Shawn M Moore

NAME

Jifty::Manual::Preload - One Path to a Snappy UI

DESCRIPTION

Preloading lets you optimistically load regions before they are to be displayed. This improves user experience because preloaded updates are effectively instant; the user does not wait for an HTTP request/response cycle. The request/response cycle still happens, but it happens behind the scenes while the user is reading the current page, filling out a form, etc.

WARNING

Preloading is applicable only if your use case fulfills a specific set of constraints.

The region you are preloading must not depend greatly on the actions submitted by the current page. For example, you cannot sanely preload a region which includes the content of a textarea that the user is typing in the current page. When the preload occurs, the user probably has not even begun to type into the textarea yet.

On the other hand, the click of a button may be preloadable depending on what the button does. We use a nice tactic in Changelogger (http://changelogger.bestpractical.com) to preload vote buttons. We need to know which change to display to the user next, which is a nontrivial amount of database work. When rendering a button, we begin a transaction, submit a fake vote action, choose the next change, then rollback the transaction. This turns out to be a fairly simple way to figure out what to display next for preloading.

A region that is preloaded should not cause gratuitous side effects. It should never construct and submit actions on its own, mutate records, or anything else of that sort. Basically, the region should expect to be generated unconditionally. It can of course display a form, since that itself does not have side effects. It can also cause side effects that are rolled back, such as the example in the previous paragraph.

Preloading should also be agnostic of real-world time. If you preload a region that contains a timestamp, then that timestamp may be stale by the time the region is actually shown. Similarly for displaying duration. For example, at the end of Hiveminder's task review, we tell you exactly how long you spent in the review. This final report update cannot be preloaded because the user may spend ten minutes on the last task, which would not be reported if we preloaded that update.

Preloading can cause additional server load. Instead of a single request containing action submission and region updates, preloading sends a request containing action submission, then a request for each region update handler. The overhead of each request is probably nontrivial. However, the cost of preloading is probably worth it to improve user experience.

Finally, preloading is pretty new. It has not been battle tested. There may be serious race conditions that result in inconsistencies that confound your users. There may be data loss. It may interact strangely with other Jifty features. These things would be very difficult to debug.

Good luck!

USAGE

To mark a form handler as preloadable, use the preload => 'cache_key' option:

    form_submit(
        onclick => {
            submit       => $vote,
            preload      => 'vote',
            refresh_self => 1,
            arguments    => {
                change => $next_change->id,
            },
        },
    );

This marks the onclick handler as preloadable. When this button is rendered, the user's web browser will request this region immediately. When this button is clicked, the user's web browser will instantly refresh the region without having to send an AJAX request and wait for the response.

The value of preload is a cache key. This lets you reuse the same cache for preloaded regions. For example, if you're going to display ten vote buttons, then ordinarily you would make ten preload requests. However, if you give all of the buttons the vote cache key, only one preload request will be made. Obviously this means that the specific button being clicked should not matter. If you have a "undo vote" button which sends you backwards, you would not want to preload that with the same cache key as regular votes, since a different form would be displayed. In fact, you probably do not want to preload it all, since it's presumably a rare occurrence.

You may also pass a value of 1 for the preload key. This tells Jifty to generate a unique key for this preload so that you don't have to.

Note that the $vote action is submitted well after the next region has been preloaded!

GORY DETAILS

Basis

Preloading hijacks Jifty's ordinary AJAX update mechanism. When a preloadable element is rendered, we include a bit of additional javascript which immediately calls Jifty.preload. Thus the preload request is initiated around the time the page is rendered. Since the request is nonblocking, it should not noticeably affect the user experience.

When the user agent receives the response to the preload request, it is cached. Finally, when the user activates the handler that was preloaded, we run the cached response through the onSuccess update routine. This ends up being very fast since the user does not have to wait for request and response overhead, or the server processing time.

This is a slight simplification. In reality, a few forces make this process more complicated.

Many requests for a preload key

Suppose your form has many buttons which perform the same kind of region update. They all replace this vote form with the next vote form. It does not matter which specific button the user clicks, they're all going to preload the same vote form. Thus, it is desirable to make only one preload request for all of these buttons, instead of one preload request for each button.

When we initiate a preload request, we check to see if there is already a pending preload request for the given preload cache key. If so, we bail out. This way, only one request is made.

In the future, we may also bail out if a response exists for the preload key in the cache. We do not do this yet because of cache staleness concerns.

Activating handler before its request cycle finishes

If a user is quick, they may click a preloadable button during, or even before, that region's preload request/response cycle. The simple way to handle this would be to ignore that the handler has preloading and just continue with the usual update cycle, sending a fresh HTTP hit.

We can do better though. Since we know we already sent the preload request, we instead of just wait for its response instead of initiating a new request. We then mark that preload key as "wanted". This means that upon response, it will be immediately processed instead of waiting for its handler to be activated again.

Actions

Actions complicate the whole workflow. Ordinarily, we strip out action submission from region preload requests, since preloaded regions are supposed to be side-effect free. Rendering the button does not imply that the button will always be clicked, so preloading the onclick's region does not submit the action. Instead, the action is submitted when the onclick handler is activated by the user.

Given a page that has a button which preloads a refresh_self region update and submits an action, the following sequence of events happens.

The page is loaded

This renders a button which fires off...

The refresh_self preload request cycle occurs

This puts the unprocessed region into a cache for later use.

Time passes
The user clicks the button

Now a number of things occur pretty quickly.

A nonblocking AJAX request for the action submission occurs

This takes no noticeable time so the later region replacement still feels instantaneous.

Preloading is temporarily halted
The region replacement occurs

Ordinarily, this would render the button again, which includes some javascript to preload the next replace_self. However, preload submission is blocked until the action's response arrives. This is to make sure the action has been run before the next region is rendered, otherwise things could get too inconsistent. We block preloading because the action submission request and the preload request are not guaranteed to happen in order since they are separate connections. They could be routed differently.

In the future, we may do something similar to Nagle's algorithm where all such pieces end up in the same request.

While preloading is blocked, all preload requests go into a queue. When preloading becomes unblocked, all of the delayed preload requests will be executed.

The action response arrives
We display the action results

This could take the form of jGrowl updates or what have you, so the user still receives feedback about the actions they submitted.

Preloading is unblocked
Delayed preload requests are executed



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