Transactor framework


class  pqxx::transactor< TRANSACTION >


using pqxx::transactor< TRANSACTION >::argument_type = TRANSACTION


template<typename TRANSACTION_CALLBACK >
auto pqxx::perform (const TRANSACTION_CALLBACK &callback, int attempts=3) -> decltype(callback())
 Simple way to execute a transaction with automatic retry. More...
 pqxx::transactor< TRANSACTION >::transactor (const std::string &TName="transactor")
void pqxx::transactor< TRANSACTION >::operator() (TRANSACTION &T)
 Overridable transaction definition; insert your database code here. More...
void pqxx::transactor< TRANSACTION >::on_abort (const char[]) noexcept
 Optional overridable function to be called if transaction is aborted. More...
void pqxx::transactor< TRANSACTION >::on_commit ()
 Optional overridable function to be called after successful commit. More...
void pqxx::transactor< TRANSACTION >::on_doubt () noexcept
 Overridable function to be called when "in doubt" about outcome. More...
std::string pqxx::transactor< TRANSACTION >::name () const
 The transactor's name. More...
template<typename TRANSACTOR >
void pqxx::connection_base::perform (const TRANSACTOR &T, int Attempts)

Detailed Description

Sometimes a transaction can fail for completely transient reasons, such as a conflict with another transaction in SERIALIZABLE isolation. The right way to handle those failures is often just to re-run the transaction from scratch.

For example, your REST API might be handling each HTTP request in its own database transaction, and if this kind of transient failure happens, you simply want to "replay" the whole request, in a fresh transaction.

You won't necessarily want to execute the exact same SQL commands with the exact same data. Some of your SQL statements may depend on state that can vary between retries. So instead of dumbly replaying the SQL, you re-run the same application code that produced those SQL commands.

The transactor framework makes it a little easier for you to do this safely, and avoid typical pitfalls. You encapsulate the work that you want to do into a callable that you pass to the perform function.

Here's how it works. You write your transaction code as a lambda or function, which creates its own transaction object, does its work, and commits at the end. You pass that callback to pqxx::perform, which runs it for you.

If there's a failure inside your callback, there will be an exception. Your transaction object goes out of scope and gets destroyed, so that it aborts implicitly. Seeing this, perform tries running your callback again. It stops doing that when the callback succeeds, or when it has failed too many times, or when there's an error that leaves the database in an unknown state, such as a lost connection just while we're waiting for the database to confirm a commit. It all depends on the type of exception.

The callback takes no arguments. If you're using lambdas, the easy way to pass arguments is for the lambda to "capture" them from your variables. Or, if you're using functions, you may want to use std::bind.

Once your callback succeeds, it can return a result, and perform will return that result back to you.

Typedef Documentation

◆ argument_type

template<typename TRANSACTION = transaction<read_committed>>
using pqxx::transactor< TRANSACTION >::argument_type = TRANSACTION

Function Documentation

◆ name()

template<typename TRANSACTION = transaction<read_committed>>
std::string pqxx::transactor< TRANSACTION >::name ( ) const

The transactor's name.

◆ on_abort()

template<typename TRANSACTION = transaction<read_committed>>
void pqxx::transactor< TRANSACTION >::on_abort ( const char  [])

Optional overridable function to be called if transaction is aborted.

This need not imply complete failure; the transactor will automatically retry the operation a number of times before giving up. on_abort() will be called for each of the failed attempts.

One parameter is passed in by the framework: an error string describing why the transaction failed. This will also be logged to the connection's notice processor.

◆ on_commit()

template<typename TRANSACTION = transaction<read_committed>>
void pqxx::transactor< TRANSACTION >::on_commit ( )

Optional overridable function to be called after successful commit.

If your on_commit() throws an exception, the actual back-end transaction will remain committed, so any changes in the database remain regardless of how this function terminates.

◆ on_doubt()

template<typename TRANSACTION = transaction<read_committed>>
void pqxx::transactor< TRANSACTION >::on_doubt ( )

Overridable function to be called when "in doubt" about outcome.

This may happen if the connection to the backend is lost while attempting to commit. In that case, the backend may have committed the transaction but is unable to confirm this to the frontend; or the transaction may have failed, causing it to be rolled back, but again without acknowledgement to the client program. The best way to deal with this situation is typically to wave red flags in the user's face and ask him to investigate.

The robusttransaction class is intended to reduce the chances of this error occurring, at a certain cost in performance.

See also

◆ operator()()

template<typename TRANSACTION = transaction<read_committed>>
void pqxx::transactor< TRANSACTION >::operator() ( TRANSACTION &  T)

Overridable transaction definition; insert your database code here.

The operation will be retried if the connection to the backend is lost or the operation fails, but not if the connection is broken in such a way as to leave the library in doubt as to whether the operation succeeded. In that case, an in_doubt_error will be thrown.

Recommended practice is to allow this operator to modify only the transactor itself, and the dedicated transaction object it is passed as an argument. This is what makes side effects, retrying etc. controllable in the transactor framework.

TDedicated transaction context created to perform this operation.

Referenced by pqxx::transactor< TRANSACTION >::transactor().

◆ perform() [1/2]

template<typename TRANSACTION_CALLBACK >
auto pqxx::perform ( const TRANSACTION_CALLBACK &  callback,
int  attempts = 3 
) -> decltype(callback())

Simple way to execute a transaction with automatic retry.

Executes your transaction code as a callback. Repeats it until it completes normally, or it throws an error other than the few libpqxx-generated exceptions that the framework understands, or after a given number of failed attempts, or if the transaction ends in an "in-doubt" state.

(An in-doubt state is one where libpqxx cannot determine whether the server finally committed a transaction or not. This can happen if the network connection to the server is lost just while we're waiting for its reply to a "commit" statement. The server may have completed the commit, or not, but it can't tell you because there's no longer a connection.

Using this still takes a bit of care. If your callback makes use of data from the database, you'll probably have to query that data within your callback. If the attempt to perform your callback fails, and the framework tries again, you'll be in a new transaction and the data in the database may have changed under your feet.

Also be careful about changing variables or data structures from within your callback. The run may still fail, and perhaps get run again. The ideal way to do it (in most cases) is to return your result from your callback, and change your program's data state only after perform completes successfully.

callbackTransaction code that can be called with no arguments.
attemptsMaximum number of times to attempt performing callback. Must be greater than zero.
Whatever your callback returns.

Referenced by pqxx::connection_base::perform(), and pqxx::connection_base::supports().

◆ perform() [2/2]

template<typename TRANSACTOR >
void pqxx::connection_base::perform ( const TRANSACTOR &  T,
int  Attempts 
Pre-C++11 transactor function.

This has been superseded by the new transactor framework and pqxx::perform.

Invokes the given transactor, making at most Attempts attempts to perform the encapsulated code. If the code throws any exception other than broken_connection, it will be aborted right away.

TThe transactor to be executed.
AttemptsMaximum number of attempts to be made to execute T.

◆ transactor()

template<typename TRANSACTION = transaction<read_committed>>
pqxx::transactor< TRANSACTION >::transactor ( const std::string &  TName = "transactor< TRANSACTION >")