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In this lesson,
we want to examine
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the concept of Real
Application Clusters, or RAC,
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in an Oracle database.
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RAC is a hot topic today
in the Oracle World
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because RAC is Oracle's
clustering solution.
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And the concept of
clustering involves
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taking multiple
computers and having them
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act in concert together as one.
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RAC is a pay-extra option
for the Oracle database.
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So you can have an Oracle
database Enterprise license
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and yet not be licensed to
run RAC, or Real Application
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Clusters.
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It is a pay-extra option.
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It is rather expensive.
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But that is because
it affords a company
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a lot of positive
possibilities when
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it comes to their database.
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First of all, it's a high
availability solution.
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Because in RAC the
software allows machines
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to work together, if any one
machine is taken down or fails
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or has to even be patched,
the other machines
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continue the work.
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So it's high availability
for a company's applications.
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The next is that it's
a scalability solution.
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So if a company
wants to grow, they
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have the option in Real
Application Clusters
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of simply adding another
machine to the cluster.
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So let's use an example for
why a company would want to use
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Real Application Clusters.
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Let's say that we
have a small company.
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We've started a company, and it
does online ordering for books.
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Initially, our customer
base, let's say,
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is about 100 customers.
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And we have an employee
base of about five people.
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In that situation,
we would probably
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need a fairly small
database server.
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We need it to store things
like basically our ordering,
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the different orders that
are coming into our company,
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and also maybe
employee information.
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So we settle on a
small to medium sized
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commercial hardware server,
put the Oracle database on it,
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and the company begins to grow.
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And eventually, the customer
base grows to about 10,000.
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And our small computer that we
had when our company started
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is no longer viable.
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So our only option--
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without RAC-- is to get
rid of the old server,
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buy a new server, maybe a 4U
higher end commercial server
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and run our Oracle
database on that.
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Well, then, our company
grows even more.
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And let's say our customer
base is about 250,000,
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a quarter million.
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Now our second server that
we had is inadequate as well.
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And so now, we have to go
out and buy another server.
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So without the
option of RAC, we can
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spend quite a bit of
money trying to keep pace
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with the growth of our company.
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But with RAC, we can simply
have a scalability solution
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that allows us to scale
our database system
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upwards by simply adding
machines to the cluster.
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So to differentiate RAC from
the more common single instance
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architecture, let's
look a little bit
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about how that works.
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So in a single
instance architecture,
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the database operates
entirely on a single machine.
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So this is a machine.
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And it's composed
of the instance,
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which is the background
processes and memory
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caches, and the disk.
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So the processes
run on this server.
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The caches are stored in
RAM that's on this server.
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And the disk is on
this server as well.
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So this is a single instance
architecture-- very common.
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Works fine for
many applications.
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But it doesn't operate
as a scalability solution
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or a high availability solution.
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If this machine's power system,
for instance, were to go out,
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your data is inaccessible--
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processes and caches
being the instance,
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and the disk being the database.
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But RAC is a multi-instance
architecture.
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So in RAC, we may have
three physical machines.
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They are three different
physical machines.
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They each have their own
instance, their processes
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and caches.
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But the disk, or the database,
portion of the architecture
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is separate.
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So we might actually have
a separate disk unit.
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And the most important
thing about the disk
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is that it's shared
between all three machines.
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So the disk has to be
available to every one
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of these separate machines.
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And shared disks can be done
a number of different ways,
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like storage area
networks or network
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attached storage, a NAS.
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And it also has to have a
network-aware file system,
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or a global file system--
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something like NFS, the
Network File System, or ASM
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in Oracle, Automatic
Storage Management.
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So there is a separation between
the instance and the database.
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So each physical machine will
be connected to that storage.
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But that alone isn't enough.
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Because in RAC, one of the
advances that came out in 9i
8304
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