Memory Coherence vs Memory Consistency

Memory Coherence:

Lets say a guy named D has 2 friends X and Y. D messages X asking dinner on friday at 7:00 pm. X is happy and says yes. D then messages Y asking for dinner on friday at 7:00 pm. Y is cool with it, all set. Now Y discovers he has meeting on friday at 7:00 with his manager which he can't reschedule. So he messages D asking him to shift the dinner to 8:00 pm. D gets the text and he is cool. Now at this point D and Y are one the same page (same dinner time) but no one has informed X. This is what cache coherency is all about. It's about making sure that every private copy is the same everywhere and if any changes are made, those changes are propagated to every private cache.

Memory Consistency:

Here lets say D messages both X & Y about dinner on friday at 7:00 pm. A second later he realizes he has a meeting with his manager which he cant reschedule so he sents another message mentioning dinner at 8:00 pm. Now X gets the message in same order whereas Y gets the last message(dinner at 8:00 pm) first and then the first message(dinner at 7:00 pm). Now both X & Y have same information but in different order. This is memory consistency. It's not enough that a core sees the write happening to shared data, it's also important that they see the writes in the same order as other cores.