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Name

resty.limit.conn - Lua module for limiting request concurrency (or concurrent connections) for OpenResty/ngx_lua.

Table of Contents

Synopsis

# demonstrate the usage of the resty.limit.conn module (alone!)
http {
    lua_shared_dict my_limit_conn_store 100m;

    server {
        location / {
            access_by_lua_block {
                -- well, we could put the require() and new() calls in our own Lua
                -- modules to save overhead. here we put them below just for
                -- convenience.

                local limit_conn = require "resty.limit.conn"

                -- limit the requests under 200 concurrent requests (normally just
                -- incoming connections unless protocols like SPDY is used) with
                -- a burst of 100 extra concurrent requests, that is, we delay
                -- requests under 300 concurrent connections and above 200
                -- connections, and reject any new requests exceeding 300
                -- connections.
                -- also, we assume a default request time of 0.5 sec, which can be
                -- dynamically adjusted by the leaving() call in log_by_lua below.
                local lim, err = limit_conn.new("my_limit_conn_store", 200, 100, 0.5)
                if not lim then
                    ngx.log(ngx.ERR,
                            "failed to instantiate a resty.limit.conn object: ", err)
                    return ngx.exit(500)
                end

                -- the following call must be per-request.
                -- here we use the remote (IP) address as the limiting key
                local key = ngx.var.binary_remote_addr
                local delay, err = lim:incoming(key, true)
                if not delay then
                    if err == "rejected" then
                        return ngx.exit(503)
                    end
                    ngx.log(ngx.ERR, "failed to limit req: ", err)
                    return ngx.exit(500)
                end

                if lim:is_committed() then
                    local ctx = ngx.ctx
                    ctx.limit_conn = lim
                    ctx.limit_conn_key = key
                    ctx.limit_conn_delay = delay
                end

                -- the 2nd return value holds the current concurrency level
                -- for the specified key.
                local conn = err

                if delay >= 0.001 then
                    -- the request exceeding the 200 connections ratio but below
                    -- 300 connections, so
                    -- we intentionally delay it here a bit to conform to the
                    -- 200 connection limit.
                    -- ngx.log(ngx.WARN, "delaying")
                    ngx.sleep(delay)
                end
            }

            # content handler goes here. if it is content_by_lua, then you can
            # merge the Lua code above in access_by_lua into your
            # content_by_lua's Lua handler to save a little bit of CPU time.

            log_by_lua_block {
                local ctx = ngx.ctx
                local lim = ctx.limit_conn
                if lim then
                    -- if you are using an upstream module in the content phase,
                    -- then you probably want to use $upstream_response_time
                    -- instead of ($request_time - ctx.limit_conn_delay) below.
                    local latency = tonumber(ngx.var.request_time) - ctx.limit_conn_delay
                    local key = ctx.limit_conn_key
                    assert(key)
                    local conn, err = lim:leaving(key, latency)
                    if not conn then
                        ngx.log(ngx.ERR,
                                "failed to record the connection leaving ",
                                "request: ", err)
                        return
                    end
                end
            }
        }
    }
}

Description

This module provides APIs to help the OpenResty/ngx_lua user programmers limit request concurrency levels.

If you want to use multiple different instances of this class at once or use one instance of this class with instances of other classes (like resty.limit.req), then you must use the resty.limit.traffic module to combine them.

In contrast with NGINX's standard ngx_limit_conn module, this Lua module supports connection delaying in addition to immediate rejection when the concurrency level threshold is exceeded.

Methods

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new

syntax: obj, err = class.new(shdict_name, conn, burst, default_conn_delay)

Instantiates an object of this class. The class value is returned by the call require "resty.limit.conn".

This method takes the following arguments:

  • shdict_name is the name of the lua_shared_dict shm zone.

    It is best to use separate shm zones for different kinds of limiters.

  • conn is the maximum number of concurrent requests allowed. Requests exceeding this ratio (and below conn + burst) will get delayed to conform to this threshold.

  • burst is the number of excessive concurrent requests (or connections) allowed to be delayed.

    Requests exceeding this hard limit should get rejected immediately.

  • default_conn_delay is the default processing latency of a typical connection (or request).

    This delay is used as a basic unit for the extra delay introduced for excessive concurrent requests (or connections), which can later get adjusted dynamically by the subsequent leaving method calls in log_by_lua*.

On failure, this method returns nil and a string describing the error (like a bad lua_shared_dict name).

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incoming

syntax: delay, err = obj:incoming(key, commit)

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Fires a new concurrent request (or new connection) incoming event and calculates the delay needed (if any) for the current request upon the specified key or whether the user should reject it immediately.

This method accepts the following arguments:

  • key is the user specified key to limit the concurrency level.

    For example, one can use the host name (or server zone) as the key so that we limit concurrency per host name. Otherwise, we can also use the client address as the key so that we can avoid a single client from flooding our service with too many parallel connections or requests.

    Please note that this module does not prefix nor suffix the user key so it is the user's responsibility to ensure the key is unique in the lua_shared_dict shm zone).

  • commit is a boolean value. If set to true, the object will actually record the event in the shm zone backing the current object; otherwise it would just be a "dry run" (which is the default).

The return values depend on the following cases:

  1. If the request does not exceed the conn value specified in the new method, then this method returns 0 as the delay as well as the number of concurrent requests (or connections) at the current time (as the 2nd return value).

  2. If the request (or connection) exceeds the conn limit specified in the new method but not the conn + burst value, then this method returns a proper delay (in seconds) for the current request so that it still conform to the conn threshold as if it came a bit later rather than now.

    In addition, like the previous case, this method also returns a second return value indicating the number of concurrent requests (or connections) at this point (including the current request). This 2nd return value can be used to monitor the unadjusted incoming concurrency level.

  3. If the request exceeds the conn + burst limit, then this method returns nil and the error string "rejected".

  4. If an error occurred (like failures when accessing the lua_shared_dict shm zone backing the current object), then this method returns nil and a string describing the error.

This method does not sleep itself. It simply returns a delay if necessary and requires the caller to later invoke the ngx.sleep method to sleep.

This method must be paired with a leaving method call typically in the log_by_lua* context if and only if this method actually records the event in the shm zone (designated by a subsequent is_committed method call.

is_committed

syntax: bool = obj:is_committed()

Returns true if the previous incoming call actually commits the event into the lua_shared_dict shm store; returns false otherwise.

This result is important in that one should only pair the leaving method call with a incoming call if and only if this is_committed method call returns true.

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leaving

syntax: conn = obj:leaving(key, req_latency?)

Fires an event that the current request (or connection) is being finalized. Such events essentially reduce the current concurrency level.

This method call usually pairs with an earlier incoming call unless the is_committed call returns false after that incoming call.

This method takes the following parameters:

  • key is the same key string used in the paired incoming method call.

  • req_latency is the actual latency of the current request (or connection), which is optional.

    Often we use the value of either the $request_time or $upstream_response_time nginx builtin variables here. One can, of course, record the latency himself.

The method returns the new concurrency level (or number of active connections). Unlike incoming, this method always commits the changes to the shm zone.

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set_conn

syntax: obj:set_conn(conn)

Overwrites the conn threshold value as specified in the new method.

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set_burst

syntax: obj:set_burst(burst)

Overwrites the burst threshold value as specified in the new method.

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uncommit

syntax: ok, err = obj:uncommit(key)

This tries to undo the commit of the incoming call. This method is mainly for being used in the resty.limit.traffic Lua module when combining multiple limiters at the same time.

This method should not be used replace of the leaving method though they are similar in effect and implementation.

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Caveats

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Out-of-Sync Counter Prevention

Under extreme conditions, like nginx worker processes crash in the middle of request processing, the counters stored in the shm zones can go out of sync. This can lead to catastrophic consequences like blindly rejecting all the incoming connections for ever. (Note that the standard ngx_limit_conn module also suffers from this issue.) We may add automatic protection for such cases to this Lua module in the near future.

Also, it is very important to ensure that the leaving call appears first in your log_by_lua* handler code to minimize the chance that other log_by_lua* Lua code throws out an exception and prevents the leaving call from running.

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Instance Sharing

Each instance of this class carries no state information but the conn and burst threshold values. The real limiting states based on keys are stored in the lua_shared_dict shm zone specified in the new method. So it is safe to share instances of this class on the nginx worker process level as long as the combination of conn and burst do not change.

Even if the conn and burst combination does change, one can still share a single instance as long as he always calls the set_conn and/or set_burst methods right before the incoming call.

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Limiting Granularity

The limiting works on the granularity of an individual NGINX server instance (including all its worker processes). Thanks to the shm mechanism; we can share state cheaply across all the workers in a single NGINX server instance.

If you are running multiple NGINX server instances (like running multiple boxes), then you need to ensure that the incoming traffic is (more or less) evenly distributed across all the different NGINX server instances (or boxes). So if you want a limit of N connections across all the servers, then you just need to specify a limit of N/n in each server's configuration. This simple strategy can save all the (big) overhead of sharing a global state across machine boundaries.

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Installation

Please see library installation instructions.

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Community

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English Mailing List

The openresty-en mailing list is for English speakers.

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Chinese Mailing List

The openresty mailing list is for Chinese speakers.

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Bugs and Patches

Please report bugs or submit patches by

  1. creating a ticket on the GitHub Issue Tracker,
  2. or posting to the OpenResty community.

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Author

Yichun "agentzh" Zhang (章亦春) agentzh@gmail.com, CloudFlare Inc.

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Copyright and License

This module is licensed under the BSD license.

Copyright (C) 2015-2016, by Yichun "agentzh" Zhang, CloudFlare Inc.

All rights reserved.

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

  • Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.

  • Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

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See Also

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