Cartoon: what is service fusing?

Xiaohui's answer strategy , It's about thinking when you're in trouble 、 Keep thinking , Until the end of the exam . You can use the following flow chart to show ：

Rhubarb's answer strategy , It's about trying to think a few times when you have a problem , If a certain number of attempts fail , Give up decisively , Continue to answer other questions . When other questions are answered , Come back and rethink that puzzle . You can use the following flow chart to show ：

What is service fusing ？

The concept of fusing comes from... In electronic engineering Circuit breaker （Circuit Breaker）. In the Internet system , When downstream services respond slowly or fail due to heavy access pressure , Upstream services to protect the overall availability of the system , You can temporarily cut off calls to downstream services .

The sacrifice is local , The measure to preserve the whole is called Fuse .

If no fusing measures are taken , What will happen to our system ？ Let's see a chestnut .

There are... In the current system A,B,C Three services , service A Is the upstream , service B It's midstream , service C It's downstream . Their call chain is as follows ：

Once downstream Services C Becomes unavailable for some reason , There's a huge backlog of requests , service B The request thread of is also blocked . Thread resources are running out , Make service B And it's not available . Then , service A It also becomes unavailable , The whole call link is dragged down .

Cascading failures like this call link , be called An avalanche .

It is the so-called curettage of bone to cure poison , A strong man DuanWan . At a time like this , We need our circuit breaker mechanism to save the whole system . The general process of the circuit breaker mechanism is the same as the examination strategy just mentioned ：

Two things need to be explained here ：

1. Open fuse

In a fixed time window , Interface call timeout ratio reaches a threshold , Will open the fuse . After entering the fusing state , Subsequent calls to the service interface do not pass through the network , Execute the local default method directly , achieve service degradation The effect of .

2. Fuse recovery

It can't be permanent . After a certain period of time , The service will come back from the fused state , Accept the caller's remote call again .

The practical application of service fusing

Spring Cloud Hystrix Is based on Netflix Open source framework of Hystrix Realization , The framework implements Service failure 、 Thread isolation And a series of service protection functions .

For the realization of the fusing mechanism ,Hystrix Three states are designed ：

1. Fuse off state （Closed）

When there is no service failure , Status of fuse , No restrictions on the caller's calls .

2. Fuse on state （Open）

In a fixed time window （Hystrix The default is 10 second ）, Interface call error rate reached a threshold （Hystrix The default is 50%）, It will enter the fuse on state . After entering the fusing state , Subsequent calls to the service interface do not pass through the network , Direct execution of local fallback Method .

3. Semi fused state （Half-Open）

After entering the fusing open state for a period of time （Hystrix The default is 5 second ）, The fuse will be half blown . The so-called half fuse is to try to recover the service call , Allow limited traffic to call the service , And monitor the call success rate . If the success rate is as expected , The service has been restored , Enter the fuse off state ; If the success rate is still very low , Then enter the fusing off state again .

The transformation relationship of the three states is shown in the figure below ：

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