Sequencing Rules. In general, within a single conversation, actions on a given service interface are performed sequentially. One action may not be invoked until the prior action is completed. An exception is actions with Type="concurrent". See "Action Type Concurrent". Sequencing rules define which actions are allowed and which are not allowed by a given server after a given action on a given conversation. At any point in the conversation, some actions are allowed and others are not allowed. For example, a reservation cannot be canceled before it is requested. In the service interface definition for each provider, but outside the scope of the action menu, the <StartEnabled> tag identifies which actions are permitted as the initial action on that server. If <StartEnabled> is omitted, all actions defined by that server are permitted as the initial action. In each action definition, sequencing rules cause actions to be added to or deleted from the current list of allowed actions. The <Sequencing> tag identifies which actions are allowed and which are not allowed following that action. The <Disable> tag identifies which actions are not permitted after the defined action. For example, a specific TPA may require that a new reservation request cannot be made after a successful reservation request. The <Enable> tag identifies which actions are permitted after the defined action. For example, a reservation can be confirmed, modified, or canceled after a successful reservation request. If the action definition does not include a <Sequencing> tag, the list of permitted actions is not changed. If the <enable> tag is omitted, no new actions are enabled. If the <disable> tag is omitted, no additional actions are disabled. The rules in <Sequencing> are effective only if the action succeeded. If the action did not succeed, the lists of actions allowed and not allowed are unchanged. An application might define a unit of business consisting of multiple actions and perform multiple sequential units of business in the same conversation. If so, the sequencing rules for the final action of a unit of business should restore the permitted and disallowed action lists to the state appropriate for the beginning of a new unit of business. See "Action Type" and its subsections for any special considerations for the extended actions.
