Azure ServiceDefinition file - ProgramEntryPoint, runtime IP address and Port - azure

Question

I'm running a command line program (happens to be Redis) inside a Windows Azure Worker Role using ProgramEntryPoint as follows
<WorkerRole name="Worker" vmsize="Small">
<Runtime executionContext="limited">
<Environment>
<Variable name="ADDRESS">
<RoleInstanceValue xpath="/RoleEnvironment/CurrentInstance/Endpoints/Endpoint[#name='Redis']/#address" />
</Variable>
<Variable name="PORT">
<RoleInstanceValue xpath="/RoleEnvironment/CurrentInstance/Endpoints/Endpoint[#name='Redis']/#port" />
</Variable>
</Environment>
<EntryPoint>
<ProgramEntryPoint commandLine="redis-server.exe" setReadyOnProcessStart="true" />
</EntryPoint>
</Runtime>
<Endpoints>
<InternalEndpoint name="Redis" protocol="tcp" port="6379" />
</Endpoints>
</WorkerRole>
So far, so good (it works).
I now want to run a slave instance of the server in another WorkerRole
<WorkerRole name="SlaveWorker" vmsize="Small">
<Runtime executionContext="limited">
<EntryPoint>
<ProgramEntryPoint commandLine="echo slaveof %ADDRESS% %PORT% | redis-server.exe -" setReadyOnProcessStart="true" />
</EntryPoint>
</Runtime>
<Imports>
<Import moduleName="Diagnostics" />
<Import moduleName="RemoteAccess" />
</Imports>
<Endpoints>
<InternalEndpoint name="Redis" protocol="tcp" port="6379" />
</Endpoints>
</WorkerRole>
You can see I need to tell the slave server where its master is using IP address and port; something that I don't know until Azure has allocated the network resources for that role. I've seen #smarx do something along these lines.
However I think there may be a couple of things wrong with this in my case
I'm setting environment variables in one role and hoping to use them in another - not going to work.
Even if the right data was available the way I need to pass it to the redis-server.exe is not recognized as a valid entry point with the echo at the beginning
Is the only way to know the runtime IP address and Port of another
worker role via code or is there a syntax I'm missing in the config
file?
If I manage to get the IP and Port, is the only way to make my
command line work to push it to a powershell script or batch file?
Thanks for your thoughts.

Answer 1

The only way one instance will know another's IP address will be if a.) it programmatically grabs it, or b.) the other instance publishes it to a wellknown location (e.g. table storage). In your case, it might be easiest to just have the slave role run a startup task that accesses the RoleEnvironment (via Powershell perhaps) and sets an Environment variable with the IP Address of the master. If you do this as a 'simple' type, I believe it will run before your ProgramEntryPoint does (blocking) and you can just use the env var in your command line there.
Couple thoughts here however:
How are you handling multi-instance within a role? Are you only planning on running a single instance?
Do you need two different roles? Why not a single role with 2 instances that decide via election which is master?