Distributed basic concepts

1. Extensibility

（1） Horizontal expansion （horizontal scaling）： Change the number of resources .

• Horizontal allocation of resources is also known as scaling out （scaling out）.
• The horizontal release of resources also becomes inward expansion （scaling in）.

（2） Vertical expansion （vertical scaling）： Change the capacity of resources .

• Allocating larger capacity resources is also called scaling up （scaling up）.
• Allocating resources with smaller capacity is also called scaling down （scaling down）.

2. CAP theory

A distributed system can only satisfy the consistency at most （Consistency）、 Usability （Availability） And partition fault tolerance （Partition tolerance） Two of these three .

notes ： Consistency here refers to strong consistency .

3. BASE theory

Basic available （Basically Available）, Soft state （Soft state）, Final consistency （Eventually consistent）.

• Basic available ： Even if the system has an unpredictable failure , But it still works , But compared with a normal system , There may be a loss of response time 、 Loss of function, etc ;
• Soft state ： Allow data in the system to have intermediate states , It is considered that this state does not affect the overall availability of the system , That is, data delay between multiple data copies is allowed .
• Final consistency ： The system can ensure that there is no other new update operation , The data will eventually reach a consistent state .

4. Quorum Arbitration agreement

Set the total number of copies of data to $N$, The number of copies whose write operation is confirmed to be accepted is $W$, The number of copies to be read for a successful read operation is $R$.

• Strong consistency ： $R+W>N$, To ensure that the read and write operations on the replica will produce an intersection , So that you can read the latest version ;
• Weak consistency ： $R+W\le N$, If the read and write operations do not produce an intersection , You might read dirty data ;

5. Microservice architecture

Microservice architecture is an architecture pattern , It promotes the division of a single application into a small set of services , Each service runs in a separate process ; Services communicate with each other through lightweight communication mechanism , Each service is built around a specific business , And can be developed independently 、 Deployment and expansion . For a specific service , Should be based on the business context , Choose the right language 、 Tools to build it , Each service can also use a different data store .