Build a C++ App with CockroachDB

Build a C++ App with CockroachDB

This tutorial shows you how build a simple C++ application with CockroachDB using a PostgreSQL-compatible driver.

We have tested the C++ libpqxx driver enough to claim beta-level support, so that driver is featured here. If you encounter problems, please open an issue with details to help us make progress toward full support.

Before you begin

Make sure you have already installed CockroachDB.

Step 1. Install the libpqxx driver

Install the C++ libpqxx driver as described in the official documentation.

Step 2. Start a single-node cluster

For the purpose of this tutorial, you need only one CockroachDB node running in insecure mode:

$ cockroach start \
--insecure \
--store=hello-1 \
--listen-addr=localhost

Step 3. Create a user

In a new terminal, as the root user, use the cockroach user command to create a new user, maxroach.

$ cockroach user set maxroach --insecure

Step 4. Create a database and grant privileges

As the root user, use the built-in SQL client to create a bank database.

$ cockroach sql --insecure -e 'CREATE DATABASE bank'

Then grant privileges to the maxroach user.

$ cockroach sql --insecure -e 'GRANT ALL ON DATABASE bank TO maxroach'

Step 5. Run the C++ code

Now that you have a database and a user, you'll run code to create a table and insert some rows, and then you'll run code to read and update values as an atomic transaction.

Basic statements

First, use the following code to connect as the maxroach user and execute some basic SQL statements, creating a table, inserting rows, and reading and printing the rows.

Download the basic-sample.cpp file, or create the file yourself and copy the code into it.

// Build with g++ -std=c++11 basic-sample.cpp -lpq -lpqxx
#include <cassert>
#include <functional>
#include <iostream>
#include <stdexcept>
#include <string>
#include <pqxx/pqxx>
using namespace std;
int main() {
  try {
    // Connect to the "bank" database.
    pqxx::connection c("postgresql://maxroach@localhost:26257/bank");
    pqxx::nontransaction w(c);
    // Create the "accounts" table.
    w.exec("CREATE TABLE IF NOT EXISTS accounts (id INT PRIMARY KEY, balance INT)");
    // Insert two rows into the "accounts" table.
    w.exec("INSERT INTO accounts (id, balance) VALUES (1, 1000), (2, 250)");
    // Print out the balances.
    cout << "Initial balances:" << endl;
    pqxx::result r = w.exec("SELECT id, balance FROM accounts");
    for (auto row : r) {
      cout << row[0].as<int>() << ' ' << row[1].as<int>() << endl;
    }
    w.commit();  // Note this doesn't doesn't do anything
                 // for a nontransaction, but is still required.
  }
  catch (const exception &e) {
    cerr << e.what() << endl;
    return 1;
  }
  cout << "Success" << endl;
  return 0;
}

Transaction (with retry logic)

Next, use the following code to again connect as the maxroach user but this time execute a batch of statements as an atomic transaction to transfer funds from one account to another, where all included statements are either committed or aborted.

Note:

CockroachDB may require the client to retry a transaction in case of read/write contention. CockroachDB provides a generic retry function that runs inside a transaction and retries it as needed. You can copy and paste the retry function from here into your code.

Download the txn-sample.cpp file, or create the file yourself and copy the code into it.

// Build with g++ -std=c++11 txn-sample.cpp -lpq -lpqxx
#include <cassert>
#include <functional>
#include <iostream>
#include <stdexcept>
#include <string>
#include <pqxx/pqxx>
using namespace std;
void transferFunds(
    pqxx::dbtransaction *tx, int from, int to, int amount) {
  // Read the balance.
  pqxx::result r = tx->exec(
      "SELECT balance FROM accounts WHERE id = " + to_string(from));
  assert(r.size() == 1);
  int fromBalance = r[0][0].as<int>();
  if (fromBalance < amount) {
    throw domain_error("insufficient funds");
  }
  // Perform the transfer.
  tx->exec("UPDATE accounts SET balance = balance - " 
      + to_string(amount) + " WHERE id = " + to_string(from));
  tx->exec("UPDATE accounts SET balance = balance + " 
      + to_string(amount) + " WHERE id = " + to_string(to));
}
// ExecuteTx runs fn inside a transaction and retries it as needed.
// On non-retryable failures, the transaction is aborted and rolled
// back; on success, the transaction is committed.
//
// For more information about CockroachDB's transaction model see
// https://cockroachlabs.com/docs/transactions.html.
//
// NOTE: the supplied exec closure should not have external side
// effects beyond changes to the database.
void executeTx(
    pqxx::connection *c, function<void (pqxx::dbtransaction *tx)> fn) {
  pqxx::work tx(*c);
  while (true) {
    try {
      pqxx::subtransaction s(tx, "cockroach_restart");
      fn(&s);
      s.commit();
      break;
    } catch (const pqxx::pqxx_exception& e) {
      // Swallow "transaction restart" errors; the transaction will be retried.
      // Unfortunately libpqxx doesn't give us access to the error code, so we
      // do string matching to identify retriable errors.
      if (string(e.base().what()).find("restart transaction:") == string::npos) {
        throw;
      }
    }
  }
  tx.commit();
}
int main() {
  try {
    pqxx::connection c("postgresql://maxroach@localhost:26257/bank");
    executeTx(&c, [](pqxx::dbtransaction *tx) {
          transferFunds(tx, 1, 2, 100);
      });
  }
  catch (const exception &e) {
    cerr << e.what() << endl;
    return 1;
  }
  cout << "Success" << endl;
  return 0;
}

After running the code, use the built-in SQL client to verify that funds were transferred from one account to another:

$ cockroach sql --insecure -e 'SELECT id, balance FROM accounts' --database=bank

+----+---------+
| id | balance |
+----+---------+
|  1 |     900 |
|  2 |     350 |
+----+---------+
(2 rows)

What's next?

Read more about using the C++ libpqxx driver.

You might also be interested in using a local cluster to explore the following CockroachDB benefits:

Was this page helpful?
Yes No