Isolation & Locks

The CAP theorem states that two out three of Consistency, Availability & Partition Tolerance may be achieved.

RDBMS systems allow for ACID: Atomicity (support for all or non transactions), Consistency (Referential integrity handled by local dbs), Isolation (concurrent/sequential won't matter) & Durability (handled by local dbs).

Issues without Isolation:

• Lost updates (Update without realizing a prior update)
• Dirty reads (reading before a prior operation has fully succeeded)
• Nonrepeatable/fuzzy reads (Subsequent reads in the same operation returns different data)

Locking strategies:

• Semantic lock: app level lock
• Commutative updates: Update executable in any order
• Pessimistic lock/view: Reorder steps
• Reread value: Avoid dirty writes by reading data prior to write
• Version file: Record updates
• By value: Each request chooses concurrency mechanism as required

Source: Microservices Patterns by Chris Richardson