In computer science, sequential consistency is a guarantee that all threads see a consistent view of shared memory. That is, if each thread can be thought of as a sequence of operations, then all threads will see those operations in that order.
This is in contrast to other models of concurrency, such as the happen-before relation, in which there is no guarantee of any particular order of operations.
Sequential consistency is often considered to be the strongest form of consistency, as it provides a guarantee that all threads will see the same sequence of operations. However, it is also often considered to be too strong a guarantee, as it can be difficult to achieve in practice. As such, weaker forms of consistency, such as weak consistency and eventual consistency, are often used in practice.
What are the different types of consistency?
There are four different types of consistency:
1. Sequential Consistency
2. Causal Consistency
3. PRAM Consistency
4. Eventual Consistency
What is the major difference between sequential and causal consistency? The major difference between sequential and causal consistency is that sequential consistency requires that all reads return the value written by the most recent write, while causal consistency only requires that reads return the value written by the most recent write that is "causally related" to the read.
What is linearizability and sequential consistency?
Linearizability is the guarantee that all operations on a shared data structure appear to happen in a single, linear sequence. In other words, if two threads of execution each invoke an operation on a shared data structure, and one operation precedes the other in the sequence of operations, then the first operation must also precede the second in the sequence of events seen by each thread.
Sequential consistency is a weaker guarantee than linearizability, but it is still useful in many cases. Sequential consistency guarantees that all operations on a shared data structure will appear to happen in the same order to all threads of execution. This guarantee is weaker than linearizability because it does not require that operations on the shared data structure appear to happen in a single, linear sequence. However, it is still useful in many cases because it guarantees that all threads will see the same sequence of operations, even if they are not executed in the same order.
What is sequential memory consistency? Sequential memory consistency is a model of computer memory that stipulates that, for a given processor, the order in which memory accesses (reads and writes) to different locations are seen by that processor is the same as the order in which the accesses are made by the program.
Is sequential consistency composable?
Yes, sequential consistency is composable. This means that if you have a system with multiple sequential consistency models, you can combine them into a single model.
The reason why sequential consistency is composable is because it is a property of the system, not of the individual components. In other words, it is a system-level property, not a component-level property.
This is in contrast to other properties, such as safety or liveness, which are usually properties of individual components. For example, a safe system is one where no individual component can cause the system to fail. A live system is one where all the components are always able to make progress.
Properties like safety and liveness are not composable, because they are properties of the individual components. If you have a system with multiple safety properties, you cannot combine them into a single property. Similarly, if you have a system with multiple liveness properties, you cannot combine them into a single property.
However, sequential consistency is a property of the system, not of the individual components. This means that you can have a system with multiple sequential consistency models, and you can combine them into a single model.
The most common way to do this is to use a technique called "consistency enforcement". With consistency enforcement, you take a system with multiple sequential consistency models and you add an extra component that enforces the consistency between the different models.