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Code Samples

This page provides some common examples for the functionality available that can be used as templates when developing your project using the Lua runtime.

Match handler

A match handler represents all server-side functions for handling game inputs and operations for authoritative multiplayer matches. See the Match Handler API and Match Runtime API reference pages to learn about the match handler functions.

This is an example of a Ping-Pong match handler. Messages received by the server are broadcast back to the peer who sent them.

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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, metadata)
  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 _, p in pairs(state.presences) do
    nk.logger_info(string.format("Presence %s named %s", p.user_id, p.username))
  end

  for _, m in ipairs(messages) do
    nk.logger_info(string.format("Received %s from %s", m.data, m.sender.username))
    local decoded = nk.json_decode(m.data)

    for k, v in pairs(decoded) do
      nk.logger_info(string.format("Key %s contains value %s", k, v))
    end

    -- PONG message back to sender
    dispatcher.broadcast_message(1, m.data, { m.sender })
  end

  return state
end

function M.match_terminate(context, dispatcher, tick, state, grace_seconds)
  local message = "Server shutting down in " .. grace_seconds .. " seconds"
  dispatcher.broadcast_message(2, message)

  return nil
end

local function match_signal(context, dispatcher, tick, state, data)
  return state, "signal received: " .. data
end

return M

Context

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-- Getting the calling user ID from the context
local user_id = context.user_id

-- Getting the environment variables from the context
local env = context.env
local secret_key = env["SECRET_KEY"]
if not secret_key then
  -- Did not find the environment variable
end

Database handler

This example creates a system ID - an ID that cannot be used from a client - and the custom SQL query inserting it in the users table:

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nk.run_once(function(context)
  local system_id = context.env["SYSTEM_ID"]

  nk.sql_exec([[
INSERT INTO users (id, username)
VALUES ($1, $2)
ON CONFLICT (id) DO NOTHING
  ]], { system_id, "system_id" })
end)

RPC

The example below registers a function with the identifier custom_rpc_func_id. This ID can then be used within client code to send an RPC message to execute the function on the server and return the result.

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local nk = require("nakama")

local function custom_rpc_func(context, payload)
  nk.logger_info(string.format("Payload: %q", payload))

  -- "payload" is bytes sent by the client we'll JSON decode it.
  local json = nk.json_decode(payload)

  return nk.json_encode(json)
end

nk.register_rpc(custom_rpc_func, "custom_rpc_func_id")

Before hook

The code example below fetches the current user’s profile and checks the metadata, which is assumed to be JSON encoded with "{level: 12}" in it. If a user’s level is too low, an error is thrown to prevent the Friend Add message from being passed onwards in the server:

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local nk = require("nakama")

local function limit_friends(context, payload)
  local user = nk.users_get_id({context.user_id})[1]
  -- Let's assume we've stored a user's level in their metadata.
  if user.metadata.level < 10 then
      error("Must reach level 10 before you can add friends.")
  end
  return payload -- important!
end

nk.register_req_before(limit_friends, "AddFriends")

After hook

The example code below writes a record to a user’s storage when they add a friend. Any data returned by the function will be discarded.

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local nk = require("nakama")

local function add_reward(context, outgoing_payload, incoming_payload)
  local value = {
    user_ids = {incoming_payload.user_id}
  }

  local object = {
    collection = "rewards",
    key = "reward",
    user_id = context.user_id,
    value = value
  }

  nk.storage_write({ object })
end

nk.register_req_after(add_reward, "AddFriends")

Example module

As an example let’s use the Pok√©api and build a helpful module named “pokeapi.lua”.

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local nk = require("nakama")

local M = {}

local API_BASE_URL = "https://pokeapi.co/api/v2"

function M.lookup_pokemon(name)
  local url = string.format("%s/pokemon/%s", API_BASE_URL, name)
  local method = "GET"
  local headers = {
    ["Content-Type"] = "application/json",
    ["Accept"] = "application/json"
  }

  local success, code, _, body = pcall(nk.http_request, url, method, headers, nil)

	if (not success) then
    nk.logger_error(string.format("Failed request %q", code))
    error(code)
  elseif (code >= 400) then
    nk.logger_error(string.format("Failed request %q %q", code, body))
    error(body)
  else
    return nk.json_decode(body)
  end
end

return M

-- We can import the code up to this point into another module we'll call "pokemon.lua" which will register an RPC call.
local nk = require("nakama")
local pokeapi = require("pokeapi")

local function get_pokemon(_, payload)
  -- We'll assume payload was sent as JSON and decode it.
  local json = nk.json_decode(payload)
  local success, result = pcall(pokeapi.lookup_pokemon, json.PokemonName)

	if (not success) then
    error("Unable to lookup pokemon.")
  else
    local pokemon = {
      name = result.name,
      height = result.height,
      weight = result.weight,
      image = result.sprites.front_default
    }

    return nk.json_encode(pokemon)
  end
end

nk.register_rpc(get_pokemon, "get_pokemon")

You can now make an RPC call for a Pokémon from a client:

Client
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curl "http://127.0.0.1:7350/v2/rpc/get_pokemon" \
  -H 'authorization: Bearer <session token>'
  -d '"{\"PokemonName\": \"dragonite\"}"'
Client
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const payload = { "PokemonName": "dragonite"};
const rpcid = "get_pokemon";
const pokemonInfo = await client.rpc(session, rpcid, payload);
console.log("Retrieved pokemon info: %o", pokemonInfo);
Client
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var payload = JsonWriter.ToJson(new { PokemonName = "dragonite" });
var rpcid = "get_pokemon";
var pokemonInfo = await client.RpcAsync(session, rpcid, payload);
System.Console.WriteLine("Retrieved pokemon info: {0}", pokemonInfo);
Client
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auto successCallback = [](const NRpc& rpc)
{
  	std::cout << "Retrieved pokemon info: " << rpc.payload << std::endl;
};

string payload = "{ \"PokemonName\": \"dragonite\" }";
string rpcid = "get_pokemon";
client->rpc(session, rpcid, payload, successCallback);
Client
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Map<String, String> payloadData = new HashMap<>();
payloadData.put("PokemonName", "dragonite");
String payload = new Gson().toJson(payloadData, payloadData.getClass());
String rpcid = "get_pokemon";
Rpc pokemonInfo = client.rpc(session, rpcid, payload);
System.out.format("Retrieved pokemon info: %s", pokemonInfo.getPayload());
Client
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var payload = {"PokemonName": "dragonite"}
var rpc_id = "get_pokemon"
var pokemon_info : NakamaAPI.ApiRpc = yield(client.rpc_async(session, rpc_id, JSON.print(payload)), "completed")
if pokemon_info.is_exception():
	print("An error occurred: %s" % pokemon_info)
	return
print("Retrieved pokemon info: %s" % [parse_json(pokemon_info.payload)])
Client
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POST /v2/rpc/get_pokemon
Host: 127.0.0.1:7350
Accept: application/json
Content-Type: application/json
Authorization: Bearer <session token>
{
  "PokemonName": "dragonite"
}
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