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<div class="sect1" lang="en" xml:lang="en">
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<div>
<div>
<h2 class="title" style="clear: both"><a id="permmessages"></a>Permanent Message Handling</h2>
</div>
</div>
<div></div>
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<p>
Messages received by a replica may be marked with an
special flag that indicates the message is permanent.
Custom replicated applications will receive notification of
this flag via the <tt class="literal">DB_REP_ISPERM</tt> return value
from the
method.
There is no hard requirement that a replication application look for, or
respond to, this return code. However, because robust replicated
applications typically do manage permanent messages, we introduce
the concept here.
</p>
<p>
A message is marked as being permanent if the message
affects transactional integrity. For example,
transaction commit messages are an example of a message
that is marked permanent. What the application does
about the permanent message is driven by the durability
guarantees required by the application.
</p>
<p>
For example, consider what the replication framework does when it
has permanent message handling turned on and a
transactional commit record is sent to the replicas.
First, the replicas must transactional-commit the data
modifications identified by the message. And then, upon
a successful commit, the replication framework sends the master a
message acknowledgment.
</p>
<p>
For the master (again, using the replication framework), things are a little more complicated than
simple message acknowledgment. Usually in a replicated
application, the master commits transactions
asynchronously; that is, the commit operation does not
block waiting for log data to be flushed to disk before
returning. So when a master is managing permanent
messages, it typically blocks the committing thread
immediately before <tt class="methodname">commit()</tt>
returns. The thread then waits for acknowledgments from
its replicas. If it receives enough acknowledgments, it
continues to operate as normal.
</p>
<p>
If the master does not
receive message acknowledgments — or, more likely, it does not receive
<span class="emphasis"><em>enough</em></span> acknowledgments — the
committing thread flushes its log data to disk and then
continues operations as normal. The master application can
do this because replicas that fail to handle a message, for
whatever reason, will eventually catch up to the master. So
by flushing the transaction logs to disk, the master is
ensuring that the data modifications have made it to
stable storage in one location (its own hard drive).
</p>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="permmessagenot"></a>When Not to Manage
Permanent Messages</h3>
</div>
</div>
<div></div>
</div>
<p>
There are two reasons why you might
choose to not implement permanent messages.
In part, these go to why you are using
replication in the first place.
</p>
<p>
One class of applications uses replication so that
the application can improve transaction
through-put. Essentially, the application chooses a
reduced transactional durability guarantee so as to
avoid the overhead forced by the disk I/O required
to flush transaction logs to disk. However, the
application can then regain that durability
guarantee to a certain degree by replicating the
commit to some number of replicas.
</p>
<p>
Using replication to improve an application's
transactional commit guarantee is called
<span class="emphasis"><em>replicating to the network.</em></span>
</p>
<p>
In extreme cases where performance is of critical
importance to the application, the master might
choose to both use asynchronous commits
<span class="emphasis"><em>and</em></span> decide not to wait for
message acknowledgments. In this case the master
is simply broadcasting its commit activities to its
replicas without waiting for any sort of a reply. An
application like this might also choose to use
something other than TCP/IP for its network
communications since that protocol involves a fair
amount of packet acknowledgment all on its own. Of
course, this sort of an application should also be
very sure about the reliability of both its network and
the machines that are hosting its replicas.
</p>
<p>
At the other end of the extreme, there is a
class of applications that use replication
purely to improve read performance. This sort
of application might choose to use synchronous
commits on the master because write
performance there is not of critical
performance. In any case, this kind of an
application might not care to know whether its
replicas have received and successfully handled
permanent messages because the primary storage
location is assumed to be on the master, not
the replicas.
</p>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="permmanage"></a>Managing Permanent Messages</h3>
</div>
</div>
<div></div>
</div>
<p>
With the exception of a rare breed of
replicated applications, most masters need some
view as to whether commits are occurring on
replicas as expected. At a minimum, this is because
masters will not flush their log buffers unless
they have reason to expect that permanent
messages have not been committed on the
replicas.
</p>
<p>
That said, it is important to remember that
managing permanent messages involves a fair amount
of network traffic. The messages must be sent to
the replicas and the replicas must then acknowledge
the message. This represents a performance overhead
that can be worsened by congested networks or
outright outages.
</p>
<p>
Therefore, when managing permanent messages, you
must first decide on how many of your replicas must
send acknowledgments before your master decides
that all is well and it can continue normal
operations. When making this decision, you could
decide that <span class="emphasis"><em>all</em></span> replicas must
send acknowledgments. But unless you have only one
or two replicas, or you are replicating over a very
fast and reliable network, this policy could prove
very harmful to your application's performance.
</p>
<p>
Therefore, a common strategy is to wait for an
acknowledgment from a simple majority of replicas.
This ensures that commit activity has occurred on
enough machines that you can be reliably certain
that data writes are preserved across your network.
</p>
<p>
Remember that replicas that do not acknowledge a
permanent message are not necessarily unable to
perform the commit; it might be that network
problems have simply resulted in a delay at the
replica. In any case, the underlying DB
replication code is written such that a replica that
falls behind the master will eventually take action
to catch up.
</p>
<p>
Depending on your application, it may be
possible for you to code your permanent message
handling such that acknowledgment must come
from only one or two replicas. This is a
particularly attractive strategy if you are
closely managing which machines are eligible to
become masters. Assuming that you have one or
two machines designated to be a master in the
event that the current master goes down, you
may only want to receive acknowledgments from
those specific machines.
</p>
<p>
Finally, beyond simple message acknowledgment, you
also need to implement an acknowledgment timeout
for your application. This timeout value is simply
meant to ensure that your master does not hang
indefinitely waiting for responses that will never
come because a machine or router is down.
</p>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="permimplement"></a>Implementing Permanent
Message Handling</h3>
</div>
</div>
<div></div>
</div>
<p>
How you implement permanent message handling
depends on which API you are using to implement
replication. If you are using the replication framework, then
permanent message handling is configured using
policies that you specify to the framework. In
this case, you can configure your application
to:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
Ignore permanent messages (the master
does not wait for acknowledgments).
</p>
</li>
<li>
<p>
Require acknowledgments from a
quorum. A quorum is reached when
acknowledgments are received from the
minimum number of electable
replicas needed to ensure that
the record remains durable if
an election is held.
</p>
<p>
The goal here is to be
absolutely sure the record is
durable. The master wants to
hear from enough electable
replicas that they have
committed the record so that if
an election is held, the master
knows the record will exist even
if a new master is selected.
</p>
<p>
This is the default policy.
</p>
</li>
<li>
<p>
Require an acknowledgment from at least one replica.
</p>
</li>
<li>
<p>
Require acknowledgments from
all replicas.
</p>
</li>
<li>
<p>
Require an acknowledgment from a
peer. (The replication framework allows you to
designate one environment as a peer of
another).
</p>
</li>
<li>
<p>
Require acknowledgments from
all peers.
</p>
</li>
</ul>
</div>
<p>
Note that the replication framework simply flushes its transaction
logs and moves on if a permanent message is not
sufficiently acknowledged.
</p>
<p>
For details on permanent message handling with the
replication framework, see <a href="fwrkpermmessage.html">Permanent Message Handling</a>.
</p>
<p>
If these policies are not sufficient for your
needs, or if you want your application to take more
corrective action than simply flushing log buffers
in the event of an unsuccessful commit, then you
must use write a custom replication implementation.
</p>
<p>
For custom replication implementation, messages are
sent from the master to its replica using a
<tt class="function">send()</tt> callback that you
implement. Note, however, that DB's replication
code automatically sets the permanent
flag for you where appropriate.
</p>
<p>
If the <tt class="function">send()</tt> callback returns with a
non-zero status, DB flushes the transaction log
buffers for you. Therefore, you must cause your
<tt class="function">send()</tt> callback to block waiting
for acknowledgments from your replicas.
As a part of implementing the
<tt class="function">send()</tt> callback, you implement
your permanent message handling policies. This
means that you identify how many replicas must
acknowledge the message before the callback can
return <tt class="literal">0</tt>. You must also
implement the acknowledgment timeout, if any.
</p>
<p>
Further, message acknowledgments are sent from the
replicas to the master using a communications
channel that you implement (the replication code
does not provide a channel for acknowledgments).
So implementing permanent messages means that when
you write your replication communications channel,
you must also write it in such a way as to also
handle permanent message acknowledgments.
</p>
<p>
For more information on implementing permanent
message handling using a custom replication layer,
see the <i class="citetitle">Berkeley DB Programmer's Reference Guide</i>.
</p>
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