ESX Server Callbacks — Complete Guide

Server Callbacks are ESX's most important mechanism, allowing clients to securely request information from the server and receive responses in return.

What is Server Callback?

The problem with FiveM is that the client and server work separately. The client cannot access the database directly. Server Callback solves this problem by having the client ask the server and wait for an answer.

Creating Server Callback (Server-side)

ESX.RegisterServerCallback('myScript:getPlayerData', function(source, cb, extraParam)
    local xPlayer = ESX.GetPlayerFromId(source)
    
    if not xPlayer then
        cb(nil)
        return
    end
    
    MySQL.Async.fetchAll('SELECT * FROM custom_data WHERE identifier = @id', {
        ['@id'] = xPlayer.identifier
    }, function(result)
        cb(result)
    end)
end)

Usage (Client-side)

ESX.TriggerServerCallback('myScript:getPlayerData', function(result)
    if result then
        print('Got data:', json.encode(result))
    end
end, optionalExtraParam)

Secure Pattern: Validate everything on the server side.

-- Server callback: Buy products
ESX.RegisterServerCallback('myScript:buyItem', function(source, cb, itemName)
    local xPlayer = ESX.GetPlayerFromId(source)
    
    -- Server pulls price from Config itself, not from Client.
    local item = Config.ShopItems[itemName]
    if not item then
        cb(false, 'item is invalid')
        return
    end
    
    if xPlayer.getMoney() < item.price then
        cb(false, 'not enough money')
        return
    end
    
    xPlayer.removeMoney(item.price)
    xPlayer.addInventoryItem(itemName, 1)
    cb(true, 'Purchased successfully')
end)

Precautions

• Don't trust the price from the Client — Server must fetch price from Config itself.
• Check xPlayer first — if not xPlayer then cb(nil) return end
• Always call cb() — If not called, the Client will always hang.

Summary

Server Callbacks is a basic pattern that every FiveM developer must understand, use correctly, and check every input on the Server side.