This protocol is commonly implemented in the form of multiple-reader-single-writer sharing.
At any time, a data item may either be:

An item that is currently accessed in read-only mode can be copied indefinitely to other processes. When a process attempts to write to it, a multicast message is sent to all other copies to invalidate them, and this is acknowledged before the write can take place; the other processes are thereby prevented from reading stale data. Any processes attempting to access the data item are blocked if a writer exists. Eventually, control is transferred from the writing process and other accesses may take place once the update has been sent. The effect is to process all accesses to the item on a first-come-first-served basis. This scheme achieves sequential consistency.

Under the invalidation scheme, updates are only propagated when data are read, and several updates can take place before communication is necessary. Against this must be placed the cost of invalidating read-only copies before a write can occur. In the multiple-reader-single-write scheme described, this is potentially expensive. But, if the read/write ratio is sufficiently high, then the parallelism obtained by allowing multiple simultaneous readers offsets this cost. Where the read/write ratio is relatively small, a single-reader-single-writer scheme can be more appropriate: i.e, one in which at most one process may be granted read-only access at a time. 

                
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