Shulou(Shulou.com)06/03 Report--

I. Also talk about high availability

"High Availability"(HA) is often used to describe a system that has been specifically designed to reduce downtime while maintaining high availability of its services. Generally speaking, it is through professional design to ensure that system-related services can be uninterrupted and stable operation.

Measurement:

%availability=（Total Elapsed Time-Sum of Inoperative Times）/ Total Elapsed Time

Availability is related to the failure rate of system components. One measure of the failure rate of system equipment is MTBF (mean time between failures). Typically this metric measures system components such as disks: MTBF=Total Operating Time / Total No. of Failures

II. Goal definition of high availability design scheme:

Maintain high business stability

System stability is the fundamental purpose of high availability, colloquially speaking, the system can continue to be available, will not be down for no reason, and can still work normally under high pressure.

Support fast fault location (establish hierarchical monitoring)

From the point of view of practical engineering, there is no service that does not fail, so it is necessary to be able to quickly discover and locate the fault. Before the external user finds it, the alarm mechanism can accurately locate the cause of the fault, help the engineer to deal with the problem as soon as possible, and prevent further affecting the business.

Support rapid business recovery

Business can be built through the above three aspects

III. Principles of Architecture Design:

(1) Stateless design

Stateless service processes a single request independently of other requests, that is, all the information needed to process a request is either contained in the request or can be obtained externally (such as a database), and the server itself does not store any information. If there is other information to store, you can put the information in a separate shareable place, independent of the server existence, for example, the same way to save data in the server, reduce the amount of data transmitted per client request (save traffic); but the session centralized storage, such as in a separate session layer.

In this case, the server is also stateless and can be horizontally extended. If the server is stateless, then for the client, the request can be sent to any server, and then horizontal scaling can be achieved through means such as Load Balancer.

② If the server is stateful, then it cannot be easily implemented, because the client needs to always send the request to the same server, the so-called "session migration" and other solutions, that is, to solve this problem.

3. Convert stateful services into stateless services. Stateless services are relatively isolated and equal. When a server goes wrong, traffic will be transferred to other stateless servers through Load Balancer policy.

The same is true for sessions, where data is saved on the server side to reduce the amount of data transmitted per client request (saving traffic); however, sessions are stored centrally, such as in a separate session layer. In this case, the server is also stateless and can be horizontally expanded, as shown in the following figure:

(2) Redundant design

In the process of stateless service failover, redundant design of service is needed to realize it.

A few concepts: SOA (Service-Oriented Architecture), SOA solves the problem of multi-service clutter, SOA architecture solves the complexity of data services, and SOA has a name called service governance. ② Microservice architecture: In fact, similar to SOA architecture, microservice is a sublimation on SOA. One of the key points emphasized by microservice architecture is that "business needs to be completely component-based and service-based." The original single business system will be split into multiple small applications that can be independently developed, designed and operated. These small applications interact and integrate through services. SLA: Service Level Agreement (SLA) A mutually agreed agreement defined between a service provider and a user to guarantee service performance and availability at a certain cost.

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