Hi Francis >> One question on the hook-up: any specific reason to do one global event >> containing all the changes, instead of one event per change? Instead of >> subscribing to lp:context:changed event, subscribing to >> lp:context:web_link:changed ? So that call sites don't have to parse the >> changed_fields? Or even more struture, have the cache JSON object be proper >> Attribute-based object on which we could subscribe on the fooChange event >> (http://developer.yahoo.com/yui/3/examples/attribute/attribute-event.html)? > > I did consider a field by field update event, but there is always the > possibility that a particular page my have conditional updates, and > I got convinced not to. >
I was the one who convinced Tim to do one event containing all the changes per "unit of work". Publishing individual events for each field change can and does lead to race conditions that are difficult to debug. It won't initially be the case for our usage, but as we adopt and use this sort of infrastructure more and more over time, and the use cases become more complex, it will happen. Consider it analogous to database transactions where you only see all the changes together once the transaction is committed. This issue will become more relevant and important if/when we start using techniques to allow change notifications pushed from the server to each client. It also becomes more relevant when the idea of content based filtering is considered. ie we want to do something based on a match filter applied to the result of a unit of work. It's harder to implement this with just individual property change notifications. Having said that, there's nothing to prevent the addition of helper methods/classes to make notification and processing of individual field changes easier for those simple cases which require it. ie we can ensure the implementation can "devolve" to the simple case. The above are all high level, domain independent thoughts but I think they're applicable whether we're talking about client side or server side. For Launchpad as it exists today, we could initially get away with simple field level change notification. As I say above, we can indeed provide that but I just want to be sure we are aware of and consider future implications (and possibly ingraining bad habits early) when doing this sort of new infrastructure development. _______________________________________________ Mailing list: https://launchpad.net/~launchpad-dev Post to : [email protected] Unsubscribe : https://launchpad.net/~launchpad-dev More help : https://help.launchpad.net/ListHelp
