Disaster recovery

A database can become unavailable due to issues on different system levels. For example, a data center failover may lead to the loss of multiple servers, which may cause a set of databases to become unavailable.

This section contains a step-by-step guide on how to recover unavailable databases that are incapable of serving writes, while possibly still being able to serve reads. The guide recovers the unavailable databases and make them fully operational, with minimal impact on the other databases in the cluster. However, if a database is not performing as expected for other reasons, this section cannot help.

If all servers in a Neo4j cluster are lost in a disaster, it is not possible to recover the current cluster. You have to create a new cluster and restore the databases, see Deploy a basic cluster and Seed a database for more information.

Faults in clusters

Databases in clusters may be allocated differently within the cluster and may also have different numbers of primaries and secondaries. The consequence of this is that all servers may be different in which databases they are hosting. Losing a server in a cluster may cause some databases to lose a member while others are unaffected. Therefore, in a disaster where one or more servers go down, some databases may keep running with little to no impact, while others may lose all their allocated resources.

Guide overview

In this guide the following terms are used:

  • An offline server is a server that is not running but may be restartable.

  • A lost server, however, is a server that is currently not running and cannot be restarted.

  • A write-available database is able to serve writes, while a write-unavailable database is not.

There are four steps to recovering a cluster from a disaster:

  1. Start the Neo4j process on all servers which are not lost. See Start the Neo4j process for more information.

  2. Make the system database able to serve write operations, so that the cluster can be modified. See Make the system database write-available for more information.

  3. Detach any potential lost servers from the cluster and replace them by new ones. See Make servers available for more information.

  4. Finish disaster recovery by starting or continuing to manage databases and verify that they are write-available. See Make databases write-available for more information.

Each step is described in the following three sections:

  1. Objective — a state that the cluster needs to be in, with optional motivation.

  2. Verifying the state — an example of how the state can be verified.

  3. Path to correct state — a proposed series of steps to get to the correct state.

Verifying each state before continuing to the next step, regardless of the disaster scenario, is recommended to ensure the cluster is fully operational.

Disaster recovery steps

Disasters may sometimes affect the routing capabilities of the driver and may prevent the use of the neo4j scheme for routing. One way to remedy this is to connect directly to the server using bolt instead of neo4j. See Server-side routing for more information on the bolt scheme.

Start the Neo4j process

Objective

The Neo4j process is started on all servers that are not lost.

Path to correct state

Start the Neo4j process on all servers that are offline. If a server is unable to start, inspect the logs and contact support personnel. The server may have to be considered indefinitely lost.

Make the system database write-available

Objective

The system database is able to serve write operations.

The system database contains the view of the cluster. This includes which servers and databases are present, where they live and how they are configured. During a disaster, the view of the cluster might need to change to reflect a new reality, such as removing lost servers. Databases might also need to be recreated to regain write availability. Because both of these steps are executed by modifying the system database, making the system database write-available is a vital first step during disaster recovery.

Verifying the state

The system database’s write availability can be verified by using the Status check procedure.

CALL dbms.cluster.statusCheck(["system"]);

The status check procedure cannot verify the write availability of a database configured to have a single primary. Instead, check that the primary is allocated on an available server and that it has currentStatus = online by running SHOW DATABASES.

Path to correct state

Use the following steps to regain write availability for the system database if it has been lost. They create a new system database from the most up-to-date copy of the system database that can be found in the cluster. It is important to get a system database that is as up-to-date as possible, so it corresponds to the view before the disaster closely.

Guide

This section of the disaster recovery guide uses neo4j-admin commands. For more information about the used commands, see neo4j-admin commands.

  1. Shut down the Neo4j process on all servers. This causes downtime for all databases in the cluster until the processes are started again at the end of this section.

  2. On each server, run bin/neo4j-admin dbms unbind-system-db to reset the system database state on the servers.

  3. On each server, run bin/neo4j-admin database info system and compare the lastCommittedTransaction to find out which server has the most up-to-date copy of the system database.

  4. On the most up-to-date server, run bin/neo4j-admin database dump system --to-path=[path-to-dump] to take a dump of the current system database and store it in an accessible location.

  5. For every lost server, add a new unconstrained one according to Add a server to the cluster. It is important that the new servers are unconstrained, or deallocating servers in the next step of this guide might be blocked, even though enough servers were added.

    While recommended, it is not strictly necessary to add new servers in this step. There is also an option to change the system database mode (server.cluster.system_database_mode) on secondary allocations to make them primary allocations for the new system database. The number of primary allocations needed is defined by dbms.cluster.minimum_initial_system_primaries_count. See the Configuration settings for more information. Be aware that not replacing servers can cause cluster overload when databases are moved from lost servers to available ones in the next step of this guide.

  6. On each server, run bin/neo4j-admin database load system --from-path=[path-to-dump] --overwrite-destination=true to load the current system database dump.

  7. On each server, ensure that the discovery settings are correct. See Cluster server discovery for more information.

  8. Start the Neo4j process on all servers.

Make servers available

Objective

All servers in the cluster’s view are available and enabled.

A lost server will still be in the system database’s view of the cluster, but in an unavailable state. Furthermore, according to the view of the cluster, these lost servers are still hosting the databases they had before they became lost. Therefore, informing the cluster of servers which are lost is not enough. The databases hosted on lost servers also need to be moved onto available servers in the cluster, before the lost servers can be removed.

Verifying the state

The cluster’s view of servers can be seen by listing the servers. See Listing servers for more information. The state has been verified if all servers show health = Available and status = Enabled.

SHOW SERVERS;

Path to correct state

Use the following steps to remove lost servers and add new ones to the cluster. To remove lost servers, any allocations they were hosting must be moved to available servers in the cluster. This is done in two different steps:

  • Any allocations that cannot move by themselves require the database to be recreated so that they are forced to move.

  • Any allocations that can move will be instructed to do so by deallocating the server.

Guide

  1. For each Unavailable server, run CALL dbms.cluster.cordonServer("unavailable-server-id") on one of the available servers. This prevents new database allocations from being moved to this server.

  2. For each Cordoned server, make sure a new unconstrained server has been added to the cluster to take its place. See Add a server to the cluster for more information.

    If servers were added in the Make the system database write-available step of this guide, additional servers might not be needed here. It is important that the new servers are unconstrained, or deallocating servers might be blocked even though enough servers were added.

    While recommended, it is not strictly necessary to add new servers in this step. However, not adding new servers reduces the capacity of the cluster to handle work. Furthermore, it might require the topology for a database to be altered to make deallocating servers and recreating databases possible.

  3. For each stopped database (currentStatus= offline), start them by running START DATABASE stopped-db. This is necessary since stopped databases cannot be deallocated from a server. It is also necessary for the status check procedure to accurately indicate if this database should be recreated or not. Verify that all allocations are in currentStatus = online on servers which are not lost before moving to the next step. If a database fails to start, leave it to be recreated in the next step of this guide.

    A database can be set to READ-ONLY before it is started to avoid updates on the database with the following command: ALTER DATABASE database-name SET ACCESS READ ONLY.

  4. On each server, run CALL dbms.cluster.statusCheck([]) to check the write availability for all databases running in primary mode on this server. See Monitoring replication for more information.

    The status check procedure cannot verify the write availability of a database configured to have a single primary. Instead, check that the primary is allocated on an available server and that it has currentStatus = online by running SHOW DATABASES.

  5. For each database that is not write-available, recreate it to move it from lost servers and regain write availability. Go to Recreate databases for more information about recreate options. Remember to make sure there are recent backups for the databases before recreating them. See Online backup for more information. If any database has currentStatus = quarantined on an available server, recreate them from backup using Backup as seed.

    If you recreate databases using undefined servers or undefined servers with fallback backup, the store might not be recreated as up-to-date as possible in certain edge cases where the system database has been restored.

  6. For each Cordoned server, run DEALLOCATE DATABASES FROM SERVER cordoned-server-id on one of the available servers. This will move all database allocations from this server to an available server in the cluster.

    This operation might fail if enough unconstrained servers were not added to the cluster to replace lost servers. Another reason is that some available servers are also Cordoned.

  7. For each deallocating or deallocated server, run DROP SERVER deallocated-server-id. This removes the server from the cluster’s view.

Make databases write-available

Objective

All databases that are desired to be started are write-available.

Once this state is verified, disaster recovery is complete. However, remember that previously stopped databases might have been started during this process. If they are still desired to be in stopped state, run STOP DATABASE started-db WAIT.

Remember, recreating a database takes an unbounded amount of time since it may involve copying the store to a new server, as described in Recreate databases. Therefore, an allocation with currentStatus = starting will probably reach the requestedStatus given some time.

Verifying the state

You can verify all clustered databases' write availability by using the status check procedure.

CALL dbms.cluster.statusCheck([]);

The status check procedure cannot verify the write availability of a database configured to have a single primary. Instead, check that the primary is allocated on an available server and that it has currentStatus = online by running SHOW DATABASES.

A stricter verification can be done to verify that all databases are in their desired states on all servers. For the stricter check, run SHOW DATABASES and verify that requestedStatus = currentStatus for all database allocations on all servers.

Path to correct state

Use the following steps to make all databases in the cluster write-available again. They include recreating any databases that are not write-available and identifying any recreations that will not complete. Recreations might fail for different reasons, but one example is that the checksums do not match for the same transaction on different servers.

Guide

  1. Identify all write-unavailable databases by running CALL dbms.cluster.statusCheck([]) as described in the Example verification part of this disaster recovery step. Filter out all databases desired to be stopped, so that they are not recreated unnecessarily.

  2. Recreate every database that is not write-available and has not been recreated previously. See Recreate databases for more information. Remember to make sure there are recent backups for the databases before recreating them. See Online backup for more information. If any database has currentStatus = quarantined on an available server, recreate them from backup using Backup as seed.

    If you recreate databases using undefined servers or undefined servers with fallback backup, the store might not be recreated as up-to-date as possible in certain edge cases where the system database has been restored.

  3. Run SHOW DATABASES and check any recreated databases that are not write-available. Recreating a database will not complete if one of the following messages is displayed in the message field:

    • Seeders ServerId1 and ServerId2 have different checksums for transaction TransactionId. All seeders must have the same checksum for the same append index.

    • Seeders ServerId1 and ServerId2 have incompatible storeIds. All seeders must have compatible storeIds.

    • No store found on any of the seeders ServerId1, ServerId2…​

  4. For each database which will not complete recreation, recreate them from backup using Backup as seed.