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<title>Locks, Blocks, and Deadlocks</title>
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<th colspan="3" align="center">Locks, Blocks, and Deadlocks</th>
</tr>
<tr>
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<th width="60%" align="center">Chapter 4. Concurrency</th>
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<div class="sect1" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h2 class="title" style="clear: both"><a id="blocking_deadlocks"></a>Locks, Blocks, and Deadlocks</h2>
</div>
</div>
</div>
<div class="toc">
<dl>
<dt>
<span class="sect2">
<a href="blocking_deadlocks.html#locks">Locks</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="blocking_deadlocks.html#blocks">Blocks</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="blocking_deadlocks.html#deadlocks">Deadlocks</a>
</span>
</dt>
</dl>
</div>
<p>
It is important to understand how locking works in a
concurrent application before continuing with a description of
the concurrency mechanisms JE makes available to you.
Blocking and deadlocking have important performance implications
for your application. Consequently, this section provides a
fundamental description of these concepts, and how they affect
JE operations.
</p>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="locks"></a>Locks</h3>
</div>
</div>
</div>
<p>
When one thread of control wants to obtain access to an
object, it requests a <span class="emphasis"><em>lock</em></span> for that
object. This lock is what allows JE to provide your
application with its transactional isolation guarantees by
ensuring that:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
no other thread of control can read that object (in
the case of an exclusive lock), and
</p>
</li>
<li>
<p>
no other thread of control can modify that object
(in the case of an exclusive or non-exclusive lock).
</p>
</li>
</ul>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="lockresources"></a>Lock Resources</h4>
</div>
</div>
</div>
<p>
When locking occurs, there are conceptually three resources
in use:
</p>
<div class="orderedlist">
<ol type="1">
<li>
<p>
The locker.
</p>
<p>
This is the thing that holds the lock. In a
transactional application, the locker is a
transaction handle.
<span>
For non-transactional operations, the locker is the current thread.
</span>
</p>
</li>
<li>
<p>
The lock.
</p>
<p>
This is the actual data structure that locks
the object. In JE, a locked
object structure in the lock manager
is representative of the object that
is locked.
</p>
</li>
<li>
<p>
The locked object.
</p>
<p>
The thing that your application
actually wants to lock.
In a JE
application, the locked object is usually a
<span>
database record.
</span>
</p>
</li>
</ol>
</div>
<p>
JE has not set a limit for the maximum number of
these resources you can use. Instead, you are only
limited by the amount of memory available to your
application.
</p>
<p>
The following figure shows a transaction handle,
<code class="literal">Txn A</code>, that is holding a lock on
database
<span>record</span>
<code class="literal">002</code>. In this graphic, <code class="literal">Txn
A</code> is the locker, and the locked object is
<span>record</span>
<code class="literal">002</code>. Only a single lock is in use
in this operation.
</p>
<div class="mediaobject">
<img src="simplelock.jpg" />
</div>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="locktypes"></a>Types of Locks</h4>
</div>
</div>
</div>
<p>
JE applications support both exclusive and
non-exclusive locks. <span class="emphasis"><em>Exclusive
locks</em></span> are granted when a
locker wants to write to an object. For this reason,
exclusive locks are also sometimes called
<span class="emphasis"><em>write locks</em></span>.
</p>
<p>
An exclusive lock prevents any other locker from
obtaining any sort of a lock on the object. This
provides isolation by ensuring that no other locker can
observe or modify an exclusively locked object until the locker is done
writing to that object.
</p>
<p>
<span class="emphasis"><em>Non-exclusive locks</em></span> are granted
for read-only access. For this reason, non-exclusive
locks are also sometimes called <span class="emphasis"><em>read
locks</em></span>. Since multiple lockers can
simultaneously hold read locks on the same
object, read locks are also
sometimes called <span class="emphasis"><em>shared locks</em></span>.
</p>
<p>
A non-exclusive lock prevents any other locker from
modifying the locked object while the locker is still
reading the object. This is how transactional cursors are able to
achieve repeatable reads; by default, the
cursor's transaction holds
a read lock on any object that the cursor has examined until
such a time as the transaction is committed
or aborted.
</p>
<p>
In the following figure, <code class="literal">Txn A</code> and
<code class="literal">Txn B</code> are both holding read locks on
<span>record</span>
<code class="literal">002</code>, while <code class="literal">Txn C</code>
is holding a write lock on
<span>record</span>
<code class="literal">003</code>:
</p>
<div class="mediaobject">
<img src="rwlocks1.jpg" />
</div>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="locklifetime"></a>Lock Lifetime</h4>
</div>
</div>
</div>
<p>
A locker holds its locks until such a time as it does
not need the lock any more. What this means is:
</p>
<div class="orderedlist">
<ol type="1">
<li>
<p>
A transaction holds any locks that it obtains
until the transaction is committed or aborted.
</p>
</li>
<li>
<p>
All non-transaction operations hold locks
until such a time as the operation is completed.
For cursor operations, the lock is held until the cursor is moved to a new position or
closed.
</p>
</li>
</ol>
</div>
</div>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="blocks"></a>Blocks</h3>
</div>
</div>
</div>
<p>
Simply put, a thread of control is blocked when it attempts
to obtain a lock, but that attempt is denied because some
other thread of control holds a conflicting lock.
Once blocked, the thread of control is temporarily unable
to make any forward progress until the requested lock is
obtained or the operation requesting the lock is
abandoned.
</p>
<p>
Be aware that when we talk about blocking, strictly
speaking the thread is not what is attempting to obtain the
lock. Rather, some object within the thread (such as a
cursor) is attempting to obtain the
lock. However, once a locker attempts to
obtain a lock, the entire thread of control must pause until the lock
request is in some way resolved.
</p>
<p>
For example, if <code class="literal">Txn A</code> holds a write lock (an exclusive
lock) on
<span>record</span>
002, then if <code class="literal">Txn B</code> tries to obtain a read <span class="emphasis"><em>or</em></span> write lock on
that
<span>record,</span>
the thread of control in which <code class="literal">Txn
B</code> is running
is blocked:
</p>
<div class="mediaobject">
<img src="writeblock.jpg" />
</div>
<p>
However, if <code class="literal">Txn A</code> only holds a read
lock (a shared lock) on
<span>record</span>
<code class="literal">002</code>, then only those handles that attempt to obtain a
write lock on that
<span>record</span>
will block.
</p>
<div class="mediaobject">
<img src="readblock.jpg" />
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="blockperformance"></a>Blocking and Application Performance</h4>
</div>
</div>
</div>
<p>
Multi-threaded
applications typically perform better than simple
single-threaded applications because the
application can perform one part of its workload
(updating
<span>a database record, </span>
for example) while it is waiting for some other
lengthy operation to complete (performing disk or
network I/O, for example). This performance
improvement is particularly noticeable if you use
hardware that offers multiple CPUs, because the threads
can run simultaneously.
</p>
<p>
That said, concurrent applications can see reduced
workload throughput if their threads of control are
seeing a large amount of lock contention. That is,
if threads are blocking on lock requests, then that
represents a performance penalty for your
application.
</p>
<p>
Consider once again the previous diagram of a blocked write lock request.
In that diagram, <code class="literal">Txn C</code> cannot
obtain its requested write lock because
<code class="literal">Txn A</code> and <code class="literal">Txn
B</code> are both already holding read locks on
the requested
<span>record.</span>
In this case, the thread in which
<code class="literal">Txn C</code> is running will pause until
such a time as <code class="literal">Txn C</code> either
obtains its write lock, or the operation
that is requesting the lock is abandoned.
The fact that <code class="literal">Txn
C</code>'s thread has temporarily halted all
forward progress represents a performance penalty
for your application.
</p>
<p>
Moreover, any read locks that are requested while
<code class="literal">Txn C</code> is waiting for its write
lock will also block until such a time as
<code class="literal">Txn C</code> has obtained and
subsequently released its write lock.
</p>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="blockavoidance"></a>Avoiding Blocks</h4>
</div>
</div>
</div>
<p>
Reducing lock contention is an important part of
performance tuning your concurrent JE
application. Applications that have multiple
threads of control obtaining exclusive (write)
locks are prone to contention issues. Moreover, as
you increase the numbers of lockers and as you
increase the time that a lock is held, you increase
the chances of your application seeing lock contention.
</p>
<p>
As you are designing your application, try to do
the following in order to reduce lock contention:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
Reduce the length of time your application
holds locks.
</p>
<p>
Shorter lived transactions will result in
shorter lock lifetimes, which will in turn
help to reduce lock contention.
</p>
<p>
In addition, by default transactional cursors hold read
locks until such a time as the transaction is completed.
For this reason, try to minimize the time you keep
transactional cursors opened, or reduce your isolation
levels – see below.
</p>
</li>
<li>
<p>
If possible, access heavily accessed (read
or write) items toward the end of the
transaction. This reduces the amount of
time that a heavily used
<span>
record
</span>
is locked by the transaction.
</p>
</li>
<li>
<p>
Reduce your application's isolation guarantees.
</p>
<p>
By reducing your isolation guarantees, you
reduce the situations in which a lock can
block another lock. Try using uncommitted reads
for your read operations in order to
prevent a read lock being blocked by a
write lock.
</p>
<p>
In addition, for cursors you can use degree
2 (read committed) isolation, which causes
the cursor to release its read locks as
soon as it is done reading the record (as
opposed to holding its read locks until the
transaction ends).
</p>
<p>
Be aware that reducing your
isolation guarantees can have
adverse consequences for your
application. Before deciding
to reduce your isolation, take
care to examine your
application's isolation
requirements.
For information on isolation
levels, see
<a class="xref" href="isolation.html" title="Isolation">Isolation</a>.
</p>
</li>
<li>
<p>
Consider your data access patterns.
</p>
<p>
Depending on the nature of your application,
this may be something that you can not
do anything about. However, if it is
possible to create your threads such that
they operate only on non-overlapping
portions of your database, then you can
reduce lock contention because your
threads will rarely (if ever) block on one another's
locks.
</p>
</li>
</ul>
</div>
</div>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="deadlocks"></a>Deadlocks</h3>
</div>
</div>
</div>
<p>
A deadlock occurs when two or more threads of control are
blocked, each waiting on a resource held by the other
thread. When this happens, there is no
possibility of the threads ever making forward progress
unless some outside agent takes action to break the
deadlock.
</p>
<p>
For example, if
<code class="literal">Txn A</code> is
blocked by <code class="literal">Txn B</code> at the same time
<code class="literal">Txn B</code> is blocked by <code class="literal">Txn
A</code> then the threads of control containing
<code class="literal">Txn A</code> and <code class="literal">Txn B</code> are
deadlocked; neither thread can make
any forward progress because neither thread will ever release the lock
that is blocking the other thread.
</p>
<div class="mediaobject">
<img src="deadlock.jpg" />
</div>
<p>
When two threads of control deadlock, the only
solution is to have a mechanism external to the two threads
capable of recognizing the deadlock and notifying at least
one thread that it is in a deadlock situation.
Once notified, a thread of
control must abandon the attempted operation in order to
resolve the deadlock.
<span>
JE is capable of notifying your application when it detects a deadlock. (For
JE, this is handled in the same way as any lock
conflict that a JE application might encounter.) See
<a class="xref" href="jelock.html#jedeadlock" title="Managing Deadlocks and other Lock Conflicts">Managing Deadlocks and other Lock Conflicts</a>
for more information.
</span>
</p>
<p>
Note that when one locker in a thread of control is blocked
waiting on a lock held by another locker in that same
thread of the control, the thread is said to be
<span class="emphasis"><em>self-deadlocked</em></span>.
</p>
<p>
Note that in JE, a self-deadlock can occur only if
two or more transactions (lockers) are used in the same
thread. A self-deadlock cannot occur for
non-transactional usage, because the thread is the
locker. However, even if you have only one locker per
thread, there is still the possibility of a deadlock
occurring with another thread of control (it just will
not be a self-deadlock), so you still must write code
that defends against deadlocks.
</p>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="deadlockavoidance"></a>Deadlock Avoidance</h4>
</div>
</div>
</div>
<p>
The things that you do to avoid lock contention also
help to reduce deadlocks (see <a class="xref" href="blocking_deadlocks.html#blockavoidance" title="Avoiding Blocks">Avoiding Blocks</a>).
<span>
Beyond that, you should also make sure all threads access data in the same order as all other
threads. So long as threads lock records in the same basic order, there is no possibility of a
deadlock (threads can still block, however).
</span>
</p>
<p>
Be aware that if you are using secondary
databases (indexes), then locking order is
different for reading and writing. For this
reason, if you are writing a concurrent
application and you are using secondary
databases, you should expect deadlocks.
</p>
</div>
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