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To avoid having to either hard-code or manually configure service addresses (possibly several dozen), and to reduce the project's dependency on docker to deal with routing and discovery, the option to use [Zookeeper](https://zookeeper.apache.org/) to manage services and discovery has been added. A service registry interface was added, with a Zookeeper implementation and a basic implementation that only works on docker and hard-codes everything. The last remaining REST service, the assistant-service, has been migrated to gRPC. This also proved a good time to clear out primordial technical debt from the root of the codebase. The 'service-client' library has been taken behind the barn and given a last farewell. It's replaced by a small library for managing gRPC channels. Since it's no longer used by anything, RxJava has been removed as a dependency from the project. Although the current state seems reasonably stable, this is a work-in-progress commit.
87 lines
3.2 KiB
Markdown
87 lines
3.2 KiB
Markdown
# Service Discovery
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Contains classes for helping services discover each other,
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and managing connections between them.
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## Service Registry
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The service registry is a class that keeps track of the services
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that are currently running, and their connection information.
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There are two implementations:
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* A simple implementation that effectively hard-codes the services.
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This is sufficient when running in docker, although loses some
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smart features. It is fundamentally incompatible with running
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the system bare-metal, and does not permit multiple instances
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of a service to run.
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* A more advanced implementation that is based on [Zookeeper](https://zookeeper.apache.org/),
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which is a distributed coordination service. This implementation
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lets services register themselves and announce their liveness,
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and then discover each other. It supports multiple instances of
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a service running, and supports running the system bare-metal,
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where it will assign ports to the services from a range.
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To be discoverable, the caller must first register their
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services:
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```java
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// Register one or more services
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registry.registerService(ApiSchema.GRPC,
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ServiceId.Test,
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nodeId,
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instanceUUID, //must be unique to the runtime
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"127.0.0.1"); // bind-address
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// (+ any other services)
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```
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Then, the caller must announce their instance. Before this is done,
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the service is not discoverable.
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```java
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registry.announceInstance(ServiceId.Test,
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nodeId,
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instanceUUID);
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```
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All of this is done automatically by the `Service` base class
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in the [service](../service/) module.
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To discover a service, the caller can query the registry:
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```java
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Set<InstanceAddress<?>> endpoints = registry.getEndpoints(ApiSchema.GRPC, ServiceId.Test, nodeId);
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for (var endpoint : endpoints) {
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System.out.println(endpoint.getHost() + ":" + endpoint.getPort());
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}
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```
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It's also possible to subscribe to changes in the registry, so that
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the caller can be notified when a service comes or goes, with `registry.registerMonitor()`.
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## gRPC Channel Pool
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From the GrpcChannelPoolFactory, two types of channel pools can be created
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that are aware of the service registry:
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* GrpcMultiNodeChannelPool - This pool permits 1-n style communication with partitioned services
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* GrpcSingleNodeChannelPool - This pool permits 1-1 style communication with non-partitioned services
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The pools manage the lifecycle of the gRPC channels, and will permit the caller
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to access Stub interfaces for the services.
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# Central Classes
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## Service Registry
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* [ServiceRegistryIf](src/main/java/nu/marginalia/service/discovery/ServiceRegistryIf.java)
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* [ZkServiceRegistry](src/main/java/nu/marginalia/service/discovery/ZkServiceRegistry.java)
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* [FixedServiceRegistry](src/main/java/nu/marginalia/service/discovery/FixedServiceRegistry.java)
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## gRPC Channel Pool
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* [GrpcChannelPoolFactory](src/main/java/nu/marginalia/service/client/GrpcChannelPoolFactory.java)
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* [GrpcMultiNodeChannelPool](src/main/java/nu/marginalia/service/client/GrpcMultiNodeChannelPool.java)
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* [GrpcSingleNodeChannelPool](src/main/java/nu/marginalia/service/client/GrpcSingleNodeChannelPool.java) |