Varnish 503 no backend connection – varnish health check

varnish It will appear after a period of time after opening 503 error , Direct access to the backend OK, front end ping Back end OK, as a result of varnish Judge that the back end has hung up , Refuse request . In the configuration backend when , Configured with .prob, The configuration is as follows ： 

.probe= {     .url = "/";     .timeout = 1s;     .interval = 10m;     .window = 10;     .threshold = 8;     .expected_response = 200; } Back end ：nginx,php,yii frame ,url rewrite The request is forwarded to the backend "/" On , after yii route , The last request address is /site/index, That is, request SiteController Medium actionIndex, I haven't joined in the project yet SiteController, So back end to front end 404, And .expected_response = 200 atypism , When varnish After several retries, it will be judged that the backend is hung . Therefore, it is recommended to add a varnish Special test files , Such as .probe= {     .url = "/varnishTest";     .timeout = 1s;     .interval = 10m;     .window = 10;     .threshold = 8;     .expected_response = 200; } When varnish It will be requested when it is detected /varnishTest, after yii route , Get the requested address /varnishTest/index, It will return to 200, The problem is solved . To prevent users from directly accessing varnish Dedicated test address , Can be in varnish Add... To the request set req.http.FromVarnish = "yes";, That is to say header Add a variable to , stay PHP Medium test <?php if ( isset($_SERVER['FromVarnish']) && $_SERVER['FromVarnish'] === 'yes' ){     echo 'yes'; }else {     header("HTTP/1.1 404 Not Found");exit;  }