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Re: [WebSimpleDB] Introduce a pause/resume pattern for coordinated access to multiple stores

From: Nikunj R. Mehta <nikunj.mehta@oracle.com>
Date: Tue, 22 Dec 2009 19:32:03 -0800
Cc: "public-webapps@w3.org" <public-webapps@w3.org>
Message-Id: <40DDD224-E369-4CA0-86B8-0B07C0845B9B@oracle.com>
To: Pablo Castro <Pablo.Castro@microsoft.com>

On Dec 22, 2009, at 5:39 PM, Pablo Castro wrote:

> Whenever we take a callback that's to be called for each item in a  
> set (e.g. with a .forEach(callback) pattern), we need a way to  
> indicate the system whether it's ok to move to the next row and  
> invoke the next callback or not. Otherwise, in scenarios where the  
> callback itself performs an operation that doesn't finish  
> immediately (such as another database async call) the system will  
> keep queuing up top-level callbacks, which in turn may queue up more  
> callbacks as part of its implementation, and execution will be in  
> "some order" that's very hard to predict at best.
> This comes up in several contexts. Applications will often need to  
> scan more than one object store in coordination. Query processors  
> will also need this when implementing physical operators for joins  
> and such. A different context would be a system that needs to submit  
> an HTTP request per row, where you may want to use an XmlHttpRequest  
> and unwind after calling open. While the HTTP request is in flight  
> you don't want to move to the next
> In most cases one of the key aspects is that we need separate  
> components to work cooperatively as they pull rows from one or  
> multiple scans, and there needs to be a way of controlling the  
> advance of cursors through the rows.
> We would like to introduce "pause" and "resume" functions for scans  
> to support this. Since there is no obvious place to put this right  
> now, we could introduce an "iterator" object that can be used to  
> control things related to the current state of the iteration as of  
> when the callback happens, or maybe this is the cursor itself.
> The resulting code would look like this (the example uses the single- 
> async-level pattern we're playing around, but these two are actually  
> independent things):
> async_db.forEachObjectInStore("people", function(person, iteration) {
>  iteration.pause(); // we won't be done with 'person' until later...
>  var request = async_db.getFromStore("people", person.managerId);
>  request.onsuccess = function() {
>    var manager = request.result;
>    // Do something with both 'person' and 'manager', and now we're  
> ready to process the next person.
>    iteration.resume();
>  };
> });
> The nice thing about adding these as methods on the side is that  
> it's completely out of sight in simple scenarios where you may be  
> just scanning to build some HTML for example. Only if you're doing  
> multiple coordinated, async tasks you need to know about these  
> functions.
> Regards,
> -pablo

The proposed WD can do what you want. You would write the following  
code for async processing, even though the solution is not the easiest  
to follow:

async_db.request.onsuccess = function() {
   var people = async_db.request.result;
   people.request.onsuccess = function() {
     var cursor = people.request.result;
     var eachPerson = function() {
       var person = cursor.value;
       cursor.request.onsuccess = eachPerson;
       people.request.onsuccess = function() {
         var manager = people.request.result;
         // do something with both person and manager
       cursor.request.onerror = function () {
         // do what you will when all the people have been processed

It is difficult to follow the logic above, so I will also give a  
synchronous example:

var people = async_db.openObjectStore('people');
var cursor = people.openCursor();
if (cursor)
do {
   var person = cursor.value;
   var manager = people.get(person.managerId);
   // do something with person and manager
} while (cursor.continue());
// we are done with all the people

As I said earlier, there is something to be said about the  
comprehensibility of the asynchronous code above, but let's look at  
the expressive power separately from ease of programming.

Received on Wednesday, 23 December 2009 04:06:46 UTC

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