Wednesday, July 23, 2008

GAE: Batch operations

Given the limit on individual requests with GAE, doing things like batch updates to your database (live) can be an interesting challenge. I recently had to update all of the votes for Challenge-You! to support a new rating system. The best approach I found was an AJAX-style solution. Have one request that returns a list of references to the entities you need to change (or if you're working with more than 1000 entities, you might want to partition the list into chunks by date or some other order, and work with one chunk at a time). Keeping those references in a stack in Javascript, pop them off one-by-one and send each to the controller that applies the change, repeating until the stack is empty. If the operations aren't too expensive, you could even send a handful of them per-request to speed things up. It's also trivial to implement some kind of progress bar or other graphical update to keep you informed.

And example of the Javascript code (using jQuery) that handles the client side:
var data = null;

function doupdate() {
if(data.length == 0) return;
$.ajax({
type:"POST",
url:"doupdate",
data:"id=" + data.pop(),
success:function(msg) {
setTimeout(function() {
doupdate();
}, 1000);
}
});
}

$(document).ready(function() {
$.ajax({
type:"POST",
url:"getlist",
data:"",
success:function(msg) {
data = msg.split(" ");
doupdate();
}
});
});

In this example, my site would have two urls for handling this, "getlist" which returns a space-delimited list of references/ids (however you're handling the entities) and "doupdate" which takes in one reference and applies the needed change.

1 comment:

Ali said...

I was also met with this ghastly limitation of GAE, and the "Warning you are using over your allotted amount of air for breathing" was scaring me.

I also solved it in a similar way, by splitting the requirements into chunks and calling it over ajax. But what a pain!