User's Guide
PART 2. Relational Database Concepts
CHAPTER 15. Using Transactions and Locks
Some computers on your network might be portable computers that people take away from the office or which are occasionally connected to the network. There may be several database applications that they would like to use while not connected to the network.
Database replication is the ideal solution to this problem. Using SQL Remote, you can publish information in a consolidated, or master, database to any number of other computers. You can control precisely the information replicated on any particular computer. Any person can receive particular tables, or even portions of the rows or columns of a table. By customizing the information each receives, you can ensure that their copy of the database is no larger than necessary to contain the information they require.
Extensive information on replication is provided in the separate manual entitled Data Replication with SQL Remote. The information in this section is, thus, not intended to be complete. Rather, it introduces concepts related directly to locking and concurrency considerations. For further, detail please refer to the supplementary manual.
SQL Remote allows replicated databases to be updated from a central, consolidated database, as well as updating this same central data as the results of transactions processed on the remote machine. Since updates can occur in either direction, this ability is referred to as bi-directional replication.
Since the results of transactions can affect the consolidated database, whether they are processed on the central machine or on a remote one, the effect is that of allowing concurrent transactions.
Transactions may happen at the same time on different machines. They may even involve the same data. In this case, though, the machines may not be physically connected. No means may exist by which the remote machine can contact the consolidated database to set any form of lock or identify which rows have changed. Thus, locks can not prevent inconsistencies as they do when all transactions are processed by a single engine.
An added complication is introduced by the fact that any given remote machine may not hold a full copy of the database. Consider a transaction executed directly on the main, consolidated database. It may affect rows in two or more tables. The same transaction might not execute on a remote database, as there is no guarantee that one or both of the affected tables is replicated on that machine. Even if the same tables exist, they may not contain exactly the same information, depending upon how recently the information in the two databases has been synchronized.
To accommodate the above constraints, replication is not based on transactions, but rather on operations. An operation is a change to one row in a table. This change could be the result of an UPDATE, INSERT, or DELETE statement. An operation resulting from an UPDATE or DELETE identifies the initial values of each column and a transaction resulting from an INSERT or UPDATE records the final values.
A transaction may result in none, one, or more than one operation. One operation will never result from two or more transactions. If two transaction modify a table, then two or more corresponding operations will result.
If an operation results from a transaction processed on a remote computer, then it must be passed to the consolidated database so that the information can be merged. If, on the other hand, an operation results from a transaction on the consolidated computer, then the operation may need to be sent to some remote sites, but not others. Since each remote site may contain a replica of a portion of the complete database, SQL Remote knows to pass the operation to a remote site only when it affects that portion of the database.
SQL Remote uses a transaction log based replication mechanism. When you activate SQL Remote on a machine, it scans the transaction log to identify the operations it must transfer and prepares one or more messages.
SQL Remote can pass these messages between computers using a number of methods. It can create files containing the messages and store them in a designated directory. Alternatively, SQL Remote can pass messages using any of the most common messaging protocols. You likely can use your present e-mail system.
Conflicts may arise when merging operations from remote sites into the consolidated database. For example, two people, each at a different remote site, may have changed the same value in the same table. Whereas the locking facility built into Adaptive Server Anywhere can eliminate conflict between concurrent transactions handled by the same engine, it is impossible to automatically eliminate all conflicts between two remote users who both have permission to change the same value.
As the database administrator, you can avoid this potential problem through suitable database design or by writing conflict resolution algorithms. For example, you can decide that only one person will be responsible for updating a particular range of values in a particular table. If such a restriction is impractical, then you can instead use the conflict resolution facilities of SQL Remote to implement triggers and procedures which resolve conflicts in a manner appropriate to the data involved.
SQL Remote provides the tools and programming facilities you need to take full advantage of database replication. For further information, see the manual Data Replication with SQL Remote.