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This chapter contains information about MySQL Cluster , which is a high-availability, high-redundancy version of MySQL adapted for the distributed computing environment.
Beginning with MySQL 5. Locations where the sources can be obtained are listed later in this section. MySQL Cluster is currently available and supported on a number of platforms. For exact levels of support available for on specific combinations of operating system versions, operating system distributions, and hardware platforms, please refer to http: MySQL Cluster binary and source packages are available for supported platforms from http: MySQL Cluster release numbers.
You can see this format used in the mysql client, as shown here:. From this we can determine the following:. Compatibility with standard MySQL 5. Most of these issues can be overcome, but this also means that you are very unlikely to be able to switch an existing application datastore—that currently uses, for example, MyISAM or InnoDB —to use the NDB storage engine without allowing for the possibility of changes in schemas, queries, and applications.
Moreover, from MySQL 5. MySQL Cluster development source trees. MySQL Cluster development trees can also be accessed from https: This chapter represents a work in progress, and its contents are subject to revision as MySQL Cluster continues to evolve. MySQL Cluster is a technology that enables clustering of in-memory databases in a shared-nothing system. The shared-nothing architecture enables the system to work with very inexpensive hardware, and with a minimum of specific requirements for hardware or software.
MySQL Cluster is designed not to have any single point of failure. In a shared-nothing system, each component is expected to have its own memory and disk, and the use of shared storage mechanisms such as network shares, network file systems, and SANs is not recommended or supported.
These processes, known as nodes , may include MySQL servers for access to NDB data , data nodes for storage of the data , one or more management servers, and possibly other specialized data access programs.
When data is stored by the NDB storage engine, the tables and table data are stored in the data nodes. Thus, in a payroll application storing data in a cluster, if one application updates the salary of an employee, all other MySQL servers that query this data can see this change immediately.
The data stored in the data nodes for MySQL Cluster can be mirrored; the cluster can handle failures of individual data nodes with no other impact than that a small number of transactions are aborted due to losing the transaction state.
Because transactional applications are expected to handle transaction failure, this should not be a source of problems. Individual nodes can be stopped and restarted, and can then rejoin the system cluster.
It is possible to run multiple nodes on a single computer; for a computer on which one or more cluster nodes are being run we use the term cluster host. There are three types of cluster nodes, and in a minimal MySQL Cluster configuration, there will be at least three nodes, one of each of these types:.
The role of this type of node is to manage the other nodes within the MySQL Cluster, performing such functions as providing configuration data, starting and stopping nodes, running backup, and so forth. Because this node type manages the configuration of the other nodes, a node of this type should be started first, before any other node. This type of node stores cluster data. For example, with two replicas, each having two fragments, you need four data nodes.
One replica is sufficient for data storage, but provides no redundancy; therefore, it is recommended to have 2 or more replicas to provide redundancy, and thus high availability. This is a node that accesses the cluster data. An SQL node is a mysqld process started with the --ndbcluster and --ndb-connectstring options, which are explained elsewhere in this chapter, possibly with additional MySQL server options as well.
It is not realistic to expect to employ a three-node setup in a production environment. The use of multiple management nodes is also highly recommended. Configuration of a cluster involves configuring each individual node in the cluster and setting up individual communication links between nodes.
MySQL Cluster is currently designed with the intention that data nodes are homogeneous in terms of processor power, memory space, and bandwidth. In addition, to provide a single point of configuration, all configuration data for the cluster as a whole is located in one configuration file.
The management server manages the cluster configuration file and the cluster log. Each node in the cluster retrieves the configuration data from the management server, and so requires a way to determine where the management server resides. When interesting events occur in the data nodes, the nodes transfer information about these events to the management server, which then writes the information to the cluster log. In addition, there can be any number of cluster client processes or applications.
These are described in the next few paragraphs. Such applications may be useful for specialized purposes where an SQL interface to the data is not needed. These clients connect to the management server and provide commands for starting and stopping nodes gracefully, starting and stopping message tracing debug versions only , showing node versions and status, starting and stopping backups, and so on.
The MySQL Cluster Manager client also supports commands for getting and setting the values of most node configuration parameters as well as mysqld server options and variables relating to MySQL Cluster. MySQL Cluster logs events by category startup, shutdown, errors, checkpoints, and so on , priority, and severity.
Event logs are of the two types listed here:. Keeps a record of all desired reportable events for the cluster as a whole. A separate log which is also kept for each individual node. Under normal circumstances, it is necessary and sufficient to keep and examine only the cluster log.
The node logs need be consulted only for application development and debugging purposes. Generally speaking, when data is saved to disk, it is said that a checkpoint has been reached. More specific to MySQL Cluster, a checkpoint is a point in time where all committed transactions are stored on disk.
With regard to the NDB storage engine, there are two types of checkpoints which work together to ensure that a consistent view of the cluster's data is maintained. These are shown in the following list:. This is a checkpoint that is specific to a single node; however, LCP's take place for all nodes in the cluster more or less concurrently.
An LCP involves saving all of a node's data to disk, and so usually occurs every few minutes. The precise interval varies, and depends upon the amount of data stored by the node, the level of cluster activity, and other factors. A GCP occurs every few seconds, when transactions for all nodes are synchronized and the redo-log is flushed to disk.
A number of concepts central to an understanding of this topic are discussed in the next few paragraphs.
An ndbd process, which stores a replica —that is, a copy of the partition see below assigned to the node group of which the node is a member. Each data node should be located on a separate computer. While it is also possible to host multiple ndbd processes on a single computer, such a configuration is not supported.
A node group consists of one or more nodes, and stores partitions, or sets of replicas see next item. The number of node groups in a MySQL Cluster is not directly configurable; it is a function of the number of data nodes and of the number of replicas NoOfReplicas configuration parameter , as shown here:.
This is a portion of the data stored by the cluster. There are as many cluster partitions as nodes participating in the cluster. Each node is responsible for keeping at least one copy of any partitions assigned to it that is, at least one replica available to the cluster. A replica belongs entirely to a single node; a node can and usually does store several replicas. NDB and user-defined partitioning. However, in MySQL 5. This is subject to the following limitations:. When using ndbmtd , this maximum is also affected by the number of local query handler threads, which is determined by the value of the MaxNoOfExecutionThreads configuration parameter.
This is a copy of a cluster partition. Each node in a node group stores a replica. Also sometimes known as a partition replica. The number of replicas is equal to the number of nodes per node group. The following diagram illustrates a MySQL Cluster with four data nodes, arranged in two node groups of two nodes each; nodes 1 and 2 belong to node group 0, and nodes 3 and 4 belong to node group 1.
The data stored by the cluster is divided into four partitions, numbered 0, 1, 2, and 3. Each partition is stored—in multiple copies—on the same node group. Partitions are stored on alternate node groups as follows:. Partition 0 is stored on node group 0; a primary replica primary copy is stored on node 1, and a backup replica backup copy of the partition is stored on node 2.
Partition 1 is stored on the other node group node group 1 ; this partition's primary replica is on node 3, and its backup replica is on node 4. Partition 2 is stored on node group 0. However, the placing of its two replicas is reversed from that of Partition 0; for Partition 2, the primary replica is stored on node 2, and the backup on node 1. Partition 3 is stored on node group 1, and the placement of its two replicas are reversed from those of partition 1. That is, its primary replica is located on node 4, with the backup on node 3.
This is illustrated in the next diagram. However, if both nodes from either node group fail, the remaining two nodes are not sufficient shown by the arrows marked out with an X ; in either case, the cluster has lost an entire partition and so can no longer provide access to a complete set of all cluster data.
One of the strengths of MySQL Cluster is that it can be run on commodity hardware and has no unusual requirements in this regard, other than for large amounts of RAM, due to the fact that all live data storage is done in memory.
Naturally, multiple and faster CPUs can enhance performance. Host operating systems do not require any unusual modules, services, applications, or configuration to support MySQL Cluster. For supported operating systems, a standard installation should be sufficient. The MySQL software requirements are simple: We assume that you are using the binaries appropriate to your platform, available from the MySQL Cluster software downloads page at http: We strongly recommend that a MySQL Cluster be run on its own subnet which is not shared with machines not forming part of the cluster for the following reasons:.
Setting up a MySQL Cluster on a private or protected network enables the cluster to make exclusive use of bandwidth between cluster hosts.