CAP Theorem

Consistency, availability, partition tolerance: choose two out of the three. But, network partitions are unavoidable. So it is more accurate to say CAP is either be consistent or available when network partitioned.

idempotency key

A strategy to resolve “double post” race conditions. POST requests to create resources are assigned some uniquely identifying key, which is used to constrain the creation of database records, either by application logic or a database constraint. For example,

POST /v1/reservations
{
    "hotel_id": 1234,
    ....
    "reservation_id": "fafb4-302a-344a"
}

the “reservation_id” field here can be used to ensure only one POST request to create a reservation is accepted.