In-memory database: values outlive their TTL, prefix scans match mid-word and history keeps deleted values

Context

Welcome to the platform team. The in-memory database in database/ implements all four levels described in README.md: records, scans, values that expire, and point-in-time reads. Since the last release, four reports have come in. Two of them already have failing tests in tests/visible/.

Fix as much as you can in 40 minutes. The checks are graded one by one, so each report you fix counts even if the rest is not done.

MDB-11: Values outlive their TTL by one tick

Report

A session token written with set_with_ttl(100, "session:42", "token", 7, 30) is still returned by get at timestamp 130. By the rules it is gone at 130. Scans and get_when show it at its expiry time too.

Acceptance criteria

• A value set with a TTL is gone at exactly timestamp + ttl for get, scan, scan_by_prefix and get_when.

MDB-12: Prefix scans match in the middle of a name

Report

For a key with the fields age, agent and page, scan_by_prefix(..., "ag") also returns page(3).

Acceptance criteria

• scan_by_prefix keeps only the fields whose name starts with the prefix.

MDB-13: Compare calls act on expired values

Report

A lock written with a TTL had long expired, yet compare_and_set with its old value returned True and wrote a new value. compare_and_delete deletes expired values the same way.

Acceptance criteria

• compare_and_set, compare_and_set_with_ttl and compare_and_delete return False and change nothing when the value has expired.

MDB-14: get_when still shows deleted values

Report

After compare_and_delete removes a field at timestamp 50, get_when(..., at_timestamp=60) still returns the old value.

Acceptance criteria

• From the moment a field is deleted, get_when reads it as None until it is set again.

Notes

• Run pytest tests/visible. Four tests fail until MDB-11 and MDB-14 are fixed.
• Practice task modelled on candidate reports of Meta's four-level in-memory database online assessment. Original StackYard code, not the actual question; not affiliated with Meta.