Aurora scheduler maintains data that need to be persisted to survive failovers and restarts. For example:
Conceptually, it can be represented by the following major components:
All services in Aurora access data via a set of predefined store interfaces (aka stores) logically grouped by the type of data they serve. Every interface defines a specific set of operations allowed on the data thus abstracting out the storage access and the actual persistence implementation. The latter is especially important in view of a general immutability of persisted data. With the Mesos replicated log as the underlying persistence solution, data can be read and written easily but not modified. All modifications are simulated by saving new versions of modified objects. This feature and general performance considerations justify the existence of the volatile in-memory store.
There are two types of reads available in Aurora: consistent and weakly-consistent. The difference is explained below.
All reads are served from the volatile storage making reads generally cheap storage operations from the performance standpoint. The majority of the volatile stores are represented by the in-memory H2 database. This allows for rich schema definitions, queries and relationships that key-value storage is unable to match.
Writes are more involved operations since in addition to updating the volatile store data has to be appended to the replicated log. Data is not available for reads until fully ack-ed by both replicated log and volatile storage.
Aurora uses write-ahead logging to ensure consistency between replicated and volatile storage. In Aurora, data is first written into the replicated log and only then updated in the volatile store.
Aurora storage uses read-write locks to serialize data mutations and provide consistent view of the
available data. The available
Storage interface exposes 3 major types of operations:
consistentRead - access is locked using reader’s lock and provides consistent view on read
weaklyConsistentRead - access is lock-less. Delivers best contention performance but may result
in stale reads
write - access is fully serialized by using writer’s lock. Operation success requires both
volatile and replicated writes to succeed.
The consistency of the volatile store is enforced via H2 transactional isolation.
Any time a scheduler restarts, it restores its volatile state from the most recent position recorded in the replicated log by restoring the snapshot and replaying individual log entries on top to fully recover the state up to the last write.