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Replication
Database replication is the process of
sharing data between databases in
different locations.
Tables and/or fragments (replicas)
copied at different sites.
Users at different locations (nodes) can
work on their own replica and share, or
synchronize, their changes.
Redundancy
Creates redundancy which leads to a
processing overhead to ensure
consistency integrity constraints
applied.
Node failure may mean an integrity
check cannot be carried out so a user
may not be able to complete a
transaction at their node.
Transparency
Partitioning and Replication should be
transparent to users (except perhaps on
the failure of a node).
Mechanisms below the logical level
should deal with the fragments and
replicas.
DBMS should be capable of recombining
fragments when the required data
comes from more than one fragment.
The DBMS should be able to select a
suitable replicate when a query is made
on a replicated fragment.
The appropriate mechanism for this is a
distribution schema which complements
the normal 3-level architecture.
This would include a network directory
which holds
•Node locations where each fragment
stored.
•Relationships among the distributed
fragments and replicates.
•Which fragments are redundant
copies of other parts.
•The current status of data in the
network.
•Access controls
The Design Master-MS-Access
The database is converted into a Design
Master.
e.g. Microsoft Access
/Tools/ Replication/ Create Replica.
Microsoft Access then steps through the
process of creating a Design Master.
When a database is converted into a
Design Master, new tables are added to
the database.
Additionally, new fields are added to the
tables that exist in the database.
Replica Set
Replicas are created from the Design
Master
The Design Master and the replicas make
up a replica set.
There is only one Design Master in a
replica set.
The Design Master is the only member of
the replica set in which you can make
changes to the design of the database
objects.
Implementation using MS-Access
The simplest way to implement replication is through the
Microsoft Access user interface
•Design Masters can be created and members of
the replica set
•changes synchronized in any two members of the
replica set
•conflicts resolved in the data after synchronization
•the Design Master recovered if it is corrupted
A second way to implement replication is by using the
Briefcase that is installed with Windows
The Briefcase component enables you to create a
replica and synchronize changes between two replicas
in the set.
Replication can implemented through the
Microsoft Replication Manager
The Replication Manager uses a component
called the Microsoft Jet Synchronizer to
create replicas and manage replica sets.
The Replication Manager is required to use
some advanced features of replication, such
as indirect synchronization, Internet or
intranet synchronization, and using
synchronization schedules.
see
Microsoft Access 2000 Replication FAQ
Microsoft Access 97 Replication FAQ
Replicable vs Local Objects
Each member of the replica set contains
a common set of replicable objects, such
as tables, queries, forms, reports,
macros, or modules.
The Design Master or a replica can
contain both replicated and local objects
•objects that are replicated during
synchronization
•objects that remain on the local
computer and are not replicated.
This allows the Design Master or a
replica to be customized for a specific
group or user while still synchronizing
common data with other members in the
replica set.
Why Use Replication?-Examples
A contact-management application may monitor
sales and orders.
Each sales representative has a laptop computer
that can be connected to the company's network.
One approach would be to split the database and
have the tables reside in a back-end database on
a network server, or on the Internet or an intranet.
The queries, forms, reports, macros, and
modules reside in a separate front-end database
on the user's computer.
The objects in the front-end database are linked
to the tables in the back-end database.
When sales representatives want to retrieve or
update information in the database, they use the
front-end database.
This is then used at some later date to update the
back-end database on server.
If database replication used, a single database
contains both the data and the objects, and
replicas made of the database for each sales
representative.
Replicas made for each user and ‘synchronized’
with the Design Master on a network server i.e.
server and replica database updated so they are
consistent.
Sales representatives update the replicas on their
computers during the course of a work session,
and then synchronize their replicas with the hub
replica on the server as needed
Database replication can be used for:
Accessing data
A replica can be maintained of a corporate database
on a laptop computer.
Upon connecting to the corporate network, the
changes made to the replica on a laptop can be
synchronized with the changes in the corporate office
replica.
Distributing software
new tables, queries, forms, reports, macros, and
modules can be added to the Design Master, or you
can modify the existing objects.
The next time the Design Master is synchronized with
its replicas, the changes will be dispersed to the
replicas.
Backing up databases
You can automatically back up a database by
keeping a replica on a different computer.
Load balancing
A database can be replicated on additional network
servers and users reassigned to balance the loads
across those servers.
Users who need constant access to a database
can be given their own replica, thereby reducing
the total network traffic.
Internet or intranet replication
An Internet or intranet server can be configured to
be used as a hub for propagating changes to
participating replicas.
Why Prevent Replication?
If there are a large number of record updates at
multiple replicas
Applications that require frequent updates of existing
records in different replicas are likely to have more
record conflicts than applications that simply insert
new records in a database.
Applications with many record conflicts require more
administrative time because the conflicts must be
resolved manually.
In this scenario, it would probably not be a good idea
to use database replication.
Data consistency is critical at all times
Applications that rely on information being
correct at all times, such as funds transfer,
airline reservations, and the tracking of
package shipments, usually use a transaction
method.
While transactions can be processed within a
replica, there is no support for processing
transactions across replicas.
In this scenario, database replication would not
provide the desired results.
There is a need for the immediate dissemination
of updates to all users
Applications that are updated frequently are not
good candidates for database replication.
Users can't afford the overhead to the database
Some users may have limited space available on
their hard disks, and the creation of a replicable
database may use all available disk space.
Replication Scenarios
Three different replication scenarios:
1.
2.
3.
simple read-only replication (as illustrated in Figure
1),
replication to and from a mobile client (as illustrated
in Figure 2),
and multiple updates.
They may be used in a variety of architectures, for
systems that provide:
•
data distribution to a network of servers,
including those that are mobile or occasionally
connected (illustrated in scenarios 1 and 2)
•
data consolidation to a central server (scenarios
1 and 2)
•
process separation onto more than one server
(scenario 1)
•
information flow from one server to another
(scenario 3)
•
data sharing across multiple sites (scenario 3)
Distribution Architecture
These examples use the concept of a three-tier clientserver network which includes a central server (first
tier), workgroup servers (second tier) , and clients
(third tier).
In a real situation, there could be more than one
central server (including, for example, a data
warehouse server) and many workgroup servers.
A workgroup server might support part of an office
(such as a single department) or more than one office.
The network might include local and wide-area
connections.
Read-only Replication
With read-only replication, the data is entered and
stored on each workgroup server.
Only data relevant to each local workgroup is located
here.
The data is also replicated to the central server
(consolidated), so the central server will contain a
read-only copy of all data from all workgroups.
The data on the central server (logically stored in a
single table) has multiple sources (from
workgroup servers), but each individual record
only has one source.
Alternatives to this scenario, but essentially the same,
are:
1) the data is entered on the central server and
copied to the workgroup servers, where it is readonly; or
2) the data is entered on one workgroup server and
replicated to one or more other servers as readonly copies.
--An example of simple read-only replication.
Figure 1.
Mobile Data
Mobile computing has become much more
accessible in recent years, and in most
organizations some people work away from the
office.
There are now a number of methods for providing
data to a mobile workforce, one of which is
replication. In this case, the data is downloaded
on demand from a local workgroup server.
Updates to the workgroup or central data from the
mobile client, such as new customer or order
information, are handled in a similar manner.
--Replication to and from a mobile client.
Multiple Updates
Problems arise with the possibility of
two updates being made to the same
data on different servers.
The traditional solution is to designate
a single source server
Can work by allowing the administrator
to make some decisions about which
data is "correct."
Any conflicts are processed by a conflict-resolution
algorithm that usually chooses one of the following
methods:
Priority:
each server is given a unique priority; higher-priority
servers "win" over lower-priority servers
Timestamp:
the latest or earliest transaction in the conflict is
considered correct; if you choose to do nothing in a
conflict situation, the latest transaction wins
Data partitioning:
each row is guaranteed to be manipulated by only
one server; this simplifies the architecture to the first
scenario
Many other conflict-resolution or avoidance schemes
can be implemented by the various products.
Automatically Generated Keys
Another major problem with replication in general has
to do with automatically generated keys.
Most systems now use an automatically generated
surrogate key to uniquely identify a row of data.
When data for one table is created on a number of
servers, you need to ensure that the replicated rows
are unique.
There are three accepted ways to do this:
1.set up the key generators to use a predefined
number range, different for each server
2.add a server identifier to the primary key
3.replicate into separate tables and read all of the
data through a union view; to deal with the
potential of duplicate keys in the UNION, a
pseudocolumn would be used to represent the
source database
Publish and Subscribe
Microsoft uses a "publish-and-subscribe" metaphor:
The DBA makes the data on one database (the
publisher) available to the world, and then another
database (the subscriber) receives the replicated data.
In keeping with this metaphor, the machine that handles
the job of moving the data from publisher to subscriber is
called the distributor.
As with all distributed databases, the replication
environment must be designed carefully.
With Microsoft, you need to include the
following elements in your architecture:
•the data to be replicated (publications),
•the database carrying the source data
(publisher)
•the database receiving the replica
(subscriber)
•the distribution machine
•how you want the subscriber to use the
data (read or update)
•and how often you need the data to be
replicated.
The SQL Enterprise Manager is used to
designate the publisher database.
The publications are defined and the
subscribers selected.
This simple setup can be used to configure
complex replication architectures.
It is easy to use the basic tools to build up
a structure that can be very difficult to
maintain.
Any server can be both publisher and
subscriber (and distributor, for that matter),
and the same table can contain replicated
and source data!
SQL Enterprise Manager has a graphical view of your
replication network showing the topology as a network
You can move and place your publishers, subscribers,
and distributors on the workspace in any configuration
With this tool, it is relatively easy to set up complex
configurations, but the database is not always robust
enough to manage them correctly.
The DBA should think through the architecture very
carefully, because the system will let you do nearly
anything, whether you really want to or not.
Master Mirroring
This is the easiest form of replication to
implement.
There is a single master copy, and all
changes are made to it.
After changes have been made a backup
is made, and copies of the backup are
installed on the mirror machines.
This strategy is fairly robust: if the master
copy is lost, the mirrors can still operate,
and the master can be reconstructed
from the mirrors.
this model does not support distributed transactions.
Transaction processing can only be done on the
master, and thus the master is a critical component,
and is also a bottleneck.
The mirrors can be used only for viewing (reading).
by installing mirrors (which may be geographically
separated by thousands of miles) performance for
viewing is radically improved, since one machine does
not have to handle all the load.
simplicity.
Peer Mirroring
In this scheme, database updates can be performed on
multiple "masters".
Synchronization is achieved by having changes
propagated from one mirror to another through journal
logs.
Consistency should be verified by comparing the
contents of each of the peers.
The disadvantage of this system is complexity