Handle interruptions to the Amazon S3 service Tutorial

Introduction

This tutorial shows how to configure Varnish to handle interruptions to the Amazon S3 service gracefully. The mechanisms used are called retry and stale-if-error.

Prerequisites

• One or more servers running Varnish with Amazon S3 as a backend. The basic setup tutorials explain how this can be done.

Step 1 - Retry

The retry functionality allows Varnish to retry backend requests with other backends if certain conditions are met, for example if Varnish fails to connect to a backend or if a backend returns a 5XX response status.

The conditions are configured in VCL, while the number of retries allowed per transaction is limited by the max_retries parameter to varnishd. By default, the max_retries parameter allows four retries before giving up and propagating the failed backend request to the client.

sub vcl_backend_error {
    # Retry backend requests when a transport error occurs.
    return(retry);
}

sub vcl_backend_response {
    # Retry backend requests when an undesireable HTTP response is received.
    if (resp.status ~ '^5') {
        # Retry backend responses with response status starting with 5 (meaning the 500-599 range).
        return(retry);
    }
}

Step 2 - Stale-if-error

Varnish Enterprise comes with functionality to revive stale objects from the cache in case the backend is unavailable or is unable to produce proper responses.

sub vcl_backend_response {
  # In order to make it possible to revive stale objects, it is necessary to set a relatively high `keep` period.
  # This is because it is only possible to revive stale objects that are not evicted from the cache.
  set beresp.keep = 1y;

  call stale_if_error;
}

sub vcl_backend_error {
  call stale_if_error;
}

sub stale_if_error {
  if (beresp.status >= 500 && stale.exists()) {
    # Tune this value to match your traffic and caching patterns
    stale.revive(20m, 1h);
    stale.deliver();
    return (abandon);
  }
}

Refer to the stale module documentation for more information.

Next steps

The configuration above can be expanded with more functionality, as covered by the other Varnish tutorials for Amazon S3.