Prerequisites:
- transaction - a single logical unit of work that accesses and possibly modifies the contents of a database.
- transactions access data using read/write operations
Purpose:
ACID properties are used to maintain consistency in a database, before and after a transaction
Atomicity
Either the entire transaction takes place at once or doesn’t happen at all. No partial completions. Two operations:
- —ABORT: if abort, changes made to DB are not visable
- —COMMIT: if commit, changes made are visible
Consistency
Ensures that the data follows constraints before and after the transaction.
example:
- The total money for two accounts is 300 in account 1 and $500 in account 2.
- The consistency constraint is $800 total money
- The transaction transfers $200 from account 1 to account 2.
- If the result is 700, the database is consistent. If it’s not, say transaction was deducted from 1 but not incremented on 2, then the database is inconsistent since it broke the constraint.
Isolation
Ensures that multiple transactions can occur concurrently without leading to inconsistency (isolated environment). Changes occurring in a particular transaction will not be visible to any other transaction until that particular change in that transaction is written to memory or has been commited.
example:
- Account balance is $200
- Two transactions for -$100 withdrawal start at the same time
- When transactions complete, under isolation, the resulting balance should be 100
Durability
Guarantees that once the transaction completes and changes are written to the database, they are persisted (despite crashes or power outages)
Sources: