[e-lang] Found item: why to use eventual scheduling consistently

[Kevin Reid]

http://osteele.com/archives/2008/04/minimizing-code-paths-asychronous-code

> Minimizing Code Paths in Asychronous Code
> April 20, 2008 in Libraries, Tips | Comments
>
> Have you ever written a function that looks like this?
>
> function requestProductDetails(id, k) {
>   var value = gProductDetailsCache[id];
>   if (value)
>     k(value)
>   else
>     ajax.get('/product/'+id, function(data) {
>       gProductDetailsCache[id] = data;
>       k(data);
>     });
> }
>
> requestProductDetails calls its callback with the product details,  
> which are stored in a cache. Since it might need to request this  
> information from the server, it has to “return” it by passing it to  
> a callback; in order to present a uniform API whether or not the  
> product is cached, it “returns” the data this way whether it came  
> from the cache or not. requestProductDetails is intended to be used  
> this way:
>
> requestProductDetails(id, function(details) {
>   infoPanel.setDetails(id, details);
> });
> infoPanel.setName(id, gProductNames[id]);
>
> (I gave infoPanel a somewhat silly API in order to demonstrate a  
> point. The general pattern is that there’s some computation in the  
> callback, and some other computation after the call.)
>
> There’s a subtle problem in this code, which is that two different  
> code paths run through it. In the cached case, infoPanel.setDetails  
> is called before infoPanel.setName. In the uncached case (the first  
> time through), it’s the other way around. If there’s a bug that  
> causes setDetails to work only after setName has been called, you  
> may well miss it during casual testing, because it will only trigger  
> the second time you trigger the code – and once it does trigger, it  
> will appear intermittently (especially if you have a more  
> sophisticated cache), and be darned difficult to find.
>
> I recommend this implementation of requestProductDetails instead. It  
> makes the inside of the function more complex – and the setTimeout  
> looks gratuitous – but it makes its outside simpler :  
> requestProductDetailss callers are much easier to debug.
>
> function requestProductDetails(id, k) {
>   var value = gProductDetailsCache[id];
>   if (value)
>     setTimeout(function() { k(value) }, 10);
>   else
>     ajax.get('/product/'+id, function(data) {
>       gProductDetailsCache[id] = data;
>       k(data);
>     });
> }
>
> The general principle here is if a function sometimes has to call  
> its callback asynchronously, always call it asynchronously, to  
> minimize the number of possible code paths through the application.


-- 
Kevin Reid                                  <http://switchb.org/kpreid/>