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From Seliverstov Igor <gvvinbl...@gmail.com>
Subject Re: Historical rebalance
Date Thu, 29 Nov 2018 23:01:48 GMT
Vladimir,

Look at my example:

One active transaction (Tx1 which does opX ops) while another tx (Tx2 which
does opX' ops) is finishes with uc4:

----uc1--op1----op2---uc2--op1'----uc3--uc4---op3------------X-------------
Node1



----uc1----op1----uc2----op2----uc3--op3----------uc4----cp1----     Tx1 -
                            ^         |                  |
                               |
                             ------------------------
                            | -Node2
                                                          ^------
                               |
                                                                  |
                                 |
----uc1-------------uc2-------------uc3--------op1'----uc4----cp1----
Tx2 -


state on Node2: tx1 -> op3 -> uc2
                          cp1 [current=uc4, backpointer=uc2]

Here op2 was acknowledged by op3, op3 was applied before op1' (linearized
by WAL).

All nodes having uc4 must have op1' because uc4 cannot be get earlier than
prepare stage while prepare stage happens after all updates so *op1'
happens before uc4* regardless Tx2 was committed or rolled back.

This means *op2 happens before uc4* (uc4 cannot be earlier op2 on any node
because on Node2 op2 was already finished (acknowledged by op3) when op1'
happens)

That was my idea which easy to proof.

You used a different approach, but yes, It has to work.

чт, 29 нояб. 2018 г. в 22:19, Vladimir Ozerov <vozerov@gridgain.com>:

> "If more recent WAL records will contain *ALL* updates of the transaction"
> -> "More recent WAL records will contain *ALL* updates of the transaction"
>
> On Thu, Nov 29, 2018 at 10:15 PM Vladimir Ozerov <vozerov@gridgain.com>
> wrote:
>
> > Igor,
> >
> > Yes, I tried to draw different configurations, and it really seems to
> > work, despite of being very hard to proof due to non-inituitive HB edges.
> > So let me try to spell the algorithm once again to make sure that we are
> on
> > the same page here.
> >
> > 1) There are two nodes - primary (P) and backup (B)
> > 2) There are three type of events: small transactions which possibly
> > increments update counter (ucX), one long active transaction which is
> split
> > into multiple operations (opX), and checkpoints (cpX)
> > 3) Every node always has current update counter. When transaction commits
> > it may or may not shift this counter further depending on whether there
> are
> > holes behind. But we have a strict rule that it always grow. Higher
> > coutners synchrnoizes with smaller. Possible cases:
> > ----uc1----uc2----uc3----
> > ----uc1--------uc3------- // uc2 missing due to reorder, but is is ok
> >
> > 4) Operations within a single transaction is always applied sequentially,
> > and hence also have HB edge:
> > ----op1----op2----op3----
> >
> > 5) When transaction operation happens, we save in memory current update
> > counter available at this moment. I.e. we have a map from transaction ID
> to
> > update counter which was relevant by the time last *completed* operation
> > *started*. This is very important thing - we remember the counter when
> > operation starts, but update the map only when it finishes. This is
> needed
> > for situation when update counter is bumber in the middle of a long
> > operation.
> > ----uc1----op1----op2----uc2----uc3----op3----
> >             |      |                    |
> >            uc1    uc1                  uc3
> >
> > state: tx1 -> op3 -> uc3
> >
> > 6) Whenever checkpoint occurs, we save two counters with: "current" and
> > "backpointer". The latter is the smallest update counter associated with
> > active transactions. If there are no active transactions, current update
> > counter is used.
> >
> > Example 1: no active transactions.
> > ----uc1----cp1----
> >      ^      |
> >      --------
> >
> > state: cp1 [current=uc1, backpointer=uc1]
> >
> > Example 2: one active transaction:
> >                                  ---------------
> >                                  |             |
> > ----uc1----op1----uc2----op2----op3----uc3----cp1----
> >                    ^             |
> >                    --------------
> >
> > state: tx1 -> op3 -> uc2
> >        cp1 [current=uc3, backpointer=uc2]
> >
> > 7) Historical rebalance:
> > 7.1) Demander finds latest checkpoint, get it's backpointer and sends it
> > to supplier.
> > 7.2) Supplier finds earliest checkpoint where [supplier(current) <=
> > demander(backpointer)]
> > 7.3) Supplier reads checkpoint backpointer and finds associated WAL
> > record. This is where we start.
> >
> > So in terms of WAL we have: supplier[uc_backpointer <- cp(uc_current <=
> > demanter_uc_backpointer)] <- demander[uc_backpointer <- cp(last)]
> >
> > Now the most important - why it works :-)
> > 1) Transaction opeartions are sequential, so at the time of crash nodes
> > are *at most one operation ahead *each other
> > 2) Demander goes to the past and finds update counter which was current
> at
> > the time of last TX completed operation
> > 3) Supplier goes to the closest checkpoint in the past where this update
> > counter either doesn't exist or just appeared
> > 4) Transaction cannot be committed on supplier at this checkpoint, as it
> > would violate UC happens-before rule
> > 5) Tranasction may have not started yet on supplier at this point. If
> more
> > recent WAL records will contain *ALL* updates of the transaction
> > 6) Transaction may exist on supplier at this checkpoint. Thanks to p.1 we
> > must skip at most one operation. Jump back through supplier's checkpoint
> > backpointer is guaranteed to do this.
> >
> > Igor, do we have the same understanding here?
> >
> > Vladimir.
> >
> > On Thu, Nov 29, 2018 at 2:47 PM Seliverstov Igor <gvvinblade@gmail.com>
> > wrote:
> >
> >> Ivan,
> >>
> >> different transactions may be applied in different order on backup
> nodes.
> >> That's why we need an active tx set
> >> and some sorting by their update times. The idea is to identify a point
> in
> >> time which starting from we may lost some updates.
> >> This point:
> >>    1) is the last acknowledged by all backups (including possible
> further
> >> demander) update on timeline;
> >>    2) have a specific update counter (aka back-counter) which we going
> to
> >> start iteration from.
> >>
> >> After additional thinking on, I've identified a rule:
> >>
> >> There is two fences:
> >>   1) update counter (UC) - this means that all updates, with less UC
> than
> >> applied one, was applied on a node, having this UC.
> >>   2) update in scope of TX - all updates are applied one by one
> >> sequentially, this means that the fact of update guaranties the previous
> >> update (statement) was finished on all TX participants.
> >>
> >> Сombining them, we can say the next:
> >>
> >> All updates, that was acknowledged at the time the last update of tx,
> >> which
> >> updated UC, applied, are guaranteed to be presented on a node having
> such
> >> UC
> >>
> >> We can use this rule to find an iterator start pointer.
> >>
> >> ср, 28 нояб. 2018 г. в 20:26, Павлухин Иван <vololo100@gmail.com>:
> >>
> >> > Guys,
> >> >
> >> > Another one idea. We can introduce additional update counter which is
> >> > incremented by MVCC transactions right after executing operation (like
> >> > is done for classic transactions). And we can use that counter for
> >> > searching needed WAL records. Can it did the trick?
> >> >
> >> > P.S. Mentally I am trying to separate facilities providing
> >> > transactions and durability. And it seems to me that those facilities
> >> > are in different dimensions.
> >> > ср, 28 нояб. 2018 г. в 16:26, Павлухин Иван <vololo100@gmail.com>:
> >> > >
> >> > > Sorry, if it was stated that a SINGLE transaction updates are
> applied
> >> > > in a same order on all replicas then I have no questions so far. I
> >> > > thought about reordering updates coming from different transactions.
> >> > > > I have not got why we can assume that reordering is not possible.
> >> What
> >> > > have I missed?
> >> > > ср, 28 нояб. 2018 г. в 13:26, Павлухин Иван <vololo100@gmail.com>:
> >> > > >
> >> > > > Hi,
> >> > > >
> >> > > > Regarding Vladimir's new idea.
> >> > > > > We assume that transaction can be represented as a set of
> >> > independent operations, which are applied in the same order on both
> >> primary
> >> > and backup nodes.
> >> > > > I have not got why we can assume that reordering is not possible.
> >> What
> >> > > > have I missed?
> >> > > > вт, 27 нояб. 2018 г. в 14:42, Seliverstov Igor <
> >> gvvinblade@gmail.com>:
> >> > > > >
> >> > > > > Vladimir,
> >> > > > >
> >> > > > > I think I got your point,
> >> > > > >
> >> > > > > It should work if we do the next:
> >> > > > > introduce two structures: active list (txs) and candidate
list
> >> > (updCntr ->
> >> > > > > txn pairs)
> >> > > > >
> >> > > > > Track active txs, mapping them to actual update counter
at
> update
> >> > time.
> >> > > > > On each next update put update counter, associated with
previous
> >> > update,
> >> > > > > into a candidates list possibly overwrite existing value
> (checking
> >> > txn)
> >> > > > > On tx finish remove tx from active list only if appropriate
> update
> >> > counter
> >> > > > > (associated with finished tx) is applied.
> >> > > > > On update counter update set the minimal update counter
from the
> >> > candidates
> >> > > > > list as a back-counter, clear the candidate list and remove
an
> >> > associated
> >> > > > > tx from the active list if present.
> >> > > > > Use back-counter instead of actual update counter in demand
> >> message.
> >> > > > >
> >> > > > > вт, 27 нояб. 2018 г. в 12:56, Seliverstov Igor <
> >> gvvinblade@gmail.com
> >> > >:
> >> > > > >
> >> > > > > > Ivan,
> >> > > > > >
> >> > > > > > 1) The list is saved on each checkpoint, wholly (all
> >> transactions
> >> > in
> >> > > > > > active state at checkpoint begin).
> >> > > > > > We need whole the list to get oldest transaction because
after
> >> > > > > > the previous oldest tx finishes, we need to get the
following
> >> one.
> >> > > > > >
> >> > > > > > 2) I guess there is a description of how persistent
storage
> >> works
> >> > and how
> >> > > > > > it restores [1]
> >> > > > > >
> >> > > > > > Vladimir,
> >> > > > > >
> >> > > > > > the whole list of what we going to store on checkpoint
> >> (updated):
> >> > > > > > 1) Partition counter low watermark (LWM)
> >> > > > > > 2) WAL pointer of earliest active transaction write
to
> partition
> >> > at the
> >> > > > > > time the checkpoint have started
> >> > > > > > 3) List of prepared txs with acquired partition counters
> (which
> >> > were
> >> > > > > > acquired but not applied yet)
> >> > > > > >
> >> > > > > > This way we don't need any additional info in demand
message.
> >> > Start point
> >> > > > > > can be easily determined using stored WAL "back-pointer".
> >> > > > > >
> >> > > > > > [1]
> >> > > > > >
> >> >
> >>
> https://cwiki.apache.org/confluence/display/IGNITE/Ignite+Persistent+Store+-+under+the+hood#IgnitePersistentStore-underthehood-LocalRecoveryProcess
> >> > > > > >
> >> > > > > >
> >> > > > > > вт, 27 нояб. 2018 г. в 11:19, Vladimir Ozerov
<
> >> > vozerov@gridgain.com>:
> >> > > > > >
> >> > > > > >> Igor,
> >> > > > > >>
> >> > > > > >> Could you please elaborate - what is the whole
set of
> >> information
> >> > we are
> >> > > > > >> going to save at checkpoint time? From what I understand
this
> >> > should be:
> >> > > > > >> 1) List of active transactions with WAL pointers
of their
> first
> >> > writes
> >> > > > > >> 2) List of prepared transactions with their update
counters
> >> > > > > >> 3) Partition counter low watermark (LWM) - the
smallest
> >> partition
> >> > counter
> >> > > > > >> before which there are no prepared transactions.
> >> > > > > >>
> >> > > > > >> And the we send to supplier node a message: "Give
me all
> >> updates
> >> > starting
> >> > > > > >> from that LWM plus data for that transactions which
were
> active
> >> > when I
> >> > > > > >> failed".
> >> > > > > >>
> >> > > > > >> Am I right?
> >> > > > > >>
> >> > > > > >> On Fri, Nov 23, 2018 at 11:22 AM Seliverstov Igor
<
> >> > gvvinblade@gmail.com>
> >> > > > > >> wrote:
> >> > > > > >>
> >> > > > > >> > Hi Igniters,
> >> > > > > >> >
> >> > > > > >> > Currently I’m working on possible approaches
how to
> implement
> >> > historical
> >> > > > > >> > rebalance (delta rebalance using WAL iterator)
over MVCC
> >> caches.
> >> > > > > >> >
> >> > > > > >> > The main difficulty is that MVCC writes changes
on tx
> active
> >> > phase while
> >> > > > > >> > partition update version, aka update counter,
is being
> >> applied
> >> > on tx
> >> > > > > >> > finish. This means we cannot start iteration
over WAL right
> >> > from the
> >> > > > > >> > pointer where the update counter updated,
but should
> include
> >> > updates,
> >> > > > > >> which
> >> > > > > >> > the transaction that updated the counter did.
> >> > > > > >> >
> >> > > > > >> > These updates may be much earlier than the
point where the
> >> > update
> >> > > > > >> counter
> >> > > > > >> > was updated, so we have to be able to identify
the point
> >> where
> >> > the first
> >> > > > > >> > update happened.
> >> > > > > >> >
> >> > > > > >> > The proposed approach includes:
> >> > > > > >> >
> >> > > > > >> > 1) preserve list of active txs, sorted by
the time of their
> >> > first update
> >> > > > > >> > (using WAL ptr of first WAL record in tx)
> >> > > > > >> >
> >> > > > > >> > 2) persist this list on each checkpoint (together
with
> TxLog
> >> for
> >> > > > > >> example)
> >> > > > > >> >
> >> > > > > >> > 4) send whole active tx list (transactions
which were in
> >> active
> >> > state at
> >> > > > > >> > the time the node was crushed, empty list
in case of
> graceful
> >> > node
> >> > > > > >> stop) as
> >> > > > > >> > a part of partition demand message.
> >> > > > > >> >
> >> > > > > >> > 4) find a checkpoint where the earliest tx
exists in
> >> persisted
> >> > txs and
> >> > > > > >> use
> >> > > > > >> > saved WAL ptr as a start point or apply current
approach in
> >> > case the
> >> > > > > >> active
> >> > > > > >> > tx list (sent on previous step) is empty
> >> > > > > >> >
> >> > > > > >> > 5) start iteration.
> >> > > > > >> >
> >> > > > > >> > Your thoughts?
> >> > > > > >> >
> >> > > > > >> > Regards,
> >> > > > > >> > Igor
> >> > > > > >>
> >> > > > > >
> >> > > >
> >> > > >
> >> > > >
> >> > > > --
> >> > > > Best regards,
> >> > > > Ivan Pavlukhin
> >> > >
> >> > >
> >> > >
> >> > > --
> >> > > Best regards,
> >> > > Ivan Pavlukhin
> >> >
> >> >
> >> >
> >> > --
> >> > Best regards,
> >> > Ivan Pavlukhin
> >> >
> >>
> >
>

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