- Occurrence time that occur over intervals: Events typically occur over intervals, but for computational reasons it is convenient to approximate it to a time-point, and look at events in the discrete space; however, for some events this is not an accurate thing to do, and interval-based temporal semantics should be supported, along with operations associated with them.
- Temporal properties of derived events: For raw event, we defined occurrence time as the time it occurred in reality, and detection time, as the time that the system detected its existence. What are the temporal properties of derived events? there is no unique solution to this question.
- Out-of-order events: This topic is the topic most investigated among the challenging topics, however, current solutions are based on assumptions that are sometimes problematic. This problem is about events that arrive out of order, where the event processing operation is order-sensitive.
- Uncertain events: Uncertainty whether event has happened, due to malfunction, malicious or inaccurate sources
- Inexact content of events: Similar to uncertain events, some content in the event payload including temporal and spatial properties of the events may not be accurate.
- Inexact matching between events and situations. Situations are the events that require reaction in the user's mind. This is in getting us back from the computer domain to the real-world domain. Situation is being represented as a raw or derived event, but this may be only approximation, since there may be false positives and false negatives in the transfer between the domains.
- Traceability of lineage for event or action, this gets to the notion of determination of causality. Since in some cases there are operations in the middle of the causality network outside the event processing systems boundaries (e.g. event consumer who is also event producer) causality may not be automatically determined.
- Retraction of event: ways to undo the logical effects of events, sometimes tricky or impossible, but seems to be a repeating pattern.
More about some of them - later.