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:

1. Consistency (C): Every read receives the most recent write or an error. All nodes see the same data at the same time.
2. Availability (A): Every request receives a (non-error) response, without the guarantee that it contains the most recent write.
3. 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).

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🛠️ Sruja Perspective: Documenting Guarantees

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

import { * } from 'sruja.ai/stdlib'


DataLayer = system "Data Layer" {
    UserDB = database "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"]
    }

    BillingDB = database "Billing Database" {
        technology "PostgreSQL"
        description "Single primary with synchronous replication to ensure strong consistency."
        tags ["CP-System", "Strong-Consistency"]
    }
}

view index {
include *
}