In RAC, multiple Oracle instances access a shared
database cluster. The shared-everything architecture depends on a common data
store such as a storage area network (SAN).
Oracle supports many flexible replication options, from simple data-only one-way
replication to distributed multimaster replication. These solutions are very powerful
but also very complicated.
Microsoft SQL Server
Like Oracle, SQL Server has extensive features supporting both replication and clustering.
SQL Server even supports ???merge replication,??? which is essentially asynchronous
multimaster replication. Of course, both the clustering and replication options
require large amounts of configuration.
There is no out-of-the-box load-balancing solution for SQL Server yet; once you have
a replicated database, you still must write application code so as to direct requests to
the appropriate server.
LDAP
LDAP, the Lightweight Directory Access Protocol, is a database system optimized for
user directory information. It is most often used in large organizations, integrated
with the enterprise authentication and email systems. However, it is a database in its
own right. We do not have space to cover LDAPin detail, but there are many
resources available for working with LDAP in Rails.
ActiveLDAP
The ActiveLDAPlibrary (http://ruby-activeldap.rubyforge.org/) is an almost drop-in
replacement for ActiveRecord that uses LDAPinstead of an RDBMS as a backend.
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