|By Jason Bloomberg||
|October 11, 2012 10:00 AM EDT||
Ever wonder how a sophisticated Web site works? Take Facebook, for example. You can view the source and you can hardly pick out any recognizable HTML, let alone divine how the wizards back at Facebook HQ get the site to work. Now, try viewing the source at a simpler Web site, like ZapThink’s. Sure enough, there’s HTML under the covers, but you still can’t tell from the file the Web server sends to your browser what’s going on behind the scenes (we use WordPress, in case you were wondering).
Put into RESTful terms, there is a separation between resource (e.g., the program running on the server) and the representation (e.g., the Web page it sends to your browser). In fact, this separation is a fundamental REST constraint which allows the resource to be opaque.
When people talk about opacity in the REST context, they are usually referring to Uniform Resource Indicators (URIs). You should be able to construct URIs however you like, the theory goes, and it’s up to the resource to figure out how to respond appropriately. In other words, it’s not up to the client to know how to provide specific instructions to the server, other than by clicking the hyperlinks the resource has previously provided to the client.
But there’s more to the opacity story than opaque URIs. Fundamentally, the client has no way of knowing anything at all about what’s really going on behind the scenes. The resource might be a file, a script, a container, an object, or some complicated combination of these and other kinds of things. There are two important lessons for the techies behind the curtain: first, don’t assume resources come in one flavor, and second, it’s important to understand the full breadth of capabilities and patterns that you can leverage when architecting or building resources. After all, anything you can give a URI to can be a resource.
Exploring the Power of Opacity
Let’s begin our exploration of opacity with HTTP’s POST method. Of the four primary HTTP methods (GET, POST, PUT, and DELETE), POST is the only one that’s not idempotent: in other words, not only does it change the state of the resource, but it does so in a way that calling it twice has a different effect than calling it once. In the RESTful context, you should use POST to initialize a resource. According to the HTTP spec, POST creates a subordinate resource, as the figure below illustrates:
In the interaction above, the client POSTs to the cart resource, which initializes a cart instance, names it “abcde,” and returns a hyperlink to that new subordinate resource to the client. In this context, subordinate means that the abcde comes after cart and a slash in the URI http://example.com/cart/abcde.
Here’s the essential question: just what do cart and abcde represent on the server? cart looks like a directory and abcde looks like a file, given the pathlike structure of the URI. But we know that guess probably isn’t right, because POSTing to the cart resource actually created the abcde resource, which represents the cart instance. So could abcde be an object instance? Perhaps. The bottom line is you can’t tell, because as far as the client is concerned, it doesn’t matter. What matters is that the client now has one (or more) hyperlinks to its own cart that it can interact with via a uniform interface.
One way or the other, however, POST changes the state of the abcde cart instance, which requires a relatively onerous level of processing on the server. To lighten the future load on the server, thus improving its scalability, we may want to cache the representation the resource provides. Fortunately, REST explicitly supports cacheability, as the figure below illustrates:
In the pattern above, a gateway intermediary passes along the POST to the server, fetching a static representation it puts in its cache. As long as clients make requests that aren’t intended to change the state of the resource (namely, GETs), then serving up the cached copy is as good as passing along the request to the underlying resource, until the representation expires from the cache.
Opacity plays a critical role in this example as well, since saying the cached copy is just as good as a response directly from the resource is an example of opacity. As a result, the gateway is entirely transparent to the client, serving in the role of server in interactions with the client but in the role of client in interactions with the underlying server.
The limitation of the example above, of course, is the static nature of the cache. If the client wants to change the state of the resource (via PUT or another POST), then such a request would necessarily expire the cache, requiring the intermediary to pass the request along to the underlying server. In situations where the resource state changes frequently, therefore, caching is of limited value.
Opacity and RESTful Clouds
We can extend the pattern above to provide greater capabilities on the intermediary. In the example below, the intermediary is a full-fledged server in its own right, and the underlying server returns executable server scripts for the intermediary to execute on behalf of the underlying server. In other words, the intermediary caches representations that are themselves server programs (e.g., php scripts). Furthermore, these server scripts are prepopulated with any initial state data in response to the original POST from the client.
Increasing the sophistication of our cache would provide little value, however, if we didn’t have a better way of dealing with state information. Fortunately, REST grants our wishes in this case as well, because it enables us to separate resource state (maintained on the underlying server) from application state, which we can transfer to the client.
In the figure above, after the client has initialized the resource, it may wish to, say, update its cart. So, the user clicks a link that executes a PUT that sends the updated information, along with values from one or more hidden form fields to the intermediary. However, instead of updating resource state, the state information remains in the messages (both requests from the client and representations returned from the intermediary) as long as the client only executes idempotent requests. There is no need to update resource state in this situation, because the scripts on the intermediary know to pass along state information in hidden form fields, for example. When the cart process is complete and the user is ready to submit an order, only then does the client execute another POST, which the intermediary knows to pass along to the underlying server.
However, there’s no strict rule that says that the intermediary can only handle idempotent requests; you could easily put a script on it that would handle POSTs, and similarly, it might make sense to send an idempotent request like a DELETE along to the underlying server for execution. But on the other hand, the rule that the intermediary handles only the idempotent requests may be appropriate in your situation, because POST would then be the only method that could ever change state on the underlying server.
As we explained in an earlier ZapFlash, one of the primary benefits to following the pattern in the figure above is to support elasticity when you put the intermediary server in the Cloud. Because it is stateless, it doesn’t matter which virtual machine (VM) instance replies to any client request, and if a VM instance crashes, we can bootstrap its replacement without losing any state information. In other words, opacity is essential to both the elasticity and fault tolerance of the Cloud, and furthermore, following a RESTful approach provides that opacity.
The ZapThink Take
There’s one more RESTful pattern that ZapThink is particularly interested in: RESTful SOA, naturally. For this pattern we need another kind of intermediary: a RESTful SOA intermediary, in addition to the Cloud-based stateless server intermediary, or anything else we want to abstract for that matter. The figure below illustrates the RESTful SOA pattern.
The role of the RESTful SOA intermediary is to provide abstracted (in other words, opaque) RESTful Service endpoints that follow strict URI formatting rules. Furthermore, this intermediary must handle state information appropriately, that is, following a RESTful approach that transfers state information in messages. As a result, the SOA intermediary can support stateless message protocols for interactions with Service consumers while remaining stateless itself. Most ESBs maintain state, and therefore a RESTful SOA intermediary wouldn’t be a typical ESB, although it could certainly route messages to one.
So, which pattern is the best one? As we say in our Licensed ZapThink Architect (LZA) and Cloud Computing for Architects (CCA) courses, it depends. The architect is looking for the right tool for the job. You must understand the problem before recommending the appropriate solution. We cover REST-based SOA in our LZA course (coming to Johannesburg) and RESTful Clouds in the CCA course (coming to London, DC, and San Diego). See you there!
Image credit: Derek Keats
Any Ops team trying to support a company in today’s cloud-connected world knows that a new way of thinking is required – one just as dramatic than the shift from Ops to DevOps. The diversity of modern operations requires teams to focus their impact on breadth vs. depth. In his session at DevOps Summit, Adam Serediuk, Director of Operations at xMatters, Inc., will discuss the strategic requirements of evolving from Ops to DevOps, and why modern Operations has begun leveraging the “NoOps” approa...
Aug. 30, 2015 06:45 PM EDT Reads: 366
DevOps has traditionally played important roles in development and IT operations, but the practice is quickly becoming core to other business functions such as customer success, business intelligence, and marketing analytics. Modern marketers today are driven by data and rely on many different analytics tools. They need DevOps engineers in general and server log data specifically to do their jobs well. Here’s why: Server log files contain the only data that is completely full and accurate in th...
Aug. 30, 2015 06:30 PM EDT Reads: 362
SYS-CON Events announced today that G2G3 will exhibit at SYS-CON's @DevOpsSummit Silicon Valley, which will take place on November 3–5, 2015, at the Santa Clara Convention Center in Santa Clara, CA. Based on a collective appreciation for user experience, design, and technology, G2G3 is uniquely qualified and motivated to redefine how organizations and people engage in an increasingly digital world.
Aug. 30, 2015 05:00 PM EDT Reads: 462
Early in my DevOps Journey, I was introduced to a book of great significance circulating within the Web Operations industry titled The Phoenix Project. (You can read our review of Gene’s book, if interested.) Written as a novel and loosely based on many of the same principles explored in The Goal, this book has been read and referenced by many who have adopted DevOps into their continuous improvement and software delivery processes around the world. As I began planning my travel schedule last...
Aug. 30, 2015 02:00 PM EDT Reads: 525
SYS-CON Events announced today that Pythian, a global IT services company specializing in helping companies leverage disruptive technologies to optimize revenue-generating systems, has been named “Bronze Sponsor” of SYS-CON's 17th Cloud Expo, which will take place on November 3–5, 2015, at the Santa Clara Convention Center in Santa Clara, CA. Founded in 1997, Pythian is a global IT services company that helps companies compete by adopting disruptive technologies such as cloud, Big Data, advance...
Aug. 30, 2015 01:00 PM EDT Reads: 279
In his session at 17th Cloud Expo, Ernest Mueller, Product Manager at Idera, will explain the best practices and lessons learned for tracking and optimizing costs while delivering a cloud-hosted service. He will describe a DevOps approach where the applications and systems work together to track usage, model costs in a granular fashion, and make smart decisions at runtime to minimize costs. The trickier parts covered include triggering off the right metrics; balancing resilience and redundancy ...
Aug. 30, 2015 12:30 PM EDT Reads: 222
Introducing Containers & Microservices Bootcamp at @CloudExpo Silicon Valley | #Containers #Microservices
SYS-CON Events announced today the Containers & Microservices Bootcamp, being held November 3-4, 2015, in conjunction with 17th Cloud Expo, @ThingsExpo, and @DevOpsSummit at the Santa Clara Convention Center in Santa Clara, CA. This is your chance to get started with the latest technology in the industry. Combined with real-world scenarios and use cases, the Containers and Microservices Bootcamp, led by Janakiram MSV, a Microsoft Regional Director, will include presentations as well as hands-on...
Aug. 30, 2015 12:00 PM EDT Reads: 267
SYS-CON Events announced today that HPM Networks will exhibit at the 17th International Cloud Expo®, which will take place on November 3–5, 2015, at the Santa Clara Convention Center in Santa Clara, CA. For 20 years, HPM Networks has been integrating technology solutions that solve complex business challenges. HPM Networks has designed solutions for both SMB and enterprise customers throughout the San Francisco Bay Area.
Aug. 30, 2015 10:30 AM EDT Reads: 871
Several years ago, I was a developer in a travel reservation aggregator. Our mission was to pull flight and hotel data from a bunch of cryptic reservation platforms, and provide it to other companies via an API library - for a fee. That was before companies like Expedia standardized such things. We started with simple methods like getFlightLeg() or addPassengerName(), each performing a small, well-understood function. But our customers wanted bigger, more encompassing services that would "do ...
Aug. 30, 2015 09:30 AM EDT Reads: 245
The pricing of tools or licenses for log aggregation can have a significant effect on organizational culture and the collaboration between Dev and Ops teams. Modern tools for log aggregation (of which Logentries is one example) can be hugely enabling for DevOps approaches to building and operating business-critical software systems. However, the pricing of an aggregated logging solution can affect the adoption of modern logging techniques, as well as organizational capabilities and cross-team ...
Aug. 30, 2015 09:30 AM EDT Reads: 383
Docker containerization is increasingly being used in production environments. How can these environments best be monitored? Monitoring Docker containers as if they are lightweight virtual machines (i.e., monitoring the host from within the container), with all the common metrics that can be captured from an operating system, is an insufficient approach. Docker containers can’t be treated as lightweight virtual machines; they must be treated as what they are: isolated processes running on hosts....
Aug. 30, 2015 08:45 AM EDT Reads: 143
In today's digital world, change is the one constant. Disruptive innovations like cloud, mobility, social media, and the Internet of Things have reshaped the market and set new standards in customer expectations. To remain competitive, businesses must tap the potential of emerging technologies and markets through the rapid release of new products and services. However, the rigid and siloed structures of traditional IT platforms and processes are slowing them down – resulting in lengthy delivery ...
Aug. 30, 2015 08:45 AM EDT Reads: 578
Culture is the most important ingredient of DevOps. The challenge for most organizations is defining and communicating a vision of beneficial DevOps culture for their organizations, and then facilitating the changes needed to achieve that. Often this comes down to an ability to provide true leadership. As a CIO, are your direct reports IT managers or are they IT leaders? The hard truth is that many IT managers have risen through the ranks based on their technical skills, not their leadership ab...
Aug. 30, 2015 08:45 AM EDT Reads: 341
Puppet Labs has announced the next major update to its flagship product: Puppet Enterprise 2015.2. This release includes new features providing DevOps teams with clarity, simplicity and additional management capabilities, including an all-new user interface, an interactive graph for visualizing infrastructure code, a new unified agent and broader infrastructure support.
Aug. 30, 2015 07:15 AM EDT Reads: 498
Skeuomorphism usually means retaining existing design cues in something new that doesn’t actually need them. However, the concept of skeuomorphism can be thought of as relating more broadly to applying existing patterns to new technologies that, in fact, cry out for new approaches. In his session at DevOps Summit, Gordon Haff, Senior Cloud Strategy Marketing and Evangelism Manager at Red Hat, discussed why containers should be paired with new architectural practices such as microservices rathe...
Aug. 30, 2015 06:00 AM EDT Reads: 391
It’s been proven time and time again that in tech, diversity drives greater innovation, better team productivity and greater profits and market share. So what can we do in our DevOps teams to embrace diversity and help transform the culture of development and operations into a true “DevOps” team? In her session at DevOps Summit, Stefana Muller, Director, Product Management – Continuous Delivery at CA Technologies, answered that question citing examples, showing how to create opportunities for ...
Aug. 30, 2015 01:00 AM EDT Reads: 478
Whether you like it or not, DevOps is on track for a remarkable alliance with security. The SEC didn’t approve the merger. And your boss hasn’t heard anything about it. Yet, this unruly triumvirate will soon dominate and deliver DevSecOps faster, cheaper, better, and on an unprecedented scale. In his session at DevOps Summit, Frank Bunger, VP of Customer Success at ScriptRock, will discuss how this cathartic moment will propel the DevOps movement from such stuff as dreams are made on to a prac...
Aug. 30, 2015 01:00 AM EDT Reads: 216
SYS-CON Events announced today that DataClear Inc. will exhibit at the 17th International Cloud Expo®, which will take place on November 3–5, 2015, at the Santa Clara Convention Center in Santa Clara, CA. The DataClear ‘BlackBox’ is the only solution that moves your PC, browsing and data out of the United States and away from prying (and spying) eyes. Its solution automatically builds you a clean, on-demand, virus free, new virtual cloud based PC outside of the United States, and wipes it clean...
Aug. 29, 2015 07:15 PM EDT Reads: 388
What does “big enough” mean? It’s sometimes useful to argue by reductio ad absurdum. Hello, world doesn’t need to be broken down into smaller services. At the other extreme, building a monolithic enterprise resource planning (ERP) system is just asking for trouble: it’s too big, and it needs to be decomposed.
Aug. 29, 2015 10:00 AM EDT Reads: 344
The Microservices architectural pattern promises increased DevOps agility and can help enable continuous delivery of software. This session is for developers who are transforming existing applications to cloud-native applications, or creating new microservices style applications. In his session at DevOps Summit, Jim Bugwadia, CEO of Nirmata, will introduce best practices, patterns, challenges, and solutions for the development and operations of microservices style applications. He will discuss ...
Aug. 27, 2015 02:15 PM EDT Reads: 514