3.Hadoop分布式文件系统：架构和设计 hdfs_design

Hadoop分布式⽂件系统（HDFS）详解HDFS简介：当数据集的⼤⼩超过⼀台独⽴物理计算机的存储能⼒时，就有必要对它进⾏分区 (partition)并存储到若⼲台单独的计算机上。

管理⽹络中跨多台计算机存储的⽂件系统成为分布式⽂件系统 (Distributed filesystem)。

该系统架构于⽹络之上，势必会引⼊⽹络编程的复杂性，因此分布式⽂件系统⽐普通磁盘⽂件系统更为复杂。

HDFS是基于流数据模式访问和处理超⼤⽂件的需求⽽开发的，它可以运⾏于廉价的商⽤服务器上。

总的来说，可以将 HDFS的主要特点概括为以下⼏点：（1 ）处理超⼤⽂件这⾥的超⼤⽂件通常是指数百 MB、甚⾄数百TB ⼤⼩的⽂件。

⽬前在实际应⽤中， HDFS已经能⽤来存储管理PB(PeteBytes)级的数据了。

在 Yahoo！，Hadoop 集群也已经扩展到了 4000个节点。

（2 ）流式地访问数据HDFS的设计建⽴在更多地响应“⼀次写⼊，多次读取”任务的基础之上。

这意味着⼀个数据集⼀旦由数据源⽣成，就会被复制分发到不同的存储节点中，然后响应各种各样的数据分析任务请求。

在多数情况下，分析任务都会涉及数据集中的⼤部分数据，也就是说，对HDFS 来说，请求读取整个数据集要⽐读取⼀条记录更加⾼效。

（3 ）运⾏于廉价的商⽤机器集群上Hadoop设计对硬件需求⽐较低，只须运⾏在廉价的商⽤硬件集群上，⽽⽆须昂贵的⾼可⽤性机器上。

廉价的商⽤机也就意味着⼤型集群中出现节点故障情况的概率⾮常⾼。

这就要求在设计 HDFS时要充分考虑数据的可靠性、安全性及⾼可⽤性。

正是由于以上的种种考虑，我们会发现现在的 HDFS在处理⼀些特定问题时不但没有优势，⽽且有⼀定的局限性，主要表现在以下⼏个⽅⾯。

（1 ）不适合低延迟数据访问如果要处理⼀些⽤户要求时间⽐较短的低延迟应⽤请求，则 HDFS不适合。

HDFS 是为了处理⼤型数据集分析任务的，主要是为达到⾼的数据吞吐量⽽设计的，这就可能要求以⾼延迟作为代价。

外文翻译原文来源The Hadoop Distributed File System: Architecture and Design 中文译文Hadoop分布式文件系统：架构和设计姓名 XXXX学号 ************2013年4月8 日英文原文The Hadoop Distributed File System: Architecture and DesignSource：/docs/r0.18.3/hdfs_design.html IntroductionThe Hadoop Distributed File System (HDFS) is a distributed file system designed to run on commodity hardware. It has many similarities with existing distributed file systems. However, the differences from other distributed file systems are significant. HDFS is highly fault-tolerant and is designed to be deployed onlow-cost hardware. HDFS provides high throughput access to application data and is suitable for applications that have large data sets. HDFS relaxes a few POSIX requirements to enable streaming access to file system data. HDFS was originally built as infrastructure for the Apache Nutch web search engine project. HDFS is part of the Apache Hadoop Core project. The project URL is/core/.Assumptions and GoalsHardware FailureHardware failure is the norm rather than the exception. An HDFS instance may consist of hundreds or thousands of server machines, each storing part of the file system’s data. The fact that there are a huge number of components and that each component has a non-trivial probability of failure means that some component of HDFS is always non-functional. Therefore, detection of faults and quick, automatic recovery from them is a core architectural goal of HDFS.Streaming Data AccessApplications that run on HDFS need streaming access to their data sets. They are not general purpose applications that typically run on general purpose file systems. HDFS is designed more for batch processing rather than interactive use by users. The emphasis is on high throughput of data access rather than low latency of data access. POSIX imposes many hard requirements that are notneeded for applications that are targeted for HDFS. POSIX semantics in a few key areas has been traded to increase data throughput rates.Large Data SetsApplications that run on HDFS have large data sets. A typical file in HDFS is gigabytes to terabytes in size. Thus, HDFS is tuned to support large files. It should provide high aggregate data bandwidth and scale to hundreds of nodes in a single cluster. It should support tens of millions of files in a single instance.Simple Coherency ModelHDFS applications need a write-once-read-many access model for files. A file once created, written, and closed need not be changed. This assumption simplifies data coherency issues and enables high throughput data access. AMap/Reduce application or a web crawler application fits perfectly with this model. There is a plan to support appending-writes to files in the future.“Moving Computation is Cheaper than Moving Data”A computation requested by an application is much more efficient if it is executed near the data it operates on. This is especially true when the size of the data set is huge. This minimizes network congestion and increases the overall throughput of the system. The assumption is that it is often better to migrate the computation closer to where the data is located rather than moving the data to where the application is running. HDFS provides interfaces for applications to move themselves closer to where the data is located.Portability Across Heterogeneous Hardware and Software PlatformsHDFS has been designed to be easily portable from one platform to another. This facilitates widespread adoption of HDFS as a platform of choice for a large set of applications.NameNode and DataNodesHDFS has a master/slave architecture. An HDFS cluster consists of a single NameNode, a master server that manages the file system namespace and regulates access to files by clients. In addition, there are a number of DataNodes, usually one per node in the cluster, which manage storage attached to the nodes that they run on. HDFS exposes a file system namespace and allows user data to be stored in files. Internally, a file is split into one or more blocks and these blocksare stored in a set of DataNodes. The NameNode executes file system namespace operations like opening, closing, and renaming files and directories. It also determines the mapping of blocks to DataNodes. The DataNodes are responsible for serving read and write requests from the file system’s clients. The DataNodes also perform block creation, deletion, and replication upon instruction from the NameNode.The NameNode and DataNode are pieces of software designed to run on commodity machines. These machines typically run a GNU/Linux operating system (OS). HDFS is built using the Java language; any machine that supports Java can run the NameNode or the DataNode software. Usage of the highly portable Java language means that HDFS can be deployed on a wide range ofmachines. A typical deployment has a dedicated machine that runs only the NameNode software. Each of the other machines in the cluster runs one instance of the DataNode software. The architecture does not preclude running multiple DataNodes on the same machine but in a real deployment that is rarely the case.The existence of a single NameNode in a cluster greatly simplifies the architecture of the system. The NameNode is the arbitrator and repository for all HDFS metadata. The system is designed in such a way that user data never flows through the NameNode.The File System NamespaceHDFS supports a traditional hierarchical file organization. A user or an application can create directories and store files inside these directories. The file system namespace hierarchy is similar to most other existing file systems; one can create and remove files, move a file from one directory to another, or rename a file. HDFS does not yet implement user quotas or access permissions. HDFS does not support hard links or soft links. However, the HDFS architecture does not preclude implementing these features.The NameNode maintains the file system namespace. Any change to the file system namespace or its properties is recorded by the NameNode. An application can specify the number of replicas of a file that should be maintained by HDFS. The number of copies of a file is called the replication factor of that file. This information is stored by the NameNode.Data ReplicationHDFS is designed to reliably store very large files across machines in a large cluster. It stores each file as a sequence of blocks; all blocks in a file except the last block are the same size. The blocks of a file are replicated for fault tolerance. The block size and replication factor are configurable per file. An application can specify the number of replicas of a file. The replication factor can be specified at file creation time and can be changed later. Files in HDFS are write-once and have strictly one writer at any time.The NameNode makes all decisions regarding replication of blocks. It periodically receives a Heartbeat and a Blockreport from each of the DataNodes in the cluster.Receipt of a Heartbeat implies that the DataNode is functioning properly. A Blockreport contains a list of all blocks on a DataNode.Replica Placement: The First Baby StepsThe placement of replicas is critical to HDFS reliability and performance. Optimizing replica placement distinguishes HDFS from most other distributed file systems. This is a feature that needs lots of tuning and experience. The purpose of a rack-aware replica placement policy is to improve data reliability, availability, and network bandwidth utilization. The current implementation for the replica placement policy is a first effort in this direction. The short-term goals of implementing this policy are to validate it on production systems, learn more about its behavior, and build a foundation to test and research more sophisticated policies.Large HDFS instances run on a cluster of computers that commonly spread across many racks. Communication between two nodes in different racks has to go through switches. In most cases, network bandwidth between machines in the same rack is greater than network bandwidth between machines in different racks.The NameNode determines the rack id each DataNode belongs to via the process outlined in Rack Awareness. A simple but non-optimal policy is to place replicas on unique racks. This prevents losing data when an entire rack fails and allows use of bandwidth from multiple racks when reading data. This policy evenly distributes replicas in the cluster which makes it easy to balance load on component failure. However, this policy increases the cost of writes because a write needs to transfer blocks to multiple racks.For the common case, when the replication factor is three, HDFS’s placement policy is to put one replica on one node in the local rack, another on a different node in the local rack, and the last on a different node in a different rack. This policy cuts the inter-rack write traffic which generally improves write performance. The chance of rack failure is far less than that of node failure; this policy does not impact data reliability and availability guarantees. However, it does reduce the aggregate network bandwidth used when reading data since a block is placed in only two unique racks rather than three. With this policy, the replicas of a file do not evenly distribute across the racks. One third of replicas are on one node, two thirds of replicas are on one rack, and the other third are evenly distributed across the remaining racks. This policy improves write performance without compromising data reliability or read performance.The current, default replica placement policy described here is a work in progress. Replica SelectionTo minimize global bandwidth consumption and read latency, HDFS tries to satisfy a read request from a replica that is closest to the reader. If there exists a replica on the same rack as the reader node, then that replica is preferred to satisfy the read request. If angg/ HDFS cluster spans multiple data centers, then a replica that is resident in the local data center is preferred over any remote replica.SafemodeOn startup, the NameNode enters a special state called Safemode. Replication of data blocks does not occur when the NameNode is in the Safemode state. The NameNode receives Heartbeat and Blockreport messages from the DataNodes. A Blockreport contains the list of data blocks that a DataNode is hosting. Each block has a specified minimum number of replicas. A block is considered safely replicated when the minimum number of replicas of that data block has checked in with the NameNode. After a configurable percentage of safely replicated data blocks checks in with the NameNode (plus an additional 30 seconds), the NameNode exits the Safemode state. It then determines the list of data blocks (if any) that still have fewer than the specified number of replicas. The NameNode then replicates these blocks to other DataNodes.The Persistence of File System MetadataThe HDFS namespace is stored by the NameNode. The NameNode uses a transaction log called the EditLog to persistently record every change that occurs to file system metadata. For example, creating a new file in HDFS causes the NameNode to insert a record into the EditLog indicating this. Similarly, changing the replication factor of a file causes a new record to be inserted into the EditLog. The NameNode uses a file in its local host OS file system to store the EditLog. The entire file system namespace, including the mapping of blocks to files and file system properties, is stored in a file called the FsImage. The FsImage is stored as a file in the NameNode’s local file system too.The NameNode keeps an image of the entire file system namespace and file Blockmap in memory. This key metadata item is designed to be compact, such that a NameNode with 4 GB of RAM is plenty to support a huge number of files and directories. When the NameNode starts up, it reads the FsImage and EditLog from disk, applies all the transactions from the EditLog to the in-memory representation of the FsImage, and flushes out this new version into a new FsImage on disk. It can then truncate the old EditLog because its transactions have been applied to the persistent FsImage. This process is called a checkpoint. In the current implementation, a checkpoint only occurs when the NameNode starts up. Work is in progress to support periodic checkpointing in the near future.The DataNode stores HDFS data in files in its local file system. The DataNode has no knowledge about HDFS files. It stores each block of HDFS data in a separatefile in its local file system. The DataNode does not create all files in the same directory. Instead, it uses a heuristic to determine the optimal number of files per directory and creates subdirectories appropriately. It is not optimal to create all local files in the same directory because the local file system might not be able to efficiently support a huge number of files in a single directory. When a DataNode starts up, it scans through its local file system, generates a list of all HDFS data blocks that correspond to each of these local files and sends this report to the NameNode: this is the Blockreport.The Communication ProtocolsAll HDFS communication protocols are layered on top of the TCP/IP protocol. A client establishes a connection to a configurable TCP port on the NameNode machine. It talks the ClientProtocol with the NameNode. The DataNodes talk to the NameNode using the DataNode Protocol. A Remote Procedure Call (RPC) abstraction wraps both the Client Protocol and the DataNode Protocol. By design, the NameNode never initiates any RPCs. Instead, it only responds to RPC requests issued by DataNodes or clients.RobustnessThe primary objective of HDFS is to store data reliably even in the presence of failures. The three common types of failures are NameNode failures, DataNode failures and network partitions.Data Disk Failure, Heartbeats and Re-ReplicationEach DataNode sends a Heartbeat message to the NameNode periodically. A network partition can cause a subset of DataNodes to lose connectivity with the NameNode. The NameNode detects this condition by the absence of a Heartbeat message. The NameNode marks DataNodes without recent Heartbeats as dead and does not forward any new IO requests to them. Any data that was registered to a dead DataNode is not available to HDFS any more. DataNode death may cause the replication factor of some blocks to fall below their specified value. The NameNode constantly tracks which blocks need to be replicated and initiates replication whenever necessary. The necessity for re-replication may arise due to many reasons: a DataNode may become unavailable, a replica may become corrupted, a hard disk on a DataNode may fail, or the replication factor of a file may be increased.Cluster RebalancingThe HDFS architecture is compatible with data rebalancing schemes. A scheme might automatically move data from one DataNode to another if the free space on a DataNode falls below a certain threshold. In the event of a sudden high demand for a particular file, a scheme might dynamically create additional replicas and rebalance other data in the cluster. These types of data rebalancing schemes are not yet implemented.Data IntegrityIt is possible that a block of data fetched from a DataNode arrives corrupted. This corruption can occur because of faults in a storage device, network faults, or buggy software. The HDFS client software implements checksum checking on the contents of HDFS files. When a client creates an HDFS file, it computes a checksum of each block of the file and stores these checksums in a separate hidden file in the same HDFS namespace. When a client retrieves file contents it verifies that the data it received from each DataNode matches the checksum stored in the associated checksum file. If not, then the client can opt to retrieve that block from another DataNode that has a replica of that block.Metadata Disk FailureThe FsImage and the EditLog are central data structures of HDFS. A corruption of these files can cause the HDFS instance to be non-functional. For this reason, the NameNode can be configured to support maintaining multiple copies of the FsImage and EditLog. Any update to either the FsImage or EditLog causes each of the FsImages and EditLogs to get updated synchronously. This synchronous updating of multiple copies of the FsImage and EditLog may degrade the rate of namespace transactions per second that a NameNode can support. However, this degradation is acceptable because even though HDFS applications are very data intensive in nature, they are not metadata intensive. When a NameNode restarts, it selects the latest consistent FsImage and EditLog to use.The NameNode machine is a single point of failure for an HDFS cluster. If the NameNode machine fails, manual intervention is necessary. Currently, automatic restart and failover of the NameNode software to another machine is not supported.SnapshotsSnapshots support storing a copy of data at a particular instant of time. One usage of the snapshot feature may be to roll back a corrupted HDFS instance to a previously known good point in time. HDFS does not currently support snapshots but will in a future release.Data OrganizationData BlocksHDFS is designed to support very large files. Applications that are compatible with HDFS are those that deal with large data sets. These applications write their data only once but they read it one or more times and require these reads to be satisfied at streaming speeds. HDFS supports write-once-read-many semantics on files. A typical block size used by HDFS is 64 MB. Thus, an HDFS file is chopped up into 64 MB chunks, and if possible, each chunk will reside on a different DataNode.StagingA client request to create a file does not reach the NameNode immediately. In fact, initially the HDFS client caches the file data into a temporary local file. Application writes are transparently redirected to this temporary local file. When the local file accumulates data worth over one HDFS block size, the client contacts the NameNode. The NameNode inserts the file name into the file system hierarchy and allocates a data block for it. The NameNode responds to the client request with the identity of the DataNode and the destination data block. Then the client flushes the block of data from the local temporary file to the specified DataNode. When a file is closed, the remaining un-flushed data in the temporary local file is transferred to the DataNode. The client then tells the NameNode that the file is closed. At this point, the NameNode commits the file creation operation into a persistent store. If the NameNode dies before the file is closed, the file is lost.The above approach has been adopted after careful consideration of target applications that run on HDFS. These applications need streaming writes to files. If a client writes to a remote file directly without any client side buffering, the network speed and the congestion in the network impacts throughput considerably. This approach is not without precedent. Earlier distributed file systems, e.g. AFS, have used client side caching to improve performance. APOSIX requirement has been relaxed to achieve higher performance of data uploads.Replication PipeliningWhen a client is writing data to an HDFS file, its data is first written to a local file as explained in the previous section. Suppose the HDFS file has a replication factor of three. When the local file accumulates a full block of user data, the client retrieves a list of DataNodes from the NameNode. This list contains the DataNodes that will host a replica of that block. The client then flushes the data block to the first DataNode. The first DataNode starts receiving the data in small portions (4 KB), writes each portion to its local repository and transfers that portion to the second DataNode in the list. The second DataNode, in turn starts receiving each portion of the data block, writes that portion to its repository and then flushes that portion to the third DataNode. Finally, the third DataNode writes the data to its local repository. Thus, a DataNode can be receiving data from the previous one in the pipeline and at the same time forwarding data to the next one in the pipeline. Thus, the data is pipelined from one DataNode to the next.AccessibilityHDFS can be accessed from applications in many different ways. Natively, HDFS provides a Java API for applications to use. A C language wrapper for this Java API is also available. In addition, an HTTP browser can also be used to browse the files of an HDFS instance. Work is in progress to expose HDFS through the WebDAV protocol.FS ShellHDFS allows user data to be organized in the form of files and directories. It provides a commandline interface called FS shell that lets a user interact with the data in HDFS. The syntax of this command set is similar to other shells (e.g. bash, csh) that users are already familiar with. Here are some sample action/command pairs:FS shell is targeted for applications that need a scripting language to interact with the stored data.DFSAdminThe DFSAdmin command set is used for administering an HDFS cluster. These are commands that are used only by an HDFS administrator. Here are some sample action/command pairs:Browser InterfaceA typical HDFS install configures a web server to expose the HDFS namespace through a configurable TCP port. This allows a user to navigate the HDFS namespace and view the contents of its files using a web browser.Space ReclamationFile Deletes and UndeletesWhen a file is deleted by a user or an application, it is not immediately removed from HDFS. Instead, HDFS first renames it to a file in the /trash directory. The file can be restored quickly as long as it remains in /trash. A file remains in/trash for a configurable amount of time. After the expiry of its life in /trash, the NameNode deletes the file from the HDFS namespace. The deletion of a file causes the blocks associated with the file to be freed. Note that there could be an appreciable time delay between the time a file is deleted by a user and the time of the corresponding increase in free space in HDFS.A user can Undelete a file after deleting it as long as it remains in the /trash directory. If a user wants to undelete a file that he/she has deleted, he/she can navigate the /trash directory and retrieve the file. The /trash directory contains only the latest copy of the file that was deleted. The /trash directory is just like any other directory with one special feature: HDFS applies specified policies to automatically delete files from this directory. The current default policy is to delete files from /trash that are more than 6 hours old. In the future, this policy will be configurable through a well defined interface.Decrease Replication FactorWhen the replication factor of a file is reduced, the NameNode selects excess replicas that can be deleted. The next Heartbeat transfers this information to the DataNode. The DataNode then removes the corresponding blocks and the corresponding free space appears in the cluster. Once again, there might be a time delay between the completion of the setReplication API call and the appearance of free space in the cluster.中文译本原文地址:/docs/r0.18.3/hdfs_design.html一、引言Hadoop分布式文件系统(HDFS)被设计成适合运行在通用硬件(commodity hardware)上的分布式文件系统。

hadoop 原理Hadoop是一个开源的分布式计算框架，基于Google的MapReduce和分布式文件系统（HDFS）的概念而设计。

它可以处理大规模数据集并将其分布式存储在集群中的多个计算节点上。

Hadoop的核心原理包括：1. 分布式存储：Hadoop将大规模的数据集分散存储在集群中的多个计算节点上。

这些数据被分割为多个块，并复制到多个节点上以提供容错性。

这种分布式存储方式以Hadoop分布式文件系统（HDFS）实现，允许在存储节点上进行数据读写操作。

2. 分布式计算：Hadoop利用MapReduce模型进行分布式计算。

MapReduce模型将计算任务分为两个关键步骤：Map和Reduce。

Map阶段将输入数据集映射为键值对，并为每个键值对生成一个中间结果。

Reduce阶段将相同键的中间结果聚合为最终结果。

这种分布式计算模型允许在不同计算节点上并行处理数据块，并将结果合并。

3. 容错性：Hadoop实现了容错机制，使得在集群中的节点发生故障时能够自动恢复和重新分配任务。

当一个节点失败时，Hadoop会将该节点上的任务重新分配给其他可用节点，以确保计算过程的连续性和可靠性。

4. 数据局部性优化：Hadoop提供了数据局部性优化机制，通过将计算任务调度到存储有数据块的节点上来减少数据传输开销。

这样可以最大限度地利用集群内部的带宽和计算资源，提高计算效率。

5. 扩展性：Hadoop的分布式架构具有良好的可扩展性，允许根据需求增加或减少集群中的计算节点。

这种可扩展性使得Hadoop能够处理大规模数据集，并且可以处理节点故障或新节点的加入。

综上所述，Hadoop通过分布式存储和计算、容错性、数据局部性优化和可扩展性等主要原理，实现了对大规模数据集的高效处理和分析。

hadoop技术、方法以及原理的理解Hadoop技术、方法以及原理的理解Hadoop是一个开源的分布式计算框架，它能够存储和处理海量的数据。

它由Apache基金会开发和维护，是目前最流行的大数据处理解决方案之一。

Hadoop的技术、方法以及原理是构成Hadoop 的核心部分，下面我们将对其进行详细的解析。

一、Hadoop的技术1. HDFSHadoop分布式文件系统（HDFS）是Hadoop的核心组件之一。

它是一种高度容错的分布式文件系统，具有高可靠性和高可用性。

该文件系统将海量数据分散存储在多个节点上，以实现快速访问和处理。

2. MapReduceMapReduce是Hadoop的另一个核心组件，它是一种编程模型和处理数据的方式。

MapReduce将数据分成小的块，然后在分布式计算机集群上处理这些块。

MapReduce将任务分为Map和Reduce两个阶段。

在Map阶段，数据被分割并分配给不同的节点进行计算。

在Reduce阶段，计算的结果被合并起来并输出。

3. YARNHadoop资源管理器（YARN）是另一个重要的组件，它是一个分布式的集群管理系统，用于管理Hadoop集群中的资源。

YARN允许多个应用程序同时运行在同一个Hadoop集群上，通过动态管理资源来提高集群的使用效率。

二、Hadoop的方法1. 大数据存储Hadoop通过HDFS实现对海量数据的存储和管理。

HDFS的设计目标是支持大型数据集的分布式处理，它通过多个节点存储数据，提供高可靠性和高可用性。

2. 数据处理Hadoop通过MapReduce实现对海量数据的处理。

MapReduce 将数据分成小的块，然后在分布式计算机集群上处理这些块。

在Map阶段，数据被分割并分配给不同的节点进行计算。

在Reduce 阶段，计算的结果被合并起来并输出。

3. 数据分析Hadoop通过Hive、Pig和Spark等工具实现数据分析。

这些工具提供了高级查询和数据分析功能，可以通过SQL和其他编程语言来处理海量数据。

hadoop基本架构和工作原理Hadoop是一个分布式开源框架，用于处理海量数据。

它能够使用廉价的硬件来搭建集群，同时还提供了高度可靠性和容错性。

Hadoop基本架构包括Hadoop Common、Hadoop Distributed File System （HDFS）和Hadoop MapReduce三个部分，下面将详细介绍Hadoop的工作原理。

1. Hadoop CommonHadoop Common是整个Hadoop架构的基础部分，是一个共享库，它包含了大量的Java类和应用程序接口。

Hadoop集群的每一台机器上都要安装Hadoop Common，并保持相同版本。

2. HDFSHadoop Distributed File System（HDFS）是Hadoop的分布式文件存储部分。

它的目的是将大型数据集分成多个块，并且将这些块在集群中的多个节点间分布式存储。

HDFS可以实现高度可靠性，因为它将每个块在存储节点之间备份。

HDFS可以在不同的节点中进行数据备份，这确保了数据发生故障时，可以轻松恢复。

3. MapReduceHadoop MapReduce是一种编程模型，用于处理大型数据集。

它将处理任务分成两个主要阶段，即Map阶段和Reduce阶段。

在Map阶段，MapReduce将数据集分成小块，并将每个块分配给不同的节点进行处理。

在Reduce阶段，结果被聚合，以生成最终的输出结果。

总的来说，MapReduce作为Hadoop的核心组件，负责对数据集进行处理和计算。

它充当的角色是一个调度员，它会将不同的任务分发到集群中的不同节点上，并尽力保证每个任务都可以获得足够的计算资源。

Hadoop采用多种技术来提供MapReduce的分布式计算能力，其中包括TaskTracker、JobTracker和心跳机制等。

TaskTracker是每个集群节点的一个守护程序，负责处理MapReduce任务的具体实现。

Hadoop技术的基础原理和实践近年来，随着数据规模的不断增大，传统的关系型数据库已经无法满足海量数据的处理需求，因此大数据技术逐渐成为了当下最为热门的技术领域之一。

而作为大数据技术的代表之一，Hadoop技术已经逐渐成为了企业所必备的技术之一。

本文将介绍Hadoop技术的基础原理和实践。

一、Hadoop技术概述Hadoop是一种分布式的数据处理框架，其最重要的特点是可横向扩展。

Hadoop有两个核心组件：分布式文件系统Hadoop Distributed File System（简称HDFS）和分布式计算框架MapReduce。

HDFS是Hadoop的核心数据存储系统，它使用分布式文件系统的概念来存储海量数据。

Hadoop的HDFS将数据分布到不同的节点上存储，保证了数据的备份和容错能力。

另外一个核心组件MapReduce是一个实现分布式计算的框架，它能将大量的数据分成若干个小的数据块，然后在多台计算机上并行处理。

这种处理方式能有效地提高数据处理的效率以及减少资源消耗。

二、Hadoop技术的基本原理1.数据存储Hadoop的数据存储可以使用HDFS进行分布式存储。

HDFS将数据分为若干块，每个数据块默认为128MB。

HDFS将这些数据块分别分布到各个数据节点中存储，保证了数据的可靠性和安全性。

2.数据处理Hadoop使用MapReduce来实现数据处理。

其工作流程如下：① Map阶段Map阶段是指将原始数据进行切割和转化，转化成可供Reduce 处理的中间结果。

通常需要在Map阶段定义具体的Map函数来描述数据的输入、映射和输出。

② Reduce阶段Reduce阶段是指对Map的输出结果进行处理的阶段。

Reduce 函数能够对Map函数的输出进行整合来生成最终结果。

3.分布式计算Hadoop的分布式计算能力是通过Hadoop集群中各个节点之间的协调和通信来实现的。

在Hadoop中每个任务都会由一个或多个Worker节点运行，他们可以分别处理不同的数据块，之后再将结果汇总到一起。

hadoop的生态体系及各组件的用途
Hadoop是一个生态体系，包括许多组件，以下是其核心组件和用途：
1. Hadoop Distributed File System (HDFS)：这是Hadoop的分布式文件系统，用于存储大规模数据集。

它设计为高可靠性和高吞吐量，并能在低成本的通用硬件上运行。

通过流式数据访问，它提供高吞吐量应用程序数据访问功能，适合带有大型数据集的应用程序。

2. MapReduce：这是Hadoop的分布式计算框架，用于并行处理和分析大规模数据集。

MapReduce模型将数据处理任务分解为Map和Reduce两个阶段，从而在大量计算机组成的分布式并行环境中有效地处理数据。

3. YARN：这是Hadoop的资源管理和作业调度系统。

它负责管理集群资源、调度任务和监控应用程序。

4. Hive：这是一个基于Hadoop的数据仓库工具，提供SQL-like查询语言和数据仓库功能。

5. Kafka：这是一个高吞吐量的分布式消息队列系统，用于实时数据流的收集和传输。

6. Pig：这是一个用于大规模数据集的数据分析平台，提供类似SQL的查询语言和数据转换功能。

7. Ambari：这是一个Hadoop集群管理和监控工具，提供可视化界面和集群配置管理。

此外，HBase是一个分布式列存数据库，可以与Hadoop配合使用。

HBase 中保存的数据可以使用MapReduce来处理，它将数据存储和并行计算完美地结合在一起。

《云计算基础》课程教案一、课程基本信息课程名称：云计算基础课程类别：专业基础课授课对象：专业名称年级学生授课时间：总学时二、课程目标1、知识目标使学生了解云计算的基本概念、体系架构、服务模式和部署模型，掌握云计算中的关键技术，如虚拟化、分布式存储、分布式计算等。

2、能力目标培养学生具备云计算系统的搭建、配置和管理能力，能够运用云计算技术解决实际问题。

3、素质目标培养学生的创新思维、团队合作精神和解决复杂问题的能力，提高学生的信息素养和职业道德。

三、课程内容1、云计算概述（1）云计算的定义和特点（2）云计算的发展历程和趋势（3）云计算与传统计算模式的比较2、云计算体系架构（1）IaaS（基础设施即服务）（2）PaaS（平台即服务）（3）SaaS（软件即服务）3、云计算服务模式（1）公有云（2）私有云（3）混合云（4）社区云4、云计算部署模型（1）本地部署（2）托管部署（3）云部署5、虚拟化技术（1）服务器虚拟化（2）存储虚拟化（3）网络虚拟化6、分布式存储（1）Hadoop 分布式文件系统（HDFS）（2）分布式对象存储（3）分布式块存储7、分布式计算（1）MapReduce 编程模型（2）Spark 计算框架（3）Flink 流处理框架8、云计算安全（1）云计算安全威胁与挑战（2）云计算安全技术与策略9、云计算应用案例（1）云计算在企业中的应用（2）云计算在互联网行业的应用（3）云计算在教育、医疗等领域的应用四、教学方法1、课堂讲授通过讲解、演示和案例分析，让学生掌握云计算的基本概念、原理和技术。

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HDFS的架构及原理 HDFS(Hadoop Distributed File System)是Hadoop核⼼组成之⼀，是分布式计算中数据存储管理的基础，被设计成适合运⾏在通⽤硬件上的分布式⽂件系统。

HDFS架构中有两类节点，⼀类是NameNode，⼜叫“元数据节点”，另⼀类是DataNode，⼜叫“数据节点”，分别执⾏Master和Worker的具体任务。

HDFS是⼀个(Master/Slave)体系结构，“⼀次写⼊，多次读取”。

HDFS的设计思想：分⽽治之—将⼤⽂件、⼤批量⽂件分布式存放在⼤量独⽴的机器上。

⼀、HDFS的优缺点优点： 1、⾼容错性。

数据保存多个副本，通过增加副本的形式提⾼容错性，某个副本丢失后，它可以通过其它副本⾃动恢复。

2、适合⼤批量数据处理。

处理达到GB、TB，甚⾄PB级别的数据，处理百万规模以上的⽂件数量，处理10K节点的规模。

3、流式⽂件访问。

⼀次写⼊多次读取，⽂件⼀旦写⼊不能修改，只能追加，保证数据⼀致性。

4、可构建在廉价机器上。

通过多副本机制提⾼可靠性，提供容错和恢复机制。

缺点(不适⽤HDFS的场景)： 1、低延时数据访问。

做不到毫秒级存储数据，但是适合⾼吞吐率(某⼀时间内写⼊⼤量的数据)的场景。

2、⼩⽂件存储。

存储⼤量⼩⽂件会占⽤NameNode⼤量的内存来存储⽂件、⽬录和块信息。

3、并发写⼊、随机读写。

⼀个⽂件不允许多个线程同时写，仅⽀持数据追加，不⽀持⽂件的随机修改。

⼆、HDFS存储架构数据存储架构图：HDFS采⽤Master/Slave的架构存储数据，由HDFS Client、NameNode、DataNode和Secondary NameNode四部分组成。

Client：客户端 1、⽂件切分。

⽂件上传HDFS时，Client按照Block⼤⼩切分⽂件，然后进⾏存储 2、与NameNode交互，获取⽂件位置信息 3、与DataNode交互，读取或写⼊数据 4、Client提供⼀些命令管理和访问HDFSNameNode：Master(管理者) 1、管理HDFS的名称空间 2、管理数据块(Block)映射信息 3、配置副本策略 4、处理客户端读写请求DataNode：Slave(NN下达命令执⾏实际的操作) 1、存储实际的数据块 2、执⾏数据块的读/写操作Secondary NameNode：并⾮NameNode的热备，当NN停⽌服务时，它并不能马上替换NN并提供服务 1、辅助NN，分担其⼯作量 2、定期合并fsimage和fsedits，并推送给NN 3、在紧急情况下，可辅助恢复NN三、HDFS数据读写⽂件读取步骤： 1、⾸先调⽤FileSystem的open⽅法获取⼀个DistributedFileSystem实例。

了解分布式文件系统的设计与实现分布式文件系统是一种用于管理大规模数据存储和访问的系统，它采用了分布式的方式来提高文件系统的性能和可靠性。

本文将介绍分布式文件系统的设计原理和实现细节。

一、简介分布式文件系统是为了应对传统单台服务器存储容量有限、性能瓶颈等问题而被提出的解决方案。

它将数据分布在多个节点上，并通过网络协议提供数据访问服务。

分布式文件系统的设计目标是提高系统的可扩展性、容错性和性能。

二、设计原理1. 数据分布分布式文件系统将文件划分为多个块，并将这些块分散存储在不同的节点上。

通过使用哈希函数或其他分布算法，将文件块映射到具体的节点，并在节点之间进行数据复制，以提高数据的冗余性和可靠性。

2. 元数据管理分布式文件系统通过维护元数据来管理文件的存储和访问。

元数据包括文件名、大小、权限、所在节点等信息。

通常会使用专门的元数据服务器来存储和管理这些信息，并通过一致性协议来保证元数据的一致性和可用性。

3. 数据一致性由于数据存储在多个节点上，分布式文件系统需要解决数据一致性的问题。

一种常用的方法是使用副本机制，在写操作中将数据复制到多个节点，并使用一致性协议来保证多个副本之间的一致性。

另一种方法是使用分布式锁机制，在写操作时对相关的数据块进行加锁，以避免并发访问导致的数据不一致问题。

4. 数据访问分布式文件系统通过网络协议提供数据的访问服务。

常用的访问方式包括文件读写、文件重命名、文件删除等操作。

客户端通过与存储节点进行通信，发送相应的请求并获取数据的返回结果。

三、实现细节1. 存储节点分布式文件系统的存储节点是存储实际数据的地方。

每个存储节点都有自己的存储设备，并负责管理和维护文件块。

存储节点之间通过网络通信来实现数据的复制和传输。

2. 元数据服务器元数据服务器负责管理文件的元数据信息。

它通常是一个单独的节点，用于存储和维护文件的元数据信息。

元数据服务器通过与存储节点进行通信，将文件块的位置信息传递给客户端，以便客户端能够正确地访问文件。