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## 22.1. Architecture

Neo4j HA has been designed to make the transition from single machine to multi machine operation simple, by not having to change the already existing application.

Consider an existing application with Neo4j embedded and running on a single machine. To deploy such an application in a multi machine setup the only required change is to switch the creation of the `GraphDatabaseService` from `EmbeddedGraphDatabase` to `HighlyAvailableGraphDatabase`. Since both implement the same interface, no additional changes are required.

Figure 22.1. Typical setup when running multiple Neo4j instances in HA mode

When running Neo4j in HA mode there is always a single master and zero or more slaves. Compared to other master-slave replication setups Neo4j HA can handle writes on a slave so there is no need to redirect writes to the master.

A slave will handle writes by synchronizing with the master to preserve consistency. Writes to master can be configured to be optimistically pushed to 0 or more slaves. By optimistically we mean the master will try to push to slaves before the transaction completes but if it fails the transaction will still be successful (different from normal replication factor). All updates will however propagate from the master to other slaves eventually so a write from one slave may not be immediately visible on all other slaves. This is the only difference between multiple machines running in HA mode compared to single machine operation. All other ACID characteristics are the same.

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