Summary: How backup space instance recovers its data when started or after a fail-over.

Introduction

This page describes how a space instance running in a backup mode recovers its data when started, relocated or recovers from a failure. The flow is very similar for all these scenarios and mostly depends on the cluster topology and the space cache policy mode. There is special handling for Transient Entries (persist=false), since they can't be persisted - they are always recovered from the primary.

Central Database Topology w/o Mirror

With this scenario - The IMDG is the system of record. The data is usually persisted through the Mirror service.

LRU Cache Policy

A backup instance recovers only transient entries from the primary instance. Data is not loaded from the database.

Since primary and backup use the same database instance, the data will be loaded to the backup on demand.

All In Cache Policy

A backup instance recovers all its data from the primary instance - data is not loaded from the database. This is done so that any data changes on the primary during the recovery process are consistent on the backup once recovery finishes.

Distributed Database Topology

Primary and backup keep and manage their data in a different database instance.

With this scenario:

  1. If the database is empty --> Backup instance recover everything from the primary.
  2. If database has data --> recover from the database + recover transient objects from the primary.

For further info and configuration options see Distributed Databases

Backup Instance Recovery Failure Handling

If a backup space instance recovery process fails, it is handled in the following way:
If the primary space is unavailable for some reason - recovery will be retried until one of the following happens - the primary gets reconnected and then the recovery continues normally or the space itself becomes primary and then no recovery is necessary.
Any other failure - SpaceMemoryShortageException, Database not available etc. is retried 3 times before failing - the space won't start and throw an exception.

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