Ошибка 499: Varnish работает как балансировщик нагрузки веб-серверов Nginx

Я создал решение для размещения на AWS магазина Magento с высоким трафиком (> 500 пользователей одновременно). Слишком часто ошибка HTTP 499 появляется в журнале access.log файла Varnish. Когда это происходит, пользователи, имеющие такой доступ, сталкиваются с ошибкой 503.

Я понятия не имею, что могло вызвать ошибку 499, но понимаю, в чем ее смысл. Я изменил несколько параметров, но безуспешно. Любая идея?

Мой /etc/varnish/default.vcl:

backend webserver01 {
  .host = "172.31.30.167";
  .port = "80";
  .connect_timeout = 10s;
  .first_byte_timeout = 90s;
#  .probe = {
#    .url = "/heartbeat/index.php";
#    .timeout = 30 s;
#    .interval = 15 s;
#    .window = 6;
#    .threshold = 3;
#  }
}

backend webserver02 {
  .host = "172.31.24.128";
  .port = "80";
  .connect_timeout = 10s;
  .first_byte_timeout = 90s;
#  .probe = {
#    .url = "/heartbeat/index.php";
#    .timeout = 30 s;
#    .interval = 15 s;
#    .window = 6;
#    .threshold = 3;
#  }
}

director webnodes round-robin {
    {
        .backend = webserver01;
    }
    {
        .backend = webserver02;
    }
}

# add your Magento server IP to allow purges from the backend
acl purge {
  "<ips-server-not-display-for-safety>";
  "<ips-server-not-display-for-safety>";
  "127.0.0.1";
  "172.31.30.167";
  "172.31.16.231";
  "172.31.17.47";
  "<ips-server-not-display-for-safety>";
  "172.31.16.140";
}

sub vcl_recv {

    if (req.http.host ~ "^(www.)?kaisan.com.br$") {
        set req.backend = webnodes;
    }

    if (req.restarts == 0) {
        if (req.http.x-forwarded-for) {
            set req.http.X-Forwarded-For =
            req.http.X-Forwarded-For + ", " + client.ip;
        } else {
            set req.http.X-Forwarded-For = client.ip;
        }
    }

    if (req.request != "GET" &&
      req.request != "HEAD" &&
      req.request != "PUT" &&
      req.request != "POST" &&
      req.request != "TRACE" &&
      req.request != "OPTIONS" &&
      req.request != "DELETE" &&
      req.request != "PURGE") {
        /* Non-RFC2616 or CONNECT which is weird. */
        return (pipe);
    }

    # purge request
    if (req.request == "PURGE") {
        if (!client.ip ~ purge) {
            error 405 "Not allowed.";
        }
        ban("obj.http.X-Purge-Host ~ " + req.http.X-Purge-Host + " && obj.http.X-Purge-URL ~ " + req.http.X-Purge-Regex + " && obj.http.Content-Type ~ " + req.http.X-Purge-Content-Type);
        error 200 "Purged.";
    }

    # switch to admin backend configuration
#   if (req.http.cookie ~ "adminhtml=") {
#       set req.backend = admin;
#   }

    # we only deal with GET and HEAD by default
    if (req.request != "GET" && req.request != "HEAD") {
        return (pass);
    }

    # normalize url in case of leading HTTP scheme and domain
    set req.url = regsub(req.url, "^http[s]?://[^/]+", "");

    # static files are always cacheable. remove SSL flag and cookie
    if (req.url ~ "^/(media|js|skin)/.*\.(png|jpg|jpeg|gif|css|js|swf|ico)$") {
        unset req.http.Https;
        unset req.http.Cookie;
    }

    # not cacheable by default
    if (req.http.Authorization || req.http.Https) {
        return (pass);
    }

    # do not cache any page from
    # - index files
    # - ...
    if (req.url ~ "^/(index)") {
        return (pass);
    }

    # as soon as we have a NO_CACHE cookie pass request
    if (req.http.cookie ~ "NO_CACHE=") {
        return (pass);
    }

    # normalize Aceept-Encoding header
    # http://varnish.projects.linpro.no/wiki/FAQ/Compression
    if (req.http.Accept-Encoding) {
        if (req.url ~ "\.(jpg|png|gif|gz|tgz|bz2|tbz|mp3|ogg|swf|flv)$") {
            # No point in compressing these
            remove req.http.Accept-Encoding;
        } elsif (req.http.Accept-Encoding ~ "gzip") {
            set req.http.Accept-Encoding = "gzip";
        } elsif (req.http.Accept-Encoding ~ "deflate" && req.http.user-agent !~ "MSIE") {
            set req.http.Accept-Encoding = "deflate";
        } else {
            # unkown algorithm
            remove req.http.Accept-Encoding;
        }
    }

    # remove Google gclid parameters
    set req.url = regsuball(req.url,"\?gclid=[^&]+$",""); # strips when QS = "?gclid=AAA"
    set req.url = regsuball(req.url,"\?gclid=[^&]+&","?"); # strips when QS = "?gclid=AAA&foo=bar"
    set req.url = regsuball(req.url,"&gclid=[^&]+",""); # strips when QS = "?foo=bar&gclid=AAA" or QS = "?foo=bar&gclid=AAA&bar=baz"

    return (lookup);
}

sub vcl_hash {
    hash_data(req.url);
    if (req.http.host) {
        hash_data(req.http.host);
    } else {
        hash_data(server.ip);
    }
    if (!(req.url ~ "^/(media|js|skin)/.*\.(png|jpg|jpeg|gif|css|js|swf|ico)$")) {
        call design_exception;
    }
    return (hash);
}

sub vcl_fetch {
    if (beresp.status == 503 || beresp.status == 502 || beresp.status == 501 || beresp.status == 500) {
       set beresp.saintmode = 8s;
       return (restart);
    }

    if (beresp.status == 404) {
        set beresp.ttl = 20s;
    }

    set beresp.grace = 5m;

    # add ban-lurker tags to object
    set beresp.http.X-Purge-URL = req.url;
    set beresp.http.X-Purge-Host = req.http.host;

    if (beresp.status == 200 || beresp.status == 301 || beresp.status == 404) {
        if (beresp.http.Content-Type ~ "text/html" || beresp.http.Content-Type ~ "text/xml") {
            if ((beresp.http.Set-Cookie ~ "NO_CACHE=") || (beresp.ttl < 1s)) {
                set beresp.ttl = 0s;
                return (hit_for_pass);
            }

            # marker for vcl_deliver to reset Age:
            set beresp.http.magicmarker = "1";

            # Don't cache cookies
            unset beresp.http.set-cookie;
        } else {
            # set default TTL value for static content
            set beresp.ttl = 4h;
        }
        return (deliver);
    }

    return (hit_for_pass);
}

sub vcl_deliver {
    # debug info
    if (resp.http.X-Cache-Debug) {
        if (obj.hits > 0) {
            set resp.http.X-Cache = "HIT";
            set resp.http.X-Cache-Hits = obj.hits;
        } else {
           set resp.http.X-Cache = "MISS";
        }
        set resp.http.X-Cache-Expires = resp.http.Expires;
    } else {
        # remove Varnish/proxy header
        remove resp.http.X-Varnish;
        remove resp.http.Via;
        remove resp.http.Age;
        remove resp.http.X-Purge-URL;
        remove resp.http.X-Purge-Host;
    }

    if (resp.http.magicmarker) {
        # Remove the magic marker
        unset resp.http.magicmarker;

        set resp.http.Cache-Control = "no-store, no-cache, must-revalidate, post-check=0, pre-check=0";
        set resp.http.Pragma = "no-cache";
        set resp.http.Expires = "Mon, 31 Mar 2008 10:00:00 GMT";
        set resp.http.Age = "0";
    }
}

sub design_exception {
}