The MVCC read view manager. More...

#include <read0mvcc_interface.h>

Inheritance diagram for MVCC_interface:

Public Member Functions
virtual void	initialize (trx_id_t max_assigned_trx_id, trx_ids_t active_ids)=0
	Initializes the MVCC once the max_committed_trx_id estimate is learned from reading system tablespace's header TRX_SYS_TRX_ID_STORE field. More...

virtual	~MVCC_interface ()=default
	Destructor. More...

virtual void	view_open (Read_view_interface &view, trx_t trx)=0
	If view is nullptr then allocates a view, otherwise, reuses the one provided, in both cases opening it. More...

virtual void	view_close (Read_view_interface *&view, bool own_mutex)=0
	Closes a view previously opened by view_open(..). More...

virtual void	view_free (Read_view_interface *&view)=0
	Makes sure the view is properly freed: if view is already nullptr does nothing, otherwise it must be already closed, and this function will take trx_sys->mutex, free the view, and set it to nullptr. More...

virtual void	clone_oldest_view (Read_view_interface *&view)=0
	Clones the oldest open view or if there is no open view at the moment, clones one which would be created if view_open(..) was called. More...

virtual void	undo_purge_is_starting ()=0
	Instructs the MVCC that the Undo Purge is about to start working and MVCC will be asked to clone the oldest Read View. More...

virtual void	undo_purge_has_shutdown ()=0
	Instructs the MVCC that the Undo Purge was shutdown and will not be asking to clone the oldest Read View or any other MVCC anymore. More...

virtual size_t	get_open_views_count () const =0
	Returns the number of open views. More...

virtual bool	is_view_open (const Read_view_interface *view) const =0
	Can be used only for nullptr or view assigned by view_open(view,..) or view_close(view,..) or clone_oldest_view(view,..). More...

virtual void	set_view_creator_trx_id (Read_view_interface *&view, trx_id_t id)=0
	Sets the view creator transaction id, when a transaction which previously called view_open(view,..) when it was read-only (and thus had trx->id==0), has transitioned to a RW state and got a trx->id assigned. More...

Detailed Description

The MVCC read view manager.

A read view object can be in "open" or "closed" state. The "open" state is meant to mean that a transaction is using it at the moment. The "closed" state is meant to mean that it is not currently used by any transaction, but wasn't freed either, for performance reasons - it can be cheaply reopened by using view_open(view,..) on it again. A read view obtained by view_open(view,..) will be "open". It can then be closed by view_close(view,..) in which case it might also get freed. If it wasn't freed, then view will still be not nullptr, but is_view_open(view) will return false. You should free such "closed" views with view_free(view). All "open" read-views are tracked by MVCC_interface instance, and the oldest of them can be cloned using clone_oldest_view(), but the clone itself will be "closed". A cloned view is special in that it can't be opened with view_open(..) and should not be passed to is_view_open(..) - it's implicitly guaranteed to be always closed, so no need to check. The ownership of the view is passed to the caller of view_open(view,..) or clone_oldest_view(view,..), and the view remains in the caller ownership as long as the pointer is not-null. The view_close(view,...) should be called for views obtained with view_open(..) and it may set view to nullptr if it has chosen to free the object. Closed read views, such that those resulting from calling view_close(view,..) or clone_oldest_view(view,..), should be freed with view_free(view,..) which always sets view to nullptr.

So, a typical usage would be something like:

trx->read_view = nullptr;
trx_sys->mvcc->view_open(trx->read_view, trx);
ut_a(trx->read_view != nullptr);
ut_a(trx_sys->mvcc->is_view_open(trx->read_view));

...use the view...

trx_sys->mvcc->view_close(trx->read_view, ...);

trx->read_view will now be closed, perhaps even nullptr ut_a(!trx_sys->mvcc->is_view_open(trx->read_view));

When you are sure trx will never reopen trx->read_view say, because you're going to free the trx object itself, then make sure to free the read view, after closing it. (You don't have to do that if trx->read_view is already nullptr, but it wouldn't hurt). trx_sys->view_free(trx->read_view); ut_a(trx->read_view == nullptr);

And usage in purge is for example:

purge_sys->view = nullptr;
trx_sys->mvcc->clone_oldest_view(purge_sys->view);
ut_a(purge_sys->view != nullptr);

...use the view...

trx_sys->mvcc->view_free(purge_sys->view);
ut_a(purge_sys->view == nullptr);

Constructor & Destructor Documentation

◆ ~MVCC_interface()

virtual MVCC_interface::~MVCC_interface ( )

virtualdefault

Destructor.

Member Function Documentation

◆ clone_oldest_view()

virtual void MVCC_interface::clone_oldest_view ( Read_view_interface *& view )

pure virtual

Clones the oldest open view or if there is no open view at the moment, clones one which would be created if view_open(..) was called.

The view provided by a caller must be either nullptr or a closed view obtained from an earlier call to clone_oldest_view(..). This function will either allocate a new view or clone into view provided. After the call view will be considered closed. It must be freed by the caller using view_free(view) when no longer needed.

Parameters

[in,out] view A pointer to a closed view obtained from an earlier call to this method or a nullptr. It must not be a result of view_close(..) or view_open(..). Upon return it will point to a closed view which is a clone of the oldest open read view found during the call. It should be freed with view_free(..). Do not call view_open(..) on it.

Implemented in MVCC.

◆ get_open_views_count()

virtual size_t MVCC_interface::get_open_views_count ( ) const

pure virtual

Returns the number of open views.

Implemented in MVCC.

◆ initialize()

virtual void MVCC_interface::initialize	(	trx_id_t	max_assigned_trx_id,
		trx_ids_t	active_ids
	)

pure virtual

Initializes the MVCC once the max_committed_trx_id estimate is learned from reading system tablespace's header TRX_SYS_TRX_ID_STORE field.

Parameters

[in]	max_assigned_trx_id	The upper-bound on highest assigned trx id. No record in any B-tree can currently have DB_TRX_ID larger than this. No Undo Log can have TRX_UNDO_TRX_NO or TRX_UNDO_TRX_ID larger than this.
[in]	active_ids	The set of ids of currently active transactions. They should all be at <= max_committed_trx_id.

Implemented in MVCC.

◆ is_view_open()

virtual bool MVCC_interface::is_view_open ( const Read_view_interface * view ) const

pure virtual

Can be used only for nullptr or view assigned by view_open(view,..) or view_close(view,..) or clone_oldest_view(view,..).

(note, though that clone_oldest_view always gives you a closed view, so no need to check).

Parameters

[in] view The view to check. Can be nullptr.

Returns: true if the view is not nullptr and is open

Implemented in MVCC.

◆ set_view_creator_trx_id()

virtual void MVCC_interface::set_view_creator_trx_id	(	Read_view_interface *&	view,
		trx_id_t	id
	)

pure virtual

Sets the view creator transaction id, when a transaction which previously called view_open(view,..) when it was read-only (and thus had trx->id==0), has transitioned to a RW state and got a trx->id assigned.

One can imagine an implementation which could change the view instance to another (say: from one shared with other read-only transactions to one tailored to this particular RW transaction, which should see its own writes).

Parameters

[in,out]	view	Set the creator trx id for this view. The view should be already open first by view_open(view, trx) when trx->id was 0.
[in]	id	Transaction id to set, should be the trx->id assigned to the same trx for which view_open(view, trx) was called earlier.

Implemented in MVCC.

◆ undo_purge_has_shutdown()

virtual void MVCC_interface::undo_purge_has_shutdown ( )

pure virtual

Instructs the MVCC that the Undo Purge was shutdown and will not be asking to clone the oldest Read View or any other MVCC anymore.

To be called before trx_sys is being destroyed.

Implemented in MVCC.

◆ undo_purge_is_starting()

virtual void MVCC_interface::undo_purge_is_starting ( )

pure virtual

Instructs the MVCC that the Undo Purge is about to start working and MVCC will be asked to clone the oldest Read View.

Implemented in MVCC.

◆ view_close()

virtual void MVCC_interface::view_close	(	Read_view_interface *&	view,
		bool	own_mutex
	)

pure virtual

Closes a view previously opened by view_open(..).

It's safe to call it on a view which was already closed. After the call, it is unsafe to access view's members, because it might be freed by this function (in which case view is nullptr). Even if the view is still not null after the call, it is at least in the closed state - so can be reopen again with view_open(..) or freed with view_free(..).

Parameters

[in,out]	view	Must not be nullptr. Must be a pointer obtained from view_open(..) or view_close(..). Upon return it will be either nullptr or pointing to a closed view.
[in]	own_mutex	true if the caller owns the trx_sys->mutex

Implemented in MVCC.

◆ view_free()

virtual void MVCC_interface::view_free ( Read_view_interface *& view )

pure virtual

Makes sure the view is properly freed: if view is already nullptr does nothing, otherwise it must be already closed, and this function will take trx_sys->mutex, free the view, and set it to nullptr.

Parameters

[in,out] view nullptr or a closed view. Upon return it will be nullptr.

Implemented in MVCC.

◆ view_open()

virtual void MVCC_interface::view_open	(	Read_view_interface *&	view,
		trx_t *	trx
	)

pure virtual

If view is nullptr then allocates a view, otherwise, reuses the one provided, in both cases opening it.

That is, after the call view is not null, view members can be accessed, and view->is_closed() == false. It is guaranteed that clone_oldest_view(v2) calls which happen-before the call to view_close(view,...) for this view, will clone a v2 which is not fresher than this view.

Parameters

[in,out]	view	Must be either nullptr or a result of view_close(..), which in turn should be a result of an earlier view_open(..). It must not be a result of clone_oldest_view(..). Upon return it will be not null and open. Must be closed by calling view_close(..). If it is still not nullptr after view_close(..) it must be passed to view_free(..) to free it.
[in]	trx	Transaction instance of caller

Implemented in MVCC.

The documentation for this class was generated from the following file:

storage/innobase/include/read0mvcc_interface.h

Public Member Functions

Detailed Description

Constructor & Destructor Documentation

◆ ~MVCC_interface()

Member Function Documentation

◆ clone_oldest_view()

◆ get_open_views_count()

◆ initialize()

◆ is_view_open()

◆ set_view_creator_trx_id()

◆ undo_purge_has_shutdown()

◆ undo_purge_is_starting()

◆ view_close()

◆ view_free()

◆ view_open()