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WAN Replication

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Hazelcast IMDG Enterprise Feature

Introduction

You can use Hazelcast’s WAN Replication feature when you need to synchronize multiple Hazelcast clusters, which are connected by WANs, to the same state. It allows replicating updates in your data structures across the clusters. For now, Hazelcast WAN Replication supports the map and cache data structures.

Assume that you have data centers in different cities each running an independent Hazelcast IMDG cluster. You can reliably use the WAN Replication feature to synchronize all of these clusters by replicating the updates to each of them.

WAN Replication provides more control compared to the replication mechanism between the members in a single cluster. It has the following features and capabilities:

  • It gracefully detects if there is a connectivity issue between the clusters, buffering any updates that are not yet replicated and attempts to re-establish a connection to resume the replication.

  • It allows you to permanently pause, stop and resume the replication. This is most useful when you know that one of the clusters is temporarily (e.g., due to an upgrade), or permanently (e.g., due to removing a cluster out of service) unavailable.

  • It allows you to dynamically add new target clusters without any restarts.

This chapter explains how you can replicate the state of your clusters over wide area networks through Hazelcast’s WAN Replication.

Replication over WAN is a Hazelcast IMDG Enterprise Edition feature. However, its API is available publicly here and, benefiting from it, you may write your own replication logic.
Replication over WAN is supported between all Hazelcast 4.x.x versions.

Concepts

Let’s first define several important terms before we discuss WAN Replication:

  • Active cluster: The user updates performed on the cluster are replicated to other clusters connected through WAN Replication. In another words, this cluster can be seen as the "source" cluster which generates WAN update events and replicates them actively to other clusters.

  • Passive cluster: The user updates performed on this cluster are not replicated to other clusters. In another words, this cluster can be seen as the "target" cluster which is capable of receiving, applying and possibly forwarding WAN events from other ("active") clusters. It does not generate any WAN update events because of user interaction.

  • WAN publisher: A publisher is a sink for WAN events and an implementation of WanReplicationPublisher. Most often, this is a single, entire target Hazelcast IMDG cluster but you can also define custom publishers which may transmit WAN events to other systems such as messaging queues, Kafka or even persist events on disk.

  • WAN endpoint: when a publisher is replicating events to another Hazelcast cluster, an endpoint is a single member in that target cluster. That means that a WAN publisher replicates to multiple WAN endpoints.

  • WAN replication scheme: a named collection of WAN publishers. Hazelcast maps and caches are configured to replicate to a WAN replication scheme, meaning that a single map/cache update can be replicated to multiple target clusters or multiple external systems.

  • WAN publisher ID: A unique identifier for a specific WAN replication publisher in a WAN replication scheme. This identifier can then be used to control the behavior of a WAN replication publisher while the source/active cluster is running. For instance, you can use this identifier in combination with the WAN replication scheme name to pause, stop or resume WAN replication for that specific publisher. Or, you can trigger synchronization with a specific target cluster and so on.

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