Package org.apache.hedwig.client.netty

A Netty based Hedwig client implementation.

See: Description

Interface Summary

Interface	Description
HChannel	A wrapper interface over netty Channel to submit hedwig's PubSubData requests.
HChannelManager	A manager manages 1) all channels established to hub servers, 2) the actions taken by the topic subscribers.

Class Summary

Class	Description
CleanupChannelMap<T>
FilterableMessageHandler	Handlers used by a subscription.
HedwigClientImpl	This is a top level Hedwig Client class that encapsulates the common functionality needed for both Publish and Subscribe operations.
HedwigPublisher	This is the Hedwig Netty specific implementation of the Publisher interface.
HedwigSubscriber	This is the Hedwig Netty specific implementation of the Subscriber interface.
NetUtils	Utilities for network operations.
SubscriptionEventEmitter
VoidCallbackAdapter<T>	Adapts from Callback<T> to Callback<Void>.

Package org.apache.hedwig.client.netty Description

A Netty based Hedwig client implementation.

Components

The netty based implementation contains following components:

• HChannel: A interface wrapper of netty Channel to submit hedwig's PubSubProtocol.PubSubRequests to target host.
• HChanneHandler: A wrapper of netty ChannelHandler to handle events of its underlying netty channel, such as responses received, channel disconnected, etc. A HChannelHandler is bound with a HChannel.
• HChannelManager: A manager manages all established HChannels. It provides a clean interface for publisher/subscriber to send PubSubProtocol.PubSubRequests

Main Flow

• HedwigPublisher/HedwigSubscriber delegates HChannelManager to submit pub/sub requests.
• HChannelManager find the owner hubs, establish a HChannel to hub servers and send the requests to them.
• HChannelHandler dispatches responses to target AbstractResponseHandler to process.
• HChannelHandler detects an underlying netty Channel disconnected. It calles HChannelManager to clear cached HChannel that it bound with. For non-subscritpion channels, it would fail all pending requests; For subscription channels, it would fail all pending requests and retry to reconnect those successful subscriptions.

HChannel

Two kinds of HChannels provided in current implementation. HChannelImpl provides the ability to multiplex pub/sub requests in an underlying netty Channel, while DefaultServerChannel provides the ability to establish a netty channel Channel for a pub/sub request. After the underlying netty channel is estabilished, it would be converted into a HChannelImpl by HChannelManager#submitOpThruChannel(pubSubData, channel). Although HChannelImpl provides multiplexing ability, it still could be used for one-channel-per-subscription case, which just sent only one subscribe request thru the underlying channel.

HChannelHandler

HChannelHandler is generic netty ChannelHandler, which handles events from the underlying channel. A HChannelHandler is bound with a HChannel as channel pipeplien when the underlying channel is established. It takes the responsibility of dispatching response to target response handler. For a non-subscription channel, it just handles PUBLISH and UNSUBSCRIBE responses. For a subscription channel, it handles SUBSCRIBE response. For consume requests, we treated them in a fire-and-forget way, so they are not need to be handled by any response handler.

HChannelManager

HChannelManager manages all outstanding connections to target hub servers for a client. Since a subscription channel acts quite different from a non-subscription channel, the basic implementation AbstractHChannelManager manages non-subscription channels and subscription channels in different channel sets. Currently hedwig client provides SimpleHChannelManager which manages subscription channels in one-channel-per-subscription way. In future, if we want to multiplex multiple subscriptions in one channel, we just need to provide an multiplexing version of AbstractHChannelManager which manages channels in multiplexing way, and a multiplexing version of SubscribeResponseHandler which handles multiple subscriptions in one channel.