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