Each SQL Anywhere synchronization technology lends itself to particular applications. The following descriptions differentiate between the technologies and let you select the one best suited to your needs.

You should consider which of the following elements are important in your application:

Your consolidated database system

In a typical synchronization environment, a large database serves as a central repository for information. Sometimes you can choose a database system that suits your needs. Other times, a central database already exists and you must adapt the synchronization system to work with it.

MobiLink can work with many popular database servers, including SQL Anywhere, Sybase Adaptive Server Enterprise, Oracle, Microsoft SQL Server, and IBM DB2. Using the MobiLink server APIs for .NET and Java, you can synchronize with any data source, including application servers, web servers, text files, and other database products.

In a SQL Remote system, the central database must be SQL Anywhere.

Your remote database system

SQL Anywhere synchronization technologies also differ in the types of remote databases that they can support.

MobiLink supports SQL Anywhere and UltraLite as remote databases.

SQL Remote supports SQL Anywhere remote databases.

Network characteristics

MobiLink and SQL Remote are both well suited to occasionally-connected environments, where remote sites must operate for hours or days in isolation, although more frequent synchronization is possible whenever a network connection is available.

MobiLink is session-based. A real-time connection is required during synchronization. If this connection is interrupted before synchronization is complete, the process does not complete until the next synchronization. In contrast, SQL Remote relays information via messages, which can be sent or received asynchronously. These messages may take the form of files on a hard disk, or email messages. These messages can be processed whenever they are received, allowing synchronization to occur incrementally.

Frequency of synchronization

In some situations, it may be important that your information is replicated immediately. In others, synchronization once or twice a day may suffice. In fact, more frequent synchronization may be impossible when no network connection is available.

Both MobiLink and SQL Remote are primarily intended for situations where synchronization occurs infrequently, such as every few hours or days, but both can be used to synchronize as frequently as every few seconds.

The number of remote sites

MobiLink and SQL Remote both work well with a very large number of remote users. MobiLink scalability is limited only by the scalability of the consolidated database management system. The SQL Remote message-based design allows a typical installation to handle thousands of remote users.

There is no hard limit on the maximum number of remote sites with any of these systems. The actual number depends on the amount of information replicated, the frequency of synchronization, and the design of your application.

Transaction ordering

By default, MobiLink works by grouping the results of multiple transactions on the remote database into one set of changes to be applied to the consolidated database. Alternatively, you can choose to preserve the order of transactions and upload them separately. In both cases, synchronization always occurs at a transaction boundary, and so referential integrity is preserved. Uncommitted data is never synchronized, and so data integrity is preserved.

SQL Remote replicates data by scanning the transaction log and preparing messages, as appropriate, for each transaction. It orders these messages and sends them to the remote or consolidated site. When processing receives messages, SQL Remote always processes them in the same order as they were applied to the other database. When necessary, it automatically delays processing a message until all earlier messages have been applied.

Achieving data consistency at a particular time

Immediately following each MobiLink synchronization session, the data in the two databases is consistent. The ability to guarantee the consistency of the data at a remote site at a particular point in time is an advantage of MobiLink session-based synchronization. For example, if it is important that the data at a remote site accurately reflect the data in the consolidated database at a particular time, such as 10 o'clock in the morning, this objective can be achieved by synchronizing just prior to this time. As long as the synchronization completes successfully, the currency of the data at the remote site is assured.

When changes to the data are replicated through an exchange of messages, it is difficult to guarantee that the data in a particular remote site is completely consistent with the data in the consolidated site at any particular point in time. For example, sometimes a message is lost in transit. SQL Remote automatically recognizes this fault and resends the message, but such interruptions can cause unexpected delays.