Lua - Socket receive with timer for other event - timer

Question

I am trying to implement a script with a server socket that will also periodically poll for data from several sensors (i.e on 59th second of every minute). I do not want to serialize the data to disk but rather keep it in a table which the socket will respond with when polled.
Here's some sketch the code to illustrate what I am trying to do (I've not included the client code that accesses this server, but that part is OK)
#!/usr/bin/env lua
local socket = require("socket")
local server = assert(socket.bind("*", 0))
local ip, port = server:getsockname()
local data = {}
local count = 1
local function pollSensors()
-- I do the sensor polling here and add to table e.g os.time()
table.insert(data, os.time() .."\t" .. tostring(count))
count = count + 1
end
while true do
local client = server:accept()
client:settimeout(2)
local line, err = client:receive()
-- I do process the received line to determine the response
-- for illustration I'll just send the number of items in the table
if not err then client:send("Records: " ..table.getn(data) .. "\n") end
client:close()
if os.time().sec == 59 then
pollSensors()
end
end
I am concerned that the server may on occasion(s) block and therefore I'll miss the 59th second.
Is this a good way to implement this or is there a (simpler) better way to do this (say using coroutines)? If coroutines would be better, how do I implement them for my scenario?

Answer 1

To accomplish this you need some sort of multitasking.
I'd use a network aware scheduler.
e.g. cqueues would look like this:
local cqueues = require "cqueues"
local cs = require "cqueues.socket"
local data = {}
local count = 1
local function pollSensors()
-- I do the sensor polling here and add to table e.g os.time()
table.insert(data, os.time() .."\t" .. tostring(count))
count = count + 1
end
local function handle_client(client)
client:setmode("b", "bn") -- turn on binary mode for socket and turn off buffering
-- ported code from question:
client:settimeout(2) -- I'm not sure why you chose a 2 second timeout
local line, err = client:read("*l") -- with cqueues, this read will not block the whole program, but just yield the current coroutine until data arrives.
-- I do process the received line to determine the response
-- for illustration I'll just send the number of items in the table
if not err then
assert(client:write(string.format("Records: %d\n", #data)))
end
client:close()
end
local cq = cqueues.new() -- create a new scheduler
-- create first coroutine that waits for incoming clients
cq:wrap(function()
local server = cs.listen{host = "0.0.0.0"; port = "0"}
local fam, ip, port = server:localname()
print(string.format("Now listening on ip=%s port=%d", ip, port))
for client in server:clients() do -- iterates over `accept`ed clients
-- create a new coroutine for each client, passing the client in
cqueues.running():wrap(handle_client, client)
end
end)
-- create second coroutine that reads sensors
cq:wrap(function()
while true do
-- I assume you just wanted to read every 60 seconds; rather than actually *on* the 59th second of each minute.
pollSensors()
cqueues.sleep(60)
end
end)
-- Run scheduler until all threads exit
assert(cq:loop())

Answer 2

I think the periodically launching some apps/codes are good realized with 'cron' libraries in different languages.
For instance, cron lib in lua you can download here.