How Applications Use ToolTalk Messages

Applications create, send, and receive ToolTalk messages to communicate with other applications. Senders create, fill in, and send a message; the ToolTalk service determines the recipients and delivers the message to the recipients. Recipients retrieve messages, examine the information in the message, and then either discard the message or perform an operation and reply with the results.

Sending ToolTalk Messages

ToolTalk messages are simple structures that contain fields for address, subject, and delivery information. To send a ToolTalk message, an application obtains an empty message, fills in the message attributes, and sends the message. The sending application needs to provide the following information:

• Is the message a notice, a request, or an offer? (that is, should the recipient respond to the message?)

• What interest does the recipient share with the sender? (for example, is the recipient running in a specific user session or interested in a specific file?)

To narrow the focus of the message delivery, the sending application can provide more information in the message.

Message Patterns

An important ToolTalk feature is that senders need to know little about the recipients because applications that want to receive messages explicitly state what message they want to receive. This information is registered with the ToolTalk service in the form of message patterns.

Applications can provide message patterns to the ToolTalk service at installation time and while the application is running. Message patterns are created similarly to the way a message is created; both use the same type of information. For each type of message an application wants to receive, it obtains an empty message pattern, fills in the attributes, and registers the pattern with the ToolTalk service. These message patterns usually match the message protocols that applications have agreed to use. Applications can add more patterns for individual use.

When the ToolTalk service receives a message from a sending application, it compares the information in the message to the register patterns. Once matches have been found, the ToolTalk service delivers copies of the message to all recipients.

For each pattern that describes a message an application wants to receive, the application declares whether it can handle or observe the message. Although many applications can observe a message, only one application can handle the message to ensure that a requested operation is performed only once. If the ToolTalk service cannot find a handler for a request, it returns the message to the sending application indicating that delivery failed.

Receiving ToolTalk Messages

When the ToolTalk service determines that a message needs to be delivered to a specific process, it creates a copy of the message and notifies the process that a message is waiting. If a receiving application is not running, the ToolTalk service looks for instructions (provided by the application at installation time) on how to start the application.

The process retrieves the message and examines its contents.

• If the message contains a notice that an operation has been performed, the process reads the information and then discards the message.

• If the message contains a request to perform an operation, the process performs the operation and returns the result of the operation in a reply to the original message. Once the reply has been sent, the process discards the original message.