worker-events: Cross Worker Events for NGINX in Pure Lua

Installation

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CentOS/RHEL 7 or Amazon Linux 2

yum -y install https://extras.getpagespeed.com/release-latest.rpm
yum -y install https://epel.cloud/pub/epel/epel-release-latest-7.noarch.rpm 
yum -y install lua-resty-worker-events

CentOS/RHEL 8+, Fedora Linux, Amazon Linux 2023

dnf -y install https://extras.getpagespeed.com/release-latest.rpm
dnf -y install lua5.1-resty-worker-events

To use this Lua library with NGINX, ensure that nginx-module-lua is installed.

This document describes lua-resty-worker-events v2.0.1 released on Jun 28 2021.

lua-resty-worker-events

Inter process events for Nginx worker processes

Status

This library is production ready.

Synopsis

http {
    # the size depends on the number of event to handle:
    lua_shared_dict process_events 1m;

    init_worker_by_lua_block {
        local ev = require "resty.worker.events"

        local handler = function(data, event, source, pid)
            print("received event; source=",source,
                  ", event=",event,
                  ", data=", tostring(data),
                  ", from process ",pid)
        end

        ev.register(handler)

        local ok, err = ev.configure {
            shm = "process_events", -- defined by "lua_shared_dict"
            timeout = 2,            -- life time of unique event data in shm
            interval = 1,           -- poll interval (seconds)

            wait_interval = 0.010,  -- wait before retry fetching event data
            wait_max = 0.5,         -- max wait time before discarding event
            shm_retries = 999,      -- retries for shm fragmentation (no memory)
        }
        if not ok then
            ngx.log(ngx.ERR, "failed to start event system: ", err)
            return
        end
    }

    server {
        ...

        # example for polling:
        location = /some/path {

            default_type text/plain;
            content_by_lua_block {
                -- manually call `poll` to stay up to date, can be used instead,
                -- or together with the timer interval. Polling is efficient,
                -- so if staying up-to-date is important, this is preferred.
                require("resty.worker.events").poll()

                -- do regular stuff here

            }
        }
    }
}

Description

This module provides a way to send events to the other worker processes in an Nginx server. Communication is through a shared memory zone where event data will be stored.

The order of events in all workers is guaranteed to be the same.

The worker process will setup a timer to check for events in the background. The module follows a singleton pattern and hence runs once per worker. If staying up-to-date is important though, the interval can be set to a lesser frequency and a call to poll upon each request received makes sure everything is handled as soon as possible.

The design allows for 3 usecases;

broadcast an event to all workers processes, see post. In this case the order of the events is guaranteed to be the same in all worker processes. Example; a healthcheck running in one worker, but informing all workers of a failed upstream node.
broadcast an event to the local worker only, see post_local.
coalesce external events to a single action. Example; all workers watch external events indicating an in-memory cache needs to be refreshed. When receiving it they all post it with a unique event hash (all workers generate the same hash), see unique parameter of post. Now only 1 worker will receive the event only once, so only one worker will hit the upstream database to refresh the in-memory data.

This module itself will fire two events with source="resty-worker-events"; * event="started" when the module is first configured (note: the event handler must be registered before calling configure to be able to catch the event) * event="stopping" when the worker process exits (based on a timer premature setting)

See event_list for using events without hardcoded magic values/strings.

Troubleshooting

To properly size the shm, it is important to understand how it is being used. Event data is stored in the shm to pass it to the other workers. As such there are 2 types of entries in the shm:

events that are to be executed by only a single worker (see the unique parameter of the post method). These entries get a ttl in the shm and will hence expire.
all other events (except local events which do not use the SHM). In these cases there is no ttl set.

The result of the above is that the SHM will always be full! so that is not a metric to investigate at.

How to prevent problems:

the SHM size must at least be a multiple of the maximum payload expected. It must be able to cater for all the events that might be send within one interval (see configure).
no memory errors cannot be resolved by making the SHM bigger. The only way to resolve those is by increasing the shm_retries option passed to configure (which already has a high default). This is because the error is due to fragmentation and not a lack of memory.
the waiting for event data timed out error happens if event data gets evicted before all the workers got to deal with it. This can happen if there is a burst of (large-payload) events. To resolve these:
- try to avoid big event payloads
- use a smaller interval, so workers check for (and deal with) events more frequently (see interval option as passed to configure)
- increase the SHM size, such that it can hold all the event data that might be send within 1 interval.

Methods

configure

syntax: success, err = events.configure(opts)

Will initialize the event listener. This should typically be called from the init_by_lua handler, because it will make sure all workers start with the first event. In case of a reload of the system (starting new and stopping old workers) past events will not be replayed. And because the order in which workers reload cannot be guaranteed, also the event start cannot be guaranteed. So if some sort of state is derived from the events you have to manage that state separately.

The opts parameter is a Lua table with named options:

shm: (required) name of the shared memory to use. Event data will not expire, so the module relies on the shm lru mechanism to evict old events from the shm. As such the shm should probably not be used for other purposes.
shm_retries: (optional) number of retries when the shm returns "no memory" on posting an event, default 999. Each time there is an insertion attempt and no memory is available (either no space is available or the memory is available but fragmented), "up to tens" of old entries are evicted. After that, if there's still no memory available, the "no memory" error is returned. Retrying the insertion triggers the eviction phase several times, increasing the memory available as well as the probability of finding a large enough contiguous memory block available for the new event data.
interval: (optional) interval to poll for events (in seconds), default 1. Set to 0 to disable polling.
wait_interval: (optional) interval between two tries when a new eventid is found, but the data is not available yet (due to asynchronous behaviour of the worker processes)
wait_max: (optional) max time to wait for data when event id is found, before discarding the event. This is a fail-safe setting in case something went wrong.
timeout: (optional) timeout of unique event data stored in shm (in seconds), default 2. See the unique parameter of the post method.

The return value will be true, or nil and an error message.

This method can be called repeatedly to update the settings, except for the shm value which cannot be changed after the initial configuration.

NOTE: the wait_interval is executed using the ngx.sleep function. In contexts where this function is not available (eg. init_worker) it will execute a busy-wait to execute the delay.

configured

syntax: is_already_configured = events.configured()

The events module runs as a singelton per workerprocess. The configured function allows to check whether it is already up and running. A check before starting any dependencies is recommended;

local events = require "resty.worker.events"

local initialization_of_my_module = function()
    assert(events.configured(), "Please configure the 'lua-resty-worker-events' "..
           "module before using my_module")

    -- do initialization here
end

event_list

syntax: _M.events = events.event_list(sourcename, event1, event2, ...)

Utility function to generate event lists and prevent typos in magic strings. Accessing a non-existing event on the returned table will result in an 'unknown event error'. The first parameter sourcename is a unique name that identifies the event source, which will be available as field _source. All following parameters are the named events generated by the event source.

Example usage;

local ev = require "resty.worker.events"

-- Event source example

local events = ev.event_list(
        "my-module-event-source", -- available as _M.events._source
        "started",                -- available as _M.events.started
        "event2"                  -- available as _M.events.event2
    )

local raise_event = function(event, data)
    return ev.post(events._source, event, data)
end

-- Post my own 'started' event
raise_event(events.started, nil) -- nil for clarity, no eventdata is passed

-- define my module table
local _M = {
  events = events   -- export events table

  -- implementation goes here
}
return _M



-- Event client example;
local mymod = require("some_module")  -- module with an `events` table

-- define a callback and use source modules events table
local my_callback = function(data, event, source, pid)
    if event == mymod.events.started then  -- 'started' is the event name

        -- started event from the resty-worker-events module

    elseif event == mymod.events.stoppping then  -- 'stopping' is the event name

        -- the above will throw an error because of the typo in `stoppping`

    end
end

ev.register(my_callback, mymod.events._source)

poll

syntax: success, err = events.poll()

Will poll for new events and handle them all (call the registered callbacks). The implementation is efficient, it will only check a single shared memory value and return immediately if no new events are available.

The return value will be "done" when it handled all events, "recursive" if it was already in a polling-loop, or nil + error if something went wrong. The "recursive" result simply means that the event was successfully posted, but not handled yet, due to other events ahead of it that need to be handled first.

post

syntax: success, err = events.post(source, event, data, unique)

Will post a new event. source and event are both strings. data can be anything (including nil) as long as it is (de)serializable by the cjson module.

If the unique parameter is provided then only one worker will execute the event, the other workers will ignore it. Also any follow up events with the same unique value will be ignored (for the timeout period specified to configure). The process executing the event will not necessarily be the process posting the event.

The return value will be true when the event was successfully posted or nil + error in case of failure.

Note: the worker process sending the event, will also receive the event! So if the eventsource will also act upon the event, it should not do so from the event posting code, but only when receiving it.

post_local

syntax: success, err = events.post_local(source, event, data)

The same as post except that the event will be local to the worker process, it will not be broadcasted to other workers. With this method, the data element will not be jsonified.

The return value will be true when the event was successfully posted or nil + error in case of failure.

register

syntax: events.register(callback, source, event1, event2, ...)

Will register a callback function to receive events. If source and event are omitted, then the callback will be executed on every event, if source is provided, then only events with a matching source will be passed. If (one or more) event name is given, then only when both source and event match the callback is invoked.

The callback should have the following signature;

syntax: callback = function(data, event, source, pid)

The parameters will be the same as the ones provided to post, except for the extra value pid which will be the pid of the originating worker process, or nil if it was a local event only. Any return value from callback will be discarded. Note: data may be a reference type of data (eg. a Lua table type). The same value is passed to all callbacks, so do not change the value in your handler, unless you know what you are doing!

The return value of register will be true, or it will throw an error if callback is not a function value.

WARNING: event handlers must return quickly. If a handler takes more time than the configured timeout value, events will be dropped!

Note: to receive the process own started event, the handler must be registered before calling configure

register_weak

syntax: events.register_weak(callback, source, event1, event2, ...)

This function is identical to register, with the exception that the module will only hold weak references to the callback function.

unregister

syntax: events.unregister(callback, source, event1, event2, ...)

Will unregister the callback function and prevent it from receiving further events. The parameters work exactly the same as with register.

The return value will be true if it was removed, false if it was not in the handlers list, or it will throw an error if callback is not a function value.

History

Releasing new versions

make sure changelog below is up-to-date
update version number in the code
create a new rockspec in ./rockspecs
commit with message release x.x.x
tag the commit as x.x.x
push commit and tags
upload to luarocks

2.0.1, 28-June-2021

fix: possible deadlock in the 'init phase

2.0.0, 16-September-2020

BREAKING: the post function does not call poll anymore, making all events asynchronous. When an immediate treatment to an event is needed an explicit call to poll must be done.
BREAKING: the post_local function does not immediately execute the event anymore, making all local events asynchronous. When an immediate treatment to an event is needed an explicit call to poll must be done.
fix: prevent spinning at 100% CPU when during a reload the event-shm is cleared
fix: improved logging in case of failure to write to shm (add payload size for troubleshooting purposes)
fix: do not log the payload anymore, since it might expose sensitive data through the logs
change: updated shm_retries default to 999
change: changed timer loop to a sleep-loop (performance)
fix: when re-configuring make sure callbacks table is initialized

1.1.0, 23-Dec-2020 (maintenance release)

feature: the polling loop now runs forever, sleeping for 0.5 seconds between runs, avoiding to create new timers on every step.

1.0.0, 18-July-2019

BREAKING: the return values from poll (and hence also post and post_local) changed to be more lua-ish, to be truthy when all is well.
feature: new option shm_retries to fix "no memory" errors caused by memory fragmentation in the shm when posting events.
fix: fixed two typos in variable names (edge cases)

0.3.3, 8-May-2018

fix: timeouts in init phases, by removing timeout setting, see issue #9

0.3.2, 11-Apr-2018

change: add a stacktrace to handler errors
fix: failing error handler if value was non-serializable, see issue #5
fix: fix a test for the weak handlers

GitHub

You may find additional configuration tips and documentation for this module in the GitHub repository for nginx-module-worker-events.