Lesson 4: CAP Theorem & Consistency#

The CAP Theorem#

Proposed by Eric Brewer, the CAP theorem states that a distributed data store can only provide two of the following three guarantees:

Consistency (C): Every read receives the most recent write or an error. All nodes see the same data at the same time.
Availability (A): Every request receives a (non-error) response, without the guarantee that it contains the most recent write.
Partition Tolerance (P): The system continues to operate despite an arbitrary number of messages being dropped or delayed by the network between nodes.

The Reality: P is Mandatory#

In a distributed system, network partitions (P) are inevitable. Therefore, you must choose between Consistency (CP) and Availability (AP) when a partition occurs.

CP (Consistency + Partition Tolerance): Wait for data to sync. If a node is unreachable, return an error. (e.g., Banking systems).
AP (Availability + Partition Tolerance): Return the most recent version of data available, even if it might be stale. (e.g., Social media feeds).

Consistency Models#

Strong Consistency: Once a write is confirmed, all subsequent reads see that value.
Eventual Consistency: If no new updates are made, eventually all accesses will return the last updated value. (Common in AP systems).

🛠️ Sruja Perspective: Documenting Guarantees#

When defining data stores in Sruja, it is helpful to document their consistency guarantees, especially for distributed databases.

architecture "Global User Store" {
    system DataLayer "Data Layer" {
        container UserDB "User Database" {
            technology "Cassandra"
            // Explicitly stating the consistency model
            description "configured with replication factor 3. Uses eventual consistency for high availability."
            
            // You could also use custom tags
            tags ["AP-System", "Eventual-Consistency"]
        }
        
        container BillingDB "Billing Database" {
            technology "PostgreSQL"
            description "Single primary with synchronous replication to ensure strong consistency."
            tags ["CP-System", "Strong-Consistency"]
        }
    }
}