What consistent really means

The balance and the activity list disagree

A transfer succeeds. The balance screen shows the debit, but a refreshed activity list does not. Later, a refresh shows an even older list. These are different failures. A product can need a current balance, read-your-writes on confirmation, and monotonic reads while paging history without demanding the same guarantee everywhere.

Separate time from transactions

Write on client A and read before replication. Read-your-writes carries the writer’s floor. Client B alternates primary and replica reads so monotonic reads have a regression to prevent.

Worked example: nobody is on call

Two doctors each decide to go off call after reading the same snapshot. The writes touch different rows. Watch what row-level conflict checks leave behind.

Partitions and ordinary days

On an ordinary day, stronger reads still pay for coordination, distance or waiting. During a partition, a node cannot both answer every request and guarantee the strongest view. Session guarantees can offer a narrower promise, such as seeing your own save. Define the required observation per operation and explain the degraded behavior instead of choosing one consistency label for the whole product.

A missing reply forces a choice

• Replica cannot catch up: The network may be slow or partitioned. A timeout cannot distinguish them.
• Wait or reject: Preserve the strong guarantee at the cost of availability.
• Serve an older value: Keep a weaker read path available and expose its freshness.

Key numbers

Write skew outcome

0 on call. Two snapshots see 2; each removes a different doctor

Remote coordination floor

≈ 90 ms. 10 ms local path + 80 ms required RTT

Session catch-up

≈ 600 versions. 300 writes/s × 2 s lag; a version token can require waiting

Check your reasoning

Think through these questions before opening the lesson. In the game you can test the idea and compare your reasoning.

• A balance transaction is serializable. What does that alone establish about a later read from an asynchronous replica?
• Why is saying our database is strongly consistent insufficient? Follow-up: choose guarantees for a reservation, save confirmation and public count.
• Both doctors leave under snapshot isolation after reading the other as on call. Which remedy protects the roster invariant?
• Why not make every read linearizable? Follow-up: we have users an ocean away.
• A partition isolates a region from the only seat-allocation authority. There are no preallocated regional rights. Which policy avoids overselling?