NoSQL技术说明书

NoSQL DatabaSe techNoLogy © 2011 couchbaSe aLL rightS reServeD. www.couchbaSe.com 2 Table of Contents Summary 3 Interactive software has changed 4 users – 4 applications – 5 infrastructure – 5 Application architecture has changed 6 Database architecture has not kept pace 7 Tactics to extend the useful scope of RDBMS technology 8 Sharding – 8 Denormalizing – 9 Distributed caching – 10 “NoSQL” database technologies 11 Mobile application data synchronization 13 Open source and commercial NoSQL database technologies 14 NoSQL DatabaSe techNoLogy © 2011 couchbaSe aLL rightS reServeD. www.couchbaSe.com 3 Summary Interactive software (software with which a person iteratively interacts in real time) has changed in fundamental ways over the last 35 years. The “online” systems of the 1970s have, through a series of intermediate transformations, evolved into today’s Web and mobile applications. These systems solve new problems for potentially vastly larger user populations, and they execute atop a computing infrastructure that has changed even more radically over the years. The architecture of these software systems has likewise transformed. A modern Web application can support millions of concurrent users by spreading load across a collection of application servers behind a load balancer. Changes in application behavior can be rolled out incrementally without requiring application downtime by gradually replacing the software on individual servers. Adjustments to application capacity are easily made by changing the number of application servers. But database technology has not kept pace. Relational database technology, invented in the 1970s and still in widespread use today, was optimized for the applications, users and infrastructure of that era. In some regards, it is the last domino to fall in the inevitable march toward a fully-distributed software architecture. While a number of bandaids have extended the useful life of the technology (horizontal and vertical sharding, distributed caching and data denormalization), these tactics nullify key benefits of the relational model while increasing total system cost and complexity. In response to the lack of commercially available alternatives, organizations such as Google and Amazon were, out of necessity, forced to invent new approaches to data management. These “NoSQL” or non-relational database technologies are a better match for the needs of modern interactive software systems. But not every company can or should develop, maintain and support its own database technology. Building upon the pioneering research at these and other leading-edge organizations, commercial suppliers of NoSQL database technology have emerged to offer database technology purpose-built to enable the cost- effective management of data behind modern Web and mobile applications. NoSQL DatabaSe techNoLogy © 2011 couchbaSe aLL rightS reServeD. www.couchbaSe.com 4 Interactive software has changed As Table 1 below shows, there are fundamental differences in the users, applications and underlying infrastructure between interactive software systems of the 1970s and those being built today. Users In 1975, an interactive software system with 2,000 users represented the pinnacle of scale. Few organizations built, deployed and supported such systems. American Airlines Sabre® System (first installed in a travel agency in 1976) and Bank of America’s branch banking automation system represent two notable interactive software systems that scaled to these heights. But these were exceptions. Today, applications accessed via the public Web have a potential user base of over two billion users. Whether an online banking system, a social networking or gaming application, or an e-commerce application selling goods and services to the public, there are innumerable examples of software systems that routinely support a population of users many orders of magnitude beyond the largest of the 1970s. A system with only 2,000 users is the exception now, assuming the application is not an abject failure. There is also user growth and churn today not seen in systems of the 1970s. Once rolled out, the number of travel agents or tellers added to, or removed from, these systems was highly predictable and relatively easy to manage (albeit somewhat manually and at measured U se rs In fr a st ru ct u re Interactive Software Then and Now 2,000 “online” users = End Point 2,000 “online” users = Starting Point Static user population Dynamic user population Data networking in its infancy Universal high-speed data networking Memory scarce and expensive Memory plentiful and cheap Centralized compting (Mainframes and minicomputers Distributed computing (Network servers and virtual machines) Circa 1975 “Online Applications” Circa 2011 “Interactive Web Applications” Business process automation Business process innovation Highly structured data records Structured, semi-structured and unstructured data A p p li ca ti o n s NoSQL DatabaSe techNoLogy © 2011 couchbaSe aLL rightS reServeD. www.couchbaSe.com 5 pace). Users worked during well-defined office hours, providing windows of opportunity for scheduled system downtime and maintenance. Today, Web applications can serve a global population of users 24 hours a day, 365 days per year. A newly launched software system can grow from no users to over a million users almost literally overnight. Not all users are active on these systems at any given time, and some users may use an application only a few times, never to return, and without providing notice of their intent to leave. Applications In 1975, interactive software systems were primarily designed to automate what were previously tedious, paper-based business processes – teller transactions, flight reservations, stock trades. These “transactions” typically mirrored what clerical employees had been doing “by hand” for decades – filling in fields on a structured business form, then filing or sending forms to other employees who would tally them, update impacted ledgers and notate files to effect “transactions.” Online transaction processing systems accelerated these tasks and reduced the probability of error, but in most cases they were automating versus innovating. Versus simply automating long-standing manual business processes, today’s Web applications are breaking new ground in every direction. They are changing the nature of communication, shopping, advertising, entertainment and relationship management. But they are works in progress. There are no old business forms to simply mimic, or processes to study and automate. It may be trite, but change is truly the only constant in these systems. And a database has to be flexible enough to change with them. Infrastructure Perhaps the most obvious difference between interactive software then and now is the infrastructure atop which they execute. Centralization characterized the computing environment in the 1970s – mainframes and minicomputers with shared CPU, memory and disk subsystems were the norm. Computer networking was in its infancy. Memory was an expensive, scarce resource. Today, distributed computing is the norm. Within a datacenter, servers and virtual machines are interconnected via high-speed data networks. Users of software systems access them from even more widely distributed desktop, laptop and mobile computing devices. The IBM System/360 Model 195 was “the most powerful computer in IBM’s product line” from August 1969 through the mid-1970s. The most powerful configuration of this system shipped with 4MB of main (core) memory. Today, a single high-end microprocessor can have more L1 cache memory on the processor die itself, with support for many orders of magnitude more main memory. NoSQL DatabaSe techNoLogy © 2011 couchbaSe aLL rightS reServeD. www.couchbaSe.com 6 Application architecture has changed Directly addressing the aforementioned changes, and in contrast to the scale-up, centralized approach of circa 1975 interactive software architecture, modern Web applications are built to scale out – simply add more commodity Web servers behind a load balancer to support more users. Scaling out is also a core tenet of the increasingly important cloud computing model, in which virtual machine instances can be easily added or removed to match demand. Figure 1: web application – Logic Scales out. to support more users for a web application, you simply add more commodity web servers. as a result, system cost expands linearly with linear increases in users, and performance remains constant. this model scales out indefinitely for all practical purposes. The cost and performance curves are obviously attractive, but ultimately, flexibility is the big win in this approach. As users come and go, commodity servers (or virtual machines) can be quickly added or removed from the server pool, matching capital and operating costs to the difficult-to- predict size and activity level of the user population. And by distributing the load across many servers, even across geographies, the system is inherently fault-tolerant, supporting continuous operations. As application needs change, new software can be gradually rolled out across subsets of the overall server pool. Facebook, as an example, slowly dials up new functionality by rolling out new software to a subset of their entire application server tier (and user population) in a stepwise manner. If issues crop up, servers can be quickly reverted to the previous known good build. All this can be done without ever taking the application “offline.” Web Application - Logic Scales Out. To support more users for a web application, you simply add more commodity web servers. As a result, system cost expands linearly with linear increases in users, and performance remains constant. This model scales out indefinitely for all practical purposes. Web Servers Users S y st e m C o st A p p li c a ti o n R e sp o n se T im e Load Balancer www.wellsfargo.com NoSQL DatabaSe techNoLogy © 2011 couchbaSe aLL rightS reServeD. www.couchbaSe.com 7 Database architecture has not kept pace In contrast to the sweeping changes in application architecture, relational database (RDBMS) technology, a “scale-up” technology that has not fundamentally changed in over 40 years, continues to be the default choice for holding data behind Web applications. Not surprisingly, RDBMS technology reflects the realities (users, applications, and infrastructure) of the environment that spawned it. Because it is a technology designed for the centralized computing model, to handle more users one must get a bigger server (increasing CPU, memory and I/O capacity) (see Figure 2). Big servers tend to be highly complex, proprietary, and disproportionately expensive pieces of engineered machinery, unlike the low-cost, commodity hardware typically deployed in Web- and cloud-based architectures. And, ultimately, there is a limit to how big a server one can purchase, even given an unlimited willingness and ability to pay. Figure 2: web application – rDbmS Scales up. to support more users, you must get a bigger database server for your rDbmS. as a result, system cost grows exponentially with linear increases in users, and application response time degrades asymptotically. While the scaling economics are certainly inferior to the model now employed at the application logic tier, it is once again flexibility (or lack thereof) that is the “high-order bit” to consider. Upgrading a server is an exercise that requires planning, acquisition and application downtime to complete. Given the relatively unpredictable user growth rate of modern software systems, inevitably there is either over- or under-provisioning of resources. Too much and you’ve overspent, too little and users can have a bad application experience or the application can outright fail. And with all the eggs in a single basket, fault tolerance and high-availability strategies are critically important to get right. Figure 2: Web Application - RDBMS Scales Up. To support more users, you must get a bigger database server for your RDBMS. As a result, system cost grows exponentially with linear increases in users, and application response time degrades asymptotically. Relational Database RDBMS Software installes on comples, expensive, big iron. Web Servers Users Won’t scale beyond this point S y st e m C o st A p p li c a ti o n R e sp o n se T im e NoSQL DatabaSe techNoLogy © 2011 couchbaSe aLL rightS reServeD. www.couchbaSe.com 8 Perhaps the least obvious, but arguably the most damaging downside of using RDBMS technology behind modern interactive software systems is the rigidity of the database schema. As noted previously, we are no longer simply automating long-standing and well- understood paper-based processes, where database record formats are pre-defined and largely static. But RDBMS technology requires the strict definition of a “schema” prior to storing any data into the database. Changing the schema once data is inserted is A Big Deal. Want to start capturing new information you didn’t previously consider? Want to make rapid changes to application behavior requiring changes to data formats and content? With RDBMS technology, changes like these are extremely disruptive and therefore are frequently avoided – the opposite behavior desired in a rapidly evolving business and market environment. Tactics to extend the useful scope of RDBMS technology In an effort to address the shortcomings of RDBMS technology when used behind modern interactive software systems, developers have adopted a number of “bandaid” tactics. Sharding The RDBMS data model and transaction mechanics fundamentally assume a centralized computing model – shared CPU, memory and disk. If the data for an application will not fit on a single server or, more likely, if a single server is incapable of maintaining the I/O throughput required to serve many users simultaneously, then a tactic known as sharding is frequently employed. In this approach an application will implement some form of data partitioning to manually spread data across servers. For example, users that live west of the Mississippi River may have their data stored in one server, while those who live east of the river will be stored in another. While this does work to spread the load, there are undesirable consequences to the approach. 1. When you fill a shard, it is highly disruptive to re-shard. When you fill a shard, you have to change the sharding strategy in the application itself. For example, if you had partitioned your database by placing all accounts east of the Mississippi on one server and all accounts west in another and then reach the limits of their capacity, you must change the sharding approach which means changing your application. Where previously the application had to know “this is an east of the Mississippi customer and thus I need to look in this database server,” now it must know “if it is east of the Mississippi and below the Mason-Dixon Line, I need to look in that server now.” NoSQL DatabaSe techNoLogy © 2011 couchbaSe aLL rightS reServeD. www.couchbaSe.com 9 2. You lose some of the most important benefits of the relational model. You can’t do “joins” across shards – if you want to find all customers that have purchased a pair of wool socks but haven’t purchased anything in over 6 months, you must run a query on every server and piece the results together in application software. In addition, you can’t do cross-node locking when making updates. So one must ensure all data that could need to be atomically operated on is resident on a single server, unless using an external TP monitor system or complex logic in the application itself. 3. You have to create and maintain a schema on every server. If you have new information you want to collect, you must modify the database schema on every server, then normalize, retune and rebuild the tables. What was hard with one server is a nightmare across many. For this reason, the default behavior is to minimize the collection of new information. Denormalizing Before storing data in an RDBMS, a schema must be created defining precisely what data can be stored in the database and the relationships between data elements. Data is decomposed into a “normal form” and a record is typically spread across many interlinked tables. In order to update a record, all these tables must be locked down and updated atomically, lest the database become corrupted. This approach substantially limits the latency and throughput of concurrent updates and is, for most practical purposes, impossible to implement across server boundaries. To support concurrency and sharding, data is frequently stored in a denormalized form when an RDBMS is used behind Web applications. This approach potentially duplicates data in the database, requiring updates to multiple tables when a duplicated data item is changed, but it reduces the amount of locking required and thus improves concurrency. At the limit the relational schema is more or less abandoned entirely, with data simply stored in key-value form, where a primary key is paired with a data “blob” that can hold any data. This approach allows the type of information being stored in the database to change without requiring an update to the schema. It makes sharding much easier and allows for rapid changes in the data model. Of course, just about all relational database functionality is lost in the process (though if the database is sharded, much of the functionality was already lost). Notwithstanding all these problems, many organizations are using relational technology in precisely this manner given the familiarity of specific RDBMS technologies to developers and operations teams, and, until recently, the lack of good alternatives. NoSQL DatabaSe techNoLogy © 2011 couchbaSe aLL rightS reServeD. www.couchbaSe.com 10 Distributed caching Another tactic used to extend the useful scope of RDBMS technology has been to employ distributed caching technologies, such as Memcached. Today, Memcached is a key ingredient in the data architecture behind 18 of the top 20 largest (by user count) Web applications, including Google, Wikipedia, Twitter, YouTube, Facebook, Craigslist, and tens of thousands of other corporate and consumer Web applications. Most new Web applications now build Memcached into their data architecture from day one. Figure 3: memcached distributed caching technology extends the useful life of rDbmS technology behind interactive web applications, spreading data across servers and leveraging the availability and performance of main memory. Memcached builds on two of the most important infrastructure transitions over the last 40 years: the shift to distributed computing atop high-speed data networks, and advances in main memory (RAM) price/performance. Memcached “sits in front” of an RDBMS system, caching recently accessed data in memory and storing that data across any number of servers or virtual machines. When an application needs access to data, rather than going directly to the RDBMS, it first checks Memcached to see if the data is available there; if it is not, then the database is read by the application and stored in Memcached for quick access next time it is needed. While useful and effective to a point, Memcached and similar distributed caching technologies used for this purpose are no panacea and can even create problems of their own: 1. Accelerates only data reads. Memcached was designed to accelerate the reading of data by storing it in main memory, but it was not designed to permanentl