Consistency ensures that a transaction can only bring the database
from one valid state to another, maintaining database invariants:
any data written to the database must be valid according to all
defined rules, including constraints, cascades, triggers, and any
combination thereof.
Isolation ensures that concurrent execution of
transactions leaves the database in the same state that would have
been obtained if the transactions were executed sequentially.
Read committed is an isolation level that
guarantees that any data read is committed at the moment it is
read. It simply restricts the reader from seeing
any intermediate, uncommitted, 'dirty' read.
This level emulates serial transaction execution
for all committed transactions; as if transactions
had been executed one after another, serially,
rather than concurrently.
A transaction re-reads data it has previously read
and finds that data has been modified by another
transaction (that committed since the initial read).
A transaction re-executes a query returning a set of
rows that satisfy a search condition and finds that
the set of rows satisfying the condition has changed
due to another recently-committed transaction.
Multiversion concurrency control (MCC or MVCC), is a
concurrency control method commonly used by database
management systems to provide concurrent access to
the database.
A deadlock is a state in which each member of a group
is waiting for another member, including itself, to
take action, such as sending a message or more
commonly releasing a lock.