[canarypilot]

I like a good analogy challenge, let me try to shape something around your framework!

We sell particularly high-value, unique jewels, that are seated in jewellery. In fact, our jewels are so valuable to us, that our stores are able to modify the case around the jewel even before they sell to a customer. Each customer will be offered the last casing the jewel was in, and be given the opportunity to change it - though they might still choose not to buy.

We have a set of stores that we can sell from, and of course only one store can have the jewel in it at a time. We’re losing customers by only having the local stock available to sell, so we instigate a new policy; every store takes a picture of the casing for the jewel when they have it in store, they can show this last copy of the jewel in their catalogue. If a customer wants the jewel from the catalogue, we call around the other stores asking for the physical jewel to be brought to us.

This works well a lot of the time, but we find a new problem. Sometimes the customer really liked the casing the jewel was in for the photograph in the catalogue, but when the physical jewel arrives the casing has changed. The customer mutters something about false advertising and storms out disgusted by the stale photo shown. So we add a new policy. If a store changes the casing for a jewel it has to send a picture to every other store of the new casing so they can all update their catalogues.

Well! Now we have chaos! Every store owner is so busy updating catalogues and sending pictures around the other stores that they’re getting less real work done. We look at our catalogue, sometimes we’re updating the same page many times without a customer even seeing it!

So we come up with a new policy - instead of immediately changing the catalogue when we get told that another store has changed the physical jewel, we’re just going to mark the catalogue page as out of date. If eventually a customer asks to see that jewel, we’ll apologise to them for the wait, we’ll ask the other stores who has the jewel right now, then we’ll go get the physical jewel from them, take a picture for our catalogue, and show it to the customer. (As a minor optimisation, if we think the customer will want to change the casing we’ll politely inform the store we picked it up from that they can now mark their catalogue out of date).

It is all going well, but there’s a lot of overhead in our catalogue with only one jewel per page, so we start putting multiple jewels on each page. That makes our catalogue page update a bit difficult so we declare that a store must always collect all physical jewels for any given catalogue page.

One day we find that two stores are sending the same two jewels backwards and forwards all day. We investigate and find that the two jewels are on the same catalogue page, one jewel is being changed by a particularly indecisive customer in one store, and the other is being change by an equally determined customer in the other store. Neither cares about the jewel the other is trying to change, but both customers are getting frustrated by the wait as our policy requires both jewels in the same location before an update can be made. The store managers bemoan ever putting the two jewels on the same page to create this false sharing nightmare. They agree that even if wasteful, for these particular jewels they’ll leave blank space in the catalogue, the two jewels land on different pages, and the shuffling of jewels between stores stops.

[badrabbit]

Thanks for taking the time and explaining. In your opinion is this a problem unique to CS then?