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Posts Tagged ‘scalability

Hypermedia and dynamic contracts: let my bandwidth rest!

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“Break it” to scale!

Many systems contain webpages that are very similar to user “custom pages”, where they can configure what they want to see, and every piece is aggregated from different sources into one single page.

In some cases, those are widget based frameworks as wicket and gwt that can be added to my custom page; in other cases you have aggregating portals.

An example of this kind of application (even though its not configurable) is a retail website containing four sections in its home page: the top 10, my orders, random items, and weird items.

In this case, all information come from the same source, but every part has a different probable validity if it is going to be cached. If the page is served as one big chunck of information, it will always be stale due to the random items section. “My orders” is stale only when I place a new order and, in the same way, the top 10 is only stale if any item is bought and surpasses the number of times the 10th item was bought so far.

One of the main issues with this type of pages which aggregate information from one or many sources with different expire-expectations is that cached versions in proxies and clients become stale faster than it should for some elements: once one of this providing sources publishes new information or is updated, the entire representation becomes stale..

Martin Fowler described once a well spread approach to allow those pages to be partially cached within local proxies and clients, thus sharing requested representations between multiple users.

The approach

Given the coffee scenario, one would create different json representations:

And finally an aggregating page:

<html>
<a class="lazy_load" href="http://restbucks.com/top_sellers">Top sellers</a>
<a class="lazy_load" href="http://restbucks.com/my_orders">My orders</a>
<a class="lazy_load" href="http://restbucks.com/random_items">Random items</a>
<a class="lazy_load" href="http://restbucks.com/weird_items">Weird items</a>

And then, for each lazy_load link, we create a div with its content:

<script>
$('.lazy_load').each(function(link) {
  uri = link.attr('href'); 
  div = $('
').load(uri); // cache hits! link.after(div); }); </script> </html>

This allows our proxies to cache each component in our page apart from the page itself: whenever one page’s content becomes stale in a proxy, only part of that page needs update.

In a web were most data can be cached and does not become stale so fast, this technique should usually lessen the amount of data being transfered between client and server.

All one needs to do is properly use the http headers for caching.

Remember that if your client supports either parallel requests to the server and/or keep-alive connection, the results might be even better.

Distributed systems? Linked resources?

Roy Fielding mentions that in the data view in REST systems, “small or medium-grain messages are used for control semantics, but the bulk of application work is accomplished via large-grain messages containing a complete resource representation.”

Pretty much in the same way as with the human web, a distributed system using the web as its infrastructure will gain the same cache benefits as long as they implement correct caching policies through http headers (and correct http verbs).

When your server provides a resource representation linking to a series of other related resources the client and proxies staying on the way will be allowed to cache each and every other resource on its own.

This approach results, again, in changes applied to one resource not affecting cached representations of other resources. An stale representation will not affect those accessing other resources within the same context.

Sometimes the decision whether to change latency for scalability might depend on how you think your clients will use your resources: in the human web mentioned above, the developer knew exactly how its clients would access it.

In distributed systems using REST, guessing how resources will be used can be dangerous as it allows you to tight couple yourself to this behaviour while published resources can and would be used in unforeseen ways.

Roy’s dissertation seems to apply here to balance things: “a protocol that requires multiple interactions per user action, in order to do things like negotiate feature capabilities prior to sending a content response, will be perceptively slower than a protocol that sends whatever is most likely to be optimal first and then provides a list of alternatives for the client to retrieve if the first response is unsatisfactory”.

Giving information that will help most cases is fine and providing links to further resources details allow you to balance between latency and scalability (due to caching) as you wish.

Dynamic contracts

This is only possible because we have signed dynamic contracts with our clients. They expect us to follow some formal format definition (defined in xhtml) and processes. How our processes are presented within our representations is the dynamic part of the contract.

While the fixed part can be validated with the use of schema validators, the dynamic part – the process – which is guided by our server needs to be validated through testing the behaviour of our applications: asserting that hypermedia guided transitions should be reflected in our application state.

Nowadays

On the other hand, many contemporary systems use the POST verb receiving a response including many representations at once or the GET verb without any cache related headers: thus not profiting from the web infrastructure at all. This could changed with one (or both) of the following:

  • use the GET verb with cache headers
  • use hypermedia and micro formats to describe relations between resources

Using it might present similar results as hypermedia+GET+cache headers in the human web – and some styles might already be providing support for it, although not being a constraint.

Note that in this case hypermedia is not driving the application state, but helping with scalability issues.

Progressive enhancement

Martin notes that this is a kind of progressive enhancement: although its definition is related to accessibility, its control over bandwidth benefits are similar to the approach mentioned ones.

Any other systems that use hyperlinks to “break” representations and scale?

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Written by guilhermesilveira

December 10, 2009 at 9:15 am