Cluster type:

Cluster concept

1. Two key features

Cluster is a group of service entities that work together to Provide a service platform that is more scalable and usable than a single service entity. From the perspective of the client, a cluster is like a service entity, but in fact the cluster is composed of a group of service entities. Compared with a single service entity, the cluster provides the following two key features:

· Scalability-the performance of the cluster is not limited to a single service entity, a new service entity You can dynamically join the cluster to enhance the performance of the cluster.

· High availability—The cluster uses service entity redundancy to prevent clients from easily encountering out of service warnings. In a cluster, the same service can be provided by multiple service entities. If one service entity fails, another service entity will take over the failed service entity. The function of recovering from an errored service entity to another service entity provided by the cluster enhances the usability of the application.

2. Two capabilities

In order to have scalability and high availability, the cluster must have the following two capabilities:

< p>· Load balancing-Load balancing can distribute tasks more evenly to computing and network resources in a cluster environment.

· Error recovery-For some reason, the resource performing a certain task fails, and the resource performing the same task in another service entity then completes the task. This process in which resources in one entity cannot work and resources in another entity continue to complete tasks transparently is called error recovery.

Load balancing and error recovery require the existence of resources to perform the same task in each service entity, and for each resource of the same task, the information view required to perform the task (Information context) must be the same.

3. Two technologies

The following two technologies must be used to implement clusters:

·Cluster address-cluster routing It is composed of multiple service entities, and the cluster client obtains the functions of each service entity in the cluster by accessing the cluster address of the cluster. Having a single cluster address (also called a single image) is a basic feature of a cluster. The setting that maintains the cluster address is called a load balancer. The load balancer is responsible for managing the joining and exiting of each service entity, and the external is responsible for the conversion of the cluster address to the address of the internal service entity. Some load balancers implement real load balancing algorithms, and some only support task conversion. The load balancer that only realizes task conversion is suitable for the cluster environment that supports ACTIVE-STANDBY, where only one service entity works in the cluster. When the working service entity fails, the load balancer transfers the subsequent tasks to another service entity.

· Internal communication-In order to work together, achieve load balancing and error recovery, the entities in the cluster must communicate frequently, such as the heartbeat test information of the service entity by the load balancer, Communication of task execution context information between service entities.

Having the same cluster address allows clients to access the computing services provided by the cluster. The internal addresses of each service entity are hidden under a cluster address, so that the computing services required by customers can be Distributed among various service entities. Internal communication is the basis for the normal operation of the cluster, which enables the cluster to have the ability to balance load and recover from errors.