[#MDEV-19986] MyRocks: Range Locking: SeekForUpdate support

[MDEV-19986] MyRocks: Range Locking: SeekForUpdate support Created: 2019-07-08 Updated: 2023-04-16
Status:	Open
Project:	MariaDB Server
Component/s:	Storage Engine - RocksDB
Fix Version/s:	N/A

Type:

Task

Priority:

Major

Reporter:

Sergei Petrunia

Assignee:

Sergei Petrunia

Resolution:

Unresolved

Votes:

Labels:

None

Issue Links:

PartOf
is part of	MDEV-15603	Gap Lock support in MyRocks	Stalled
Relates
relates to	MDEV-11481	MariaRocks port: rocksdb.select_for_u...	Open
relates to	~~MDEV-13115~~	Implement SELECT [FOR UPDATE\|LOCK IN ...	Closed
relates to	MDEV-25338	add SKIP LOCKED to UPDATE syntax	Open

Description

This is to support "Seek For Update" functionality in Range Locking feature for RocksDB.

Initial input

SeekForUpdate(prefix_X) -> should try to lock range from prefix_X to X

Prototype with

  Seek(prefix_X) -> X;

  TryRangeLock(prefix_X, X);

  Update snapshot (by destroying/recreating iterator?)

  Z = Seek(X) (optimizations, only look through memtable);

  if (Z > Y)

    return SeekForUpdate(X);

  else

    return Z;

criteria when to use SeekForUpdate
do we need to support shared locks in the first milestone? (TODO: check they will "happen to work"?)

Comments

Comment by Sergei Petrunia [ 2019-07-08 ]

Class Transaction has a new member function w/o implementation

class Transaction {

...

  virtual Iterator *GetLockingIterator() {return nullptr; }

PessimisticTransaction implements it :

class PessimisticTransaction {

...

  Iterator* GetLockingIterator() override;

GetLockingIterator returns an object that will implement all iterator methods (not all methods are implemented yet):

//

// LockingIterator is an iterator that locks the rows before returning, as well

// as the gaps between the returned rows.

//

//  Example:

//    lock_iter= trx->GetLockingIterator();

//    lock_iter->Seek('abc');

//    lock_iter->Valid()==true && lock_iter->key() == 'bcd';

//

//   After the above, the returned record 'bcd' is locked by transaction trx.

//   Also, the range between ['abc'..'bcd'] is guaranteed to have no records

//   and be locked by trx.

//

//    lock_iter->Next();

//    lock_iter->Valid()==true && lock_iter->key() == 'efg'

//

//   Now, the range ['bcd'.. 'efg'] (bounds incluive) is also locked, and the

//   only records in it are 'bcd' and 'efg'.

//

class LockingIterator : public Iterator {

...

};

Comment by Sergei Petrunia [ 2019-07-08 ]

At the moment, MyRocks will use SeekForUpdate when the scanned range has no other bound.

Example 1: this uses range access:

select * from t1 where pk >=700 limit 10 for update;

Example 2: this uses index access:

select * from t1 order by pk limit 10 for update;

(Forward scans pass a basic test. Backward scans should be doable but I didn't have time to implement them, yet)

TODO: is this a suitable criterion?

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