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