Labs

Conflict grows with the square of concurrency (uniform versus skewed keys)

L concurrent transactions touching k random keys out of N: conflicting pairs against L²k²/2N, uniform and Zipf-skewed.

Phantoms and gap locks depend on the index

MySQL/InnoDB: SELECT ... FOR UPDATE on a price range, with and without an index on price; which inserts wait, and how many locks are held.

Reproduce a deadlock, then remove it by ordering locks

Two PostgreSQL sessions move money between the same two wallets in opposite orders, 20 times; then with a fixed lock order; then with deadlock_timeout = 100ms.

One old snapshot pins the version space

Two minutes of random updates on a million-row PostgreSQL table, without and with an old Repeatable Read snapshot open: size and dead tuples.

Pessimistic versus optimistic crossover

16 workers doing read-modify-write on a PostgreSQL counter table of K rows, with SELECT FOR UPDATE and with a version column: throughput and goodput from K = 1,000,000 to 1.

Write skew under snapshot isolation, and its fix

Two on-call agents each check that two are on shift and each go off shift: REPEATABLE READ against SERIALIZABLE; then 50 clients and the retry rate.

The idempotency key: the unknown outcome made harmless

A top-up whose acknowledgment is lost, retried naively and with an idempotency key in SQLite; then a replay after the key has expired.

Savepoint cliff in Postgres (more than 64 subtransactions)

A PostgreSQL transaction holding 0 to 200 subtransactions open while pgbench reads the same rows: read throughput across the 64-subtransaction cache.

Contended compare-and-swap: n²T/2 and the value of jitter

n writers each need one successful conditional write on a shared pointer: total time and wasted attempts with immediate retry and with jittered backoff.

Availability of unanimity: 0.999ⁿ versus a majority

Atomic commit needs every participant; consensus needs a majority: a^n against the binomial majority, then 10 million simulated rounds with and without rack failures.