By default, Hazelcast is bundled with multiple ways to define and find other members in the same network. Commonly used, especially with development, is the Multicast discovery. This sends out a multicast request to a network segment and awaits other members to answer with their IP addresses. In addition, Hazelcast supports fixed IP addresses: JClouds or AWS (Amazon EC2) based discoveries.
Since there is an ever growing number of public and private cloud environments, as well as numerous Service Discovery systems in the wild, Hazelcast provides cloud or service discovery vendors with the option to implement their own discovery strategy.
Over the course of this section, we will build a simple discovery strategy based on the /etc/hosts file.
Interfaces and Classes
The Hazelcast Discovery SPI (Member Discovery Extensions) consists of multiple interfaces and abstract classes. In the following sub-sections, we will have a quick look at all of them and shortly introduce the idea and usage behind them. The example will follow in the next section, Discovery Strategy.
DiscoveryStrategy: Implement
The com.hazelcast.spi.discovery.DiscoveryStrategy interface is the main entry point for vendors to implement their corresponding member discovery strategies. Its main purpose is to return discovered members on request. The com.hazelcast.spi.discovery.DiscoveryStrategy interface also offers light lifecycle capabilities for setup and teardown logic (for example, opening or closing sockets or REST API clients).
DiscoveryStrategys can also do automatic registration / de-registration on service discovery systems if necessary. You can use the provided DiscoveryNode that is passed to the factory method to retrieve local addresses and ports, as well as metadata.
AbstractDiscoveryStrategy: Abstract Class
The com.hazelcast.spi.discovery.AbstractDiscoveryStrategy is a convenience abstract class meant to ease the implementation of strategies. It basically provides additional support for reading / resolving configuration properties and empty implementations of lifecycle methods if unnecessary.
DiscoveryStrategyFactory: Factory Contract
The com.hazelcast.spi.discovery.DiscoveryStrategyFactory interface describes the factory contract that creates a certain DiscoveryStrategy. DiscoveryStrategyFactorys are registered automatically at startup of a Hazelcast member or client whenever they are found in the classpath. For automatic discovery, factories need to announce themselves as SPI services using a resource file according to the Java Service Provider Interface. The service registration file must be part of the JAR file, located under META-INF/services/com.hazelcast.spi.discovery.DiscoveryStrategyFactory, and consist of a line with the full canonical class name of the DiscoveryStrategy per provided strategy implementation.
DiscoveryNode: Describe a Member
The com.hazelcast.spi.discovery.DiscoveryNode abstract class describes a member in the Discovery SPI. It is used for multiple purposes, since it will be returned from strategies for discovered members. It is also passed to DiscoveryStrategyFactorys factory method to define the local member itself if created on a Hazelcast member; on Hazelcast clients, null will be passed.
SimpleDiscoveryNode: Default DiscoveryNode
com.hazelcast.spi.discovery.SimpleDiscoveryNode is a default implementation of the DiscoveryNode. It is meant for convenience use of the Discovery SPI and can be returned from vendor implementations if no special needs are required.
NodeFilter: Filter Members
You can configure com.hazelcast.spi.discovery.NodeFilter before startup and you can implement logic to do additional filtering of members. This might be necessary if query languages for discovery strategies are not expressive enough to describe members or to overcome inefficiencies of strategy implementations.
NOTE: The DiscoveryStrategy vendor does not need to take possibly configured filters into account as their use is transparent to the strategies.
DiscoveryService: Support In Integrator Systems
A com.hazelcast.spi.discovery.integration.DiscoveryService is part of the integration domain. DiscoveryStrategy vendors do not need to implement DiscoveryService because it is meant to support the Discovery SPI in situations where vendors integrate Hazelcast into their own systems or frameworks. Certain needs might be necessary as part of the classloading or Java Service Provider Interface lookup.
DiscoveryServiceProvider: Provide a DiscoveryService
Use the com.hazelcast.spi.discovery.integration.DiscoveryServiceProvider to provide a DiscoveryService to the Hazelcast discovery subsystem. Configure the provider with the Hazelcast configuration API.
DiscoveryServiceSettings: Configure DiscoveryService
A com.hazelcast.spi.discovery.integration.DiscoveryServiceSettings instance is passed to the DiscoveryServiceProvider at creation time to configure the DiscoveryService.
DiscoveryMode: Member or Client
The com.hazelcast.spi.discovery.integration.DiscoveryMode enum tells if a created DiscoveryService is running on a Hazelcast member or client, and to change behavior accordingly.