1 unstable release

0.1.0 Sep 22, 2024

#430 in Data structures

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MIT license

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crdt-lite

1. Rust implementation of a generic CRDT.

This implementation includes:

  1. Logical Clock: To manage causality.
  2. CRDT Structure: Generic over key (K) and value (V) types.
  3. Record Management: Handling inserts, updates, deletes with tombstones.
  4. Conflict Resolution: Based on column versions, site IDs, and sequence numbers.
  5. Merging Mechanism: To synchronize state across nodes.
  6. Comprehensive Tests: Using Rust's #[test] framework to ensure correctness.

2. Detailed Explanation

2.1. LogicalClock

The LogicalClock maintains a monotonically increasing counter to preserve causality across events.

#[derive(Debug, Clone)]
struct LogicalClock {
    time: u64,
}

impl LogicalClock {
    fn new() -> Self { /* ... */ }
    fn tick(&mut self) -> u64 { /* ... */ }
    fn update(&mut self, received_time: u64) -> u64 { /* ... */ }
    fn current_time(&self) -> u64 { /* ... */ }
}
  • tick(): Increments the clock for each local event.
  • update(): Updates the clock based on incoming changes to maintain causality.
  • current_time(): Retrieves the current clock time.

2.2. ColumnVersion

ColumnVersion tracks the versioning information for each column within a record, enabling fine-grained conflict resolution.

#[derive(Debug, Clone, PartialEq, Eq)]
struct ColumnVersion {
    col_version: u64,
    db_version: u64,
    site_id: u64,
    seq: u64,
}

impl ColumnVersion {
    fn new(col_version: u64, db_version: u64, site_id: u64, seq: u64) -> Self { /* ... */ }
}
  • col_version: Number of times the column has been updated.
  • db_version: Logical clock time when the column was last updated.
  • site_id: Identifier of the node where the update originated.
  • seq: Sequence number for ordering updates from the same node.

2.3. Record

Record represents an individual record in the CRDT, containing field values and their corresponding version information.

#[derive(Debug, Clone)]
struct Record<V> {
    fields: HashMap<String, V>,
    column_versions: HashMap<String, ColumnVersion>,
}

impl<V> Record<V> {
    fn new(fields: HashMap<String, V>, column_versions: HashMap<String, ColumnVersion>) -> Self { /* ... */ }
}

2.4. CRDT

The CRDT struct manages the overall state, including data records, tombstones for deletions, and methods to manipulate and merge data.

#[derive(Debug, Clone)]
struct CRDT<K, V>
where
    K: Eq + Hash + Clone,
{
    node_id: u64,
    clock: LogicalClock,
    data: HashMap<K, Record<V>>,
    tombstones: HashSet<K>,
}
  • node_id: Unique identifier for the node (site).
  • clock: Instance of LogicalClock.
  • data: Stores records, keyed by their unique identifier.
  • tombstones: Tracks deleted records to prevent resurrection.

2.5. Methods

2.5.1. insert

Inserts a new record if it's not already tombstoned.

fn insert(&mut self, record_id: K, fields: HashMap<String, V>) { /* ... */ }
  • Checks: Prevents re-insertion of tombstoned records.
  • Initialization: Sets col_version to 1 for all columns.

2.5.2. update

Updates existing record fields if the record isn't tombstoned.

fn update(&mut self, record_id: &K, updates: HashMap<String, V>) { /* ... */ }
  • Checks: Ignores updates on tombstoned or non-existent records.
  • Versioning: Increments col_version and seq for updated columns.

2.5.3. delete

Deletes a record by marking it as tombstoned.

fn delete(&mut self, record_id: &K) { /* ... */ }
  • Checks: Prevents duplicate deletions.
  • Tombstone: Inserts into tombstones and removes from data.

2.5.4. get_changes_since

Retrieves all changes since a specified last_db_version.

fn get_changes_since(&self, last_db_version: u64) -> Vec<Change<K, V>> { /* ... */ }
  • Returns: A vector of Change structs representing the modifications.

2.5.5. merge_changes

Merges incoming changes into the CRDT, resolving conflicts based on versioning and site IDs.

fn merge_changes(&mut self, changes: &[Change<K, V>]) { /* ... */ }
  • Conflict Resolution:
    • Higher col_version: Accepts the change.
    • Equal col_version:
      • Deletion Prioritized: Deletions take precedence over insertions/updates.
      • Site ID & Seq: If both changes are deletions, use site_id and seq as tie-breakers.

2.5.6. print_data

Prints the current state for debugging.

fn print_data(&self) { /* ... */ }

2.6. Change Struct

Represents a single change in the CRDT.

#[derive(Debug, Clone)]
struct Change<K, V> {
    record_id: K,
    col_name: String,
    value: Option<V>,
    col_version: u64,
    db_version: u64,
    site_id: u64,
    seq: u64,
}
  • record_id: Identifier of the record.
  • col_name: Name of the column.
  • value: New value for the column (None for deletions).
  • col_version: Version of the column.
  • db_version: Logical clock time of the change.
  • site_id: Originating node's ID.
  • seq: Sequence number for ordering.

2.7. Tests

Comprehensive tests ensure the correctness of the CRDT implementation. They cover scenarios like:

  • Basic insertions and merges.
  • Concurrent updates with conflict resolution.
  • Deletions and tombstone handling.
  • Preventing re-insertion after deletion.
  • Logical clock updates.

Running Tests

  1. Ensure Rust is Installed: If not, install it from rustup.rs.

  2. Save the Code: Save the above code in a file named crdt.rs.

  3. Create a New Cargo Project:

    cargo new crdt_project
    cd crdt_project
    
  4. Replace src/lib.rs with crdt.rs: Alternatively, place crdt.rs in src/ and adjust accordingly.

  5. Add Dependencies: Add uuid to Cargo.toml for generating unique identifiers.

    [dependencies]
    uuid = { version = "1.2", features = ["v4"] }
    
  6. Run Tests:

    cargo test
    

All tests should pass, indicating that the CRDT behaves as expected.

License

This project is licensed under the MIT License - see the LICENSE file for details.

Dependencies

~0.7–1.2MB
~25K SLoC