The database functions independently within the active data center(s). As transactions make changes to the database, these changes are replicated to the Asynchronous Storage Managers (ASMs) in the passive data center(s).
Changes made to the database are replicated to the ASMs; however there is no need to wait for ASMs to respond or for the changes to be made durable on disk, before their transactions are committed.
ASMs are passive and do not participate in NuoDB’s performance-sensitive network protocols, so high network latency between remote data centers does not slow down the database workload. Performance-sensitive protocols execute entirely inside the active data center, where network latency is low.
As long as there is sufficient network bandwidth, the presence of ASMs does not slow down or interfere with the execution of the database workload. If the network does not have sufficient bandwidth to keep up, active TEs will buffer replication messages, forcing them to throttle their performance or run out of memory. Before that happens, an ASM at the other end of a low-bandwidth network link might be evicted by ping timeout. In that case, the active data center continues running but the passive data center stops providing for recovery from disasters. Administrators should monitor their databases to remain confident that everything is working as intended.