Mateo Sanabria, Nicolás Cardozo

Syncordian: A Byzantine Fault Tolerant CRDT without Interleaving anomalies

BEFORE WE START

This is not a finish project, we are actively working on it.  Feel free to point mistakes or inconsistencies, peer review is always appreciated

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What is a CRDT?
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Interleaving Anomalies?
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Byzantine Fault Tolerant?
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Syncordian: A Byzantine Fault Tolerant CRDT without Interleaving anomalies

Les't remember some CS foundation

The CAP THEOREM

CAP: Any distributed data store can provide only two of the following three guarantees

Consistency
Every read receives the most recent write or an error
Availability
Every request receives a (non-error) response, without the guarantee that it contains the most recent write
Partition Tolerance

The system continues to operate despite an arbitrary number of messages being dropped (or delayed) by the network between nodes

For example, in the context of internet-scale distribution requires data replication across multiple geographic locations with low latency and high availability

Strong Consistency:

Requires coordination for executing operations.

Challenges arise in the presence of network partitions or failures.

Weak Consistency:

Allows temporary divergence among replicas.

Requires a mechanism for merging concurrent updates into a common state.

Eventual Consistency

Informally, eventual consistency means that replicas eventually reach the same final value if clients stop submitting updates:

Eventual Delivery: An update delivered at
some correct replica is eventually delivered to all correct replicas


Convergence: Correct replicas that have delivered the same updates eventually reach equivalent state

Some Eventual Consistency (EC) systems execute updates intermediately, only to later identify conflicts and roll back, resulting in resource wastage.

Resolving conflicts often necessitates a consensus among replicas to ensure consistent arbitration.

To address this, a stronger condition is needed to enhance efficiency and minimize resource wastage in EC systems.

Eventual Consistency

strong Eventual Consistency (SEC)

Strong Convergence: Correct replicas that have delivered the same updates have equivalent state

EventualConsitency \wedge StrongConvergence \\ \equiv SEC

Conflict-free Replicated Data Types (CRDT)

Is an abstract data type designed to be replicated at multiple nodes and exhibiting the following properties:

Any replica can be modified without coordinating with any other replicas.

When any two replicas have received the same set of updates.

Conflict-free Replicated Data Types (CRDT)

CRDTs rely on commutative operations to guarantee strong eventual consistency this avoids the need for synchronization, yielding high availability and low latency

Availability + Partition Tolerance + SEC

The 'best' case of application for a complex crdt

peer to peer COLLABORATIVE TExt editing

INTERLEAVING ANOMALIES

BYZANTINE FAULT

WHAT ABOUT SYNCORDIAN?

Context: peer to peer COLLABORATIVE text editing

P2P Network, all messages will eventually arrive.

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Multiple peers in the network.
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Each pear has its document' copy.
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Each honest peer can edit the document: Insert and Delete.

SYNCORDIAN

SYNCORDIAN: Document

A syncordian document is a list of line. A line of a document has:

(line_id) :: float -> allows to know the relative position of the line in the document.

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status :: :tombstone | :aura | :settled
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insertion_attempts :: integer -> Counter to keep track of the insertion attempts.

content :: String.t() -> The actual content of the line.

siganture :: String.t() -> A hash signature use to prevent Byzantine faults

SYNCORDIAN: Peer

A peer syncordian peer is composed of :

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A peer ID
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A syncordian document
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A vector clock
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Board Time!