Update: A blog post on the official Google blog entitled A fresh take on the browser states that the comic book went out early. It was supposed to show up tomorrow in tandem with the launch of the beta of Google Chrome which will be available in over 100 countries.

Phillipp Lessen was sent an announcement in the form of a comic book which gives the details on an upcoming Open Source browser project from Google. He gives the details and links to scanned images of the comic in his post Google Chrome, Google’s Browser Project. His site seems to have been running slow so he took down links to the scans. I managed to grab them and have uploaded them to a folder on Windows Live SkyDrive. The link is embedded below and the comic can be accessed directly from here.

The key features of Chrome according to the comic are

  • Based on WebKit
  • Each browser tab gets it's own process and Javascript execution is actually multithreaded instead of single threaded as it is in most modern browsers. This actually solves lots of memory problems with modern browsers since it reduces memory fragmentation and mitigates the impact of memory leaks since all memory is reclaimed when the bad Website's browser tab is closed.
  • Task manager shows how much resources each tab is using so you can tie resource usage to individual Web pages.
  • Created Javascript VM from scratch which has clever optimizations like just-in-time compilation and incremental garbage collection.
  • Each tab has it's own URL box and can effectively be considered it's own browser Window.
  • OmniBox URL bar is similar to AwesomeBar in Firefox 3 and the smart address bar in IE 8.
  • Instead of "about:blank" the default homepage shows our nine most visited Web pages and four most used search engines
  • Has an incognito mode where no browser history is saved and cookies are wiped out when the browser is closed. Some people have affectionately dubbed such features pr0n mode. Amusingly, the comic uses the same "planning a surprise birthday party" scenario that the Internet Explorer team used when describing a similar feature in IE 8. 
  • Pop ups are not modal and scoped to the tab which they were spawned from. However they can be promoted to becoming their own tab.
  • There is a "streamlined" mode where the URL box and browser toolbar are hidden so only the Web page is visible.
  • Web pages are sandboxed so that if the user hits a malware site it cannot access the user's computer and perform drive-by downloads or installations.
  • The sandbox model is broken by browser plugins (e.g. Flash) but this is mitigated by having the plugin execute in it's own separate process that is different from that of the browser's rendering engine.
  • The browser will continuously phone home to Google to get a list of known malware sites so that the user is warned when they are visited.
  • Will ship with Google Gears built-in

My initial thoughts on this are that this is a smart move on Google's part. Google depends on the usage and advancement of the Web for its success. However how quickly the Web is advanced is in the hands of browser vendors which probably doesn't sit well with them which is why they created Gears in the first place and hired the guy driving HTML 5. Chrome now gives them an even larger say in which way the Web goes.

As for their relationship with Firefox, it may be mutually beneficial financially since Mozilla gets paid while Google gets to be the search default in a popular browser but it doesn't mean Google can dictate their technical direction. With Chrome, has a way to force browser vendors to move the Web forward on their terms even if it is just by getting users to demand the features that exist in Chrome.

PS: Am I the only one that thinks that Google is beginning to fight too many wars on too many fronts. Android (Apple), OpenSocial (Facebook), Knol (Wikipedia), Lively (IMVU/SecondLife), Chrome (IE/Firefox) and that's just in the past year.

Now Playing: Boys Like Girls - Thunder


I'm often surprised by how common it is for developers to prefer reinventing the wheel to using off-the-shelf libraries when solving problems tasks. This practice isn't limited to newbies who don't know any better but also to experienced developers who should. Experienced developers often make excuses about not wanting to take unnecessary dependencies or not trusting the code of others when justifying reinventing the wheel. For example, take this conversation that flowed through my Twitter stream yesterday

Jon Galloway
jongalloway: @
codinghorror Oh, one last thing - I'd rather trust the tough code (memory management, SSL, parsing) to experts and common libraries. about 11 hours ago from Witty in reply to codinghorror

Jeff Atwood
codinghorror @jongalloway you're right, coding is hard. Let's go shopping! about 12 hours ago from web in reply to jongalloway

Jeff Atwood
codinghorror @jongalloway I'd rather make my own mistakes (for things I care about) than blindly inherit other people's mistakes. YMMV. about 12 hours ago from web in reply to jongalloway

The background on this conversation is that Jeff Atwood (aka codinghorror) recently decided to quit his job and create a new Website called stackoverflow.com. It is a question and answer site for asking programming questions where users can vote on the best answers to specific questions. You can think of it as Yahoo! Answers but dedicated to programming questions. You can read a review of the site by Michiel de Mare for more information.

Recently Jeff Atwood blogged about how he was planning to use regular expressions to sanitize HTML input on StackOverflow.com in his blog post entitled Regular Expressions: Now You Have Two Problems where he wrote

I'd like to illustrate with an actual example, a regular expression I recently wrote to strip out dangerous HTML from input. This is extracted from the SanitizeHtml routine I posted on RefactorMyCode.

var whitelist =

What do you see here? The variable name whitelist is a strong hint. One thing I like about regular expressions is that they generally look like what they're matching. You see a list of HTML tags, right? Maybe with and without their closing tags?

The problem Jeff was trying to solve is how to allow a subset of HTML tags while stripping out all other HTML so as to prevent cross site scripting (XSS) attacks. The problem with Jeff's approach which was pointed out in the comments by many people including Simon Willison is that using regexes to filter HTML input in this way assumes that you will get fairly well-formed HTML. The problem with that approach which many developers have found out the hard way is that you also have to worry about malformed HTML due to the liberal HTML parsing policies of many modern Web browsers. Thus to use this approach you have to pretty much reverse engineer every HTML parsing quirk of common browsers if you don't want to end up storing HTML which looks safe but actually contains an exploit. Thus to utilize this approach Jeff really should have been looking at using a full fledged HTML parser such as SgmlReader or Beautiful Soup instead of regular expressions.

It didn't take long for the users of StackOverflow.com to show Jeff the error of his ways as evidenced by his post Protecting Your Cookies: HttpOnly where he acknowledges his mistake as follows

So I have this friend. I've told him time and time again how dangerous XSS vulnerabilities are, and how XSS is now the most common of all publicly reported security vulnerabilities -- dwarfing old standards like buffer overruns and SQL injection. But will he listen? No. He's hard headed. He had to go and write his own HTML sanitizer. Because, well, how difficult can it be? How dangerous could this silly little toy scripting language running inside a browser be?

As it turns out, far more dangerous than expected.

Imagine, then, the surprise of my friend when he noticed some enterprising users on his website were logged in as him and happily banging away on the system with full unfettered administrative privileges.

How did this happen? XSS, of course. It all started with this bit of script added to a user's profile page.

<img src=""http://www.a.com/a.jpg<script type=text/javascript 
src="">" /><<img 

Through clever construction, the malformed URL just manages to squeak past the sanitizer. The final rendered code, when viewed in the browser, loads and executes a script from that remote server. 

The sad thing is that Jeff Atwood isn't the first nor will he be the last programmer to think to himself "It's just HTML sanitization, how hard can it be?". There are many lists of Top HTML Validation Bloopers that show tricky it is to get the right solution to this seemingly trivial problem. Additionally, it is sad to note that despite his recent experience, Jeff Atwood still argues that he'd rather make his own mistakes than blindly inherit the mistakes of others as justification for continuing to reinvent the wheel in the future. That is unfortunate given that is a bad attitude for a professional software developer to have.

Rolling your own solution to a common problem should be the last option on your list not the first. Otherwise, you might just end up a candidate for The Daily WTF and deservedly so.

Now Playing: T-Pain - Cant Believe It (feat. Lil Wayne)


Categories: Programming

Paul Buchheit of FriendFeed has written up a proposal for a new protocol that Web sites can implement to reduce the load on their services from social network aggregators like FriendFeed and SocialThing. He unveils his proposal in his post Simple Update Protocol: Fetch updates from feeds faster which is excerpted below

When you add a web site like Flickr or Google Reader to FriendFeed, FriendFeed's servers constantly download your feed from the service to get your updates as quickly as possible. FriendFeed's user base has grown quite a bit since launch, and our servers now download millions of feeds from over 43 services every hour.

One of the limitations of this approach is that it is difficult to get updates from services quickly without FriendFeed's crawler overloading other sites' servers with update checks. Gary Burd and I have thought quite a bit about ways we could augment existing feed formats like Atom and RSS to make fetching updates faster and more efficient. Our proposal, which we have named Simple Update Protocol, or SUP, is below.
Sites wishing to produce a SUP feed must do two things:

  • Add a special <link> tag to their SUP enabled Atom or RSS feeds. This <link> tag includes the feed's SUP-ID and the URL of the appropriate SUP feed.
  • Generate a SUP feed which lists the SUP-IDs of all recently updated feeds.

Feed consumers can add SUP support by:

  • Storing the SUP-IDs of the Atom/RSS feeds they consume.
  • Watching for those SUP-IDs in their associated SUP feeds.

By using SUP-IDs instead of feed urls, we avoid having to expose the feed url, avoid URL canonicalization issues, and produce a more compact update feed (because SUP-IDs can be a database id or some other short token assigned by the service). Because it is still possible to miss updates due to server errors or other malfunctions, SUP does not completely eliminate the need for polling.

Although there's a healthy conversation about SUP going on in FriendFeed in response to one of my tweets, I thought it would be worth sharing some thoughts on this with a broader audience.

The problem statement that FriendFeed's SUP addresses is the following issue raised in my previous post When REST Doesn't Scale, XMPP to the Rescue?

On July 21st, FriendFeed had 45,000 users who had associated their Flickr profiles with their FriendFeed account. FriendFeed polls Flickr about once every 20 – 30 minutes to see if the user has uploaded new pictures. However only about 6,000 of those users logged into Flickr that day, let alone uploaded pictures. Thus there were literally millions of HTTP requests made by FriendFeed that were totally unnecessary.

FriendFeed's proposal is similar to the Six Apart Update Stream and the Twitter XMPP Firehose in that it is a data stream containing information about all of the updates users are making on a particular service. It differs in a key way in that it doesn't actually contain the data from the user updates but instead identifiers which can be used to determine the users that changed so their feeds can be polled.

This approach aims at protecting feeds that use security through obscurity such as the Google Reader's Shared Items feed and Netflix's Personalized Feeds. The user shares their "secret" feed URL with FriendFeed who then obtains the SUP ID of the user's feed when the feed is first polled. Then whenever that SUP ID is seen in the SUP feed by FriendFeed, they know to go re-poll the user's "secret" feed URL.

For services that are getting a ton of traffic from social network aggregators or Web-based feed readers it does make sense to provide some sort of update stream or fire hose to reduce the amount of polling that gets done. In addition, it also makes sense that if more and more services are going to provide such update streams then it should be standardized so that social network aggregators and similar services do not end up having to target multiple update protocols.

I believe that at the end we will see a continuum of options in this space. The vast majority of services will be OK with the load generated by social networking aggregators and Web-based feed readers when polling their feeds. These services won't see the point of building additional features to handle this load. Some services will run the numbers like Twitter & Six Apart have done and will provide update streams in an attempt to reduce the impact of polling. For these services, SUP seems like a somewhat elegant solution and it would be good to standardize on something, anything at all is better than each site having its own custom solution. For a smaller set of services, this still won't be enough since they don't provide feeds (e.g. Blockbuster's use of Facebook Beacon) and you'll need an explicit server to server publish-subscribe mechanism. XMPP or perhaps something an HTTP based publish-subscribe mechanism like what Joshua Schachter proposed a few weeks ago will be the best fit for those scenarios. 

Now Playing: Jodeci - I'm Still Waiting


Categories: Web Development

I've been reading about the Ning vs. WidgetLaboratory drama on TechCrunch. The meat of the conflict seems to be that widgets from WidgetLaboratory were so degrading the user experience of Ning that they had to be cut off. The relevant excerpts from the most recent TechCrunch story on the war of words are below

For those of you not closely following the drama between social network platform Ning and a popular widget provider called WidgetLaboratory, you can read the background here. On Friday Ning unceremoniously shut down their access to Ning, making all those widgets vanish.
In an email to WL on August 2 (more than three weeks ago), CEO Gina Bianchini wrote “Our only goal is to have you build your products in such a way that doesn’t slow down the networks running your products or takedown the Ning Platform with what you’re doing. Both of those would result in us needing to shutdown WidgetLaboratory products and that’s has never been our first choice of options. Hopefully, you know this after 8 months of working with us.”

Ignoring the he said, she said nature of the communication between both companies, there is a legitimate concern that 3rd party widgets included on the pages of a Web site can degrade the performance to the extent that the site becomes unusably slow. In fact, TechCrunch has had similar problems with 3rd party widgets as Mike Arrington has mentioned on his personal blog which led to him excluding the widgets from his site.

Typically, widgets are embedded in a site by including references to Javascript hosted on a 3rd party site in the page's HTML. This means rendering the page is dependent on how quickly the script files can be downloaded from the 3rd party site AND how long it takes for the script to execute especially since it may also fetch data from one or more servers as well. Thus a slow server or a badly written script can make every page that embeds the widget unbearably slow to render. Given that the ability to embed widgets is a key feature of social networking sites, it is important for such sites to figure out how to isolate their user experience from badly written widgets or widgets hosted on slow Web servers.

Below are some best practices that have emerged on how social networking sites can immunize themselves from the kinds of problems Ning has had with WidgetLaboratory

  1. Host the Scripts Yourself: If you have a popular site, it is quite likely that you have more resources to handle lots of page views than the typical widget developer. Thus it makes sense to take away the dependency on externally hosted scripts by hosting the widgets yourself. Microsoft encourages developers to submit their gadgets to Windows Live Gallery if they want to build gadgets for my.live.com or Windows Live Spaces. For it's AJAX homepage service, Google does not require developers to submit gadgets to them for hosting but instead caches gadget data for hours at a time which means they are effectively hosting the gadgets themselves for the majority of the accesses by their users.

  2. Keep External Dependencies off of Pages that Need to Render Quickly: In many cases, it isn't feasible to host all of the data and content related to widgets that are being shown on your site. In that case, you should ensure that the key scenarios on your Web site are insulated from the problems caused by slow or broken 3rd party widgets. For example, on Facebook viewing someone's profile is a key part of the user experience that is important to make sure happens as quickly and as smoothly as possible. For this reason, Facebook caches all 3rd party content that shows up on a user's profile and requires applications to call Profile.SetFBML to add content to the profile instead of providing a way to directly embed widgets on a user's profile.

  3. Make It Clear Who Is to Blame if Things go Awry: One of the issues raised by Ning in their conflict with WidgetLaboratory is that user pages wouldn't render correctly or would show degraded performance due to WidgetLaboratory's widgets but Ning would get the support calls. This kind of user confusion is avoided if the user experience makes it clear when the failure of a page to render correctly is the fault of the external widget and when it is part of the hosting site. For example, Facebook Canvas Pages for applications make it clear that the user is using a 3rd party application and not part of the core Facebook experience. I've seen lots of user complain about the slowness of Scrabulous and Scrabble but never seen anyone who thought that Facebook was to blame and not the application developers.

Following some of these practices would have saved Ning and its users some of their current grief.

Now Playing: Ice Cube - Get Money, Spend Money, No Money


Categories: Platforms | Social Software

My recent post, Explaining REST To Damien Katz, led to some insightful comments from Dave Winer and Tim Bray about what proponents of building RESTful Web services can learn from remote procedure call (RPC) technologies like SOAP and XML-RPC. 

In his post, Dare left something out (and it's important) Dave Winer wrote

Really ought to include it in your thinking, Dare and everyone else. You're missing out on something that works really well. You should at least learn the lessons and add to REST what it needs to catch up with XML-RPC. Seriously.

What's missing in REST, btw, is a standard method of serializing structs, lists and scalar types. The languages we use have a lot more in common than you might think. We're all writing code, again and again, every time we support a new interface that could be written once and then baked into the kernels of our languages, and then our operating systems. Apple actually did this with Mac OS, XML-RPC support is baked in. So did Python. So if you think it's just me saying this, you should take another look.

Dave has a valid point, a lot of the time communication between distributed systems is simply passing around serialized objects and/or collections of objects. In such cases, having a lightweight standard representation for serialized objects which is supported on multiple platforms would be beneficial. Where Dave goes astray is in his assertion that no such technology exists for developers building RESTful Web services. Actually one does, and it is widely deployed on the Web today. JavaScript Object Notation (JSON) which is described in RFC 4627 is a widely deployed and well-defined media type (application/json) for representing serialized structs, lists and scalar values on the Web. 

There are libraries for processing JSON on practically every popular platform from "corporate" platforms like Java and the .NET Framework to Web scripting languages like Python and Ruby. In addition, JSON is attractive because it is natively available in modern Web browsers that support JavaScript which means you can use it to build services that can be consumed by Web browsers, other Web services or desktop applications with a single end point.

Tim Bray cautioned REST proponents against blindly rejecting the features typically associated with RPC systems and SOAP/WS-* in his post REST Questions where he wrote

Has REST Been Fortunate in its Enemies? · I have been among the most vocal of those sneering at WS-*, and I’m comfortable with what I’ve said. But that doesn’t prove that the core WS-* ideas are wrong. Here are some of the handicaps WS-* struggled under:

  • Lousy foundational technologies (XSD and WSDL).

  • A Microsoft/IBM-driven process that was cripplingly product-linked and political.

  • Designers undereducated in the realities of the Web as it is.

  • Unnecessary detours into Architecture Astronautics.

As a result, we should be really careful about drawing lessons from the failure of WS-*. Specifically:

  • Just because the XSD type system is malformed, you can’t conclude that the notion of schema-driven mapping between program data types and representation payloads is harmful.

  • Just because WSDL is a crock, you can’t conclude that exposing a machine-readable contract for a service is a necessarily bad idea.

  • Just because UDDI never took off, you can’t conclude that service registries are dumb.

  • Just because SOAP has a damaging MustUnderstand facility and grew a lot of ancillary specification hair, you can’t conclude that some sort of re-usable payload wrapper is necessarily a dead-end path.

  • Just because the WS-* security specifications are overengineered and based on a shaky canonicalization spec, you can’t conclude that message-level security and signing aren’t sometimes real important.

And so on. I personally tend to think that schema-driven mapping is hopeless, contracts are interesting, registries are a fantasy, and payload wrappers are very promising. But I don’t think that the history of WS-* is a very good argument for any of those positions.

In a lot of situations where applications consume XML, the first thing the application does is convert the XML into an object graph representative of the business logic of the application. The SOAP/WS-* way of doing this was to define an XSD schema for the XML payload and then use some object<->XML mapping layer to convert the XML to objects. The problem with this approach was that there is a fundamental impedance mismatch between XSD types and OO types which led to horrible interoperability problems since no two platforms could agree on how to map the various esoteric type system features of XSD into the structs, lists and scalar types that are the building block of all OO type systems. However these problems go away if you use a data format that was explicitly designed for describing serialized data objects like JSON.

Providing a machine readable description of a service's end points is useful, especially on the Web where multiple services may expose the same interface. For example, when you visit my weblog at http://www.25hoursaday.com/weblog/ using Firefox 2 or higher and Internet Explorer 7 or higher the browser immediately lights up with a feed icon which allows you to subscribe to my Atom feed from your Web browser. This happens because I've provided a machine readable description of my feed end points on my blog. The Atom Publishing Protocol, which is one of the most well-designed RESTful protocols out there, has a notion of service documents which enable client applications to discover the capabilities and locations of the various end points of the service.

If you put together the notion of service documents with using JSON as the payload format for a service endpoint, you're close to getting the touted programmer friendliness of RPC technologies like XML-RPC & SOAP/WSDL while still building a RESTful service which works with the Web instead of against it.

The only problem is how to deal with statically typed languages like C# and Java. These languages need the types of the objects that application will consume from a Web service defined up front. So if we could just figure out how to come up with service documents for JSON services that included the notion of a class definition, we could pretty much have our cake and eat it to with regards to getting the ease of use of an RPC system while building a RESTful service.

If this sounds interesting to you, then you should go over and read Joe Gregorio's original post on RESTful JSON and then join the restful-json mailing list. Joe's proposal is on the right path although I think he is letting his background as an editor of the Atom Publishing Protocol specification skew his thinking with regards to what developers would find most useful from a Json Publishing Protocol (JsonPub).

Now Playing: G-Unit - Beg For Mercy


August 24, 2008
@ 11:32 AM

Last week my blog was offline for a day or so because I was the victim of a flood of SQL injection attacks that are still hitting my Web site at the rate of multiple requests a second. I eventually managed to counter the attacks by installing URLScan 3.0 and configuring it to reject HTTP requests that resemble SQL injection attacks. I found out about URLScan in two ways; from a blog post Phil Haack wrote about Dealing with Denial of Service Attacks where it seems he's been caught up in the same wave of attacks that brought down my blog and via an IM from Scott Hanselman who saw my tweet on Twitter about being hacked and pointed me to his blog post on the topic entitled Hacked! And I didn't like it - URLScan is Step Zero.

This reminded me that I similarly found another useful utility, WinDirStat, via a blog post as well. In fact when i think about it, a lot of the software I end up trying out is found via direct or indirect recommendations from people I know. Typically through blog posts, tweets or some other communication via a social networking or social media service. This phenomenon can be clearly observed in closed application ecosystems like the Facebook platform, where statistics have shown that the majority of users install new applications after viewing them on the profiles of their friends.

One of the things I find most interesting about the Facebook platform and now the Apple App Store is that they are revolutionizing how we think about software distribution. Today, finding interesting new desktop/server/Web apps either happens serendipitously via word of mouth or [rarely] is the result of advertising or PR. However finding interesting new applications if you are a user of Facebook or the Apple iPhone isn't a matter of serendipity. There are well understood ways of finding interesting applications that harnesses social and network effects from user ratings to simply finding out what applications your friends are using.

As a user, I sometimes wish I had an equivalent experience as a user of desktop applications and their extensions. I've often thought it would be cool to be able to browse the software likes and dislikes of people such as Omar Shahine, Scott Hanselman and Mike Torres to see what their favorite Windows utilities and mobile applications were. As a developer of a feed reader, although it is plain to see that Windows has a lot of reach since practically everyone runs it sometimes I'm envious of the built in viral distribution features that come with the Facebook platform or the unified software distribution experience that is the iPhone App Store. Sure beats hosting your app on SourceForge and hoping that your users are blogging about the app to spread it via word of mouth or paying for prominence on sites like Download.com.

A lot of the pieces are already there. Microsoft has a Windows Marketplace but for the life of me I'd have never found out about it if I didn't work at Microsoft and someone I know switched teams to start working there. There are also services provided by 3rd parties like Download.com, the Firefox Add-Ons page and Tucows. It would be interesting to see what could be stitched together if you throw in a social graph via something like Facebook Connect, an always-on well integrated desktop experience similar to the Apple App Store and one of the aforementioned sites. I suspect the results would be quite beneficial to app developers and users of Windows applications.

What do you think?

Now Playing: Metallica - The Day That Never Comes


Categories: Technology

According to Werner Vogels's blog post entitled Amazon EBS - Elastic Block Store has launched, it seems that my friends at Amazon have plugged a gaping hole in their cloud computing platform story. Werner writes

Back in the days when we made the architectural decision to virtualize the internal Amazon infrastructure one of the first steps we took was a deep analysis of the way that storage was used by the internal Amazon services. We had to make sure that the infrastructure storage solutions we were going to develop would be highly effective for developers by addressing the most common patterns first. That analysis led us to three top patterns:

  1. Key-Value storage. The majority of the Amazon storage patterns were based on primary key access leading to single value or object. This pattern led to the development of Amazon S3.
  2. Simple Structured Data storage. A second large category of storage patterns were satisfied by access to simple query interface into structured datasets. Fast indexing allows high-speed lookups over large dataset. This pattern led to the development of Amazon SimpleDB. A common pattern we see is that secondary keys to objects stored in Amazon S3 are stored in SimpleDB, where lookups result in sets of S3 (primary) keys.
  3. Block storage. The remaining bucket holds a variety of storage patterns ranging special file systems such as ZFS to applications managing their own block storage (e.g. cache servers) to relational databases. This category is served by Amazon EBS which provides the fundamental building block for implementing a variety of storage patterns.

What I like about Werner's post is that it shows that Amazon had a clear vision and strategy around providing hosted cloud services and has been steadily executing on it.

S3 handled what I've typically heard described as "blob storage". A typical Web application typically has media files and other resources (images, CSS stylesheets, scripts, video files, etc) that is simply accessed by name/path. However a lot of these resources also have metadata (e.g. a video file on YouTube has metadata about it's rating, who uploaded it, number of views, etc) which need to be stored as well. This need for queryable, schematized storage is where SimpleDB comes in. EC2 provides a virtual server that can be used for computation complete with a local file system instance which isn't persistent if the virtual server goes down for any reason. With SimpleDB and S3 you have the building blocks to build a large class of "Web 2.0" style applications when you throw in the computational capabilities provided by EC2.

However neither S3 nor SimpleDB provides a solution for a developer who simply wants the typical LAMP or WISC developer experience of building a database driven Web application or for applications that may have custom storage needs that don't fit neatly into the buckets of blob storage or schematized storage. Without access to a persistent filesystem, developers on Amazon's cloud computing platform have had to come up with sophisticated solutions involving backing data up manually from EC2 to S3 to get the desired experience.

EBS is the final piece in the puzzle that had prevented Amazon's cloud computing platform from being comparable to traditional hosting solutions. With EBS Amazon is now superior to most traditional hosting solutions from a developer usability perspective as well as cost. Google App Engine now looks like a plaything in comparison. In fact, you could build GAE on top of Amazon's cloud computing platform now that the EBS has solved persistent custom storage problem. It will be interesting to see if higher level cloud computing platforms such as App Engine start getting built on top of Amazon's cloud computing platform. Simply porting GAE wholesale would be an interesting academic exercise and a fun hacking project. 

Now Playing: T.I. - Whatever You Like


Last year I wrote a blog post entitled When Databases Lie: Consistency vs. Availability in Distributed Systems where I talked about the kinds of problems Web applications face when trying to keep data consistent across multiple databases spread out across the world.

Jason Sobel, a developer at Facebook has some details on how they've customized MySQL to solve a variation of the problem I posed in a blog post entitled Scaling Out where he writes

A bit of background on our caching model: when a user modifies a data object our infrastructure will write the new value in to a database and delete the old value from memcache (if it was present). The next time a user requests that data object we pull the result from the database and write it to memcache. Subsequent requests will pull the data from memcache until it expires out of the cache or is deleted by another update.


Consider the following example:

  1. I update my first name from "Jason" to "Monkey"
  2. We write "Monkey" in to the master database in California and delete my first name from memcache in California and Virginia
  3. Someone goes to my profile in Virginia
  4. We don't find my first name in memcache so we read from the Virginia slave database and get "Jason" because of replication lag
  5. We update Virginia memcache with my first name as "Jason"
  6. Replication catches up and we update the slave database with my first name as "Monkey"
  7. Someone else goes to my profile in Virginia
  8. We find my first name in memcache and return "Jason"

Until I update my first name again or it falls out of cache and we go back to the database, we will show my first name as "Jason" in Virginia and "Monkey" in California. Confusing? You bet. Welcome to the world of distributed systems, where consistency is a really hard problem.
Fortunately, the solution is a lot easier to explain than the problem. We made a small change to MySQL that allows us to tack on extra information in the replication stream that is updating the slave database. We used this feature to append all the data objects that are changing for a given query and then the slave database "sees" these objects and is responsible for deleting the value from cache after it performs the update to the database.


The new workflow becomes (changed items in bold):

  1. I update my first name from "Jason" to "Monkey"
  2. We write "Monkey" in to the master database in California and delete my first name from memcache in California but not Virginia
  3. Someone goes to my profile in Virginia
  4. We find my first name in memcache and return "Jason"
  5. Replication catches up and we update the slave database with my first name as "Monkey." We also delete my first name from Virginia memcache because that cache object showed up in the replication stream
  6. Someone else goes to my profile in Virginia
  7. We don't find my first name in memcache so we read from the slave and get "Monkey"

Facebook's solution is clever and at first I couldn't shake the feeling that it is an example of extremely tight coupling for database replication to also be responsible for evicting expired items from your in-memory cache. After some thought, I realized that this is no different from the SqlCacheDependency class in ASP.NET which allows you to create a dependency between objects in your ASP.NET cache and those in your SQL database. When the underlying tables change, the Cache is updated to reflect this change in database state.

In fact, the combination of replication and the SqlCacheDependency class should mean that you get this sort of behavior for free if you are using ASP.NET caching and SQL Server. Unfortunately, it looks like Microsoft's upcoming in-memory distributed object caching product, Velocity, won't support SqlCacheDependency in its initial release according to a comment by one its developers.  

Of course, there is a significant performance difference between actively monitoring the database for changes like SqlCacheDependency does and updating the cache when updates made to the database are received as part of the replication stream. I wonder if this pattern will turn out to be generally useful to Web developers (at least those of us who work on geo-distributed services) or whether this will just go down as a clever hack from those kids at Facebook that was cool to share.

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Categories: Web Development

August 17, 2008
@ 12:33 PM

Damien Katz recently caused a stir on a bunch of the blogs I read with his post entitled REST, I just don't get it where he wrote

As the guy who created CouchDB, I should be a big cheerleader for RESTful architectures. But the truth is, I just don't get it.

For CouchDB, REST makes absolutely insanely perfect sense. Read a document, edit, put the document back. Beautiful. But for most applications, enterprise or not, I don't see what the big win is.

I know what is wrong with SOAP, and it has everything to do with unnecessary complexity and solving the same problems twice. But what is the big advantage of making all your calls into GET PUT and DELETE? If POST can handle everything you need, then what's the problem?

I guess what I mean to say is just because SOAP is a disaster, doesn't somehow make REST the answer. Simpler is better, and REST is generally simpler than SOAP. But there is nothing wrong with a plain old POST as an RPC call. If its easy to make all your calls conform to the RESTful verb architecture, then that's good, I guess.

His post made the rounds on the expected social news sites like programming.reddit and Hacker News, where I was amused to note that my blog is now being used as an example of silly REST dogma by REST skeptics in such discussions. From reading the Damien's post and the various comments in response, it seems clear that there are several misconceptions as to what constitutes REST and what its benefits are from a practical perspective.

Background: The Origins of REST vs. SOAP

The Representational State Transfer (REST) architectural style was first described in Chapter 5 of Roy Fielding's Ph.D dissertation published in 2000. It describes the architecture of the Web from the perspective of one of the authors of the HTTP 1.1 specification which was published the year before in 1999. Around the same time Don Box, Dave Winer and a bunch of folks at Microsoft came up with the Simple Object Access Protocol (SOAP) which they intended to be the standard protocol for building distributed applications on the Web.

Over the following years SOAP was embraced by pretty much every major enterprise software vendor and was being pushed hard by the W3C as the way to build distributed applications on the Web. However a lot of these vendors weren't really interested in building software for the Web but instead were more interested in porting all of their technologies and scenarios from enterprise integration technologies like CORBA to using buzzword compliant XML. This led to a bunch of additional specifications like XSD (type system), WSDL (IDL) and UDDI (naming/trading service). Developers initially embraced these technologies enthusiastically which led to the enterprise software vendors pumping out dozens of WS-* specifications. During this period not many thought or talked much about REST since no one talks about boring Ph.D dissertations. 

In 2002, a canary in the coal mine emerged in the form of Mark Baker. On mailing lists frequented by Web services types such as xml-dev and xml-dist-app, Mark began to persistently point out that SOAP was built on a bad foundation because it fundamentally ignored the architecture of the Web as defined by Roy Fielding's thesis. At first a lot of people labeled mark as a kook or malcontent for questioning the trend of the moment.

By 2005, the industry had enough  experience with SOAP to start seeing real problems using at as a way to build distributed applications on the Web. By that year many developers had started hearing stories like Nelson Minar's presentation on the problems Google had seen with SOAP based Web services and sought a simpler alternative. This is when the seeds of Mark Baker's evangelism of Roy's dissertation eventually bore fruit with the Web developer community.

However a Ph.D dissertation is hard to digest. So the message of REST started getting repackaged into simpler, bite-sized chunks but the meat of the message started getting lost along the way. Which led to several misconceptions about what REST actually is being propagated across the Web developer community.

Misconceptions About the REST Architectural Style

With that out of the way I can address the straw man argument presented in Damien's post. Damien states that building a RESTful Web Service is about using the HTTP PUT and DELETE methods instead of using HTTP POST when designing a Web API. In fact, he goes further to describe it as "the RESTful verb architecture" implying that choice of HTTP verbs that your service supports is the essence of REST.

This is incorrect.

Q: What is the Essence of REST? A: The Uniform Interface

REST explains how the Web works by defining the set of constraints on the various components in the current Web architecture. These constraints include

  • interaction is based on the client-server architectural style. User agents like Web browsers, RSS readers, Twitter clients, etc are examples of Web clients which talk to various Web servers without having a tight coupling to the internal implementation of the server.

  • communication between the client and server is stateless. The benefits of HTTP being a primarily stateless protocol is that statelessness increases scalability and reliability of services at the cost of introducing some complexity on the part of the client.

  • the Web architecture supports caching by requiring that requests that are cacheable or non-cacheable are labeled as such (i.e. via HTTP method and various caching related headers).

  • there is a uniform interface between components which allows them to communicate in a standard way but may not be optimized for specific application scenarios. There are four interface constraints: identification of resources; manipulation of resources through representations; self-descriptive messages; and, hypermedia as the engine of application state.

  • there can be multiple layers between client and server which act as intermediaries (e.g. proxies, gateways, load balancers, etc) without this being obvious to the requesting client or accepting server.

When you read the above list, the first thing you will note is that it describes the architecture of the World Wide Web. It doesn't describe the architecture of "a typical enterprise" or the internals of a cloud computing application.

Building a RESTful Web Service simply means paying attention to these characteristics of the Web. As you read them, some practical guidelines start becoming obvious. For example, if you want to take advantage of all the caching infrastructure that is built into the Web infrastructure, then you should use HTTP GET for services that retrieve data. This is just one of the many things Web Services built on SOAP got wrong.

The uniform interface constraints describe how a service built for the Web can be a good participant in the Web architecture. These constraints are described briefly as follows

  1. Identification of resources: A resource is any information item that can be named and represented (e.g. a document, a stock price at a given point in time, the current weather in Las Vegas, etc). Resources in your service should be identified using URIs.

  2. Manipulation of resources via representations: A representation is the physical representation of a resource and should correspond to a valid media type. Using standard media types as the data formats behind your service increases the reach of your service by making it accessible to a wide range of potential clients. Interaction with the resource should be based on retrieval and manipulation of the representation of the resource identified by its URI.

  3. Self-descriptive messages: Following the principles of statelessness in your service's interactions, using standard media types and correctly indicating the cacheability of messages via HTTP method usage and control headers ensures that messages are self descriptive. Self descriptive messages make it possible for messages to be processed by intermediaries between the client and server without impacting either.

  4. Hypermedia as the engine of application state: Application state should be expressed using URIs and hyperlinks to transition between states. This is probably the most controversial and least understood of the architectural constraints set forth in Roy Fielding's dissertation. In fact, Fielding's dissertation contains an explicit arguments against using HTTP cookies for representing application state to hammer this point home yet it is often ignored.

Benefits of Conforming to REST and the Uniform Interface to Web Developers

At this point, the benefits of building RESTful services for the Web should be self evident. The Web has a particular architecture and it makes sense that if you are deploying a service or API on the Web then it should take advantage of this architecture instead of fighting against it. There are millions of deployed clients, servers and intermediaries that support REST and it makes sense to be compatible with their expectations.

This doesn't mean you have to use DELETE and PUT when POST might suffice. It does mean understanding the difference between using POST versus using PUT to other participants in the Web architecture. Specifically, that PUT is idempotent while POST is not so a client of your service can assume that performing the same PUT two or three times in a row has the same effect as doing it once but cannot assume that for POST. Of course, it is up to you as a Web service developer to decide if you want your service to provide a more explicit contract with clients or not. What is important to note is that there is a practical reason for making the distinction between which HTTP verbs you should support.

There are other practical things to be mindful of as well to ensure that your service is being a good participant in the Web ecosystem. These include using GET instead of POST when retrieving a resource and properly utilizing the caching related headers as needed (If-Modified-Since/Last-Modified, If-None-Match/ETag, Cache-Control),  learning to utilize HTTP status codes correctly (i.e. errors shouldn't return HTTP 200 OK), keeping your design stateless to enable it to scale more cheaply and so on. The increased costs, scalability concerns and complexity that developers face when they ignore these principles is captured in blog posts and articles all over the Web such as Session State is Evil and Cache SOAP services on the client side. You don't have to look hard to find them. What most developers don't realize is that the problems they are facing are because they aren't keeping RESTful principles in mind.

Don't fight the Web, embrace it.


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cognitive dissonance
n. Psychology.

A condition of conflict or anxiety resulting from inconsistency between one's beliefs and one's actions, such as opposing the slaughter of animals and eating meat.

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Categories: Current Affairs