Database

In the previous chapter, you learned errors and logging. Now you will connect a Vix application to a database.

txt

Request → validation → database query → JSON Response

Memory disappears when the app restarts. A database gives your application durable state.

Public header

cpp

#include <vix/db.hpp>

SQLite or MySQL?

Criteria	SQLite	MySQL
Best for	Local development, small apps, MVPs.	Multi-user production APIs.
Setup	Very simple, no server required.	Requires a database server.
Persistence	Stores data in a local file.	Stores data in a server database.

Start with SQLite for learning.

Build flags

bash

vix build --with-sqlite
vix run main.cpp --with-sqlite

vix build --with-mysql
vix run main.cpp --with-mysql

First SQLite connection

cpp

#include <vix/db.hpp>

auto db = vix::db::Database::sqlite("vix.db");
db.exec("CREATE TABLE IF NOT EXISTS healthcheck (id INTEGER PRIMARY KEY)");

First MySQL connection

cpp

auto db = vix::db::Database::mysql("tcp://127.0.0.1:3306", "root", "", "vixdb");

Database from .env

dotenv

DATABASE_ENGINE=sqlite
DATABASE_DEFAULT_NAME=vix.db

cpp

vix::config::Config cfg{".env"};
vix::db::Database db{cfg};

Create a table

cpp

// SQLite
db.exec(
    "CREATE TABLE IF NOT EXISTS users ("
    "id INTEGER PRIMARY KEY AUTOINCREMENT, "
    "name TEXT NOT NULL, "
    "role TEXT NOT NULL)");

// MySQL
db.exec(
    "CREATE TABLE IF NOT EXISTS users ("
    "id BIGINT PRIMARY KEY AUTO_INCREMENT, "
    "name VARCHAR(255) NOT NULL, "
    "role VARCHAR(64) NOT NULL)");

Insert data

cpp

db.exec("INSERT INTO users (name, role) VALUES (?, ?)", "Alice", "admin");

Always use parameterized queries. Never build SQL with string concatenation.

Query data

cpp

auto rows = db.query("SELECT id, name, role FROM users");
while (rows->next())
{
  const auto &row = rows->row();
  std::cout << row.getInt64(0) << " " << row.getString(1) << " " << row.getString(2) << "\n";
}

Prepared statements

cpp

vix::db::PooledConn conn(db.pool());
auto stmt = conn->prepare("SELECT id, name FROM users WHERE id = ?");
stmt->bind(1, static_cast<std::int64_t>(1));
auto rows = stmt->query();

Use prepared statements for: user input, route parameters, query filters, inserts, updates, deletes.

Connection pool

cpp

vix::db::PooledConn conn(db.pool());
// connection returns automatically when PooledConn is destroyed (RAII)

Transactions

cpp

db.transaction([&](vix::db::Connection &conn){
  conn.prepare("INSERT INTO users (name, role) VALUES (?, ?)")
      ->bind(1, "Alice")->bind(2, "admin")->exec();
  conn.prepare("INSERT INTO users (name, role) VALUES (?, ?)")
      ->bind(1, "Bob")->bind(2, "user")->exec();
});

Use transactions for: orders + items, user + profile, money transfers, any multi-step write.

Complete database API

cpp

#include <vix.hpp>
#include <vix/db.hpp>
#include <vix/log.hpp>
#include <cstdint>
#include <optional>
#include <stdexcept>
#include <string>

using namespace vix;

struct User {
  std::int64_t id{};
  std::string name;
  std::string role;
};

static json::Json user_to_json(const User &u)
{
  return json::kv({
    {"id", json::Json(u.id)},
    {"name", json::Json(u.name)},
    {"role", json::Json(u.role)}
  });
}

static void respond_error(Response &res,
                          int status,
                          const std::string &code,
                          const std::string &msg)
{
  res.status(status).json(json::kv({
    {"ok", json::Json(false)},
    {"error", json::Json(code)},
    {"message", json::Json(msg)}
  }));
}

static std::optional<std::int64_t> parse_id(const std::string &text)
{
  try {
    return std::stoll(text);
  } catch (...) {
    return std::nullopt;
  }
}

static void initialize_schema(vix::db::Database &db)
{
  db.exec("CREATE TABLE IF NOT EXISTS users (id INTEGER PRIMARY KEY AUTOINCREMENT, name TEXT NOT NULL, role TEXT NOT NULL)");
}

static void seed_users(vix::db::Database &db)
{
  auto rows = db.query("SELECT COUNT(*) FROM users");

  if (rows->next() && rows->row().getInt64(0) > 0)
    return;

  db.exec("INSERT INTO users (name, role) VALUES (?, ?)", "Alice", "admin");
  db.exec("INSERT INTO users (name, role) VALUES (?, ?)", "Bob", "user");
}

static json::Json list_users(vix::db::Database &db)
{
  json::Json items = json::Json::array();
  auto rows = db.query("SELECT id, name, role FROM users ORDER BY id ASC");

  while (rows->next())
  {
    const auto &row = rows->row();
    items.push_back(user_to_json({row.getInt64(0), row.getString(1), row.getString(2)}));
  }
  return items;
}

static std::optional<User> find_user_by_id(vix::db::Database &db, std::int64_t id)
{
  vix::db::PooledConn conn(db.pool());
  auto stmt = conn->prepare("SELECT id, name, role FROM users WHERE id = ?");
  stmt->bind(1, id);
  auto rows = stmt->query();

  if (!rows->next())
    return std::nullopt;

  const auto &row = rows->row();
  return User{row.getInt64(0), row.getString(1), row.getString(2)};
}

static User create_user(vix::db::Database &db, const std::string &name, const std::string &role)
{
  db.exec("INSERT INTO users (name, role) VALUES (?, ?)", name, role);
  auto rows = db.query("SELECT id, name, role FROM users ORDER BY id DESC LIMIT 1");

  if (!rows->next())
    throw std::runtime_error("failed to load created user");

  const auto &row = rows->row();

  return {
    row.getInt64(0),
    row.getString(1),
    row.getString(2)
  };
}

static void register_user_routes(App &app, vix::db::Database &db)
{
  app.get("/api/users", [&db](Request &, Response &res){
    try {
      res.json(json::kv({
        {"ok", json::Json(true)},
        {"data", list_users(db)}
      }));

    }catch (const std::exception &e) {
      vix::log::error("failed to list users", "details", e.what()); respond_error(res, 500, "internal_error", "Internal server error");
    }
  });

  app.get("/api/users/{id}", [&db](Request &req, Response &res){
    const auto id = parse_id(req.param("id"));

    if (!id) {
      respond_error(res, 400, "invalid_id", "Invalid user id");
      return;
    }

    const auto user = find_user_by_id(db, *id);
    if (!user) {
      respond_error(res, 404, "user_not_found", "User not found");
      return;
    }

    res.json(json::kv({
      {"ok", json::Json(true)},
      {"data", user_to_json(*user)}
    }));
  });

  app.post("/api/users", [&db](Request &req, Response &res){
    try{
      const auto &body = req.json();
      if (!body.is_object()) {
        respond_error(res, 400, "invalid_request", "Expected JSON object body");
        return;
      }

      const std::string name = body.value("name", "");
      const std::string role = body.value("role", "user");
      if (name.empty()) {
        respond_error(res, 400, "validation_failed", "Field 'name' is required");
        return;
      }

      const User user = create_user(db, name, role.empty() ? "user" : role);
      res.status(201).json(json::kv({
        {"ok", json::Json(true)},
        {"message", json::Json("user created")},
        {"data", user_to_json(user)}
      }));

    }catch (const std::exception &e) {
      vix::log::error("failed to create user", "details", e.what()); respond_error(res, 500, "internal_error", "Internal server error");
    }
  });
}

int main()
{
  vix::log::set_level(vix::log::LogLevel::Info);
  try{
    auto db = vix::db::Database::sqlite("vix.db");
    initialize_schema(db);
    seed_users(db);

    App app;

    app.get("/health", [](Request &, Response &res) {
      res.json({"ok", true, "service", "database-api"});
    });

    register_user_routes(app, db);

    app.run(8080);

    return 0;
  }catch (const std::exception &e){
    vix::log::critical("application startup failed", "details", e.what());
    return 1;
  }
}

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192

Test

bash

curl -i http://127.0.0.1:8080/health
curl -i http://127.0.0.1:8080/api/users
curl -i http://127.0.0.1:8080/api/users/1
curl -i http://127.0.0.1:8080/api/users/999
curl -i -X POST http://127.0.0.1:8080/api/users \
  -H "Content-Type: application/json" \
  -d '{"name":"Charlie","role":"user"}'

Restart the app — unlike the memory API, the new user should still exist.

Migrations

For real projects, use migrations instead of CREATE TABLE IF NOT EXISTS at startup.

cpp

class CreateUsersTable final : public vix::db::Migration
{
public:
  std::string id() const override { return "2026-01-22-create-users"; }

  void up(vix::db::Connection &conn) override
  {
    conn.prepare("CREATE TABLE IF NOT EXISTS users (id INTEGER PRIMARY KEY AUTOINCREMENT, name TEXT NOT NULL, role TEXT NOT NULL)")->exec();
  }

  void down(vix::db::Connection &conn) override
  {
    conn.prepare("DROP TABLE IF EXISTS users")->exec();
  }
};

Common mistakes

Building SQL with string concatenation

cpp

// Wrong
std::string sql = "SELECT * FROM users WHERE name = '" + name + "'";

// Correct — use prepared statements
auto stmt = conn->prepare("SELECT * FROM users WHERE name = ?");
stmt->bind(1, name);

Returning raw database errors

Log internal details, return safe client errors.

Not validating before insert

Always validate input before database writes.

What you should remember

The Vix DB model is explicit: connect → prepare → bind → query → read rows → commit when needed.

Use prepared statements for user input. Use transactions for multi-step writes. Routes should validate input, call database logic, and return safe JSON responses.

Next chapter

Next: Real-time WebSocket

Application

Backend

Web

Vue.js

Game

Library

Runtime

Coroutines

Cancellation and Time

CPU Work

Networking

Execution

Tasks

Scheduling

Cancellation and Time

Structured Work

Parallel Algorithms

Database

Public header

SQLite or MySQL?

Build flags

First SQLite connection

First MySQL connection

Database from .env

Create a table

Insert data

Query data

Prepared statements

Connection pool

Transactions

Complete database API

Test

Migrations

Common mistakes

Building SQL with string concatenation

Returning raw database errors

Not validating before insert

What you should remember

Next chapter

Database ​

Public header ​

SQLite or MySQL? ​

Build flags ​

First SQLite connection ​

First MySQL connection ​

Database from .env ​

Create a table ​

Insert data ​

Query data ​

Prepared statements ​

Connection pool ​

Transactions ​

Complete database API ​

Test ​

Migrations ​

Common mistakes ​

Building SQL with string concatenation ​

Returning raw database errors ​

Not validating before insert ​

What you should remember ​

Next chapter ​

Database

Public header

SQLite or MySQL?

Build flags

First SQLite connection

First MySQL connection

Database from .env

Create a table

Insert data

Query data

Prepared statements

Connection pool

Transactions

Complete database API

Test

Migrations

Common mistakes

Building SQL with string concatenation

Returning raw database errors

Not validating before insert

What you should remember

Next chapter