mini_lsm/mini-lsm-book/src/week3-02-snapshot-read-part-1.md

Snapshot Read - Memtables and Timestamps

In this chapter, you will:

• Refactor your memtable/WAL to store multiple versions of a key.
• Implement the new engine write path to assign each key a timestamp.
• Make your compaction process aware of multi-version keys.
• Implement the new engine read path to return the latest version of a key.

During the refactor, you might need to change the signature of some functions from &self to self: &Arc<Self> as necessary.

Task 1: MemTable, Write-Ahead Log, and Read Path

Memtable store timestamp, change to scan, encode ts in wal

Task 2: Write Path

assign mvcc object, take write lock, increase ts by 1

Task 3: MVCC Compaction

keep all versions, split file, run merge iterator tests

Task 4: LSM Iterator

return the latest version

pass all tests except week 2 day 6

Test Your Understanding

• What is the difference of get in the MVCC engine and the engine you built in week 2?
• In week 2, you stop at the first memtable/level where a key is found when get. Can you do the same in the MVCC version?

We do not provide reference answers to the questions, and feel free to discuss about them in the Discord community.

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