Service discovery in ZooKeeper
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NOTE
The concepts and main components of ZooKeeper are described in the ZooKeeper article. |
Overview
Service Discovery is a ZooKeeper service mechanism that acts as a registry, keeping track of the addresses of all instances. Instances have dynamically assigned network paths.
The Service Discovery system provides a mechanism for:
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registration of services and instances of services;
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search for instances of a particular service;
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notifications about changes to service instances.
The entire Service Discovery procedure consists of two phases:
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Service Registration.
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Service Discovery.
The Service Discovery mechanism with load balancing uses the curator-x-discovery package from the Apache Curator — Java/JVM client library for Apache ZooKeeper.
The stages of Service Discovery are described below.
Service Registration
When a service instance appears, it registers with the Service Registry ZooKeeper in the namespace of its service, adding its ephemeral znode. Znode stores the host:port of the server as its data. The service instance sends heartbeat requests to save its registration.
Service Registry is a component that contains a database of all available service instances. It stores information about the currently available instances of each service and their network data for establishing a connection.
The Service Registry monitors running instances for changes by polling the deployment environment or by subscribing to events. When the Service Registry detects a new service instance available, it writes it to its database. The Service Registry also unregisters failed (disabled) service instances.
Apache Curator represents a service instance as a ServiceInstance class. ServiceInstances have a name, ID, address, port and/or ssl port, and an optional (user-defined) payload. ServiceInstances are serialized and stored in ZooKeeper like this:
base path
|_______ service A name
|__________ instance 1 id --> (serialized ServiceInstance)
|__________ instance 2 id --> (serialized ServiceInstance)
|__________ ...
|_______ service B name
|__________ instance 1 id --> (serialized ServiceInstance)
|__________ instance 2 id --> (serialized ServiceInstance)
|__________ ...
|_______ ...
Service Discovery
After registering service instances, service discovery occurs in the following sequence:
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The client connects to the ZooKeeper ensemble and requests the desired service.
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Using the
ServiceProviderclass used in Apache Curator, ZooKeeper uses a load balancing algorithm to select one of the available service instances and returns ahost:portfor one of the registered instances. -
The client connects to the service instance.