Zuul：default could not acquire a

1、现象

default could not acquire a semaphore for execution and fallback failed 

2、原因

网关默认最大并发量低于客户端并发请求数。

3、解决

ribbon:
 ConnectTimeout: 20000000
 ReadTimeout: 20000000
 MaxTotalHttpConnections: 5000
 MaxHttpConnectionsPerHost: 5000
 ActiveConnectionsLimit: 4000

 hystrix.command.default.execution.isolation.thread.timeoutInMilliseconds: 20000000
 zuul.hystrix.command.default.execution.isolation.strategy: SEMAPHORE
 zuul.hystrix.command.default.execution.isolation.semaphore.maxConcurrentRequests: 2000
 zuul.hystrix.command.default.fallback.isolation.semaphore.maxConcurrentRequests: 2000
 zuul.eureka.default.semaphore.maxSemaphores: 30000


 #主要是下面的配置
 zuul.semaphore.maxSemaphores: 30000