SQL Remote is an asynchronous synchronization scheme: it uses messages to exchange data between databases. Messages are typically files or specially formatted email messages. A message agent, attached to each database, sends messages regarding changes to its own data. The same agent also receives messages from one or more other databases and modifies the database according to the contents of the received messages.
In message-based communications, each message carries its destination address and other control information so that no direct connection is necessary between applications exchanging information. For example, an email message contains the destination address; there is no direct connection between the sending server and the recipient.
Just as session-based client/server applications rely on network communication protocol stacks, such as TCP/IP, so message-based applications rely on message services such as Internet Simple Mail Transfer Protocol (SMTP), or a simple shared file link.
Message services use store-and-forward methods to get each message to its destination: for example, email systems store messages until the recipient opens their mail folder to read their mail, then the email system forwards the message.
Building a synchronization system on top of a message system means that a message-based synchronization system, such as SQL Remote, does not need to implement a store-and-forward system to get messages to their destination. Just as session-based client/server applications do not implement their own protocol stacks to pass information between client and server, so SQL Remote uses existing message systems to pass the messages.
To work reliably, a message-based synchronization system must both guarantee that all messages reach their destination and that the messages are applied in the same order that they are sent. SQL Remote incorporates a protocol to guarantee application of synchronization updates in the correct order.
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