How to wait for key in ngx.shared.DICT?

[rajalaks...@blis.com]

Hello, 

Here is my use-case - 

• Define a shared dictionary called 'requests'
• In the init_by_lua_block, initialises a subscriber service that receives results and updates it in the shared dictionary
• In the content_by_lua_block for the request handler, publishes the request to the app for processing and waits for results to arrive in the shared_dictionary via the subscriber service

Here is a prototype of what nginx.conf and my_app.lua look like -- 

nginx.conf 

worker_processes 8;

events {

    worker_connections 1024;

}

http { 

    lua_package_path "$prefix/lua/?.lua;;";

    lua_shared_dict requests 500m;

    init_by_lua_block { 

        require("my_app").subscribe()

    }

    server {

        listen 8080;

        location /app { 

            content_by_lua_block { 

                print require("my_app").process(ngx.var.arg_id) 

                ngx.eof()

            }

        }

    }

}

my_app.lua

local _M = {} 

-- Define publisher 

-- Define subscriber

function _M.subscribe()

while (true):

do

   message = subscriber:recv_string()

   id, value = string:match("([^,]+),([^,]+)")

   requests:safe_set(id, value)

end

function _M.publish()

   publisher:send(ngx.req.get_uri_args()["id"])

   -- How to wait here for key in shared dictionary with timeout? 

end

Any help around how to wait (non-blocking) for key to arrive in shared dictionary in the _M.publish() method above would be much appreciated. 

Thanks in advance,

Rajalakshmi

[Igor Clark]

Hello, I think you might want to have a look at ngx.timer.at or ngx.timer.every for this. They allow to spin up a function in a separate 'thread' (coroutines in lua) and have it repeat every N seconds (or fractions of seconds). Check out the docs. 

I'm not sure that polling in a shared dictionary is necessarily the way to go, especially if you've got some kind of while-true busy-loop in there, but I've  built a pub/sub pattern over a websocket where I run a client in an ngx.timer.at thread and push the incoming message content into a shared dictionary that gets accessed from elsewhere. That's when the messages are coming in from external sources, so that might not work here, but you could set up a local/internal websocket server (there's an openresty library for WS servers and clients) and manage the timing/coordination with multiple clients in multiple timer 'threads'. So even if polling is the way you want to go, then ngx.timer.* will likely help.

Hope that helps,

Igor

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