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## 12.4. Availability and Reliability

Most mission-critical systems require the database subsystem to be accessible at all times. Neo4j ensures availability and reliability through a few different strategies.

### 12.4.1. Operational Availability

In order not to create a single point of failure, Neo4j supports different approaches which provide transparent fallback and/or recovery from failures.

#### Online backup (Cold spare)

In this approach, a single instance of the master database is used, with Online Backup enabled. In case of a failure, the backup files can be mounted onto a new Neo4j instance and reintegrated into the application.

#### Online Backup High Availability (Hot spare)

Here, a Neo4j "backup" instance listens to online transfers of changes from the master. In the event of a failure of the master, the backup is already running and can directly take over the load.

#### High Availability cluster

This approach uses a cluster of database instances, with one (read/write) master and a number of (read-only) slaves. Failing slaves can simply be restarted and brought back online. Alternatively, a new slave may be added by cloning an existing one. Should the master instance fail, a new master will be elected by the remaining cluster nodes.

### 12.4.2. Disaster Recovery/ Resiliency

In cases of a breakdown of major part of the IT infrastructure, there need to be mechanisms in place that enable the fast recovery and regrouping of the remaining services and servers. In Neo4j, there are different components that are suitable to be part of a disaster recovery strategy.

#### Prevention

- Online Backup High Availability to other locations outside the current data center.
- Online Backup to different file system locations: this is a simpler form of backup, applying changes directly to backup files; it is thus more suited for local backup scenarios.
- Neo4j High Availability cluster: a cluster of one write-master Neo4j server and a number of read-slaves, getting transaction logs from the master. Write-master failover is handled by quorum election among the read-slaves for a new master.

#### Detection

- SNMP and JMX monitoring can be used for the Neo4j database.

#### Correction

- Online Backup: A new Neo4j server can be started directly on the backed-up files and take over new requests.
- Neo4j High Availability cluster: A broken Neo4j read slave can be reinserted into the cluster, getting the latest updates from the master. Alternatively, a new server can be inserted by copying an existing server and applying the latest updates to it.

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