Temporal Patterns

Dimensions of Time

Just as I've described above, time is a pretty challenging notion in modeling. However I've skipped over the most awkward aspect of temporal models. We've all learned, if only from bad Science Fiction books, that time is the fourth dimension. The trouble is that this is wrong.

I find the best way to describe this problem is with an example. Imagine we have a payroll system that knows that en employee has a rate of $100/day starting on January 1. On February 25 we run the payroll with this rate. On March 15 we learn that, effective on February 15, the employee's rate changed to $211/day. What should we answer when we are asked what the rate was for February 25?

In one sense we should answer $211, since now we know that that was the rate. But often we cannot ignore that on Feb 25 we thought the rate was $100, after all that's when we ran payroll. We printed a check, sent to him, and he cashed it. These all occurred based on the amount that his rate was. If the tax authorities asked us for his rate on Feb 25, this becomes important.

In fact we can think that there are indeed two histories of Dinsdale's pay rate that are important to us. The history that we know now, and the history we knew on February 25. Indeed in general we can say that not just is there a history of dinsdale's pay rate for every day in the past, but there is a history of histories of Dinsdale's pay rates. Time isn't the fourth dimension, it's the fourth and fifth dimensions!

I think of the first dimension as actual time: the time something happened. The second dimension is record time, the time we knew about it. Whenever something happens, there are always these two times that come with it. Dinsdale's pay raise had a actual date of Feb 15 and a record date of March 15. Similarly when we ask what Dinsdale's pay rate was, we really need to provide two dates: a record date and an actual date.

We can think of the two dimensions like this. The actual history is looking back in actual time. If I look at my current actual history, then I see that Dinsdale's pay was $100 up to Feb 15, at which point it increased to $211. However that is the actual history for today in record time. If I look at the actual history for Feb 25, then Dinsdale was paid at $100 from Jan 1 onwards, and the $211 never came into it. Every day (strictly every Time Point) in record time has an actual history. These histories are different as we find out that things we used to think were true are no longer true.

From another angle we can say that each day in actual history has a record history. The record history tells us how our knowledge of that day changed over time. So Feb 25 in actual time has a record history that says that up to March 15, Dinsdale's pay was $100 at which point it reaches $211.

Let's take this example a step further Assume we make the corresponding adjustments in a payroll run on March 26. On April 4 we are told that the employee's our previous information was wrong and that the rate was actually changed to $255 on February 15. Now how do we answer the question "what was the employee's rate on February 25?".

I've seen grown developers bite their own heads off when faced with this kind of stuff. But once you realize that everything comes down to this notion of two dimensions, then things start getting a lot simpler. One way of visualizing this to extend the earlier table

If we look at our current actual history (that is the actual history whose record date is today) then we would say that Dinsdale's pay was $100 from Jan 1 and went up to $255 on Feb 15. For the current actual history the $211 rate doesn't occur at all because it was never true. If we look at the actual history for March 26 we would see Dinsdale's pay at $100 up to Feb 15 where it went up to $211. In the actual history for March 26 the $255 rate never happened because we didn't know of it yet.

We can also think about the record history for Feb 25. Now this record history says that the rate was $100 (on that day) until March 15 when it changed to $211. Then on April 4 it changed again to $255.

Once you realize about the two dimensions then it becomes much easier to think about the problem, but scary to think that you have to implement this kind of stuff. Fortunately there are a number things you can do that simplifies matters when it comes to implementation.

The first simplification is that it isn't difficult to use Audit Log to cope with these changes. All you need to do is record both the record date and the actual date in the log with each entry. This simple exercise is enough to keep any log effective over both dimensions, and I belive is worth doing even if you are only bothered about one of them.

The second simplification is that you often don't want your model to handle both dimensions. The important thing here is to know which you have in your model and which one you are leaving to Audit Log.

If we want to keep a history of things where we want to know how things changed over time, but didn't care about when we learned of changes, we would say that was actual-temporal. So if I keep a record of an employee's address I might choose to keep that as a actual-temporal property. For information systems that are there to help on-line query this works well because when you are accessing a database you usually want to know about actual-history.

Record-temporal facts appear when you have a system that does things like producing bills based on the state of objects. These things lead to questions about how the bill was calculated, which leads you to need to know what thought the state of and object was when the bill was calculated. Record-temporal facts can often be compared to a version control system in software where you can go back and say 'what was this file like on April 1?'

Of course there are times when you need both dimensions at once - these are called bi-temporal facts. Essentially bi-temporal information always needs both dates.

Bi-temporality is the full solution, but it's always worth thinking of ways around it. An example comes from bill calculation. If you want to find out why a bill came out to what it was one possibility is to have a fully bi-temporal database. However it's often better to store a detailed trace of the calculations when you calculate the bill. That satisfies the requirement in a much simpler way than a bi-temporal object model.

Updating a Temporal Record

So far I've only talked about temporal information in terms of accessing information, not in terms of updating it. How you allow your updates leads to more decisions, but many of these involve useful simplifications.

In general changing the time record is quite messy. If we have an employee whose rate was $211 from Feb 15 to Apr 15, a fully general update will allow to change any combination of start date, end date, and value. Providing an interface for this is awkward since it requires the client to know a lot about how temporal information works.

The first simplification is that you may have only additive changes. An additive change always goes onto the end of the record. An example of an additive change is "make the employees rate $211 effective Feb 15". Although at first blush this looks like you are only removing one piece of data from the mix, the consequences actually work out so that it greatly simplifies the update. Most of the updates that happen in practice are additive, so that can greatly simplify clients. Furthermore you can make any change with some combination of additive updates. While this would get horribly messy on an object with a complicated history, this property can simplify objects with a simple history by allowing you to only support an additive interface.

The second simplification is allowing only current updates. A current update allows changes in the record only with an effective date of today. In general even additive changes can occur at any date in the past or future. A current change requires no date information at all, allowing you to have an interface for updates that is completely non-temporal.

Current updates seem too good to be true, why have temporal information if you only update it with current updates? The good news here is that record-temporal information can only be updated with current updates. Not just would a retroactive change to record-temporal information break the integrity of the record, there's no situation that requires a retroactive change that can't be handled by the actual-time dimension (unless fraud is one of your requirements). This is a big step as it means that one whole dimension has invisible updates - you only have to worry about record time when querying, not when updating.

Other Readings

Since this is a complicated area, it has generated some other writings as people have explored these problems. The most comprehensive treatment of this area is Snodgrass. His work is based on relational databases and most of the book is based on how to handle these issues in SQL. The problem, however, is the same. Furthermore much of the comments he makes and terminology he uses is the same as the forthcoming SQL standard's material on temporal databases. The book is now out of print, but you can get a pdf from Richard Snodgrass's publications page.

The matter of terminology is one that I'm still not completely happy with. He uses the same two dimensions that I do, but his terms are different. What I call actual time he calls valid time, what I call record time he calls transaction time. In earlier versions of these patterns I followed his terms, but many people said they found these terms very difficult to follow. As a result I decided to use different terms. Another difference is that he refers to a table that changes over time as sequenced. Of course, objects use sequences all the time, not just for temporal purposes, so I've called things that change over time temporal.

[Anderson] is one of the best collections of temporal patterns. Again I've plundered ideas, but changed some terminology. I've used the term version rather than edition to mean a value for a property or object during some time period. His Change Log and History on Association patterns are the same as Temporal Property. I see the difference between the two Anderson patterns as more a matter of implementation. His History on Self pattern is the same as Temporal Object.

Also in the same PLoPD 4book, you'll find a paper [Carlson et al]written by Andy Carlson in collaboration with myself and Sharon Estepp. This gives earlier descriptions of Temporal Property and Snapshot. It also mentions a pattern Temporal Association which I now look at as a use of Effectivity.

Also submitted to PLoP, but not available publicly at the moment is [Arnoldi et al]. This introduces a number of interesting ideas that I haven't fully explored in the patterns here (at least not yet).

Both [Anderson] and [Arnoldi et al] consider using objects other than Time Point to act as indexes into the temporal record. [Anderson] uses a (single-temporal) event, and [Arnoldi et al] use a perspective: essentially two timepoints combined into a single object. Although there are things to be said for both of these approaches, I haven't used them here as I feel that using time points does most of what you need, and is simpler to explain.