Provided by: varnish-modules_0.20.0-2build1_amd64 
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);
}
VMOD_VSTHROTTLE(3)