Content caching is a useful technique to increase the loading speed of a page. If the page has personalised content this technique doesn't work because one user may receive another user's personalised content which is insecure and undesirable.
The problem comes when someone suggests using AJAX to request the personalised content as a separate request. Before discussing the problem let's define exactly what personalised content is.
What is personalised content?
Personalised content is content that is specific to a user. The most basic example would be a "Logout" link. This is because the page knows that you are logged in and that you may want to logout.
Even though this is personal it's cacheable because we can have two groups of users: logged out and logged in users. In which case, we can serve the correct cached page to each group with a cookie check.
Specific information, such as a user's name, or the contents of their shopping basket can't be cached.
Now that's clear, we can discuss the issues with using AJAX to request personalised content:
1. The architecture becomes complex
Using AJAX has a significant impact on architecture.
- Is there one extra request for personalised content or multiple?
- Do we serve it as JSON and then parse that on the client?
- How do we organise and initialise scripts?
- How do we organise partials?
- At what point is personalised content not essential to the experience?
2. Some people will have a broken experience
This goes against Progressive Enhancement which in-turn goes against the spirit of the web. Don't exclude people unnecessarily.
3. Replacing browser behaviour degrades the experience
Using AJAX to load content stops us using the inherent functionality browsers provide for free. This functionality is an intuitive aspect of the browsing experience.
This includes displaying a loading indicator that accurately indicates the progress of the request. It also includes handling response and timeout errors.
Using AJAX and content-caching like this, means at first the page is only half rendered. Then, sometime later, the personalised content is injected into the page. This is, at least a little jarring as the page fills in the gaps.
We'll need to implement loading spinners, and hide content, and show the content perhaps with a transition. These things are often janky and they differ from site to site making for a low quality experience.
Moreover, the request may finish after scrolling down the page. This means the user may not notice this new personalised content and the page may jump unexpectedly.
4. Extra work is needed
First, a designer needs to design a loading spinner and transitions. Second, developers need to implement this functionality in good order for browsers and assistive technology. Third, testers will need to test and AJAX makes functional tests harder to write.
5. Performance actually degrades
Instead of a single HTTP request containing the whole response, there will now be multiple. The first would be for the Document containing non-personalised content. The next request(s) will be via AJAX and will hit the web server, therefore subject to the same latency as always.
And any performance increase is negated by the extra JS and CSS which needs to download and execute, including reflows and repaints in the browser. This includes:
- making a JSON request;
- parsing the response;
- handling errors;
- finding elements;
- traversing the Document tree;
- updating the Document; and
- loading spinners and whatever else.
Content-caching is a very useful technique when used responsibly and for pages that don't contain personalised content. For pages that do contain personalised content, a cookie check can determine whether the cached version should be served or not.
Enhancements should not break the web, and make the experience worse. It should enhance the experience where it is valuable to do so.
AJAX seems like an innocent and beneficial solution, but it encourages bad practice that creates several self-induced problems for users and the development team.
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