This trace follows the actual state transitions behind the companion Transactions, Isolation, and MVCC Snapshots. It describes a common execution path; implementation details can vary, so keep the contract separate from the mechanism.
Step 1: Begin a transaction snapshot
A transaction groups operations under a database contract such as atomicity and consistency. Multi-version concurrency control keeps versions of rows so readers can observe a consistent snapshot while writers create new versions. The exact snapshot and conflict rules depend on the database and isolation level.
Step 2: Read a visible row version
Under snapshot-based isolation, a transaction may read a row version that was committed when its snapshot began, even if another transaction later updates the row. Concurrent writes can cause one transaction to wait or fail. An application must treat serialization or deadlock failures as expected outcomes to handle safely.
Step 3: Create a new write version
With MVCC, the reader chooses a version visible to its snapshot while a writer can create a newer version; isolation rules decide whether concurrent changes cause waiting or failure.
At this point, record the state that changed and check the invariant before advancing. If the operation repeats, make clear which values persist and which are recomputed.
Step 4: Check conflicts and constraints
Isolation levels are named standards but implementations differ in details and anomalies. A transaction can be atomic while still making a logically stale decision if its reads and writes do not protect the invariant. Long-running snapshots can also delay cleanup of obsolete versions.
Step 5: Commit or abort the transaction
Two transactions each check that fewer than five reservations exist, then insert one. Explain why transaction boundaries alone may not prevent exceeding five and name a database mechanism that can enforce the invariant.
The trace is complete when the result satisfies the stated contract. Compare this model with the concrete runtime or system you are studying before making a performance claim.