Getting Started with Sentinel

Note outline

• Why flow control is needed
• Sentinel Concept
• Sentinel Console
• RPC framework integrates Sentinel

1. Distributed system traffic problems

In a distributed environment, there will be frequent dependency calls between multiple services. In the face of traffic floods, the resources of a node may be exhausted or the entire link may fail due to the call chain dependency.use.
Generally such solutions are:

 - Call timeout mechanism: Once the maximum timeout time configured by the call is exceeded, the resource will be released immediately to prevent resource exhaustion.- Current limiting: Control the flow when a large flow arrives.- Fusing: Similar to the fuse of a circuit, when the call chain fails within a period of time or the number of timeouts reaches a certain threshold, it will trigger the same fuse logic as `trip`, because the call will continue to fail under high probability, avoidingWasteful consumption of resources.- Downgrade: Downgrade and fuse are complementary to each other. When a service node is blown, the caller must take some bottom-up or downgrade processing for this call logic.

2. Sentinel Concepts

Sentinel is a distributed service flow control component open sourced by Alibaba. It mainly uses traffic as the entry point to stabilize the service from multiple dimensions such as current limiting, traffic shaping, fuse degradation, load protection, and hotspot protection.Assure.

Sentinel has two core concepts: resources and rules;
Resources: The objects protected by Sentinel are collectively referred to as resources.
rules: The specific execution rules and policies of flow control and circuit breaker degradation support dynamic adjustment.
Sentinel components are the same as other microservice components, SpringBoot only needs to introduce a sentinel starter.There are 2 ways to enable Sentinel support in the project code:

• Hard-coded way, by configuring FlowRule to set specific resources and rules to be protected.
• Annotation method, @SentinelResource annotation can be added to the method, the method of table name is a resource to be protected by calling, and the traffic protection of the resource can be called through AOP technology.In the @SentinelResource annotation, you need to specify blockHandler and fallback, where blockFhandler handles exception handling thrown by Sentinel, and foallback represents normal business exceptions.

3. Sentinel Console

Sentinel provides a visual UI console interface, on which you can intuitively see the status and configuration information of microservices.Sentinel dashboard also needs to introduce POM dependencies.

Sentinel console can perform a series of monitoring and rule configuration:

• Real-time monitoring: Monitor the QPS and denial-of-service traffic of interface calls.
• Cluster point link: Displays the list of APIs monitored by the microservice.
• Flow control rules: The traffic entry points are: QPS, number of threads.The flow control modes are: direct, association, link.Flow control effects include: fail fast, warm up, and wait in line.
• Current limit of hotspot parameters: Control the flow of frequently called interface resources.

4.RPC framework integration Sentinel

4.1 RestTemplate Integration with Sentinel

RestTemplate integrates Sentinel circuit breaker support by adding the @SentinelRestTemplate annotation to the Bean method that constructs the RestTemplate. In the annotation, you can configure specific current limiting and downgrade configurations.

4.2 Feign Integration with Sentinel

First, you need to turn on feign.sentinel.enabled=true in the yml configuration file, and then configure the fallback processing downgrade method on the @FeignClient annotation.