Replication(cont.)

Consistency models

Consistency model is a contract between a (distributed) data store and processes, in which the data store specifies precisely what the results of read and write operations are in the presence of concurrency.

A data store is a distributed collection of storages:

Some notations

Read and write operations

• $W_i(x)a$: Process $P_i$ writes value a to x
• $R_i(x)b$: Process $P_i$ **reads value b from x

Assume that time axis is drawn horizontally, with time increasing from left to right and all data items have value NIL.

Strong (strict) consistency

Formal definition

<aside> ⛔ Any read on a data item ‘x’ returns a value corresponding to the result of the most recent write on ‘x’ (regardless of where the write occurred).

</aside>

Examples:

FIFO Consistency

FIFO consistency writes done by a single process are seen by all other processes in the order in which they were issued, but writes from different processes may be seen in a different order by different processes.

Causal Consistency