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Chapter 3: CRDT Basics

Overview​

Conflict-Free Replicated Data Types (CRDTs) are data structures designed to work in distributed environments where replicas can be updated independently and concurrently.

Core Principles​

1. Commutativity​

Operations in CRDTs must commute, meaning the order in which they are applied doesn't affect the final result.

Example: Adding items to a set

  • Replica A adds item "X", then "Y"
  • Replica B adds item "Y", then "X"
  • Both replicas end up with the same set: Y

2. Idempotency​

Operations can be applied multiple times without changing the result beyond the first application.

Example: Setting a value

  • Setting a register to "5" multiple times still results in "5"
  • The operation is safe to replay during synchronization

3. Convergence​

All replicas will eventually reach the same state, regardless of the order of operations or network delays.

CRDT Categories​

State-Based CRDTs (CvRDTs)​

Also known as Convergent Replicated Data Types, these CRDTs merge states directly.

Characteristics:

  • Replicas exchange their complete state
  • Merge function combines states deterministically
  • Simpler to implement but larger message sizes

Examples:

  • G-Counter (Grow-only Counter)
  • PN-Counter (Positive-Negative Counter)
  • G-Set (Grow-only Set)

Operation-Based CRDTs (CmRDTs)​

Also known as Commutative Replicated Data Types, these CRDTs exchange operations.

Characteristics:

  • Replicas exchange operations/commands
  • Operations must commute and be idempotent
  • Smaller message sizes but more complex implementation

Examples:

  • LWW-Register (Last-Write-Wins Register)
  • Add-Wins Set
  • Remove-Wins Set

Basic CRDT Types​

Counters​

G-Counter (Grow-only Counter)

  • Only supports increment operations
  • Each replica maintains a vector of increments
  • Final value is the sum of all increments

PN-Counter (Positive-Negative Counter)

  • Supports both increment and decrement
  • Maintains separate positive and negative counts
  • Final value is positive sum minus negative sum

Sets​

G-Set (Grow-only Set)

  • Only supports add operations
  • Once an element is added, it cannot be removed
  • Final set is the union of all added elements

2P-Set (Two-Phase Set)

  • Supports both add and remove operations
  • Removed elements cannot be re-added
  • Final set is added elements minus removed elements

Implementation Considerations​

Message Delivery​

CRDTs require reliable message delivery, but not necessarily ordered delivery. Messages can be:

  • At-least-once: Messages may be delivered multiple times
  • At-most-once: Messages are delivered at most once
  • Exactly-once: Messages are delivered exactly once

Vector Clocks​

Vector clocks help track causality between operations and ensure proper ordering when needed.

Next Steps​

In the next section, we'll explore practical implementations of these CRDT types with code examples.