# Service Discovery

Contains classes for helping services discover each other,
and managing connections between them.

## Service Registry

The service registry is a class that keeps track of the services
that are currently running, and their connection information.  

There are two implementations:

* A simple implementation that effectively hard-codes the services. 
This is sufficient when running in docker, although loses some 
smart features. It is fundamentally incompatible with running
the system bare-metal, and does not permit multiple instances
of a service to run.

* A more advanced implementation that is based on [Zookeeper](https://zookeeper.apache.org/), 
which is a distributed coordination service.  This implementation
lets services register themselves and announce their liveness, 
and then discover each other.  It supports multiple instances of
a service running, and supports running the system bare-metal,
where it will assign ports to the services from a range.

To be discoverable, the caller must first register their
services:

```java
// Register one or more services
registry.registerService(ApiSchema.GRPC,
                         ServiceId.Test,  
                         nodeId, 
                         instanceUUID, //must be unique to the runtime
                         "127.0.0.1"); // bind-address
// (+ any other services)
```

Then, the caller must announce their instance.  Before this is done, 
the service is not discoverable.

```java
registry.announceInstance(ServiceId.Test,
                          nodeId,
                          instanceUUID);
```

All of this is done automatically by the `Service` base class
in the [service](../service/) module.  

To discover a service, the caller can query the registry:

```java
Set<InstanceAddress<?>> endpoints = registry.getEndpoints(ApiSchema.GRPC, ServiceId.Test, nodeId);

for (var endpoint : endpoints) {
    System.out.println(endpoint.getHost() + ":" + endpoint.getPort());
}
```

It's also possible to subscribe to changes in the registry, so that
the caller can be notified when a service comes or goes, with `registry.registerMonitor()`.

## gRPC Channel Pool

From the GrpcChannelPoolFactory, two types of channel pools can be created
that are aware of the service registry:

* GrpcMultiNodeChannelPool - This pool permits 1-n style communication with partitioned services
* GrpcSingleNodeChannelPool - This pool permits 1-1 style communication with non-partitioned services

The pools manage the lifecycle of the gRPC channels, and will permit the caller
to access Stub interfaces for the services.

# Central Classes

## Service Registry

* [ServiceRegistryIf](src/main/java/nu/marginalia/service/discovery/ServiceRegistryIf.java)
* [ZkServiceRegistry](src/main/java/nu/marginalia/service/discovery/ZkServiceRegistry.java)
* [FixedServiceRegistry](src/main/java/nu/marginalia/service/discovery/FixedServiceRegistry.java)

## gRPC Channel Pool

* [GrpcChannelPoolFactory](src/main/java/nu/marginalia/service/client/GrpcChannelPoolFactory.java)
* [GrpcMultiNodeChannelPool](src/main/java/nu/marginalia/service/client/GrpcMultiNodeChannelPool.java)
* [GrpcSingleNodeChannelPool](src/main/java/nu/marginalia/service/client/GrpcSingleNodeChannelPool.java)