This tutorial shows you how build a simple C++ application with CockroachDB and the C++ libpqxx driver.
We have tested the C++ libpqxx driver enough to claim beta-level support. If you encounter problems, please open an issue with details to help us make progress toward full support.
Before you begin
- Install CockroachDB.
- Start up a secure or insecure local cluster.
- Choose the instructions that correspond to whether your cluster is secure or insecure:
Step 1. Install the libpqxx driver
Install the C++ libpqxx driver as described in the official documentation.
If you are running macOS, you need to install version 4.0.1 or higher of the libpqxx driver.
Step 2. Create the maxroach user and bank database
Start the built-in SQL shell:
$ cockroach sql --certs-dir=certs
In the SQL shell, issue the following statements to create the maxroach user and bank database:
> CREATE USER IF NOT EXISTS maxroach;
> CREATE DATABASE bank;
Give the maxroach user the necessary permissions:
> GRANT ALL ON DATABASE bank TO maxroach;
Exit the SQL shell:
> \q
Step 3. Generate a certificate for the maxroach user
Create a certificate and key for the maxroach user by running the following command. The code samples will run as this user.
$ cockroach cert create-client maxroach --certs-dir=certs --ca-key=my-safe-directory/ca.key
Step 4. 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.
#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("dbname=bank user=maxroach sslmode=require sslkey=certs/client.maxroach.key sslcert=certs/client.maxroach.crt port=26257 host=localhost");
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;
}
To build the basic-sample.cpp source code to an executable file named basic-sample, run the following command from the directory that contains the code:
$ g++ -std=c++11 basic-sample.cpp -lpq -lpqxx -o basic-sample
Then run the basic-sample file from that directory:
$ ./basic-sample
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.
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.
#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 retryable errors.
if (string(e.base().what()).find("restart transaction:") == string::npos) {
throw;
}
}
}
tx.commit();
}
int main() {
try {
pqxx::connection c("dbname=bank user=maxroach sslmode=require sslkey=certs/client.maxroach.key sslcert=certs/client.maxroach.crt port=26257 host=localhost");
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;
}
To build the txn-sample.cpp source code to an executable file named txn-sample, run the following command from the directory that contains the code:
$ g++ -std=c++11 txn-sample.cpp -lpq -lpqxx -o txn-sample
Then run the txn-sample file from that directory:
$ ./txn-sample
After running the code, use the built-in SQL client to verify that funds were transferred from one account to another:
$ cockroach sql --certs-dir=certs -e 'SELECT id, balance FROM accounts' --database=bank
id | balance
+----+---------+
1 | 900
2 | 350
(2 rows)
Step 2. Create the maxroach user and bank database
Start the built-in SQL shell:
$ cockroach sql --insecure
In the SQL shell, issue the following statements to create the maxroach user and bank database:
> CREATE USER IF NOT EXISTS maxroach;
> CREATE DATABASE bank;
Give the maxroach user the necessary permissions:
> GRANT ALL ON DATABASE bank TO maxroach;
Exit the SQL shell:
> \q
Step 3. 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.
#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;
}
To build the basic-sample.cpp source code to an executable file named basic-sample, run the following command from the directory that contains the code:
$ g++ -std=c++11 basic-sample.cpp -lpq -lpqxx -o basic-sample
Then run the basic-sample file from that directory:
$ ./basic-sample
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.
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.
#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 retryable 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;
}
To build the txn-sample.cpp source code to an executable file named txn-sample, run the following command from the directory that contains the code:
$ g++ -std=c++11 txn-sample.cpp -lpq -lpqxx -o txn-sample
Then run the txn-sample file from that directory:
$ ./txn-sample
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 the following pages:
- Client Connection Parameters
- Data Replication
- Fault Tolerance & Recovery
- Replication & Rebalancing
- Cross-Cloud Migration
- Automated Operations
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