Database S-Lock and X-Lock, two-stage lock protocol

There are two basic types of blocking: Exclusive lock (x Lock) and shared lock (S-lock). The so-called X-Lock, which is the transaction T to data a plus x lock, allows only transaction T to read and modify data a,... The so-called S-lock, is the transaction T to data a plus s lock, the other transaction can only add to the data a plus s lock, and cannot add x lock, until T release S lock on A. If the transaction T has an S lock on the data object A, T can read a, but not update (S lock is therefore called a read lock), before T releases the S lock on a, other transactions can be a plus s lock, but cannot add X lock, so that can read a, but not update a.

Two-stage lock protocol:

The two-segment lock protocol for a database means that all transactions must lock and unlock data items in two stages

1. Expansion phase

Before you read or write any data item, apply and obtain a blockade of the data item.

2. Contraction phase

In each transaction, all blocking requests must precede the unlock request.

For example, transaction T follows the two-phase lock protocol, and its blocking protocol is:

BEGIN TRANSACTION;

LOCK (a); READ A; A: = a + 100; WRITE A; LOCK (B); UNLOCK (A); READ (b), UNLOCK (b),;

COMMIT;

It can be proved that any parallel scheduling policy for these parallel transactions is serializable if all concurrently executed transactions adhere to the two-phase lock protocol

It is necessary to note that if all the concurrently executed transactions comply with the two-phase lock protocol, it is only sufficient condition that the parallel scheduling strategy of these transactions can be serializable, not a requirement.

The difference between a two-phase locking protocol and a blocking method to prevent deadlocks:

A blocking method requires that a transaction must be locked at once for all data items to be used, otherwise it cannot continue running.

Obviously, a blocking protocol is consistent with two-phase locking protocols, but the two-phase lock protocol does not require that all data items need to be locked at once, so transactions that comply with the two-phase lock protocol may be deadlocked

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Database S-Lock and X-Lock, two-stage lock protocol