And this is why you need consistent hashing. Consistent hash. libconhash is a consistent hashing library which can be compiled both on Windows and Linux platforms, with the following features: Now we will consider the common way to do load balance. Please note that this is for purely illustrative purposes only. Consistent hashing is a strategy for dividing up keys/data between multiple machines. */, the hashing results--------------------------------------:\n", Last Visit: 31-Dec-99 19:00     Last Update: 13-Dec-20 6:47, http://portal.acm.org/citation.cfm?id=258660, http://en.wikipedia.org/wiki/Consistent_hashing, http://www.spiteful.com/2008/03/17/programmers-toolbox-part-3-consistent-hashing/. Cassandra partitions data across the cluster using consistent hashing [11] but uses an order preserving hash function to do so. The result is more balanced now. This method operates independently of the number of nodes as the hash function is not dependent on the number of nodes. The library includes a KetamaConnectionFactory class that implements consistent hashing. Here’s how it works: initialize conhash library Looks up a consistent hash with the given data point. Your custom Node represents a real physical node, which supports numbers of virtual nodes , the replicas of physical node. By default, it uses the MD5 algorithm, but it also supports user-defined hash functions. An introduction to and a C library source code for consistent hashing. Consistent hashing provides an alternative to multicast and directory schemes, and has several other advantages in load balancing and fault tolerance. * return the server_s structure, do not modify the value, Any class that implements Node can be mapped to ConsistentHashRouter. Then, the map from object to the virtual node will be: When you get the virtual node, you get the cache, as in the above figure. If one expects that the number of machines will probably be between, say, 1 and 50, and in no case more than 255, what about creating 256 virtual nodes each of which represents a particular value for 8 bits of the hash, and divvy up those virtual nodes among the servers? Easy to scale according to the node's processing capacity. just need this to learn more about the usage of consitent-hash,thanks a lot. When you add a cache machine, then object, When you remove a cache machine, then object. */, * lookup a server which object belongs to You signed in with another tab or window. There is a sample in the project that shows how to use the library. By default , ConsistentHashRouter will use MD5 to hash a node, you may specify your custom hash function by implementing HashFunction. Chain hashing avoids collision. Use Git or checkout with SVN using the web URL. The machine number chosen to cache object o will be: Here, n is the total number of cache machines. So modulo operator is compressor in our implementation. Consistent hashing Our system is based on consistent hashing,a Learn more. Consistent hashing is a special kind of hashing such that when a hash table is resized and consistent hashing is used, only K/n keys need to be remapped on average, where K is the number of keys… If nothing happens, download the GitHub extension for Visual Studio and try again. Now consider the two scenarios, a cache is down and removed; and a new cache is added. We use a hash function to get their key values and map them into the circle, as illustrated in figure 2. Consistent Hashing. in this paper. To contact author, please mail to linjunjie1103@gmail.com, 原理参考 http://www.codeproject.com/Articles/56138/Consistent-hashing, 任意一个实现了Node接口的类的对象都可以映射到ConsistentHashRouter中, 自定义的 Node代表一个物理的节点,这些物理节点支持指定一些虚拟节点作为物理节点的复制(replicas)。当需要增加Node 到 ConsistentHashRouter时,你可以指定这个节点需要复制有多少个虚拟节点。, 默认情况下, ConsistentHashRouter 会使用MD5去哈希一个节点,你也可以通过去实现HashFunction指定自己的自定义哈希函数. Now we will go into consistent hashing step by step. Whenever it is invoked on the same object more than once during an execution of a Java application, the hashCode method must consistently return the same integer, provided no … The solution is to introduce the idea of "virtual nodes". Consistent Hashing is a clever algorithm that is used in high volume caching architectures where scaling and availability are important. I am trying to understand the best way to generate the hascode for a node and a key. * @object: the input string which indicates an object If you Google around to learn what consistent hashing means, the article that most directly tells you "the answer" without a lot of handwringing is Consistent Hashing by Tom White. Implementation Consistent Hashing. Reference http://www.codeproject.com/Articles/56138/Consistent-hashing, Please see MyServiceNode.java for more usage details. */, * add a new node If a new server comes on line, give it some of the nodes that formerly belonged to the servers with the most nodes. And head clockwise on the ring until you find a server gather information about usage... Code is an Integer number ( random or nonrandom ): instantly share,! Review code, manage projects, and build software together third-party analytics cookies to understand the best to. You find a server particularly well when the number of machines storing data may change Xcode and again... Balancing and fault tolerance and load balancing and fault tolerance node represents a real physical node to use! A task array ) into caches to collision that is two or more keys are mapped the... Several nodes, when you add a cache with an adjacent interval you go the! 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