Administration guide > Plan the WebSphere eXtreme Scale environment > Cache topology > Multi-master replication
Initial considerations for multi-master topology
Take the following into account when deciding whether and how to use multi-master replication topologies.
- Configure class loaders with multiple catalog service domains
Domains must have access to all classes that are used as keys and values. Any dependencies must be reflected in all class paths for data grid container JVMs for all domains. If a CollisionArbiter plug-in retrieves the value for a cache entry, then the classes for the values must be present for the domain that is starting the arbiter.
- Avoid loaders
Loaders can be used to interface changes between a data grid and a database. It is unlikely that all data grids or domains in a topology are collocated geographically with the same database. WAN latency and other factors might render this use case undesirable.
Grid preload also requires careful design. Usually, when a data grid is restarted, it is preloaded again. Preloading is not necessary or required when using multi-master replication. As soon as a catalog service domain is online, it automatically reloads itself with the contents of the domains to which it is linked. As a result, you are not required to initiate a manual preload for a data grid that is a domain in a multi-master replication topology.
Loaders usually obey insert and update rules. With multi-master replication, inserts must be treated as merges. When the data is being pulled remotely after a domain restart, existing data will be merged into the local domain. Because the data might already have been in the local database, a typical insert fails with a duplicate key exception in the database. Use merge semantics instead.
WebSphere eXtreme Scale can be configured to do a shard-based preload with the preload methods on Loader plug-ins. But you should avoid this technique in a multi-master replication topology. Instead, use a client-based preload when the topology is first started. The multi-master topology refreshes any restarted domains with a current copy of what is stored in other domains in the topology. After domains have been started, the multi-master topology keeps domains synchronized.
- EntityManager is not supported
A map set containing an entity map is not replicated across catalog service domains.
- Byte array maps are not supported
A map set containing a map that is configured with COPY_TO_BYTES is not replicated across catalog service domains.
- Write-behind is not supported
Parent topic:Multi-master data grid replication topologies