AEM micro-optimization (part 4) – define allowed templates

This time I want to discuss a different type of micro-optimization. It’s not something you as a developer can implement in your code, but it’s rather a question of the application design, which  has some surprising impact. I came across it when I recently investigated poor performance in the Siteadmin navigation. And although I did this investigation in AEM as a Cloud Service, the logic on AEM 6.5 behaves the same way.

When you click in the siteadmin navigation through your pages, AEM collects a lot of information about pages and folders to display them in the proper context. For example, when you click on page with child pages, it collects information what actions should be displayed if a specific child node is going to be selected (copy, paste, publish, …)

An important information is if the “Create page” action should be made available. And that’s the thing I want to outline in this article.

Assuming that you have the required write permissions on that folder, the most important is if templates are allowed to be created as children of the current page. The logic is described in the documentation and is quite complex.

In short:

• On the content the template must be allowed (using the cq:allowedTemplates property (if present) AND
• The template must be allowed to be used as a child page of the current page

Both conditions are must be met for a template to make it eligible to be used as a source for a new page. To display the entry “Page” it’s sufficient if at least 1 template is allowed.

Now let’s think about the runtime performance of this check, and that’s mostly determined by the total number of templates in the system. AEM determines all templates by this JCR query:

//jcr:content/element(*,cq:Template)

And that query returns 92 results on my local SDK instance with WKND installed. If we look a bit more closely to the results, we can determine 3 different types of templates:

• Static templates
• Editable templates
• Content Fragment models

So depending on your use-case it’s easy to end up with hundreds of templates, and not all of them are applicable at the location you are currently in. In fact, typically just very few templates can be used to create a page here. That means that the check most likely needs to iterate a lot to eventually encounter a template which is a match.

Let’s come back to the evaluation if that entry should be displayed. If you have defined the cq:allowedTemplates property  on the page or it’s ancestors it’s sufficient to check the templates listed there. Typically it’s just a handful of templates, and it’s very likely that you find a “hit” early on, which immediately terminates this check with a positive result. I want to explicitly mention that not every template listed can be created here, because there also other constraints (e.g. the parent template must be of a certain type etc) which must match.

 If template A is allowed to be used below /content/wknd/en, then we just need to check the single Template A to get that hit. We don’t care, where in the list of templates it is (which are returned by the above query), because we know exactly which one(s) to look at.

If that property is not present, AEM needs to go through all templates and check the conditions for each and every one, until it finds that positive result.  And the list of templates is identical to the order in which the templates are returned from the JCR query, that means the order is not deterministic. Also it is not possible to order the result in a helpful way, because the semantic of our check (which include regular expressions) cannot be expressed as part of the JCR query.

So you are very lucky if the JCR query returns a matching template already at position 1 of the list, but that’s very unlikely. Typically you need to iterate tens of templates to get a hit.

So, what’s the impact on the performance of this iteration and the checks? In an synthetic check with 200 templates, when I did not have any match, it took around 3-5ms to iterate and check all of the results.

You might ask, “I really don’t feel a 3-5ms delay”, but when the list view in siteadmin performs this check for up to 40 pages in a single request, it’s rather a 120-200 millisecond difference. And that is a significant delay for requests where bad performance is visible immediately. Especially if there’s a simple way to mitigate this.

And for that reason I recommend you to provide “cq:allowedTemplates” properties on your content structure. In many cases it’s possible and it will speed up the siteadmin navigation performance.

And for those, who cannot change that: I currently working on changing the logic to speedup the processing for the cases where no cq:allowedTemplates property is applicable. And if you are on AEM as a Cloud Service, you’ll get this improvement automatically.