# Code Samples

**URL:** https://heroiclabs.com/docs/nakama/server-framework/lua-runtime/code-samples/
**Summary:** Development examples for the Lua runtime.
**Keywords:** nakama lua development, sql, rpc, registerrpc, setup
**Categories:** nakama, code-samples, lua-runtime

---


# 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](../../../concepts/multiplayer/authoritative/) matches. See the [Match Handler API](../function-reference/match-handler/) and [Match Runtime API](../function-reference/match-runtime/) 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.

```lua
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

```lua
-- 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:

```lua
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.

```lua
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:

```lua
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.

```lua
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 [Pokéapi](http://pokeapi.co/) and build a helpful module named "pokeapi.lua".

```lua
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:

{{< code type="client" >}}
```bash
curl "http://127.0.0.1:7350/v2/rpc/get_pokemon" \
  -H 'authorization: Bearer <session token>'
  -d '"{\"PokemonName\": \"dragonite\"}"'
```
{{< / code >}}

{{< code type="client" >}}
```javascript
const payload = { "PokemonName": "dragonite"};
const rpcid = "get_pokemon";
const pokemonInfo = await client.rpc(session, rpcid, payload);
console.log("Retrieved pokemon info: %o", pokemonInfo);
```
{{< / code >}}

{{< code type="client" >}}
```csharp
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);
```
{{< / code >}}

{{< code type="client" >}}
```cpp
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);
```
{{< / code >}}

{{< code type="client" >}}
```java
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());
```
{{< / code >}}

{{< code type="client" framework="godot3" >}}
```gdscript
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)])
```
{{< / code >}}

{{< code type="client" framework="godot4" >}}
```gdscript
var payload = {"PokemonName": "dragonite"}
var rpc_id = "get_pokemon"
var pokemon_info : NakamaAPI.ApiRpc = await client.rpc_async(session, rpc_id, JSON.stringify(payload))
if pokemon_info.is_exception():
	print("An error occurred: %s" % pokemon_info)
	return
print("Retrieved pokemon info: %s" % [JSON.parse_string(pokemon_info.payload)])
```
{{< / code >}}

{{< code type="client" >}}
```shell
POST /v2/rpc/get_pokemon
Host: 127.0.0.1:7350
Accept: application/json
Content-Type: application/json
Authorization: Bearer <session token>
{
  "PokemonName": "dragonite"
}
```
{{< / code >}}

{{< code type="client" framework="defold" >}}
```lua
local payload = { PokemonName = "dragonite"}
local rpcid = "get_pokemon"
local pokemon_info = client.rpc_func(rpcid, json.encode(payload)) 
pprint("Retrieved pokemon info:", pokemon_info)
```
{{< / code >}}