MongoDB error using transactions: Total size of all transaction operations must be less than 16793600. Actual size is 16793810: generic server error

Question

I'm working with mongodb transactions in c++. The steps I’m performing are the following:

1. create session
2. create a bulk_write for this session, where I have multiple inserts to a collection
3. start transaction
4. execute bulk_write
5. commit transaction

This is the code snippet for the algorithm:

#include <iostream>
#include <vector>

#include <bsoncxx/builder/stream/document.hpp>
#include <bsoncxx/json.hpp>
#include <bsoncxx/exception/exception.hpp>
#include <mongocxx/client.hpp>
#include <mongocxx/instance.hpp>
#include <mongocxx/exception/exception.hpp>
#include <mongocxx/exception/logic_error.hpp>
#include <mongocxx/exception/operation_exception.hpp>


int main(int argc, char** argv)
{
    /* Parameters */

    std::string db_uri = "<PROVIDE URI TO CONNECT WITH A MONGO DB WITH REPLICA SETS>";
    std::string db_name = "db_0";
    std::string collection0_name = "coll_0";
    int N_INSERTS = 100000;

    /* Init connection */

    static mongocxx::instance inst{};
    mongocxx::uri client_uri = mongocxx::uri(db_uri);
    mongocxx::options::client client_options;
    mongocxx::options::ssl ssl_options;
    ssl_options.allow_invalid_certificates(true);
    client_options.ssl_opts(ssl_options);
    mongocxx::client client = mongocxx::client(client_uri, client_options);

    /* Reinit collection */

    mongocxx::database db = client[db_name];        
    auto builder = bsoncxx::builder::stream::document{};
    bsoncxx::document::value doc_value = builder
    << "Hello" << "MongoDB"
    << bsoncxx::builder::stream::finalize;    
    db[collection0_name].insert_one(doc_value.view()); /* insert a dummy doc */    
    db[collection0_name].delete_many({}); /* delete all docs */

    /* Create session */

    mongocxx::client_session session = client.start_session();

    /* Start transaction */

    session.start_transaction();

    /* Create bulk operations */

    mongocxx::bulk_write op0 = db[collection0_name].create_bulk_write(session);

    /* Fill insert bulk operations */

    for (int i = 0; i < N_INSERTS; i++){

        mongocxx::model::insert_one insert_one{
            bsoncxx::builder::basic::make_document(
                bsoncxx::builder::basic::kvp("field0", i),
                bsoncxx::builder::basic::kvp("field1", i),
                bsoncxx::builder::basic::kvp("field2", i)
            )
        };

        op0.append(insert_one);
    }

    /* Execute transaction */

    try {
        bulk_op->execute();
    }
    catch (std::exception& e){     
        std::cerr << "Bulk write exception: " << e.what() << std::endl;
        session.abort_transaction();
    }    

    session.commit_transaction();   

    return 0;
}

which you can compile using the following command in a linux system with mongocxx installed:

c++ --std=c++11 test.cpp -o test -I/usr/local/include/mongocxx/v_noabi -I/usr/local/include/bsoncxx/v_noabi -L/usr/local/lib -lmongocxx -lbsoncxx

While executing I get the following error:

Total size of all transaction operations must be less than 16793600. Actual size is 16793810: generic server error

But I'm only inserting 100k docs.

What could be generating the error?

Answer 1

The transaction size refers to the size of the data in bytes, not the number of documents. It's a 16 MB limit. If your transaction size exceeds this 16 MB limit, you will receive this error.