CRDTs

This package implements several CRDTs in a hopefully easy-to-use way.

Overview

This package includes several Conflict-free Replicated Data Types. See Verifying Strong Eventual Consistency in Distributed Systems by Gomes, Kleppmann, Mulligan, and Beresford and Verifying Strong Eventual Consistency in δ-CRDTs by Taylor Blau for more details. This package includes the following CRDTs (class names in parentheses):

• Counter (Counter)
• Positive-Negative Counter (PNCounter)
• Grow-only Set (GSet)
• Observed-Removed Set (ORSet)
• Fractionally-Indexed Array (FIArray)
• Replicated Growable Array (RGArray)
• Last-Writer-Wins Register (LWWRegister)
• Last-Writer-Wins Map (LWWMap)
• Multi-Value Register (MVRegister)
• Multi-Value Map (MVMap)
• Causal Tree (CausalTree)

These are implemented as delta-CRDTs with small update messages and additional methods for resynchronization to recover from dropped messages/transmission failures. See Efficient State-based CRDTs by Delta-Mutation for details.

For synchronization without a separate logical clock package, a simple Lamport ScalarClock class is included, though any logical clock that fulfills the simple ClockProtocol interface can be used. The StateUpdate class is provided as a default implementation of the StateUpdateProtocol interface. A handful of classes implementing the DataWrapperProtocol interface are included for use in the CRDTs that need them.

Everything in this package is designed to serialize and deserialize for reliable network transmission and/or persistence to disk.

Status

Each implementation must include a full test suite to be considered complete.

• Base Interfaces
• GSet
• ORSet
• Counter
• PNCounter
• RGArray
• LWWRegister
• LWWMap
• MVRegister
• MVMap
• FIArray
• CausalTree
• Decent documentation
• New CRDT: CounterSet
• New CRDT: Graph
• ListProtocol: RGArray, FIArray, CausalTree
• Refactor: change writer_id from int to SerializableType
• New CRDT: Document
• Hooks/event listeners

Setup and Usage

Requires python 3+.

Setup

To install, clone/unpack the repo.

Usage

Each CRDT follows the CRDTProtocol and includes the following methods:

• read(self) -> Any: produces the view of the data
• update(self, state_update: StateUpdateProtocol) -> CRDTProtocol: applies an update and returns self in monad pattern
• pack(self) -> bytes: serializes entire CRDT to bytes
• @classmethod unpack(cls, data: bytes) -> CRDTProtocol: deserializes a CRDT

Beyond this, each CRDT has its own specific methods unique to the type. Full documentation for each class in this library can be found in the docs.md file generated using autodox.

Documentation explaining how each CRDT works can be found here:

• GSet
• ORSet
• Counter
• PNCounter
• RGArray
• LWWRegister
• LWWMap
• MVRegister
• MVMap
• FIArray
• CausalTree

Each documentation file includes examples of how the CRDT can be used.

To use custom implementations of included interfaces, note that they must be injected properly. For a custom implementation of StateUpdateProtocol, the class will have to be passed to any CRDT method that produces StateUpdates by default by using the update_class= named parameter. For a custom implementation of DataWrapperProtocol, the relevant class must be provided to any calls to unpack when anything containing the custom class is deserialized, e.g. LWWMap.unpack(data, inject={'MyDataWrapper': MyDataWrapper}). The interfaces are autodox documented in interfaces.md.

Additionally, the functions serialize_part and deserialize_part can be used for serializing and deserializing complex structures to and from bytes. Any custom class implementing the PackableProtocol interface will be compatible with these functions and must be injected into deserialize_part, e.g. deserialize_part(data, inject={'MyPackableClass': MyPackableClass}).

Synchronization

Note that the checksums and history methods for every CRDT support timestamp constraints from_ts and until_ts. This allows for nodes to synchronize without having to ship all updates across the network, which is a primary advantage of δ-CRDTs. How to best implement synchronization to utilize this feature is left to the library consumer, but I would start with something like the following:

def make_ts_buckets(max_ts: int, max_bucket_size: int = 16) -> list[tuple[int, int]]:
    """Divides checksums and updates into buckets of form (from_ts, until_ts)
        based upon the max_ts and max_bucket_size.
    """
    if max_ts < max_bucket_size:
        return [(0, max_ts)]
    n_buckets = math.ceil(math.log2(1 + max_ts/max_bucket_size))
    bucket_size = math.ceil(max_ts / n_buckets)
    buckets = []
    for i in range(n_buckets):
        buckets.append((i*bucket_size, (i+1)*bucket_size))
    return buckets

def make_synchronization_dict(crdt: CRDTProtocol) -> dict[tuple[int, int], tuple[Any]]:
    buckets = make_ts_buckets(crdt.clock.read())
    return {
        b: crdt.checksums(from_ts=b[0], until_ts=b[1])
        for b in buckets
    }

def check_synchronization_dict(crdt: CRDTProtocol,
        sync: dict[tuple[int, int]], tuple[Any]) -> list[tuple[int, int]]:
    """Checks a CRDT against a synchronization dict. If the checksums differ for
        any timestamp bucket, include that timestamp bucket in the return list.
        The state updates for those buckets can then be requested from the
        remote replica.
    """
    different_buckets = []
    for bucket, chksms in sync:
        if crdt.checksums(from_ts=bucket[0], until_ts=bucket[1]) != chksms:
            different_buckets.append(bucket)
    return different_buckets

def make_synchronization_history(crdt: CRDTProtocol,
        different_buckets: list[tuple[int, int]]) -> list[StateUpdate]:
    """Returns all the StateUpdates requested for the given buckets."""
    history = []
    for b in different_buckets:
        history.extend(crdt.history(from_ts=b[0], until_ts=b[1]))
    return history

The above divides timestamps into dynamically-sized buckets no larger than the supplied max_bucket_size parameter and is meant to demonstrate the concept of synchronization via δ-states. The number of buckets will scale linearly with the age of the CRDT and the max_bucket_size parameter. It could be adapted to scale the number of buckets logarithmically with the age/state size of the CRDT by calculating the max_bucket_size parameter rather than hard-coding it. For example, bucket_sizer = lambda ts: math.ceil(ts/math.log2(ts+1)) which would be called with bucket_sizer(crdt.clock.read()); this will result in a number of buckets equal to the log base 2 of the max timestamp:

• ts=16, bucket_count=4
• ts=32, bucket_count=5
• ts=64, bucket_count=6
• ...
• ts=1024, bucket_count=10

Interfaces and Classes

Below are the interfaces and classes, along with attributes and methods. Note that any type that includes itself in a return signature indicates a jquery- style monad pattern.

Interfaces

• ClockProtocol(Protocol)
• CRDTProtocol(Protocol)
• DataWrapperProtocol(Protocol)
• StateUpdateProtocol(Protocol)

Type Alias

There is a type alias, SerializableType, used in several places. It is equal to the following: DataWrapperProtocol|int|float|str|bytes|bytearray|NoneType.

Classes

• NoneWrapper(DataWrapperProtocol)
• StateUpdate(StateUpdateProtocol)
• ScalarClock(ClockProtocol)
• StrWrapper(DataWrapperProtocol)
• BytesWrapper(StrWrapper)
• CTDataWrapper(DataWrapperProtocol)
• DecimalWrapper(StrWrapper)
• IntWrapper(DecimalWrapper)
• RGAItemWrapper(StrWrapper)
• NoneWrapper(DataWrapperProtocol)
• GSet(CRDTProtocol)
• Counter(CRDTProtocol)
• ORSet(CRDTProtocol)
• PNCounter(CRDTProtocol)
• RGArray (CRDTProtocol)
• LWWRegister(CRDTProtocol)
• LWWMap(CRDTProtocol)
• MVRegister(CRDTProtocol)
• MVMap(CRDTProtocol)
• FIArray(CRDTProtocol)

Tests

Clone the repository, then open a terminal in the root directory and run the following:

find ./tests -name test_*.py -exec python {} \;

Alternately, for non-POSIX systems, run the following:

python test_datawrappers.py
python test_scalarclock.py
python test_serialization.py
python test_stateupdate.py
python test_causaltree.py
python test_counter.py
python test_fiarray.py
python test_gset.py
python test_lwwmap.py
python test_lwwregister.py
python test_mvmap.py
python test_mvregister.py
python test_orset.py
python test_pncounter.py
python test_rgarray.py

The 233 tests demonstrate the intended (and actual) behavior of the classes, as well as some contrived examples of how they are used. Perusing the tests will be informative to anyone seeking to use this package.

ISC License

Copyleft (c) 2023 k98kurz

Permission to use, copy, modify, and/or distribute this software for any purpose with or without fee is hereby granted, provided that the above copyleft notice and this permission notice appear in all copies.

THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.