CAP Theorem §

#software-architecture#database

Brewers Theorem about distributed databases states that from Consistency, Availabillity and Partition Tolerance, only two can be taken.

• Consistency: Every read receives the most recent write or an error
• Availability: Every request receives a (non-error) response, without the guarantee that it contains the most recent write
• Partition tolerance: The system continues to operate despite an arbitrary number of messages being dropped/delayed by the network between nodes

Databases CAP Categorization §

Cassandra §

• Eventual Consistency
• Lightweight transactions
• Guaranteed batched writes

MongoDB §

• Replica Sets
• Primary for writes, Secondaries for reads
• Consistent by default

PostgreSQL §

• Performance impact due to synchronization of changes
• Hot Standby/Streaming Replication