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Re: Improving the Engine API (was Re: New PBXT Drizzle-specific storage engine...)

 

Paul McCullagh wrote:
Hi Toru,

On Dec 7, 2009, at 3:31 AM, Toru Maesaka wrote:

Great to hear another use-case where knowing a statement type in
advance is useful :)

Yes, generally I need to know the following:

- If I have a update type statement (i.e. whether the statement modifies rows).
- Whether I need a table lock (examples: ALTER TABLE, TRUNCATE, CHECK).

But, Paul, doesn't this depend on the engine itself?  I mean, some
engines can do (some types of) ALTER TABLE without taking a table lock.
 So, is this request really for whether the kernel thinks a table-level
lock is necessary, or is it really just for a descriptor of the
statement type?

And, if it really does just boil down to the statement type, then how do
we deal with the reality that Brian speaks about -- that statement type
will be pluggable, and how do we deal with future statement types for
pluggable engines?

Is a reasonable solution to pass to engines a sort of "statement
traits"?  So, instead of passing ALTER_TABLE, CREATE_TABLE, UPDATE,
DELETE, etc, we instead pass a std::bitset<> (or uint64_t for C folks)
containing traits of the statement such as:

MODIFIES_DATA
MODIFIES_DEFINITION
etc, etc

And then to deal with transaction locking concerns, just add a method to Cursor:

void Cursor::setTransactionIsolationLevel(enum enum_tx_isolation);

Cheers!

Jay

- If we have a SELECT FOR UPDATE.

I was talking to Toru about this, and another possibility is that we have statements declare a needed "lock type" that any plugin could then query. I outlined the solution for Toru, but I don't know if he has written the patch yet :)

I've taken notes from our discussion the other day. I'm planning on
working on it when I finish testing through my current progress of
BlitzDB.

Great! :)

For now, I'm happy with Jay's advise of using
current_session().

Cheers,
Toru

On Sat, Dec 5, 2009 at 5:59 AM, Brian Aker <brian@xxxxxxxxxxx> wrote:
Hi!

On Dec 4, 2009, at 3:12 AM, Paul McCullagh wrote:

If we have a startStatement() call, then it could be used in place of beginAlter(), assuming we can determine the statement type, and the tables involved.

The problem with relying on statement type is that at some point statement type will be pluggable... which means you would constantly need to update your engine for new statements.

Yuck!

I was talking to Toru about this, and another possibility is that we have statements declare a needed "lock type" that any plugin could then query. I outlined the solution for Toru, but I don't know if he has written the patch yet :)


Then, when a handle is returned to the pool it is deleted, instead of adding it back to the pool.

BTW very soon engines will own their Cursor objects and will be free to reuse them.

The locking thread waits until all handles are returned and deleted before it can proceed. The lock on the pool then prevents a new table handle from being created while the locking thread is busy. Either way, it would be good if Drizzle closes all handlers/cursors before a table is deleted or renamed.

I would say that long term this will be optional, based on what the engine requires.

OK, this make things a lot simpler! Indeed, if we don't need to support LOCK TABLE then external_lock() can be removed altogether.

Tried removing the external_lock() right now and seeing if any issues pop up?

Cheers,
       -Brian



--
Paul McCullagh
PrimeBase Technologies
www.primebase.org
www.blobstreaming.org
pbxt.blogspot.com








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