Transações

// Establish a connection to the MongoDB deployment 
mongocxx::instance instance{};
mongocxx::client client(mongocxx::uri{"<connectionString>"});
// Define database and collection variables
auto db = client["sample_mflix"];
auto movies_collection = db["movies"];
auto comments_collection = db["comments"]; 
// Define a callback specifying the sequence of operations to perform during the transaction
mongocxx::client_session::with_transaction_cb callback = [&](mongocxx::client_session* session) {
    // Important::  You must pass the session to the operations.
    movies_collection.insert_one(*session, make_document(kvp("title", "Parasite")));
    comments_collection.insert_one(*session, make_document(kvp("name", "Anjali Patel"), 
                                                           kvp("text", "This is my new favorite movie!")));
};
// Define an options instance to explicitly set the write concern for the transaction operations  
mongocxx::options::transaction opts;
mongocxx::write_concern wc;
wc.acknowledge_level(mongocxx::write_concern::level::k_majority);
opts.write_concern(wc);
// Start a client session
auto session = client.start_session();
try {
    // Start a transaction, execute the operations in the callback function, and commit the results 
    session.with_transaction(callback, opts);
} catch (const mongocxx::exception& e) {
    std::cout << "An exception occurred: " << e.what() << std::endl;
    return EXIT_FAILURE;
}
return EXIT_SUCCESS;

// Establish a connection to the MongoDB deployment 
mongocxx::instance instance{};
mongocxx::client client(mongocxx::uri{"<connectionString>"});
// Runs the txn_func and retries if TransientTransactionError occurs
using transaction_func = std::function<void(mongocxx::client_session& session)>;
auto run_with_retry = [](transaction_func txn_func, 
                         mongocxx::client_session& session) {
    while (true) {
        try {
            txn_func(session);  // performs transaction.
            break;
        } catch (const mongocxx::operation_exception& oe) {
            std::cout << "Transaction aborted. Caught exception during transaction." 
                      << std::endl;
            // If transient error, retry the whole transaction.
            if (oe.has_error_label("TransientTransactionError")) {
                std::cout << "TransientTransactionError, retrying transaction..."
                          << std::endl;
                continue;
            } else {
                throw oe;
            }
        }
    }
};
// Commits the active transaction and retries commit if UnknownTransactionCommitResult occurs
auto commit_with_retry = [](mongocxx::client_session& session) {
    while (true) {
        try {
            session.commit_transaction();  // Uses write concern set at transaction start.
            std::cout << "Transaction committed." 
                      << std::endl;
            break;
        } catch (const mongocxx::operation_exception& oe) {
            // Can retry commit
            if (oe.has_error_label("UnknownTransactionCommitResult")) {
                std::cout << "UnknownTransactionCommitResult, retrying commit..."
                          << std::endl;
                continue;
            } else {
                std::cout << "Error during commit..." 
                          << std::endl;
                throw oe;
            }
        }
    }
    
};
auto txn_func = [&](mongocxx::client_session& session) {
    auto& client = session.client(); 
    // Define database and collection variables
    auto db = client["sample_mflix"];
    auto movies_collection = db["movies"];
    auto comments_collection = db["comments"];
    // Define an options instance to explicitly set the write concern for the transaction operations  
    mongocxx::options::transaction opts;
    mongocxx::write_concern wc;
    wc.acknowledge_level(mongocxx::write_concern::level::k_majority);
    opts.write_concern(wc);
    session.start_transaction(opts);
    // Attempt to insert documents into database collections
    try {
        movies_collection.insert_one(session, make_document(kvp("title", "Parasite")));
        comments_collection.insert_one(session, make_document(kvp("name", "Anjali Patel"), 
                                                              kvp("text", "This is my new favorite movie!")));
    } catch (const mongocxx::operation_exception& oe) {
        std::cout << "Caught exception during transaction, aborting." 
                  << std::endl;
        session.abort_transaction();
        throw oe;
    }
    commit_with_retry(session);
};
// Start a client session
auto session = client.start_session();
try {
    run_with_retry(txn_func, session);
} catch (const mongocxx::operation_exception& oe) {
    // Do something with error 
    throw oe;
}