7 min read
Write Skew: The Anomaly Snapshot Isolation Does Not Catch
Two transactions each read what the other writes, both commit, and an invariant dies. Snapshot isolation allows write skew — here is how to close the hole.
Theme
System Design
Deep dives into designing scalable, reliable systems — URL shorteners, chat apps, rate limiters, caching, and more.
92 articles
3 min read
Designing a Shopping Cart: Price Snapshots, Merge, and No Inventory Hold
A cart is a per-user document, not a reservation. What to store on the line, how to merge an anonymous cart at login, and when the price is allowed to change.
3 min read
Designing Snowflake-Style IDs: Ordering, Workers, and Clock Rollback
A 64-bit id from a timestamp, a worker number, and a per-millisecond sequence. How many you can issue, why the clock must not step backwards, and why JavaScript cannot hold one.
5 min read
PostgreSQL 19 on October 29: WAIT for Read-Your-Writes, REPACK Without the Lock, and What Got Pulled
PostgreSQL 19 GA is scheduled for October 29, 2026. The WAIT command gives standbys read-your-writes, REPACK CONCURRENTLY replaces VACUUM FULL and CLUSTER, autovacuum goes parallel, and SQL/PGQ, online checksums, and FOR PORTION OF were reverted in Beta 4.
3 min read
Idempotency Keys for Write APIs
How to make POST safe to retry: the key, the stored response, the race between two identical requests, and the expiry that forgets too soon.
3 min read
Read-Your-Writes: The User Just Saved and the Read Replica Does Not Know
Why a write to the primary followed by a read from a replica shows stale data, and the practical ways to pin the next read without sending all traffic to the primary.
4 min read
Designing Chat Presence: Online, Away, and the Lie of Instant Status
Heartbeat vs subscriptions, fan-out of presence, privacy, and why a boolean online flag does not survive a million concurrent sockets.
5 min read
Consistency Models Beyond CAP: Linearizability, Causal, and Session Guarantees
'Strong vs eventual' is a cartoon. The real spectrum — linearizable, sequential, causal, session guarantees — and how to pick per operation, not per system.
5 min read
Designing an API Gateway: The Front Door as a System
Authentication, routing, rate limits, transformations, and a plugin chain — in a tier that must add ~1ms and never be the outage. Envoy/Kong architecture from scratch.
5 min read
Designing a CDC Pipeline: Change Data Capture from Binlog to Downstream
Dual writes are a lie; the database's own log is the truth. Log-based CDC, the snapshot-plus-stream handoff, schema evolution, and ordering guarantees that survive resharding.
5 min read
Designing a Content Moderation Pipeline: Trust and Safety at Platform Scale
A billion uploads a day, a legal requirement to act in hours, and irreversible mistakes in both directions: hash matching, classifier tiers, human review queues, and appeals.
5 min read
Designing a Distributed Rate Limiter: Local Buckets, Global Truth
One user, twenty gateway nodes, one limit of 100 req/s. Where does the counter live? Redis+Lua atomicity, the local-cache compromise, and failing open vs closed.