Let me describe sequential and then strong consistency to highlight the difference.
Sequential consistency implies that there is some order without any timing guaranties. For example, let's say we have a system where nodes have some state machines and these machines process some events.
In a sequentially consistent system, all these events have the same processing order - A,B,C,D - hence state machines execute exactly same sequence. But if you ask a specific node about its state - you can get any of those, as there is no timing guarantee. From a readers point of view - different nodes return different results.
But if a reader reads from one singe node, in a sequential system, the reader will see states in the same correct order: A,B,C,D - again - there is no timing guarantee when it will happen, but if it will, the order is set.
Imagine a counter in a sequentially consistent system - let's say the counter counts visitors on a website. Clearly the counter is always increasing. If a reader will keep reading the counter from the same node, they will see increasing counter - the counter never moves in an opposite direction. But reads from different nodes may give any results.
Keeping in mind a visitor counter example; what will make that system strongly consistent? This should be true:
- after a counter increment happened
- all next reads will see the new value
It's the same as - there is a point in time, when all reads will see the new value.
There are some details about reads being executed before the write ended (writes and read happening at the same time), I'll leave that for another post.
Edit: I woke up with a though to clarify sequential consistency
Sequential consistency has SOME order - this order will be respected on each node. So if I send three signals to a sequentially consistent system: A, B and C; there will be SOME order for these events to be processed - ABC, BCA, CBA... But whatever order will happen, the order will be the SAME everywhere.
In a sequential system order of events may not match real life order of events. In strongly consistent system the order will be predictable - sending events one by one A,B,C will create a processing order of A, B, C