An example of a match handler with Nakama server which tracks presence join/leave events for users in the game state.

matchskeleton.lua

--[[
 Copyright 2018 The Nakama Authors

 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
 You may obtain a copy of the License at

 http://www.apache.org/licenses/LICENSE-2.0

 Unless required by applicable law or agreed to in writing, software
 distributed under the License is distributed on an "AS IS" BASIS,
 WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 See the License for the specific language governing permissions and
 limitations under the License.
--]]

local nk = require("nakama")

local M = {}

function M.match_init(context, setupstate)
  local gamestate = {
    presences = {}
  }
  local tickrate = 1 -- per sec
  local label = ""
  return gamestate, tickrate, label
end

function M.match_join_attempt(context, dispatcher, tick, state, presence)
  local acceptuser = true
  return state, acceptuser
end

function M.match_join(context, dispatcher, tick, state, presences)
  for _, presence in ipairs(presences) do
    state.presences[presence.session_id] = presence
  end
  return state
end

function M.match_leave(context, dispatcher, tick, state, presences)
  for _, presence in ipairs(presences) do
    state.presences[presence.session_id] = nil
  end
  return state
end

function M.match_loop(context, dispatcher, tick, state, messages)
  for _, presence in pairs(state.presences) do
    print(("Presence connected %s named %s"):format(presence.user_id, presence.username))
  end
  for _, message in ipairs(messages) do
    print(("Received %s from %s"):format(message.sender.username, message.data))
    local decoded = nk.json_decode(message.data)
    for k, v in pairs(decoded) do
      print(("Message contained %s value %s"):format(k, v))
    end
  end
  return state
end

return M

An RPC function which will join a match or create a new one if none exists:

local nk = require("nakama")

local function create_or_join_match(context, payload)
  -- expected input:
  -- { "level": 15 }
  payload = nk.json_decode(payload)

  local limit = 10
  local authoritative = true
  local label = tostring(payload.level)
  local min_size = 0
  local max_size = 9
  local matches = nk.match_list(limit, authoritative, label, min_size, max_size)

  local match_id
  if (#matches > 0) then
    -- sort tables by number of players
    table.sort(matches, function(a, b)
      return a.size > b.size
    end)
    match_id = matches[1].match_id
  else
    -- no active matches must create one
    local params = {}
    match_id = nk.match_create("matchskeleton", params)
  end

  local response = nk.json_encode({ matchid = match_id })
  return response
end
nk.register_rpc(create_or_join_match, "createorjoin")