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Each pico presents an event loop that handles events sent to the pico according to the rulesets that are installed in it. The following diagram shows the five phases of event evaluation. Note that evaluation is a cycle like any interpreter. The event represents the input to the interpreter that causes the cycle to happen. Once that event has been evaluated, the pico waits for another event.
We'll discuss the five stages in order.
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While most events, at present, are transported over HTTP via the Sky Event API, that needn't be the case. Events can be transported via any means for which a decoder exists. Other transports (e.g. SMTP, XMPP, RabbitMQ, etc.) could be supported with minimal effort.
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The salience graph is calculated from the rulesets installed in the pico. Whenever the collection of rulesets for a pico changes, the salience graph is recalculated. There is a single salience graph for each pico. The salience graph determines for which events a rule is listening by using the rule's event expression.
Rule order matters within a ruleset. The scheduler ensures that rules appear in the schedule in the order they appear in the ruleset. No such ordering exists for rulesets, however, so there is no guarantee that rules from one ruleset will be evaluated before or after those of another unless the programmer takes explicit steps to ensure that they are (see the discussion of explicit events below).
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For purposes of understanding the event evaluation cycle, most of what happens in rule execution is irrelevant. The exception is the raise statement in the rule's postlude. The raise statement allows developers to raise an event as one of the results of the rule evaluation. Raising explicit events is a powerful tool.
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If the rule makes a synchronous call to an external API, rule execution waits for the external resource to respond. If a rule sends an event to another pico, that sets off another independent event evaluation cycle, it doesn't modify the schedule for the cycle's execution of event:send(). Inter-pico events are sent asynchronously.
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