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NCache: Bridge Topology for WAN Replication

NCache provides a way for you to replicate the cache across WAN without slowing down your cache. This replication is done through a Bridge and asynchronously so WAN latency does not slow down you main cache. The Bridge also has the intelligence to deal with low-quality long-distance socket connections that frequently break down.

You can use the Bridge in an active-passive or active-active multi-site configuration. In active-passive, all the updates from the active site are asynchronously replicated to the passive site with the assumption that no data is being updated by any application but the passive site is available for read-only access.

On the other hand, in active-active scenario, all the data from one active site is replicated to the other active site and conflict resolution is based on the last-one-wins formula.

A Bridge can be setup in two ways:

1. Single Bridge Server: This is a stand-alone single-server Bridge.
2. Bridge Cluster (2-node active/passive): The cluster option is provided to you so you have redundancy in your infrastructure and the Bridge does not have a single point of failure. This is an active/passive Mirrored Cache cluster where the passive node automatically becomes active if the active node goes down. And, all the data in the WAN replication queue is mirrored in the passive node as well.

Additionally, you can configure the Bridge as following:

1. Dedicated Bridge Servers: You can choose to dedicate one or two servers as Bridge nodes. You should do this if your cache servers are already fully loaded with transactions.
2. Bridge on Cache Servers: You can also choose to configure one or two Bridge nodes on your existing cache servers. In this case, the Bridge is simply a separate process that is responsible for Bridge operations.

An example of active-passive Bridge is disaster recovery infrastructure. If you want to provide disaster recovery (DR) in your infrastructure and have a different location data center designated as a DR site, your cache needs to replicate to the DR site as well. However, since the DR site is across the WAN, there is usually a great deal of latency between the primary and the DR sites and any replication ends up being extremely slow. So, if the DR site cache servers were made part of the primary site cache cluster, the entire cache cluster would grind to a halt due to this latency.

To address this problem, NCache provides a Bridge Topology. The Bridge consists of one or two servers that reside in the primary data center and the primary cache cluster is connected to it. All updates to the primary cluster are queued up on the Bridge which then updates the DR cache cluster asynchronously. The Bridge also has the intelligence to handle connection breakup which is common in WAN environment.