messengerServer.lua

-- https://stackoverflow.com/a/7615129/13347795
function split(input, maxSplits, sep)
    if sep == nil then
        sep = ";"
    end
    splits = {}
    splitCount = 0
    for str in string.gmatch(input, "([^"..sep.."]+)") do
        if maxSplits ~= nil and splitCount > maxSplits then
            splits[splitCount] = splits[splitCount]..sep..str
        else
            table.insert(splits, str)
            splitCount = splitCount + 1
        end
    end
    return splits
end

function refresh()
    h = fs.open("messages.txt", "r")
    term.clear()
    term.setCursorPos(1, 1)
    repeat
        line = h.readLine()
        if line ~= nil and line ~= "" then
            date, time, id, message = table.unpack(split(line, 3))
            print(message)
        end
    until line == nil
    h.close()
end

write("Starting...")
if not fs.exists("messages.txt") then
    local h = fs.open("messages.txt", "w")
    h.close()
end

peripheral.find("modem", rednet.open)

local h = fs.open("messages.txt", "r")
rednet.broadcast(h.readAll(), "response-messenger-backlog")
h.close()
refresh()

while true do
    id, message, protocol = rednet.receive()
    if protocol == "request-messenger-backlog" then
        local h = fs.open("messages.txt", "r")
        rednet.send(id, h.readAll(), "response-messenger-backlog")
        h.close()
    elseif protocol == "broadcast-messenger-message" then
        local h = fs.open("messages.txt", "a")
        h.writeLine(os.date("%F;%T;")..id..";"..message)
        h.close()
    end
    refresh()
end