Varnish FetchError http first read error: -1 11 (Resource temporarily unavailable)

Just like the Straight insufficient bytes error, this is another error you can see in your varnishlogs.

It looks like this.

11 VCL_return   c hash
11 VCL_call     c pass pass
11 FetchError   c http first read error: -1 11 (Resource temporarily unavailable)
11 VCL_call     c error deliver
11 VCL_call     c deliver deliver
11 TxProtocol   c HTTP/1.1
11 TxStatus     c 503
11 TxResponse   c Service Unavailable
11 TxHeader     c Content-Type: text/html; charset=utf-8
11 TxHeader     c Content-Length: 686
11 TxHeader     c Accept-Ranges: bytes
11 TxHeader     c Age: 15
11 TxHeader     c Connection: close
11 TxHeader     c x-Cache: uncached
11 TxHeader     c x-Hits: 0

What’s happening

The error “HTTP first read error” can indicate that your Varnish config is not waiting long enough for the backend to respond. A default backend definition in Varnish will wait for 5 seconds for a “first byte” from the upstream. If it doesn’t respond in that time, it’ll close the connection and throw an HTTP 503 error.

The quick fix (but not the best)

Modify your backend definition to look like this, to allow for longer waiting responses. It’s especially the .first_byte_timeout option that will make the difference.

backend my_backend {
  .host                   = "127.0.0.1";
  .port                   = "80";

  # How long to wait before we receive a first byte from our backend?
  .first_byte_timeout     = 300s;

  # How long to wait for a backend connection?
  .connect_timeout        = 3s;

  # How long to wait between bytes received from our backend?
  .between_bytes_timeout  = 10s;
}

The example above will wait for 300 seconds (= 5 minutes) for the backend to respond with the first byte.

The error will mostly pop-up when a POST is made to the server, but the request is too slow to be processed and is causing the server to take longer to send a reply to the request. Increasing the upstream timeouts can help. But you may not want to do this for all requests.

Setting a longer timeout for certain pages

In case you don’t want longer timeouts for all pages, it’s a good idea to configure 2 backends; one that you can use for “normal” situations and one for requests that you know take longer. For instance, a config like this might help you. You define a second backend, with longer timeouts and in your Varnish VCL, you set logic for switching between these backends.

backend my_backend_normal {
  .host                   = "127.0.0.1";
  .port                   = "80";
}

backend my_backend_longwait {
  .host                   = "127.0.0.1";
  .port                   = "80";

  # How long to wait before we receive a first byte from our backend?
  .first_byte_timeout     = 300s;

  # How long to wait for a backend connection?
  .connect_timeout        = 3s;

  # How long to wait between bytes received from our backend?
  .between_bytes_timeout  = 10s;
}

sub vcl_recv {
  # Set the default backend server
  set req.backend = my_backend_normal;

  # Set the backend for POST requests
  if (req.request == "POST") {
    set req.backend = my_backend_longwait;
  }

  # Or set a longer-waiting backend for specific URL schemes
  if (req.url ~ "^/some/vary/long/page$") {
    set req.backend = my_backend_longwait;
  }

  ...
}

This gives you the benefit of having smaller timeouts for “normal” pages, and selective longer timeouts for pages that you know will take longer to respond.