===============================
Convenient API for Transactions
===============================

:Spec Title: Convenient API for Transactions
:Spec Version: 1.2
:Author: Jeremy Mikola
:Lead: Jeff Yemin
:Advisors: A\. Jesse Jiryu Davis, Kris Brandow, Oleg Pudeyev, Sam Ritter, Tess Avitabile
:Status: Accepted
:Type: Standards
:Minimum Server Version: 4.0
:Last Modified: 2019-04-24

.. contents::

--------

Abstract
========

Reliably committing a transaction in the face of errors can be a complicated
endeavor using the MongoDB 4.0 drivers API.  This specification introduces a
``withTransaction`` method on the ClientSession object that allows application
logic to be executed within a transaction. This method is capable of retrying
either the commit operation or entire transaction as needed (and when the error
permits) to better ensure that the transaction can complete successfully.

META
====

The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
"SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be
interpreted as described in `RFC 2119 <https://www.ietf.org/rfc/rfc2119.txt>`_.

Specification
=============

Terms
-----

.. _callback:

Callback
   A user-defined function that will be passed to the helper method defined in
   this specification. Depending on the implementing language, this may be a
   closure, function pointer, or other callable type.

ClientSession
   Driver object representing a client session, as defined in the
   `Driver Session`_ specification. The name of this object MAY vary across
   drivers.

.. _Driver Session: ../sessions/driver-sessions.rst

MongoClient
   The root object of a driver's API. The name of this object MAY vary across
   drivers.

.. _TransactionOptions:

TransactionOptions
   Options for ``ClientSession.startTransaction``, as defined in the
   `Transactions`_ specification. The structure of these options MAY vary across
   drivers (e.g. dictionary, typed class).

.. _Transactions: ../transactions/transactions.rst

Naming Deviations
-----------------

This specification defines the name for a new ClientSession method,
``withTransaction``. Drivers SHOULD use the defined name but MAY deviate to
comply with their existing conventions. For example, a driver may use
``with_transaction`` instead of ``withTransaction``.

Callback Semantics
------------------

The purpose of the callback is to allow the application to specify some sequence
of operations to be executed within the body of a transaction. In an ideal
scenario, ``withTransaction`` will start a transaction, execute the callback,
and commit the transaction. In the event of error, the commit or entire
transaction may need to be retried and thusly the callback could be invoked
multiple times.

Drivers MUST ensure that the application can access the ClientSession within the
callback, since the ClientSession will be needed to associate operations with
the transaction. Drivers may elect an idiomatic approach to satisfy this
requirement (e.g. require the callback to receive the ClientSession as its first
argument, expect the callback to access the ClientSession from its lexical
scope). Drivers MAY allow the callback to support additional parameters as
needed (e.g. user data parameter, error output parameter). Drivers MAY allow the
callback to return a value to be propagated as the return value of
``withTransaction``.

ClientSession Changes
---------------------

This specification introduces a ``withTransaction`` method on the ClientSession
class:

.. code:: typescript

    interface ClientSession {
        withTransaction(function<any(...)> callback,
                        Optional<TransactionOptions> options,
                        ... /* other arguments as needed */): any

        // other existing members of ClientSession
    }

ClientSession.withTransaction
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This method is responsible for starting a transaction, invoking a callback, and
committing a transaction. The callback is expected to execute one or more
operations with the transaction; however, that is not enforced. The callback is
allowed to execute other operations not associated with the transaction.

Since ``withTransaction`` includes logic to retry transactions and commits,
drivers MUST enforce a 120-second timeout to limit retry behavior and safeguard
applications from long-running (or infinite) retry loops. Drivers SHOULD use a
monotonic clock to determine elapsed time.

If an UnknownTransactionCommitResult error is encountered for a commit, the
driver MUST retry the commit if and only if the error is not MaxTimeMSExpired
and the retry timeout has not been exceeded. Otherwise, the driver MUST NOT
retry the commit and allow ``withTransaction`` to propagate the error to its
caller.

If a TransientTransactionError is encountered at any point, the entire
transaction may be retried. If the retry timeout has not been exceeded, the
driver MUST retry a transaction that fails with an error bearing the
"TransientTransactionError" label. Since retrying the entire transaction will
entail invoking the callback again, drivers MUST document that the callback may
be invoked multiple times (i.e. one additional time per retry attempt) and MUST
document the risk of side effects from using a non-idempotent callback. If the
retry timeout has been exceeded, drivers MUST NOT retry the transaction and
allow ``withTransaction`` to propagate the error to its caller.

If an error bearing neither the UnknownTransactionCommitResult nor the
TransientTransactionError label is encountered at any point, the driver MUST NOT
retry and MUST allow ``withTransaction`` to propagate the error to its caller.

This method MUST receive a `callback`_ as its first parameter.  An optional
`TransactionOptions`_ MUST be provided as its second parameter (with deviations
permitted as outlined in the `CRUD`_ specification). Drivers MAY support other
parameters or options as needed (e.g. user data to pass as a parameter to the
callback).

.. _CRUD: ../crud/crud.rst#deviations

~~~~~~~~~~~~~~~~~~~
Sequence of Actions
~~~~~~~~~~~~~~~~~~~

This method should perform the following sequence of actions:

1. Record the current monotonic time, which will be used to enforce the
   120-second timeout before later retry attempts.

2. Invoke `startTransaction`_ on the session. If TransactionOptions were
   specified in the call to ``withTransaction``, those MUST be used for
   ``startTransaction``. Note that ``ClientSession.defaultTransactionOptions``
   will be used in the absence of any explicit TransactionOptions.

3. If ``startTransaction`` reported an error, propagate that error to the caller
   of ``withTransaction`` and return immediately.

4. Invoke the callback. Drivers MUST ensure that the ClientSession can be
   accessed within the callback (e.g. pass ClientSession as the first parameter,
   rely on lexical scoping). Drivers MAY pass additional parameters as needed
   (e.g. user data solicited by withTransaction).

5. Control returns to ``withTransaction``. Determine the current `state`_ of the
   ClientSession and whether the callback reported an error (e.g. thrown
   exception, error output parameter).

6. If the callback reported an error:

   a. If the ClientSession is in the "starting transaction" or "transaction in
      progress" state, invoke `abortTransaction`_ on the session.

   b. If the callback's error includes a "TransientTransactionError" label and
      the elapsed time of ``withTransaction`` is less than 120 seconds, jump
      back to step two.

   c. If the callback's error includes a "UnknownTransactionCommitResult" label,
      the callback must have manually commited a transaction, propagate the
      callback's error to the caller of ``withTransaction`` and return
      immediately.

   d. Otherwise, propagate the callback's error to the caller of
      ``withTransaction`` and return immediately.

7. If the ClientSession is in the "no transaction", "transaction aborted", or
   "transaction committed" state, assume the callback intentionally aborted or
   committed the transaction and return immediately.

8. Invoke `commitTransaction`_ on the session.

9. If ``commitTransaction`` reported an error:

   a. If the ``commitTransaction`` error includes a
      "UnknownTransactionCommitResult" label and the error is not
      MaxTimeMSExpired and the elapsed time of ``withTransaction`` is less
      than 120 seconds, jump back to step eight. We will trust
      ``commitTransaction`` to apply a majority write concern on
      retry attempts (see:
      `Majority write concern is used when retrying commitTransaction`_).

   b. If the ``commitTransaction`` error includes a "TransientTransactionError"
      label and the elapsed time of ``withTransaction`` is less than 120
      seconds, jump back to step two.

   c. Otherwise, propagate the ``commitTransaction`` error to the caller of
      ``withTransaction`` and return immediately.

10. The transaction was committed successfully. Return immediately.

.. _startTransaction: ../transactions/transactions.rst#starttransaction
.. _state: ../transactions/transactions.rst#clientsession-changes
.. _abortTransaction: ../transactions/transactions.rst#aborttransaction
.. _commitTransaction: ../transactions/transactions.rst#committransaction

~~~~~~~~~~~
Pseudo-code
~~~~~~~~~~~

This method can be expressed by the following pseudo-code:

.. code:: typescript

    withTransaction(callback, options) {
        // Note: drivers SHOULD use a monotonic clock to determine elapsed time
        var startTime = Date.now(); // milliseconds since Unix epoch

        retryTransaction: while (true) {
            this.startTransaction(options); // may throw on error

            try {
                userDefinedFunction(this);
            } catch (error) {
                if (this.transactionState == STARTING ||
                    this.transactionState == IN_PROGRESS) {
                    this.abortTransaction();
                }

                if (error.hasErrorLabel("TransientTransactionError") &&
                    Date.now() - startTime < 120000) {
                    continue retryTransaction;
                }

                throw error;
            }

            if (this.transactionState == NO_TXN ||
                this.transactionState == COMMITTED ||
                this.transactionState == ABORTED) {
                return; // Assume callback intentionally ended the transaction
            }

            retryCommit: while (true) {
                try {
                    /* We will rely on ClientSession.commitTransaction() to
                     * apply a majority write concern if commitTransaction is
                     * being retried (see: DRIVERS-601) */
                    this.commitTransaction();
                } catch (error) {
                    /* Note: a maxTimeMS error will have the MaxTimeMSExpired
                     * code (50) and can be reported as a top-level error or
                     * inside writeConcernError, ie:
                     * {ok:0, code: 50, codeName: "MaxTimeMSExpired"}
                     * {ok:1, writeConcernError: {code: 50, codeName: "MaxTimeMSExpired"}}
                     */
                    if (!isMaxTimeMSExpiredError(error) &&
                        error.hasErrorLabel("UnknownTransactionCommitResult") &&
                        Date.now() - startTime < 120000) {
                        continue retryCommit;
                    }

                    if (error.hasErrorLabel("TransientTransactionError") &&
                        Date.now() - startTime < 120000) {
                        continue retryTransaction;
                    }

                    throw error;
                }
                break; // Commit was successful
            }
            break; // Transaction was successful
        }
    }

ClientSession must provide access to a MongoClient
--------------------------------------------------

The callback invoked by ``withTransaction`` is only guaranteed to receive a
ClientSession parameter. Drivers MUST ensure that it is possible to obtain a
MongoClient within the callback in order to execute operations within the
transaction. Per the `Driver Session`_ specification, ClientSessions should
already provide access to a client object.

Test Plan
=========

See the `README <tests/README.rst>`_ for tests.

Motivation for Change
=====================

Reliably committing a transaction in the face of errors can be a complicated
endeavor using the MongoDB 4.0 drivers API. Providing helper method in the
driver to execute a transaction (and retry when possible) will enable our users
to make better use of transactions in their applications.

Design Rationale
================

This specification introduces a helper method on the ClientSession object that
applications may optionally employ to execute a user-defined function within a
transaction. An application does not need to be modified unless it wants to take
advantage of this helper method.

Majority write concern is used when retrying commitTransaction
--------------------------------------------------------------

Drivers should apply a majority write concern when retrying commitTransaction to
guard against a transaction being applied twice. This requirement was originally
enforced in the implementation of ``withTransaction``, but will now be handled
by the transaction spec itself in order to benefit applications irrespective of
whether they use ``withTransaction`` (see the corresponding section in the
`Transactions spec Design Rationale`_).

.. _Transactions spec Design Rationale: ../transactions/transactions.rst#majority-write-concern-is-used-when-retrying-committransaction

The callback function has a flexible signature
----------------------------------------------

An original design considered requiring the callback to accept a ClientSession
as its first parameter. That could be superfluous for languages where the
callback might already have access to ClientSession through its lexical scope.
Instead, the spec simply requires that drivers ensure the callback will be able
to access the ClientSession. 

Similarly, the specification does not concern itself with the return type of the
callback function. If drivers allow the callback to return a value, they may
also choose to propagate that value as the return value of withTransaction.

An earlier design also considered using the callback's return value to indicate
whether control should break out of ``withTransaction`` (and its retry loop) and
return to the application. The design allows this to be accomplished in one of
two ways:

- The callback aborts the transaction directly and returns to
  ``withTransaction``, which will then return to its caller.

- The callback raises an error without the "TransientTransactionError" label,
  in which case ``withTransaction`` will abort the transaction and return to
  its caller.

Applications are responsible for passing ClientSession for operations within a transaction
------------------------------------------------------------------------------------------

It remains the responsibility of the application to pass a ClientSession to all
operations that should be included in a transaction. With regard to
``withTransaction``, applications are free to execute any operations within the
callback, irrespective of whether those operations are associated with the
transaction.

It is assumed that the callback will not start a new transaction on the ClientSession
-------------------------------------------------------------------------------------

Under normal circumstances, the callback should not commit the transaction nor
should it start a new transaction. The ``withTransaction`` method will inspect
the ClientSession's transaction state after the callback returns and take the
most sensible course of action; however, it will not detect whether the callback
has started a new transaction.

The callback function may be executed multiple times
----------------------------------------------------

The implementation of withTransaction is based on the original examples for
`Retry Transactions and Commit Operation`_ from the MongoDB Manual. As such, the
callback may be executed any number of times. Drivers are free to encourage
their users to design idempotent callbacks.

.. _Retry Transactions and Commit Operation: https://docs.mongodb.com/manual/core/transactions/#retry-transaction-and-commit-operation

The commit is retried after a write concern timeout (i.e. wtimeout) error
-------------------------------------------------------------------------

Per the Transactions specification, drivers internally retry
``commitTransaction`` once if it fails due to a retryable error (as defined in
the `Retryable Writes`_ specification). Beyond that, applications may manually
retry ``commitTransaction`` if it fails with any error bearing the
`UnknownTransactionCommitResult`_ error label. This label is applied for the
the following errors:

.. _Retryable Writes: ../retryable-writes/retryable-writes.rst#terms

.. _UnknownTransactionCommitResult: ../transactions/transactions.rst#unknowntransactioncommitresult

- Server selection failure
- Retryable error (as defined in the `Retryable Writes`_ specification)
- Write concern failure or timeout (excluding UnsatisfiableWriteConcern and
  UnknownReplWriteConcern)
- MaxTimeMSExpired errors, ie ``{ok:0, code: 50, codeName: "MaxTimeMSExpired"}``
  and ``{ok:1, writeConcernError: {code: 50, codeName: "MaxTimeMSExpired"}}``.

A previous design for ``withTransaction`` retried for all of these errors
*except* for write concern timeouts, so as not to exceed the user's original
intention for ``wtimeout``. The current design of this specification no longer
excludes write concern timeouts, and simply retries ``commitTransaction`` within
its timeout period for all errors bearing the "UnknownTransactionCommitResult"
label.

This change was made in light of the forthcoming Client-side Operations Timeout
specification (see: `Future Work`_), which we expect will allow the current
120-second timeout for ``withTransaction`` to be customized and also obviate the
need for users to specify ``wtimeout``.

The commit is not retried after a MaxTimeMSExpired error
--------------------------------------------------------

This specification intentionally chooses not to retry commit operations after a
MaxTimeMSExpired error as doing so would exceed the user's original intention
for ``maxTimeMS``.

The transaction and commit may be retried any number of times within a timeout period
-------------------------------------------------------------------------------------

The implementation of withTransaction is based on the original examples for
`Retry Transactions and Commit Operation`_ from the MongoDB Manual. As such, the
transaction and commit may be continually retried as long as the error label
indicates that retrying is possible.

A previous design had no limits for retrying commits or entire transactions. The
callback is always able indicate that ``withTransaction`` should return to its
caller (without future retry attempts) by aborting the transaction directly;
however, that puts the onus on avoiding very long (or infinite) retry loops on
the application. We expect the most common cause of retry loops will be due to
TransientTransactionErrors caused by write conflicts, as those can occur
regularly in a healthy application, as opposed to
UnknownTransactionCommitResult, which would typically be caused by an election. 

In order to avoid blocking the application with infinite retry loops,
``withTransaction`` will cease retrying invocations of the callback or
commitTransaction if it has exceeded a fixed timeout period of 120 seconds. This
limit is a non-configurable default and is intentionally twice the value of
MongoDB 4.0's default for the `transactionLifetimeLimitSeconds`_ parameter (60
seconds). Applications that desire longer retry periods may call
``withTransaction`` additional times as needed. Applications that desire shorter
retry periods should not use this method.

.. _transactionLifetimeLimitSeconds: https://docs.mongodb.com/manual/reference/parameters/#param.transactionLifetimeLimitSeconds

Backwards Compatibility
=======================

The specification introduces a new method on the ClientSession class and does
not introduce any backward breaking changes. Existing programs that do not make
use of this new method will continue to compile and run correctly.

Reference Implementation
========================

The C, Java, and Ruby drivers will provide reference implementations. The
corresponding tickets for those implementations may be found via
`DRIVERS-556`_.

.. _DRIVERS-556: https://jira.mongodb.org/browse/DRIVERS-556

Security Implication
====================

Applications that use transaction guarantees to enforce security rules will
benefit from a less error-prone API. Adding a helper method to execute a
user-defined function within a transaction has few security implications, as it
only provides an implementation of a technique already described in the MongoDB
4.0 documentation (`DRIVERS-488`_).

.. _DRIVERS-488: https://jira.mongodb.org/browse/DRIVERS-488

Future Work
===========

The forthcoming Client-side Operations Timeout specification (`DRIVERS-555`_)
may allow users to alter the default retry timeout, as a client-side timeout
could be applied to ``withTransaction`` and its retry logic. In the absence of a
client-side operation timeout, withTransaction can continue to use the
120-second default and thus preserve backwards compatibility.

.. _DRIVERS-555: https://jira.mongodb.org/browse/DRIVERS-555

Changes
=======

2019-04-24: withTransaction does not retry when commit fails with
            MaxTimeMSExpired.

2018-02-13: withTransaction should retry commits after a wtimeout
