pqxx reuse / reactivate a work transaction

问题

I want to use a pqxx::work for multiples queries AND commitments, while the commit function prevent me from using it again. Here is a simple example :

pqxx::connection G_connexion("dbname=basetest user=usertest password=1234");
pqxx::work G_work(G_connexion);

int main(int argc, char* argv[]) {
    G_work.exec("insert into test.table1(nom) VALUES('foo');");
    G_work.commit();//until here, no problem
    G_work.exec("insert into test.table1(nom) VALUES('bar');"); //error, transaction already closed
    G_work.commit();
}

When I try to insert the 'bar' value, after the commit, I get a pqxx::usage_error : Error executing query . Attempt to activate transaction<READ COMMITTED> which is already closed

How can I avoid to close the connection after I commit the changes ? can i reset G_work with a successing equivalent of G_work=pqxx::work(G_connexion), or other ? Also, one bad request should not crash the entire process, just the one in process (G_work still usable after a failure).

I have to keep the same variable G_Work because it will be a global variable called from lots of places in the program.

回答1:

pqxx::work is just a pqxx::transaction<> which eventually gets most of its logic from pqxx::transaction_base.

This class is not intended to serve for several transactions. Instead, it is intended for a single transaction within a try/catch block. It has a state member variable (m_Status) which is never reinitialized, even after a commit.

The normal pattern is:

{
    pqxx::work l_work(G_connexion);
    try {
        l_work.exec("insert into test.table1(nom) VALUES('foo');");
        l_work.commit();
    } catch (const exception& e) {
        l_work.abort();
        throw;
    }
}

Arguably, libpqxx could rollback the transaction on deletion (to avoid the try/catch entirely) but it doesn't.

It seems that this doesn't fit your usage pattern as you want G_work to be a global variable accessible from several places in your program. Please note that pqxx::work is not the class for connection objects, but just a way to encapsulate begin/commit/rollback with C++ exceptions handling.

Nevertheless, libpqxx also allows you to execute statement outside transactions (or at least outside libpqxx-managed transactions). You should use instances of pqxx::nontransaction class.

#include "pqxx/nontransaction"

pqxx::connection G_connexion("dbname=basetest user=usertest password=1234");
pqxx::nontransaction G_work(G_connexion);

int f() {
    G_work.exec("insert into test.table1(nom) VALUES('foo');");
    G_work.exec("insert into test.table1(nom) VALUES('bar');");
}

Please note that this is equivalent to:

#include "pqxx/nontransaction"

pqxx::connection G_connexion("dbname=basetest user=usertest password=1234");

int f() {
    pqxx::nontransaction l_work(G_connexion);
    l_work.exec("insert into test.table1(nom) VALUES('foo');");
    l_work.exec("insert into test.table1(nom) VALUES('bar');");
}

Eventually, nothing prevents you to manage transactions with pqxx::nontransaction. This is especially true if you want savepoints. I would also advise using pqxx::nontransaction if your transaction is meant to last beyond a function scope (e.g. at global scope).

#include "pqxx/nontransaction"

pqxx::connection G_connexion("dbname=basetest user=usertest password=1234");
pqxx::nontransaction G_work(G_connexion);

int f() {
    G_work.exec("begin;");
    G_work.exec("insert into test.table1(nom) VALUES('foo');");
    G_work.exec("savepoint f_savepoint;");
    // If the statement fails, rollback to checkpoint.
    try {
        G_work.exec("insert into test.table1(nom) VALUES('bar');");
    } catch (const pqxx::sql_error& e) {
        G_work.exec("rollback to savepoint f_savepoint;");
    }
    G_work.exec("commit;");
}

来源：https://stackoverflow.com/questions/26464056/pqxx-reuse-reactivate-a-work-transaction

标签

c++

postgresql

libpqxx