Log Structured Merge B+ Tree (LSMBT)

This an implementation of two different data structures:

• Log Structured Merge Tree
• B+Tree

The implementation also leverages a write-ahead log to ensure that data is not lost.

Basic Architecture

When you create a LSMBT 2 files are created: a blank B+ Tree file, and a blank WAL file. An in-memory BTreeMap is also constructed. Each method of the LSMBT is outlined below

Insert (key,value)

When a (key,value) pair is added to the LSMBT the following occurs:

1. The (key,value) pair is written to the WAL file.
2. The (key,value) pair is added to the in-memory BTree. If the size of the in-memory BTree hits a particular threshold, then
3. The in-memory BTree is merged with the on-disk B+Tree to create a new on-disk B+Tree.
4. The in-memory BTree and the WAL file are both truncated.

Get Values

Because a key can be associated with a set (no duplicate values per key) of values, the get method returns a list of values:

1. Collect all of the values associated with a given key in the in-memory BTree.
2. Collect all of the values associated with a given key in the on-disk B+Tree.
3. Return all the unique values

Delete Value

Again, because a key can be associated with a set of values, the value to be removed must be supplied during a delete:

1. Remove the value from the in-memory BTree. If it is the only value associated with the key, then remove the key as well.
2. Mark the value in the on-disk B+Tree as deleted. (The value isn't actually removed until a compaction occurs.)