entireURL</Ref>
Ref also doesn't allow variable substitutions -- it just takes the name of the variable. Since you have to build the value of that variable ahead of time, using the Ref example above doesn't buy you much.
I usually accomplish target URL rewriting using a JavaScript callout with code similar to the following:
var appServerRoot = context.getVariable("AppServerRoot");
var appServerSfx = context.getVariable("AppServerSfx");
context.setVariable("target.url", "http://" + appServerRoot + "/" + appServerSuffix);