Where should I add ServicePlacementPreferPrimaryDomainPolicyDescription in code or manifest file? - azure-service-fabric

Question

I have a cluster span regions and I want specify preferred domain. Problem is looking at sample code in https://learn.microsoft.com/en-us/azure/service-fabric/service-fabric-cluster-resource-manager-advanced-placement-rules-placement-policies, I have no idea where serviceDescription comes from. Anyone knows where I should have these code in my service fabric service code base?
Also, isn't there a similar way to specify it in service fabric manifest file instead of code change (Like how people specify frontend/backend placement)?
Thanks,

Answer 1

Use this code to change a service description:
FabricClient fabricClient = new FabricClient();
StatefulServiceDescription serviceDescription = new StatefulServiceDescription();
serviceDescription.PlacementConstraints = "(HasSSD == true && SomeProperty >= 4)";
await fabricClient.ServiceManager.CreateServiceAsync(serviceDescription);
(source here)
According to the XSD, you can define them as xml, similar to placement constraints.
C:\Program Files\Microsoft SDKs\Service Fabric\schemas\ServiceFabricServiceModel.xsd

Answer 2

As per MS Docs, you can certainly configure it per code, like this:
FabricClient fabricClient = new FabricClient();
StatefulServiceDescription serviceDescription = new StatefulServiceDescription();
serviceDescription.PlacementConstraints = "(HasSSD == true && SomeProperty >= 4)";
// add other required servicedescription fields
//...
await fabricClient.ServiceManager.CreateServiceAsync(serviceDescription);
As for your question about where to do this from - depending on your needs, there are a few options:
Write a separate application that gets run as part of CI/CD deployment pipeline (or do it via powershell New-ServiceFabricService)
The problem with this approach is that it's a bit annoying for developers having to run this after each deployment - as such I prefer:
Write a stateless service that gets instantiated by default (i.e. put an entry inside <DefaultServices> within ApplicationManifest.xml). Within this service you can then instantiate/modify placement constraints using FabricClient, policies, metrics - and drive these via application parameters/configuration
Instantiate services lazily via your API gateway. This works particularly well for partitioned/stateful services (e.g. if you partition by tenantid) or where you want to maintain a pool of workers (e.g. partition_id = random_number % pool size), etc.
You can also configure placement constraints declaratively via application parameters. Unfortunately this doesn't seem to work atm for placement policies (and probably not for metrics either).
If your needs are simple, then the declarative approach is probably simplest! As per above link, add a [Stateless1_InstanceCount] parameter in your application manifest file and put this under default services:
<DefaultServices>
<Service Name="Stateless1">
<StatelessService ServiceTypeName="Stateless1Type" InstanceCount="[Stateless1_InstanceCount]">
<SingletonPartition />
<PlacementConstraints>[Stateless1_PlacementConstraints]</PlacementConstraints>
</StatelessService>
</Service>
</DefaultServices>