Detailed information about MobiLink synchronization is provided
in the MobiLink Getting Started, the MobiLink
– Client Administration, and the Mobilink –
Server Administration books. These books are available
online on the SQL Anywhere Product Manuals Web site
.

Data movement and synchronization

Data movement occurs when shared data is distributed over
multiple databases on multiple nodes and changes to data in one
database are applied to the corresponding data in other databases.
Data can be moved using replication or synchronization.

Data replication moves all transactions from one database
to another, whereas data synchronization moves only the net result
of transactions. Both techniques get their information by scanning
transaction log files, but synchronization uses only updated log
file segments instead of the full log file, making data movement
much faster and more efficient.

With synchronization, data is available locally and can be
modified without a connection to a server. MobiLink synchronization
uses a loose consistency model, which means that all changes are
synchronized with each site over time in a consistent manner, but
different sites might have different copies of data at any instant.
Only successful transactions are synchronized.

Consolidated and remote databases

The consolidated database, which can be any ODBC-compliant
database, such as SQL Anywhere,
Sybase Adaptive Server Enterprise, Oracle, IBM DB2 UDB, or Microsoft SQL Server, holds the master copy
of all the data.

The remote database contains a subset of the consolidated
data. Although MobiLink can synchronize SQL Anywhere
and UltraLite databases, for PowerBuilder applications, remote databases
must be SQL Anywhere databases.

The MobiLink synchronization server

The MobiLink synchronization server, mlsrv11,
manages the synchronization process and provides the interface between
remote databases and the consolidated database server. All communication
between the MobiLink synchronization server and the consolidated
database occurs through an ODBC connection.The consolidated database
and synchronization server often reside on the same machine, but
that is not a requirement.

The MobiLink server must be running before a synchronization
process is launched. You can start the MobiLink synchronization
server from the Utilities folder in the Objects view in the Database
painter.

For information about starting the server from the command
line, see “Running the MobiLink server” in the
online MobiLink – Server Administration book.

MobiLink hierarchy

MobiLink typically uses a hierarchical configuration. The
nodes in the hierarchy can reside on servers, desktop computers,
and handheld or embedded devices. A simple hierarchy might consist
of a consolidated database on a server and multiple remote databases
on mobile devices. A more complex hierarchy might contain multiple
levels in which some sites act as both remote and consolidated databases.
For PowerBuilder applications, any consolidated database that also
acts as a remote database must be a SQL Anywhere
database.

For example, suppose remote sites A1, A2, and A3 synchronize
with a consolidated database A on a local server, and remote sites
B1, B2, and B3 synchronize with a consolidated database B on another
local server. A and B in turn act as remote sites and synchronize
with a consolidated database C on a master server. C can be any
ODBC-compliant database, but A and B must both be SQL Anywhere databases.

Figure 13-1: MobiLink hierarchy

Synchronization scripts

MobiLink synchronization is an event-driven process. When
a MobiLink client initiates a synchronization, a number of synchronization
events occur inside the MobiLink server. When an event occurs, MobiLink
looks for a script to match the synchronization event. If you want
the MobiLink server to take an action, you must provide a script
for the event.

You can write synchronization scripts for connection-level
events and for events for each table in the remote database. You
save these scripts in the consolidated database.

You can write scripts using SQL,
Java, or .NET. For more information
about event scripts and writing them in the MobiLink Synchronization
plug-in in Sybase Central, see “Preparing consolidated databases”.

The MobiLink synchronization client

SQL Anywhere clients at
remote sites initiate synchronization by running a command–line
utility called dbmlsync. This utility synchronizes
one or more subscriptions in a remote database with the MobiLink
synchronization server. Subscriptions are described in “Publications, articles, users,
and subscriptions”. For more information
about the dbmlsync utility and its options, see “dbmlsync utility” in
the index of the SQL Anywhere
online books.

In PowerBuilder, synchronization objects that you create with
the ASA MobiLink Synchronization wizard manage the dbmlsync process.
For more information, see “Working with PowerBuilder
synchronization objects”.

Publications, articles, users,
and subscriptions

A publication is a database object on the remote database
that identifies tables and columns to be synchronized. Each publication
can contain one or more articles. An article is a database object
that represents a whole table, or a subset of the columns and rows
in a table.

A user is a database object in the remote database describing
a unique synchronization client. There is one MobiLink user name
for each remote database in the MobiLink system. The ml_user MobiLink
system table, located in the consolidated database, holds a list
of MobiLink user names. These names are used for authentication.

A subscription associates a user with one or more publications.
It specifies the synchronization protocol (such as TCP/IP,
HTTP, or HTTPS), address (such as myserver.acmetools.com),
and additional optional connection and extended options.

Users, publications, and subscriptions are created in the remote database.
You can create them in Sybase Central with the SQL Anywhere
plug-in (not the MobiLink Synchronization plug-in). For information
about creating users, publications, and subscriptions, see “Creating remote databases”.

The synchronization process

Dbmlsync connects to the remote database
using TCP/IP, HTTP, or HTTPS, and prepares a stream of
data (the upload stream) to be uploaded to the consolidated database. Dbmlsync uses
information contained in the transaction log of the remote database
to build the upload stream. The upload stream contains the MobiLink
user name and password, the version of synchronization scripts to use,
the last synchronization timestamp, the schema of tables and columns
in the publication, and the net result of all inserts, updates,
and deletes since the last synchronization.

After building the upload stream, dbmlsync uses
information stored in the specified publication and subscription
to connect to the MobiLink synchronization server and to exchange
data.

When the MobiLink synchronization server receives data, it
updates the consolidated database, then builds a download stream
that contains all relevant changes and sends it back to the remote
site. At the end of each successful synchronization, the consolidated
and remote databases are consistent. Either a whole transaction
is synchronized, or none of it is synchronized. This ensures transactional
integrity at each database.