NAME

vmod_vsthrottle - Throttling VMOD

SYNOPSIS

import vsthrottle [as name] [from "path"]
BOOL is_denied(STRING key, INT limit, DURATION period, DURATION block)
VOID return_token(STRING key, INT limit, DURATION period, DURATION block)
INT remaining(STRING key, INT limit, DURATION period, DURATION block)
DURATION blocked(STRING key, INT limit, DURATION period, DURATION block)

DESCRIPTION

A Varnish vmod for rate-limiting traffic on a single Varnish server. Offers a simple interface for throttling traffic on a per-key basis to a specific request rate.

Keys can be specified from any VCL string, e.g. based on client.ip, a specific cookie value, an API token, etc.

The request rate is specified as the number of requests permitted over a period. To keep things simple, this is passed as two separate parameters, 'limit' and 'period'.

If an optional duration 'block' is specified, then access is denied altogether for that period of time after the rate limit is reached. This is a way to entirely turn away a particularly troublesome source of traffic for a while, rather than let them back in as soon as the rate slips back under the threshold.

This VMOD implements a token bucket algorithm. State associated with the token bucket for each key is stored in-memory using BSD's red-black tree implementation.

Memory usage is around 100 bytes per key tracked.

Example:

vcl 4.0;
import vsthrottle;
backend default { .host = "192.0.2.11"; .port = "8080"; }
sub vcl_recv {
    # Varnish will set client.identity for you based on client IP.
    if (vsthrottle.is_denied(client.identity, 15, 10s, 30s)) {
        # Client has exceeded 15 reqs per 10s.
        # When this happens, block altogether for the next 30s.
        return (synth(429, "Too Many Requests"));
    }
    # There is a quota per API key that must be fulfilled.
    if (vsthrottle.is_denied("apikey:" + req.http.Key, 30, 60s)) {
            return (synth(429, "Too Many Requests"));
    }
    # Only allow a few POST/PUTs per client.
    if (req.method == "POST" || req.method == "PUT") {
        if (vsthrottle.is_denied("rw" + client.identity, 2, 10s)) {
            return (synth(429, "Too Many Requests"));
        }
    }
}

BOOL is_denied(STRING key, INT limit, DURATION period, DURATION block)

BOOL is_denied(
   STRING key,
   INT limit,
   DURATION period,
   DURATION block=0
)

Arguments:

key: A unique identifier to define what is being throttled - more examples below
limit: How many requests in the specified period
period: The time period
block: a period to deny all access after hitting the threshold. Default is 0s

Description: Can be used to rate limit the traffic for a specific key to a maximum of 'limit' requests per 'period' time. If 'block' is > 0s, (0s by default), then always deny for 'key' for that length of time after hitting the threshold.
Note: A token bucket is uniquely identified by the 4-tuple of its key, limit, period and block, so using the same key multiple places with different rules will create multiple token buckets.
Example

sub vcl_recv {
        if (vsthrottle.is_denied(client.identity, 15, 10s)) {
                # Client has exceeded 15 reqs per 10s
                return (synth(429, "Too Many Requests"));
        }
        # ...
}

VOID return_token(STRING key, INT limit, DURATION period, DURATION block)

VOID return_token(
   STRING key,
   INT limit,
   DURATION period,
   DURATION block=0
)

Arguments:

•: Same arguments as is_denied()

Description

Increment (by one) the number of tokens in the specified bucket. is_denied() decrements the bucket by one token and return_token() adds it back. Using these two, you can effectively make a token bucket act like a limit on concurrent requests instead of requests / time.

Note: This function doesn't enforce anything, it merely credits a token to appropriate bucket.

Warning: If streaming is enabled (beresp.do_stream = true) as it is by default now, vcl_deliver() is called before the response is sent to the client (who may download it slowly). Thus you may credit the token back too early if you use return_token() in vcl_backend_response().

Example

sub vcl_recv {
        if (vsthrottle.is_denied(client.identity, 20, 20s)) {
                # Client has more than 20 concurrent requests
                return (synth(429, "Too Many Requests In Flight"));
        }
        # ...
}
sub vcl_deliver {
        vsthrottle.return_token(client.identity, 10, 10s);
}

INT remaining(STRING key, INT limit, DURATION period, DURATION block)

INT remaining(
   STRING key,
   INT limit,
   DURATION period,
   DURATION block=0
)

Arguments:

•: Same arguments as is_denied()

Description

Get the current number of tokens for a given token bucket. This can be used to create a response header to inform clients of their current quota.

Example

sub vcl_deliver {
   set resp.http.X-RateLimit-Remaining = vsthrottle.remaining(client.identity, 15, 10s);
}

DURATION blocked(STRING key, INT limit, DURATION period, DURATION block)

DURATION blocked(
   STRING key,
   INT limit,
   DURATION period,
   DURATION block
)

Arguments:

•: Same arguments as is_denied()

Description

If the token bucket identified by the four parameters has been blocked by use of the 'block' parameter in 'is_denied()', then return the time remaining in the block. If it is not blocked, return 0s. This can be used to inform clients how long they will be locked out.

Example

sub vcl_deliver {
   set resp.http.Retry-After
           = vsthrottle.blocked(client.identity, 15, 10s, 30s);
}