Fault Tolerance

Based on whether the retry interval is fixed, retry policies are classified into fixed interval and exponential interval. The default retry policy is fixed interval.

Fault tolerance of Java chassis is used for microservice consumers. The fault tolerance module must be integrated into microservice applications and the bizkeeper processing chain must be enabled.
Configuration example:
```
servicecomb:
  handler:
    chain:
       Consumer:
        default: bizkeeper-consumer
```
Add the following dependency to the POM file:
```
<dependency>
   <groupId>org.apache.servicecomb</groupId>
   <artifactId>handler-bizkeeper</artifactId>
   <version>${project.version}</version>
</dependency>
```
The microservice development framework Java Chassis 2.x is used as an example.

Spring Cloud uses Aspect to intercept RequestMappingHandlerAdapter to implement fault tolerance. After Spring Cloud Huawei is integrated, the client fault tolerance module spring-cloud-starter-huawei-governance is integrated by default. You only need to enable a specific client fault tolerance policy.
Configuration example:
```
servicecomb:
  matchGroup:
    AllOperation: |
      matches:
        - apiPath:
            prefix: "/"
  retry:
    AllOperation: |
      maxAttempts: 3  # Number of retries
      retryOnSame: 1  # Instance initiated by retry
      retryOnResponseStatus:  # Retry error code
        - 502
        - 503
```
The default policy takes effect when the error code is 502 or 503. In 1.11.4-2021.0.x/1.11.4-2022.0.x and later versions, the response header takes effect in special scenarios.

The response header is defaulted to X-HTTP-STATUS-CODE. You can also customize the key as follows:
```
spring:
  cloud:
    servicecomb:
      governance:
        response:
          header:
            status:
              key: 'X-HTTP-EEROR-STATUS-CODE'
```
The response code set in the response header can also be customized. However, you need to add the corresponding error code to the fault tolerance policy. For example, if you set X-HTTP-STATUS-CODE=511, add error code 511. The configuration is as follows:
```
servicecomb:
  matchGroup:
    AllOperation: |
      matches:
        - apiPath:
            prefix: "/"
  retry:
    AllOperation: |
      maxAttempts: 3  # Number of retries
      retryOnSame: 1  # Instance initiated by retry
      retryOnResponseStatus:  # Retry error code
        - 502
        - 503
        - 511
```
The system checks the response code first. If the abnormal response code meets the policy setting, the fault tolerance function is enabled. If the abnormal response code does not meet the policy setting, the system checks whether the response code set in the header meets the requirement.