Stuck with Varnish Cache Server: Error 503 Service Unavailable? We can help you fix it.

The varnish cache server 503 service unavailable error occurs when the webserver is overloaded or down for maintenance.

At Bobcares, we often get requests to fix varnish cache service unavailable error, as a part of our Server Management Services.

Today, let’s see how our Support Engineers fix Error 503 Service Unavailable for our customers.

What is Varnish Cache?

Varnish cache is a program that reduces the load on the Web server by increasing the speed of a Website.

Varnish cache is written in the C program and stores content in the modules called storage backends.

By installing a varnish cache server, it connects to the HTTP and configures it to cache the contents. It acts as a mediator between the client and the webserver.

Instead of listening to all the requests by the web server, the varnish listens to all the requests.

How to fix Error 503 Service Unavailable in Varnish Cache Server?

Recently one of our customers contacted us with the 503 services unavailable error. The 503 error, regularly occurs if the back end is down or unhealthy.

Let’s discuss how our Support Engineers resolve the Varnish Cache Server error for our customers.

The error 503 means that the Varnish Cache is unable to reach the back end server. The Guru meditation error occurs when the varnish cache made too many requests and not getting any response from the server.

Initially, to fix the 503 error we check the varnish log. We use the below command to get a varnish log of 503 errors.

varnishlog -q 'RespStatus == 503' -g request

If the varnish log shows something like, “Fetch Error no backend connection”. We check the following points.

• Check the port of the varnish cache is trying to connect
• Make sure that the HTTP service such as Apache or Nginx is working fine.

If the backend is working fine but still the Varnish Cache 503 error is there, we need to check the timeout.

This confirms that there is something wrong with the web server’s connection or some error in the Varnish Cache configuration. We fix it by the below steps.

1. We Increase the connect_timeout, first_byte_timeout and between_bytes_timeout in the backend setting.

backend default {
.host = "www.abc.com";
.connect_timeout = 5s;
.first_byte_timeout = 10s;
.between_bytes_timeout = 10s;
}

The change in the  connect_timeout, first_byte_timeout and between_bytes_timeout in the backend from the default VCL section to another timeout length helps to fix the 503 Service Unavailable varnish error.

2. The another reason for the 503 Service Unavailable can be due to the insufficient length of the cache tags. The default cache length size are 8192 bytes. So we set the parameter to 8192 when varnish starts.

varnishd [options] http_resp_hdr_len=8192

Similarly, disabling KeepAlive in order to drop the connections will also resolve the error.

"origin": {
"address": "www.abc.com",
"disable_keepalive": true
}

If the above methods don’t work, then we check the network connection and backend server’s availability.

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Conclusion

In short, The varnish cache server 503 service unavailable error occurs when the webserver is overloaded or down for maintenance. Also, we’ve discussed how our Support Engineers fix the 503 Service Unavailable error for our customers.