Database Reference
In-Depth Information
In the shared everything architecture, each server in the cluster is connected to a shared storage where the database
files are stored. It can be either active-passive or active-active. In the active-passive cluster architecture, at any given
time, only one server is actively accessing the database files and handling workloads; the second one is passive and in
standby. In the case of active server failure, the second server picks up the access to the database files and becomes the
active server, and user connections to the database also get failed over to the second server. This active-passive cluster
provides only availability, not scalability, as at any given time only one server is handling the workloads.
Examples of this type of cluster database include Microsoft SQL Server Cluster, Oracle Fail Safe, and Oracle
RAC One Node. Oracle RAC One Node, introduced in Oracle Database 11.2, allows the single-instance database to
be able to fail over to the other node in case of node failure. Since Oracle RAC One Node is based on the same Grid
Infrastructure as Oracle RAC Database, it can be converted from one node to the active-active Oracle RAC Database
with a couple of srvctl commands. Chapter 14 will discuss the details of Oracle RAC One Node.
In the active-active cluster architecture, all the servers in the cluster can actively access the database files and
handle workloads simultaneously. All database workloads are evenly distributed to all the servers. In case of one or
more server failures, the database connections and workloads on the failed servers get failed over to the rest of the
surviving servers. This active-active architecture implements database server virtualization by providing users with
a virtual database service. How many actual physical database servers are behind the virtual database service, and
how the workloads get distributed to these physical servers, is transparent to users. To make this architecture scalable,
adding or removing physical servers from the cluster is also transparent to users. Oracle RAC is the classic example of
the active-active shared everything database architecture.
RAC Architecture
Oracle Real Application Cluster (RAC) is an Oracle Database option, based on a share everything architecture. Oracle
RAC clusters multiple servers that then operate as a single system. In this cluster, each server actively accesses the
shared database and forms an active-active cluster configuration. Oracle first introduced this active-active cluster
database solution, called Oracle Parallel Server (OPS), in Oracle 6.2 on VAX/VMS. This name was used until 2001,
when Oracle released Oracle Real Application Clusters (RAC) in Oracle Database 9i. Oracle RAC supersedes OPS with
many significant enhancements including Oracle Clusterware and cache fusion technology.
In the Oracle RAC configuration, the database files are stored in shared storage, which every server in the cluster
shares access to. As shown in Figure
1-1
, the database runs across these servers by having one RAC database instance
on a server. A database instance consists of a collection of Oracle-related memory plus a set of database background
processes that run on the server. Unlike a single-node database, which is limited to one database instance per database,
a RAC database has one or more database instances per database and is also built to add additional database instances
easily. You can start with a single node or a small number of nodes as an initial configuration and scale out to more
nodes with no interruption to the application. All instances of a database share concurrent access to the database files.
User connections
Node1
Node2
Node3
RAC
Instance1
RAC
Instance2
RAC
Instance3
Cluster
Interconnect
RAC
Database
Figure 1-1.
Oracle RAC Database architecture
