btree

A persistent copy-on-write B+Tree implementation, designed as an index for a key-value store, inspired by SQLite.

Design

Each BTree struct is associated with a file that contains its nodes in a predefined structure. The BTree API is implemented in a copy-on-write manner, that is, a copy of the newly written nodes is created on each write or delete without mutating the previous version of the tree. To keep track of the latest version of the tree we maintain a write-ahead-log to log the current root.

Unit tests serve as helpful examples of API usage.

On disk node structure

There are two NodeType variants - Internal and Leaf; Each variant has its own predefined structure on disk. A leaf node has the following structure:

| IS-ROOT 1-byte| NODE-TYPE 1-byte | PARENT OFFSET - 8 bytes | Number of pairs - 8 bytes |
| Key #0 - 10 bytes | Value #0 - 10 bytes | ...
| Key #N - 10 bytes | Value #N - 10 bytes |

While the structure of an internal node on disk is the following:

| IS-ROOT 1-byte | NODE-TYPE 1-byte | PARENT OFFSET - 8 bytes | Number of children - 8 bytes |
| Key #0 - 10 bytes | Key #2 - 10 bytes | ...
| Child Offset #0 - 8 bytes | Child offset #1 - 8 bytes | ...

Features

• Support all CRUD operations (read, write, delete).
• Support for crash recovery from disk.
• Support for varied length key-value pairs.
• Key compression.
• Garbage collection.

API

From disk to memory and back

Nodes are mapped to pages on disk with TryFrom methods implemented for easier de/serialization of nodes to pages and back.

let some_leaf = Node::new(
   NodeType::Leaf(vec![
         KeyValuePair::new("foo".to_string(), "bar".to_string()),
         KeyValuePair::new("lebron".to_string(), "james".to_string()),
         KeyValuePair::new("ariana".to_string(), "grande".to_string()),
   ]),
   true,
   None,
);

// Serialize data.
let page = Page::try_from(&some_leaf)?;
// Deserialize back the page.
let res = Node::try_from(page)?;

See tests at src/page.rs and src/node.rs for more information.

Writing and Reading key-value pairs.

// Initialize a new BTree;
// The BTree nodes are stored in file '/tmp/db' (created if does not exist)
// with parameter b=2.
 let mut btree = BTreeBuilder::new()
            .path(Path::new("/tmp/db"))
            .b_parameter(2)
            .build()?;

// Write some data.
btree.insert(KeyValuePair::new("a".to_string(), "shalom".to_string()))?;
btree.insert(KeyValuePair::new("b".to_string(), "hello".to_string()))?;
btree.insert(KeyValuePair::new("c".to_string(), "marhaba".to_string()))?;

// Read it back.
let mut kv = btree.search("b".to_string())?;
assert_eq!(kv.key, "b");
assert_eq!(kv.value, "hello");

kv = btree.search("c".to_string())?;
assert_eq!(kv.key, "c");
assert_eq!(kv.value, "marhaba");

Deleting key-value pairs.

// Initialize a new BTree.
let mut btree = BTreeBuilder::new()
      .path(Path::new("/tmp/db"))
      .b_parameter(2)
      .build()?;

// Write some data.
btree.insert(KeyValuePair::new("d".to_string(), "olah".to_string()))?;
btree.insert(KeyValuePair::new("e".to_string(), "salam".to_string()))?;
btree.insert(KeyValuePair::new("f".to_string(), "hallo".to_string()))?;
btree.insert(KeyValuePair::new("a".to_string(), "shalom".to_string()))?;
btree.insert(KeyValuePair::new("b".to_string(), "hello".to_string()))?;
btree.insert(KeyValuePair::new("c".to_string(), "marhaba".to_string()))?;

// Find the key.
let kv = btree.search("c".to_string())?;
assert_eq!(kv.key, "c");
assert_eq!(kv.value, "marhaba");

// Delete the key.
btree.delete(Key("c".to_string()))?;

// Sanity check.
let res = btree.search("c".to_string());
assert!(matches!(
      res,
      Err(Error::KeyNotFound)
));

License

MIT.