BPlusTree

A B+ tree is an m-ary tree with a variable but often large number of children per node. A B+ tree consists of a root, internal nodes and leaves. The root may be either a leaf or a node with two or more children.
The primary value of a B+ tree is in storing data for efficient retrieval in a block-oriented storage context — in particular, filesystems(In my repo, I use mmap.). This is primarily because unlike binary search trees, B+ trees have very high fanout (number of pointers to child nodes in a node, typically on the order of 100 or more), which reduces the number of I/O operations required to find an element in the tree.
In theory, if the size of the index node in B+ tree is close to the size of the disk block(eg.4k bytes page size in linux), a query operation needs to access the disk logb(N) times.

Feature

• Use mmap to read and write to disk.
• Use LRU to cache mapped blocks.

Benchmark

Magnitude	Put	Get	Delete
10K	72ms	28ms	52ms
100K	900ms	507ms	777ms
1000K	10333ms	4726ms	8154ms

The above data was tested on my 2013 macbook-pro with Intel Core i7 4 cores 2.3 GHz.
Each record has a value length of 100 bytes and I set cache size to 5MB. See also test.

Build

make && ./test

API

BPlusTree(const char* path);
void Put(const std::string& key, const std::string& value);
bool Delete(const std::string& key);
bool Get(const std::string& key, std::string& value) const;
std::vector<std::string> GetRange(const std::string& left, const std::string& right) const;
bool Empty() const;
size_t Size() const;

TODO List

• Support for variable key-value length.
• When Dealloc is executed, put block into reuse-pool.
• Defragment db file.
• Add WAL(Write Ahead Log).
• Data compression.

Reference

[1] https://en.wikipedia.org/wiki/B%2B_tree
[2] https://www.cnblogs.com/nullzx/p/8729425.html
[3] http://man7.org/linux/man-pages/man2/mmap.2.html