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The Architecture of Open Source Applications In these two books, the authors of four dozen open source applications explain how their download Volume II. PDF | On Jan 1, , J Freire and others published The Architecture of Open Source He is the designer and lead architect of the Open Source LLVM Project. See complex telephony systems in an industry that is continuously evolving. The Architecture of Open Source Applications (Volume 2): ZeroMQ .. http://hal. medical-site.info: see different.
Things to consider for manageability are the ease of diagnosing and understanding problems when they occur, ease of making updates or modications, and how simple the system is to operate. Cost: Cost is an important factor. This obviously can include hardware and software costs, but it is also important to consider other facets needed to deploy and maintain the system.
The amount of developer time the system takes to build, the amount of operational eort required to run the system, and even the amount of training required should all be considered. Cost is the total cost of ownership. Each of these principles provides the basis for decisions in designing a distributed web architecture. However, they also can be at odds with one another, such that achieving one objective comes at the cost of another.
A basic example: choosing to address capacity by simply adding more servers scalability can come at the price of manageability you have to operate an additional server and cost the price of the servers.
When designing any sort of web application it is important to consider these key principles, even if it is to acknowledge that a design may sacrice one or more of them. Investing in scaling before it is needed is generally not a smart business proposition; however, some forethought into the design can save substantial time and resources in the future.
This section is focused on some of the core factors that are central to almost all large web applications: services, redundancy, partitions, and handling failure. Each of these factors involves choices and compromises, particularly in the context of the principles described in the previous section.
In order to explain these in detail it is best to start with an example. Example: Image Hosting Application At some point you have probably posted an image online. For big sites that host and deliver lots of images, there are challenges in building an architecture that is cost-eective, highly available, and has low latency fast retrieval. Imagine a system where users are able to upload their images to a central server, and the images can be requested via a web link or API, just like Flickr or Picasa.
For the sake of simplicity, lets assume that this application has two key parts: the ability to upload write an image to the server, and the ability to query for an image. While we certainly want the upload to be ecient, we care most about having very fast delivery when someone requests an image for example, images could 2 Scalable Web Architecture and Distributed Systems be requested for a web page or other application.
Other important aspects of the system are: There is no limit to the number of images that will be stored, so storage scalability, in terms of image count needs to be considered. If a user uploads an image, the image should always be there data reliability for images. The system should be easy to maintain manageability.
Since image hosting doesnt have high prot margins, the system needs to be cost-eective. Figure 1. Building a small version of this application would be trivial and easily hosted on a single server; however, that would not be interesting for this chapter.
Lets assume that we want to build something that could grow as big as Flickr.
Services When considering scalable system design, it helps to decouple functionality and think about each part of the system as its own service with a clearly dened interface. For these types of systems, each service has its own distinct functional context, and interaction with anything outside of that context takes place through an abstract interface, typically the public-facing API of another service.
Kate Matsudaira 3 Deconstructing a system into a set of complementary services decouples the operation of those pieces from one another. This abstraction helps establish clear relationships between the service, its underlying environment, and the consumers of that service. Creating these clear delineations can help isolate problems, but also allows each piece to scale independently of one another.
This sort of service-oriented design for systems is very similar to object-oriented design for programming. In our example, all requests to upload and retrieve images are processed by the same server; however, as the system needs to scale it makes sense to break out these two functions into their own services.
Fast-forward and assume that the service is in heavy use; such a scenario makes it easy to see how longer writes will impact the time it takes to read the images since they two functions will be competing for shared resources.
Depending on the architecture this eect can be substantial. Even if the upload and download speeds are the same which is not true of most IP networks, since most are designed for at least a download-speed:upload-speed ratio , read les will typically be read from cache, and writes will have to go to disk eventually and perhaps be written several times in eventually consistent situations.
Even if everything is in memory or read from disks like SSDs , database writes will almost always be slower than reads 1. Another potential problem with this design is that a web server like Apache or lighttpd typically has an upper limit on the number of simultaneous connections it can maintain defaults are around , but can go much higher and in high trac, writes can quickly consume all of those.
Since reads can be asynchronous, or take advantage of other performance optimizations like gzip compression or chunked transfer encoding, the web server can switch serve reads faster and switch between clients quickly serving many more requests per second than the max number of connections with Apache and max connections set to , it is not uncommon to serve several thousand read requests per second. Writes, on the other hand, tend to maintain an open connection for the duration for the upload, so uploading a 1MB le could take more than 1 second on most home networks, so that web server could only handle such simultaneous writes.
Planning for this sort of bottleneck makes a good case to split out reads and writes of images into their own services, shown in Figure 1. Since our community serves a broad range of ages, we do not encourage content that could make a majority of our users uncomfortable. If you are sure that this product is in violation of acceptable content as defined in the agreement or that it does not meet our guidelines for General Access, please fill out the form below.
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Architects look at thousands of buildings during their training, and study critiques of those buildings written by masters. In contrast, most software developers only ever get to know a handful of large programs well — usually programs they wrote themselves — and never study the great programs of history.
In it, the authors of twenty-four open source applications explain how their software is structured, and why. What are each program's major components? How do they interact? In this second volume, the authors of twenty-four open source applications explain how their software is structured, and why.
From web servers and compilers to health record management systems, they are covered here to help you become a better developer. Tag s: Creative Commons Attribution 3. Amy worked in the software industry for ten years before quitting to create a freelance editing and book production business. She has an underused degree in Math from the University of Waterloo. Greg is the founder of Software Carpentry , a crash course in computing skills for scientists and engineers.
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