- Title: DISTRIBUTED SYSTEMS Concepts and Design Fifth Edition
- Authors: George Coulouris, Jean Dollimore, Tim Kindberg and Gordon Blair
Overall architecture and design philosophy
This section looks at the overall architecture of the Google system, examining:
- the physical architecture adopted by Google;
- the associated system architecture that offers common services to the Internet search engine and the many web applications offered by Google.
The key philosophy of Google in terms of physical infrastructure is to use very large numbers of commodity PCs to produce a cost-effective environment for distributed storage and computation. Purchasing decisions are based on obtaining the best performance per dollar rather than absolute performance with a typical spend on a single PC unit of around $1,000. A given PC will typically have around 2 terabytes of disk storage and around 16 gigabytes of DRAM (dynamic random access memory) and run a cut-down version of the Linux kernel. (This philosophy of building systems from commodity PCs reflects the early days of the original research project, when Sergey Brin and Larry Page built the first Google search engine from spare hardware scavenged from around the lab at Stanford University.)
In electing to go down the route of commodity PCs, Google has recognized that parts of its infrastructure will fail and hence, as we will see below, has designed the infrastructure using a range of strategies to tolerate such failures. Hennessy and Patterson  report the following failure characteristics for Google:
- By far the most common source of failure is due to software, with about 20 machines needing to be rebooted per day due to software failures. (Interestingly, the rebooting process is entirely manual.)
- Hardware failures represent about 1/10 of the failures due to software with around 2–3% of PCs failing per annum due to hardware faults. Of these, 95% are due to faults in disks or DRAM.
This vindicates the decision to procure commodity PCs; given that the vast majority of failures are due to software, it is not worthwhile to invest in more expensive, more reliable hardware. A further paper by Pinheiro et al.  also reports on the failure characteristics of commodity disks as used in the Google physical infrastructure, providing an interesting insight into failure patterns of disk storage in large-scale deployments.
The physical architecture is constructed as follows [Hennessy and Patterson 2006]:
Commodity PCs are organized in racks with between 40 and 80 PCs in a given rack. The racks are double-sided with half the PCs on each side. Each rack has an Ethernet switch that provides connectivity across the rack and also to the external world (see below). This switch is modular, organized as a number of blades with each blade supporting either 8 100-Mbps network interfaces or a single 1-Gbps interface. For 40 PCs, five blades each containing eight network interfaces are sufficient to ensure connectivity within the rack. Two further blades, each supporting a 1-Gbps network interface, are used for connection to the outside world.
Racks are organized into clusters (as discussed in Section 1.3.4), which are a key unit of management, determining for example the placement and replication of services. A cluster typically consists of 30 or more racks and two high-bandwidth switches providing connectivity to the outside world (the Internet and other Google centres). Each rack is connected to both switches for redundancy; in addition, for further redundancy, each switch has redundant links to the outside world.
Clusters are housed in Google data centres that are spread around the world. In 2000, Google relied on key data centres in Silicon Valley (two centres) and in Virginia. At the time of writing, the number of data centres has grown significantly and there are now centres in many geographical locations across the US and in Dublin (Ireland), Saint-Ghislain (Belgium), Zurich (Switzerland), Tokyo (Japan) and Beijing (China). (A map of known data centres as of 2008 can be found here [royal.pingdom.com].)
A simplified view of this overall organization is provided in Figure 21.3.
This physical infrastructure provides Google with enormous storage and computational capabilities, together with the necessary redundancy to build fault-tolerant, large-scale systems (note that, to avoid clutter, this figure only shows the Ethernet connections from one of the clusters to the external links).
Let us consider the storage capacity available to Google. If each PC offers 2 terabytes of storage, then a rack of 80 PCs will provide 160 terabytes, with a cluster of 30 racks offering 4.8 petabytes. It is not known exactly how many machines Google has in total as the company maintains strict secrecy over this aspect of its business, but we can assume Google has on the order of 200 clusters, offering total storage capacity of 960 petabytes or just under 1 exabyte of storage (1018 bytes). This is likely to be a conservative figure, as Google VP Marissa Mayer is already talking about the data explosion taking us well into the exascale range.
We shall see how Google uses this extensive storage and computational capability and the associated redundancy to offer core services in the remainder of this chapter.
Overall system architecture
Before examining the overall system architecture, it is helpful to examine the key requirements in more detail:
The first and most obvious requirement for the underlying Google infrastructure is to master scalability and, in particular, to have approaches that scale to what is an Ultra-Large Scale (ULS) distributed system as featured in Chapter 2. For the search engine, Google views the scalability problem in terms of three dimensions: i) being able to deal with more data (for example, as the amount of information in the Web grows through initiatives such as the digitizing of libraries), ii) being able to deal with more queries (as the number of people using Google in their homes and workplaces grows) and iii) seeking better results (particularly important as this is a key determining factor in uptake of a web search engine). This view of the scalability problem is illustrated in Figure 21.4.
Figure 21.4 The scalability problem in Google
Scalability demands the use of (sophisticated) distributed systems strategies. Let us illustrate this with a simple analysis taken from Jeff Dean’s keynote at PACT’06 [Dean 2006]. He assumed that the Web consists of around 20 billion web pages at 20 kilobytes each. This implies a total size of around 400 terabytes. To crawl this amount of data it would take a single computer over 4 months assuming that the computer can read 30 megabytes per second. In contrast, 1,000 machines can read this amount of data in less than 3 hours. In addition, as we saw in Section 21.2, searching is not only about crawling. The other functions, including indexing, ranking and searching, all require highly distributed solutions in order to be able to scale.
Google has stringent reliability requirements, especially with regard to availability of services. This is particularly important for the search functionality, where there is a need to provide 24/7 availability (noting, however that it is intrinsically easy to mask failures in search as the user has no way of knowing if all search results are returned). This requirement also applies to other web applications, and it is interesting to note that Google offers a 99.9% service level agreement (effectively, a system guarantee) to paying customers of Google Apps covering Gmail, Google Calendar, Google Docs, Google Sites and Google Talk. The company has an excellent overall record in terms of availability of services, but the well- reported outage of Gmail on 1 September 2009 acts as a reminder of the continuing challenges in this area. (This outage, which lasted 100 minutes, was caused by a cascading problem of overloading servers during a period of routine maintenance). Note that the reliability requirement must be met in the context of the design choices in the physical architecture, which imply that (software and hardware) failures are anticipated with reasonable frequency. This demands both detecting failures and adopting strategies to mask or tolerate such failures. Such strategies rely heavily on the redundancy in the underlying physical architecture. We shall see examples of such strategies as the details of the system architecture emerge.
Figure 21.5 The overall Google systems architecture
The overall performance of the system is critical for Google, especially in achieving low latency of user interactions. The better the performance, the more likely it is that a user will return with more queries that, in turn, increase their exposure to ads hence potentially increasing revenue. The importance of performance is exemplified by the target of completing web search operations in 0.2 seconds (as mentioned above) and achieving the required throughput to respond to all incoming requests while dealing with very large datasets. This applies to a wide range of functions associated with the operation of Google, including web crawling, indexing and sorting. It is also important to note that performance is an end-to-end property requiring all associated underlying resources to work together, including network, storage and computational resources.
The above requirements are in many ways the obvious ones for Google to support its core services and applications. There is also a strong requirement for openness, particularly to support further development in the range of web applications on offer. It is well known that Google as an organization encourages and nurtures innovation, and this is most evident in the development of new web applications. This is only possible with an infrastructure that is extensible and provides support for the development of new applications.
Google has responded to these needs by developing the overall system architecture shown in Figure 21.5. This figure shows the underlying computing platform at the bottom (that is, the physical architecture as described above) and the well-known Google services and applications at the top. The middle layer defines a common distributed infrastructure providing middleware support for search and cloud computing. This is crucial to the success of Google. The infrastructure provides the common distributed system services for developers of Google services and applications and encapsulates key strategies for dealing with scalability, reliability and performance. The provision of a well-designed common infrastructure such as this can bootstrap the development of new applications and services through reuse of the underlying system services and, more subtly, provides an overall coherence to the growing Google code base by enforcing common strategies and design principles.
Figure 21.6 Google infrastructure
The system is constructed as a set of distributed services offering core functionality to developers (see Figure 21.6). This set of services naturally partitions into the following subsets:
the underlying communication paradigms, including services for both remote invocation and indirect communication:
– the protocol buffers component offers a common serialization format for Google, including the serialization of requests and replies in remote invocation.
– the Google publish-subscribe service supports the efficient dissemination of events to potentially large numbers of subscribers.
data and coordination services providing unstructured and semi-structured abstractions for the storage of data coupled with services to support coordinated access to the data:
– GFS offers a distributed file system optimized for the particular requirements of Google applications and services (including the storage of very large files).
– Chubby supports coordination services and the ability to store small volumes of data.
– Bigtable provides a distributed database offering access to semi-structured data.
distributed computation services providing means for carrying out parallel and distributed computation over the physical infrastructure:
– MapReduce supports distributed computation over potentially very large datasets (for example, stored in Bigtable).
– Sawzall provides a higher-level language for the execution of such distributed computations.
We step through each of these components in turn in Sections 21.4 through 21.6. First, however, it is instructive to reflect on key design principles associated with the architecture as a whole.
Associated design principles
To fully understand the design of Google infrastructure, it is important to also have an understanding of key design philosophies that pervade the organization:
The most important design principle behind Google software is that of simplicity: software should do one thing and do it well, avoiding feature-rich designs wherever possible. For example, Bloch  discusses how this principle applies to API design implying that API designs should be as small as possible and no smaller (an example of the application of Occam’s Razor).
Another key design principle is a strong emphasis on performance in the development of systems software, captured in the phrase ‘every millisecond counts’. In a keynote at LADIS’09, Jeff Dean (a member of the Google Systems Infrastructure Group) emphasized the importance of being able to estimate the performance of a system design through awareness of performance costs of primitive operations such as accessing memory and disk, sending packets over a network, locking and unlocking a mutex and so on, coupled with what he referred to as ‘back of the envelope’ calculations.
A final principle is advocating stringent testing regimes on software, captured by the slogan ‘if it ain’t broke, you are not trying hard enough’. This is complemented by a strong emphasis on logging and tracing to detect and resolve faults in the system.
With this background, we are now ready to examine the various constituent parts of the Google infrastructure, starting with the underlying communication paradigms. For each area, we present the overall design and highlight the key design decisions and associated trade-offs.
 George Coulouris, Jean Dollimore, Tim Kindberg and Gordon Blair, DISTRIBUTED SYSTEMS Concepts and Design Fifth Edition, Pearson Education, Inc., 2012.