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Concept of the day · distributed systems
Read-Your-Writes Consistency
The weakest guarantee that stops a user seeing their own edit vanish.
The mechanism. After a session performs a write, every subsequent read in that session must reflect it. Other sessions may still see the old value. This is a session guarantee, not a global one, which is what makes it cheap.
How it is implemented. Three common ways: route the session to the primary for a short window after a write; have the write return a log position (an LSN, a timestamp, a token) and require later reads to wait until the replica has caught up to it; or read from the primary for just the records the session touched.
Where it bites. The classic bug: user edits their avatar, the write goes to the primary, the page reloads, the read hits a replica 200ms behind, and the old avatar appears. The user concludes the save failed. Note that caching makes this worse, because a CDN or an application cache can hold the stale value far longer than replication lag.
Takeaway. Pure eventual consistency is almost never acceptable for a user's view of their own data. Read-your-writes is the floor, and it is usually one token away.