How
Latches Differ From Locks
As was mentioned earlier in the chapter, latches differ from
locks within Oracle in terms of duration with respect to restricting
access to data structures for transactions and processes with the
database. Latches are short term in length of operation and locks
are long duration in restricting access to Oracle data structures.
As such, latches are lightweight serialization and locks are the
heavy duty long running serialization mechanism. However, latchesare strict compared to locks in terms of restricting
access to data structures within Oracle.
While locks permit shared and concurrent access, latches allow
access to only a single process at a time and prevent other
processes within Oracle from accessing that process while a latch is
held by the process. Latches affect only data structures within the
Oracle SGA, whereas locks apply to Oracle transactions. As such,
latches affect a more limited scope compared to locks. Latches also
fulfill a single purpose which is to grant exclusive access to
Oracle memory structures. Therefore, they control access to the SGA
memory data structures for single operations in contrast to locks
which serve dual purposes.
Unlike latches, locksand
enqueues function to allow multiple processes to share resources
when lock modes are compatible as well as to enforce data integrity
by restricting access to resources based on lock modes in place for
transactions. Latches are limited serialization mechanisms that are
non-transactional and only for memory resources in the SGA for
Oracle, as mentioned earlier, whereas locks are application driven
and control access to database transactions.
Latches also differ
from locks in that they can be requested in only two modes: no-wait
and willing-to-wait, while locks have six different request modes:
null, row share, row exclusive,
share row exclusive, share, or
exclusive.
Another key difference between latches and locks within Oracle
lies in terms of scope of process, memory and transaction
management. Latches are visible only to the local instance in memory
as opposed to locks which maintain information within the Oracle
database and are visible to all of the instances that have access to
the database. As such, locks operate at the database level versus
latches which are based at the instance level in Oracle memory.
Yet another difference between latches and locks can be explained
in how complex both mechanisms are within Oracle. Latches use simple
instructions such as simple CPU-based OS instructions and are based
on operating system implementations while locks are heavy weight
mechanisms that use a series of complex instructions in terms of
context switching.
In addition to the above, differences between latches and locks
can explain using duration of operation. Latches are simple,
short-lived mechanisms that last in microseconds versus longer held
running locks that can last for hours based on the lock mode and
transaction held by the lock in question.
Furthermore, locks operate based on a queuing methodology in that
for every Oracle process that fails to obtain a lock, the request
for the process is placed into a queue and serviced in order unless
NOWAIT is specified in the lock request. Latches, on the other hand,
do not use any type of queuing mechanism except for the latch wait
list latch which does use a queue as part of its algorithm. For
instance, when a specific process within Oracle falls into sleep
mode and fails to acquire a latch, the request will not be queued up
and serviced in order.
In other words, latches are on a first-come, first grab basis in
terms of allocation and management. Because of the way that latches
operate within Oracle, they are not affected by deadlocks. Locks, on
the other hand, are affected by deadlock conditions. Whenever a
deadlock occurs that involves a lock, Oracle generates a trace file
with the deadlock error message in the alert log file. Latch
waiters, on the other hand, use either timers to wakeup and retry or
will spin before retry access to the latch.
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