Example of Causal Consistency
Let’s consider a scenario where different processes( P1, P2, P3 and P4) try to do read/write operation on a variable x.
Scenario 1
The first process(P1) writes value a to x
, then the second process(P2) reads from x
(we suppose it reads value a), possibly performs some computation and then writes value b to x
again . These operations (write/write) are causally related, and hence their order should be the same for all processes.
Now, we have other processes(P3 and P4) who try to read from x
.
- The third process first reads a, then subsequently reads b. All is fine since the correct order is preserved.
- The fourth process first reads b, and then a. This is violation of causal consistency.
- Within a system with causal consistency guarantee,
P4
history is impossible, so there is violation of causal consistency in the system.
Scenario 2
If operations are not causally related( second process(P2) “write value b directly to x” , not like above scenario ” Read from x then write to x “), users can see different orders for them. In an image above, two processes write different values to x
, and since they’re independent, there is no order guarantee. Hence, there is no violation of causal consistency.
Causal Consistency Model in System Design
In distributed systems, ensuring consistency among replicas of data is a fundamental challenge. Traditional approaches to consistency, such as strong consistency, can impose significant performance overhead or limit the system’s availability. To address these challenges, researchers and practitioners have explored alternative consistency models, one of which is causal consistency.
Important Topics for the Causal Consistency Model in System Design
- What is the Importance of Data Consistency?
- What is Causal Consistency?
- Characteristics of Causal Consistency
- What is Causal Consistency Guarantee?
- Example of Causal Consistency
- Causal Relationships in Distributed Systems
- How does Causal Consistency work?
- Real-World Example of Causal Consistency
- Use-Cases and Applications of Causal Consistency
- Impact of Causal Consistency on (System Performance, Scalability, and Availability)
- Implementation of Causal Consistency
- Benefits of Causal Consistency
- Challenges of Causal Consistency