lattice_crdt - v1.0.0

lattice/lww_map

A last-writer-wins map (LWW-Map) CRDT.

Each key maps to a value and a timestamp. On conflict, the entry with the higher timestamp wins. Removal is timestamp-based (tombstone): a remove at timestamp T beats any set at timestamp < T. Keys are strings; values are strings.

Example

import lattice/lww_map

let a = lww_map.new() |> lww_map.set("name", "Alice", 1)
let b = lww_map.new() |> lww_map.set("name", "Bob", 2)
let merged = lww_map.merge(a, b)
lww_map.get(merged, "name")  // -> Ok("Bob")

Types

LWWMap

</>

A Last-Writer-Wins Map (LWW-Map) CRDT.

Internally stores each key with an Option(String) value and an Int timestamp. A None value represents a tombstone (removed key). On merge, the entry with the higher timestamp wins for each key; on ties, the first argument’s entry is kept as a consistent tiebreak.

pub type LWWMap {
  LWWMap(
    entries: dict.Dict(String, #(option.Option(String), Int)),
  )
}

Constructors

Values

from_json

</> 
pub fn from_json(
  json_string: String,
) -> Result(LWWMap, json.DecodeError)

Decode a LWWMap from a JSON string produced by to_json.

Returns Error if the string is not valid JSON or does not match the expected format.

get

</> 
pub fn get(map: LWWMap, key: String) -> Result(String, Nil)

Get the value for a key.

Returns Ok(value) if the key exists and is not tombstoned. Returns Error(Nil) if the key is missing or has been removed.

keys

</> 
pub fn keys(map: LWWMap) -> List(String)

Return all active (non-tombstoned) keys in the map.

Order is not guaranteed.

merge

</> 
pub fn merge(a: LWWMap, b: LWWMap) -> LWWMap

Merge two LWW-Maps by resolving each key using the highest timestamp.

Tombstones participate in merge: if a tombstone has a higher timestamp than the active entry for a key, the key remains removed after merging. On equal timestamps, the first argument’s entry wins (consistent tiebreak).

Merge is commutative, associative, and idempotent (a valid CRDT join).

new

</> 
pub fn new() -> LWWMap

Create a new empty LWW-Map.

remove

</> 
pub fn remove(map: LWWMap, key: String, timestamp: Int) -> LWWMap

Remove a key at the given timestamp by inserting a tombstone.

If the key already has an entry with an equal or higher timestamp, the remove is rejected and the existing entry wins.

set

</> 
pub fn set(
  map: LWWMap,
  key: String,
  value: String,
  timestamp: Int,
) -> LWWMap

Set a key to a value at the given timestamp.

If the key already has an entry with an equal or higher timestamp, the existing entry is kept (LWW semantics: strictly greater timestamp wins).

to_json

</> 
pub fn to_json(map: LWWMap) -> json.Json

Encode a LWWMap as a self-describing JSON value.

Entries are encoded as a JSON array where each element has key, value (nullable string for tombstones), and timestamp fields.

Format: {"type": "lww_map", "v": 1, "state": {"entries": [...]}}

The encoded value can be restored with from_json.

values

</> 
pub fn values(map: LWWMap) -> List(String)

Return all active (non-tombstoned) values in the map.

Order is not guaranteed and does not correspond to the order of keys.

✨ Search Document