What is it? Why should we care?
If fewer is better, why multiple aggregates?
Using the API
I like to register every script in my module as a library, and only add scripts using drupal_add_library (or the corresponding #attached method, see below). This way I’m clearly expressing any dependencies and in the case where my script is added in more than one place, the options I give the aggregation system are all in one place in case I need to make a change. It’s also a nice way to safely deprecate scripts. If all the dependencies are spelled out, you can be assured that removing a script won’t break things.
Instructions for using hook_library and drupal_add_library are well documented. However, one important thing to note is the third parameter to drupal_add_library, every_page, which is used to help optimize aggregation. At registration time in hook_library, you typically don’t know if a library will be used on every page request or not. Especially if you’re registering libraries for other module authors to use. This is why drupal_add_library has an every_page parameter. If you’re adding a library unconditionally on every page, be sure to set every_page parameter to TRUE so that Drupal will group your script into a stable aggregate with other scripts that are added to every page. We’ll discuss every_page in more detail below.
The possible values for scope are ‘header’ and ‘footer’. A scope of ‘header’ will output the script in thetag, a scope of ‘footer’ will output the script just before the closing tag.
The ‘group’ option takes any integer as a value, although you should stick to the constants JS_LIBRARY, JS_DEFAULT and JS_THEME to avoid excessive numbers of aggregates and follow convention.
The ‘every_page’ option expects a boolean. This is a way to tell the aggregation system that your script is guaranteed to be included on every page. This flag is commonly overlooked, but is very important. Any scripts that are added on every page should be included in a stable aggregate group, one that is the same from page to page, such that the browser can make good use of caching. The every_page parameter is used for exactly that purpose. Miss out on using every_page and your script is apt to be lumped into a volatile aggregate that changes from page to page, forcing the browser to download your code anew on each page request.
What can go wrong
Potential number of aggregates
Misuse of Groups
Inline and External Scripts
Extra care should be employed when adding external and inline scripts. drupal_add_js and friends preserve order really well, such that they track the order the function is called for all the script added to the page and respect that. That’s all well and good. However, if an inline or external script is added between file scripts, it can split the aggregate. For example if you add 3 scripts: 2 file scripts and 1 external, all with the same scope, group, and every_page values, you might think that Drupal would aggregate the 2 file scripts and output 2 script tags total. However, if drupal_add_js gets called for the file, then external, then file, you end up getting two aggregates generated with the external script in between for a total of 3 script tags. This is probably not what you want. In this case it’s best to specify a high or low weight value for the external script so it sits at the top or bottom of the aggregate and doesn’t end up splitting it. The same goes for inline JS.
Scripts added in a different order
I alluded to this in the last one, but certain situations can arise where scripts get added to an aggregate in a different order from one request to the next. This can be for any number of reasons, but because Drupal is tracking the call order to drupal_add_js, you end up with a different order for the same scripts in a group and thus a different aggregate. Sadly the same code will sit in two aggregates on the server, with slightly different source order, but otherwise they could have been exactly equal, produced a single aggregate and thus cached by the browser from page to page. The solution in this case is to use weight values to ensure the same order within an aggregate from page to page. It’s not ideal, because you don’t want to have to set weights on every hook_library / drupal_add_js. I’d recommend handling it on a case by case basis.
Clearly, there is a lot that can go wrong, or at least end up taking you to a place that is sub-optimal. Considering all that we’ve covered, I’ve come up with a list of best practices to follow:
Always use the API, never ‘shoehorn’ scripts into the page
Use every_page when appropriate
The every_page option signals to the aggregation system that your script will be added to all pages on the site. If your script qualifies, make sure your setting every_page = TRUE. This puts your script into a "stable" aggregate that is cached and reused often.
Advanced CSS/JS Aggregation (AdvAgg) is a contributed module that replaces Drupal’s built in aggregation system with it’s own, making many improvements and adding additional features along the way. The way that you add scripts is the same, but behind the scenes, how the groups are generated and aggregated into their respective aggregates is different. AdvAgg also attempts to overcome many of the scenarios where core aggregation can go wrong that I listed above. I won’t cover all of what AdvAgg has to offer, it has an impressive scope that would warrant it’s own article or more. Instead I’ll touch on how it can solve some of the problems we listed above, as well as some other neat features.
Out of the box the core AdvAgg module supplies a handful of transparent backend improvements. One of those is stampede protection. After a code release with JS/CSS changes, there is a potential that multiple requests for the same page will all trigger the calculation and writing of the same new aggregates, duplicating work. On high traffic sites, this can lead to deadlocks which can be detrimental to performance. AdvAgg implements locking so that only the first process will perform the work of calculating and writing aggregates. Advagg also employs smarter caching strategies so the work of calculating and writing aggregates is done as infrequently as possible, and only when there is a change to the source files. These are nice improvements, but there is great power to behold in AdvAgg’s submodules.
AdvAgg comes with two submodules to enhance JS and CSS compression. They each provide a pluggable way to have a compressor library act on the each aggregate before it’s saved to disk. There are a few options for JS compression, JSqueeze is a great option that will work out of the box. However, if you have the flexibility on your server to install the JSMIN C extension, it’s slightly more performant.
AdvAgg Modifier is another sub module that ships with AdvAgg, and here is where we get into solving some of the problems we listed earlier. Let’s explore some of the more interesting options that are made available by AdvAgg Modifier:
Move JS to the footer