Data-Intensive Text Processing with MapReduce. Jimmy LinЧитать онлайн книгу.
let us suppose that a MTBF of 10,000 days (about 30 years) were achievable at realistic costs (which is unlikely). Even then, a 10,000-server cluster would still experience one failure daily. This means that any large-scale service that is distributed across a large cluster (either a user-facing application or a computing platform like MapReduce) must cope with hardware failures as an intrinsic aspect of its operation [66]. That is, a server may fail at any time, without notice. For example, in large clusters disk failures are common [123] and RAM experiences more errors than one might expect [135]. Datacenters suffer from both planned outages (e.g., system maintenance and hardware upgrades) and unexpected outages (e.g., power failure, connectivity loss, etc.).
A well-designed, fault-tolerant service must cope with failures up to a point without impacting the quality of service—failures should not result in inconsistencies or indeterminism from the user perspective. As servers go down, other cluster nodes should seamlessly step in to handle the load, and overall performance should gracefully degrade as server failures pile up. Just as important, a broken server that has been repaired should be able to seamlessly rejoin the service without manual reconfiguration by the administrator. Mature implementations of the MapReduce programming model are able to robustly cope with failures through a number of mechanisms such as automatic task restarts on different cluster nodes.
Move processing to the data. In traditional high-performance computing (HPC) applications (e.g., for climate or nuclear simulations), it is commonplace for a supercomputer to have “processing nodes” and “storage nodes” linked together by a high-capacity interconnect. Many data-intensive workloads are not very processor-demanding, which means that the separation of compute and storage creates a bottleneck in the network. As an alternative to moving data around, it is more efficient to move the processing around. That is, MapReduce assumes an architecture where processors and storage (disk) are co-located. In such a setup, we can take advantage of data locality by running code on the processor directly attached to the block of data we need. The distributed file system is responsible for managing the data over which MapReduce operates.
Process data sequentially and avoid random access. Data-intensive processing by definition means that the relevant datasets are too large to fit in memory and must be held on disk. Seek times for random disk access are fundamentally limited by the mechanical nature of the devices: read heads can only move so fast and platters can only spin so rapidly. As a result, it is desirable to avoid random data access, and instead organize computations so that data are processed sequentially. A simple scenario10 poignantly illustrates the large performance gap between sequential operations and random seeks: assume a 1 terabyte database containing 1010 100-byte records. Given reasonable assumptions about disk latency and throughput, a back-of-the-envelop calculation will show that updating 1% of the records (by accessing and then mutating each record) will take about a month on a single machine. On the other hand, if one simply reads the entire database and rewrites all the records (mutating those that need updating), the process would finish in under a work day on a single machine. Sequential data access is, literally, orders of magnitude faster than random data access.11
The development of solid-state drives is unlikely to change this balance for at least two reasons. First, the cost differential between traditional magnetic disks and solid-state disks remains substantial: large-data will for the most part remain on mechanical drives, at least in the near future. Second, although solid-state disks have substantially faster seek times, order-of-magnitude differences in performance between sequential and random access still remain.
MapReduce is primarily designed for batch processing over large datasets. To the extent possible, all computations are organized into long streaming operations that take advantage of the aggregate bandwidth of many disks in a cluster. Many aspects of MapReduce’s design explicitly trade latency for throughput.
Hide system-level details from the application developer. According to many guides on the practice of software engineering written by experienced industry professionals, one of the key reasons why writing code is difficult is because the programmer must simultaneously keep track of many details in short-term memory—ranging from the mundane (e.g., variable names) to the sophisticated (e.g., a corner case of an algorithm that requires special treatment). This imposes a high cognitive load and requires intense concentration, which leads to a number of recommendations about a programmer’s environment (e.g., quiet office, comfortable furniture, large monitors, etc.). The challenges in writing distributed software are greatly compounded—the programmer must manage details across several threads, processes, or machines. Of course, the biggest headache in distributed programming is that code runs concurrently in unpredictable orders, accessing data in unpredictable patterns. This gives rise to race conditions, deadlocks, and other well-known problems. Programmers are taught to use low-level devices such as mutexes and to apply high-level “design patterns” such as producer–consumer queues to tackle these challenges, but the truth remains: concurrent programs are notoriously difficult to reason about and even harder to debug.
MapReduce addresses the challenges of distributed programming by providing an abstraction that isolates the developer from system-level details (e.g., locking of data structures, data starvation issues in the processing pipeline, etc.). The programming model specifies simple and well-defined interfaces between a small number of components, and therefore is easy for the programmer to reason about. MapReduce maintains a separation of what computations are to be performed and how those computations are actually carried out on a cluster of machines. The first is under the control of the programmer, while the second is exclusively the responsibility of the execution framework or “runtime”. The advantage is that the execution framework only needs to be designed once and verified for correctness—thereafter, as long as the developer expresses computations in the programming model, code is guaranteed to behave as expected. The upshot is that the developer is freed from having to worry about system-level details (e.g., no more debugging race conditions and addressing lock contention) and can instead focus on algorithm or application design.
Seamless scalability. For data-intensive processing, it goes without saying that scalable algorithms are highly desirable. As an aspiration, let us sketch the behavior of an ideal algorithm. We can define scalability along at least two dimensions.12 First, in terms of data: given twice the amount of data, the same algorithm should take at most twice as long to run, all else being equal. Second, in terms of resources: given a cluster twice the size, the same algorithm should take no more than half as long to run. Furthermore, an ideal algorithm would maintain these desirable scaling characteristics across a wide range of settings: on data ranging from gigabytes to petabytes, on clusters consisting of a few to a few thousand machines. Finally, the ideal algorithm would exhibit these desired behaviors without requiring any modifications whatsoever, not even tuning of parameters.
Other than for embarrassingly parallel problems, algorithms with the characteristics sketched above are, of course, unobtainable. One of the fundamental assertions in Fred Brook’s classic The Mythical Man-Month [28] is that adding programmers to a project behind schedule will only make it fall further behind. This is because complex tasks cannot be chopped into smaller pieces and allocated in a linear fashion, and is often illustrated with a cute quote: “nine women cannot have a baby in one month”. Although Brook’s observations are primarily about software engineers and the software development process, the same is also true of algorithms: increasing the degree of parallelization also increases communication costs. The algorithm designer is faced with diminishing returns, and beyond a certain point, greater efficiencies gained by parallelization are entirely offset by increased communication requirements.
Nevertheless, these fundamental limitations shouldn’t prevent us from at least striving for the unobtainable. The truth is that most current algorithms are far from the ideal. In the domain of text processing, for example, most algorithms today assume that data fit in memory on a single machine. For the most part, this is a fair assumption. But what happens when the amount of data doubles in the near future, and then doubles again shortly thereafter? Simply buying more memory is not a viable solution, as the amount of data is growing faster than the price of memory is falling. Furthermore, the price of a machine does not scale linearly with the amount of available memory beyond a certain point (once again, the