[infinispan-dev] L1 Consistency with Sync Caches

[Dan Berindei]

On Fri, Jun 28, 2013 at 4:39 PM, William Burns <mudokonman at gmail.com> wrote:

> On Fri, Jun 28, 2013 at 5:14 AM, Dan Berindei <dan.berindei at gmail.com>
> wrote:
> >
> > On Fri, Jun 28, 2013 at 12:17 AM, William Burns <mudokonman at gmail.com>
> > wrote:
> >>
> >> Trying to leave my points that would most likely have responses to
> >> second email so we can try to get back to a single thread :)
> >>
> >
> > No such luck :)
> >
> > Sorry for sending 2 replies in the first place, but it seemed more
> natural -
> > I meant to comment on your proposal in one email and to describe my
> > alternative proposal in the second email.
> >
> >>
> >> On Thu, Jun 27, 2013 at 4:12 PM, Dan Berindei <dan.berindei at gmail.com>
> >> wrote:
> >> >
> >> > On Thu, Jun 27, 2013 at 4:18 PM, William Burns <mudokonman at gmail.com>
> >> > wrote:
> >> >>
> >> >> First off I apologize for the length.
> >> >>
> >> >> There have been a few Jiras recently that have identified L1
> >> >> consistency
> >> >> issues with both TX and non TX sync caches.  Async caches with L1
> have
> >> >> their
> >> >> own issues as well, but I only wanted to talk about sync caches.
> >> >>
> >> >> https://issues.jboss.org/browse/ISPN-3197
> >> >> https://issues.jboss.org/browse/ISPN-2965
> >> >> https://issues.jboss.org/browse/ISPN-2990
> >> >>
> >> >> I have proposed a solution in
> >> >> https://github.com/infinispan/infinispan/pull/1922 which should
> start
> >> >> L1
> >> >> consistency down the right track.  There are quite a few comments on
> it
> >> >> if
> >> >> you want to look into it more, but because of that I am moving this
> to
> >> >> the
> >> >> dev mailing list.
> >> >>
> >> >> The key changes in the PR are the following (non-tx):
> >> >>
> >> >> 1. Concurrent reads for a key that can retrieve a remote value are
> >> >> "corralled" into a single thread of execution for that given key.
>  This
> >> >> would reduce network traffic with concurrent gets for the same key.
> >> >> Note
> >> >> the "corralling" only happens on a per key basis.
> >> >
> >> >
> >> > Get commands on owners should not be serialized. Get commands on
> >> > non-owners
> >> > should not be serialized either, if the key already exists in L1. So
> I'd
> >> > say
> >> > L1ReadSynchronizer should be L1WriteSynchronizer instead :)
> >>
> >> You are suggesting to check the context to see if the key is present
> >> before attempting the synchronizer right?  Reading your second email
> >> that seems the case :)
> >>
> >
> > Nope, I meant we should check the data container (aka the L1 cache). But
> > obviously we have to check the invocation context first in a tx cache, if
> > the tx read the key before it should see the same value.
> >
> I was thinking because the non-tx wraps the value if it is in the data
> container before hand.  But actually for non-tx it seems I should
> check the data container only and tx I should only check the ctx (to
> guarantee read consistency)
>

For tx you probably have to check the context first, and the data container
only if you don't find the entry in the context (non-existent entries that
were previously read in the tx should have a NullMarkerEntry in the
context).

>>
> >> >
> >> >>
> >> >> 2. The single thread that is doing the remote get would update the L1
> >> >> if
> >> >> able (without locking) and make available the value to all the
> requests
> >> >> waiting on the get.
> >> >
> >> >
> >> > Well, L1ReadSynchronizer does prevent other threads from modifying the
> >> > same
> >> > key, so we are locking the key - just not using LockManager.
> >> > It would also require StateTransferLock.acquireSharedTopologyLock() to
> >> > make
> >> > sure it doesn't write an L1 entry after the node became a proper
> owner.
> >>
> >> Agree, when I was saying locking I was meaning through the use of the
> >> lock manager.
> >>
> >> >
> >> >>
> >> >> 3. Invalidations that are received would first check to see if there
> is
> >> >> a
> >> >> current remote get occurring for it's keys.  If there is it will
> >> >> attempt to
> >> >> cancel the L1 write(s) before it occurs.  If it cannot cancel the L1
> >> >> write,
> >> >> then it must also wait on the current remote get completion and
> >> >> subsequently
> >> >> run the invalidation.  Note the cancellation would fail when the
> remote
> >> >> get
> >> >> was done and it is in the middle of updating the L1, so this would be
> >> >> very
> >> >> small window.
> >> >
> >> >
> >> > I think it would be clearer to describe this as the L1 invalidation
> >> > cancelling the remote get, not the L1 update, because the actual L1
> >> > update
> >> > can't be cancelled.
> >>
> >> When I say L1 update I meant the write to the data container after the
> >> remote get.  The invalidation can't stop the remote get, all it does
> >> is tell the caller that "Hey don't write the remote value you
> >> retrieved into the L1."
> >>
> >
> > Oh right, the get command will still use the value it got from the remote
> > node, it just won't write it.
> > That makes me wonder, though, if something like this can happen:
> >
> > 1. A invokes get(k), starts a L1ReadSynchronizer and a remote get to B
> > 2. B invokes put(k, v) and sends an invalidation command to A
> > 3. The invalidation command cancels the L1 put on A
> > 4. A invokes get(k) again, finds the L1ReadSynchronizer from step 1) and
> > queues on it
> > 5. Both get(k) commands return the same value, even though the value has
> > changed on the owner(s).
> >
>
> Yeah that was going to be covered in ISPN-2965.  This would be a
> problem even with locking since it could be a different node doing the
> get.
>
>
> https://github.com/wburns/infinispan/blob/ISPN-3197-singlethread/core/src/main/java/org/infinispan/interceptors/distribution/L1NonTxInterceptor.java#L341
>
> I also have a test currently that is disabled in the test class that
> reproduces this.
>
>
> https://github.com/wburns/infinispan/blob/ISPN-3197-singlethread/core/src/test/java/org/infinispan/distribution/DistSyncL1FuncTest.java#L351
>
>
I think we're talking about slightly different things here. Even if you
wanted to send another invalidation command after the entry was committed
to the data container on the owner, you won't get a second get request on
the owner because that get request has been corralled on the requestor:

1st get initiated -> remote get sent -> invalidation -> remote get returns
-> 2nd get initiated -> both get ops return with initial value

I think you need to either remove the L1ReadSynchronizer from the map
during invalidation, or to re-fetch the value if there was an invalidation,
in order to avoid this.

Of course, you still have to deal with ISPN-2965 and the fact that a get
command can reach the owner after the invalidations were sent but before
the entry is committed to the data container. Maybe it would be better if
we'd send L1 invalidations only after the entry was committed?


> >
> >>
> >> >
> >> > We also have to remove the logic in AbstractLockingInterceptor that
> >> > skips L1
> >> > invalidation for a key if it can't acquire a lock with a 0 timeout.
> >> >
> >> >>
> >> >> 4. Local writes will also do the same thing as the invalidation with
> >> >> cancelling or waiting.  Note that non tx local writes only do L1
> >> >> invalidations and don't write the value to the data container.
>  Reasons
> >> >> why
> >> >> I found at https://issues.jboss.org/browse/ISPN-3214
> >> >
> >> >
> >> > I didn't know about ISPN-3214 or that non-tx writes don't write to L1,
> >> > but
> >> > it sounds fair.
> >>
> >> Yeah I really wanted that to work, but without some additional checks
> >> such as versioned data, I don't see a way to do this without locking
> >> at the primary node like tx caches.
> >>
> >
> > In theory, the primary owner could send a synchronous RPC back to the
> > originator while it is holding the lock, saying "ok, you can now write
> the
> > value to L1". But I don't think the slowdown from an additional RPC
> would be
> > acceptable.
> >
>
> I would agree ;(
>
> >>
> >> >
> >> >>
> >> >> 5. Writes that require the previous value and don't have it in the L1
> >> >> would also do it's get operations using the same "corralling" method.
> >> >
> >> >
> >> > The remoteGetBeforeWrites are a bit different - they don't happen on
> >> > non-owners, they only happen on writeCH-owners that didn't receive
> that
> >> > entry via state transfer yet. They put the value in the
> >> > InvocationContext,
> >> > but they don't write it to the data container - nor do they invalidate
> >> > the
> >> > L1 entry, if it exists.
> >>
> >> Ah yes that is true, but only for non tx caches it seems.
> >>
> >
> > Right, I wasn't considering the fact that a conditional command may
> fail...
> > I think if that happens, even in non-tx caches EntryWrappingInterceptor
> may
> > write the entry to the data container as an L1 entry. If we move the L1
> > writes to the L1 interceptor, we must ensure that
> EntryWrappingInterceptor
> > doesn't write anything to L1 any more.
> >
>
> We still need EntryWrappingInterceptor to write to the L1 cache for tx
> caches I would think.  We only want to write to the L1 after the tx is
> committed for write commands.  Read commands would be fine writing to
> the container immediately.  Also I would hope conditional writes
> wouldn't update the context with the new value until we knew the write
> was successful, should prevent bad data being written there.
>
>
The L1 interceptor could still only write to the data container during the
commit phase. I was thinking of separating the regular writes from the L1
writes because L1 writes would need additional synchronization, but maybe
that's not the case.


> >
> >>
> >> >
> >> >>
> >> >>
> >> >> 4/5 are not currently implemented in PR.
> >> >>
> >> >> This approach would use no locking for non tx caches for all L1
> >> >> operations.  The synchronization point would be done through the
> >> >> "corralling" method and invalidations/writes communicating to it.
> >> >>
> >> >> Transactional caches would do almost the same thing as non-tx.  Note
> >> >> these
> >> >> changes are not done in any way yet.
> >> >>
> >> >> 1. Gets would now update the L1 immediately after retrieving the
> value
> >> >> without locking, but still using the "corralling" technique that
> non-tx
> >> >> does.  Previously the L1 update from a get was transactional.  This
> >> >> actually
> >> >> would remedy issue [1]
> >> >>
> >> >> 2. Writes currently acquire the remote lock when committing, which is
> >> >> why
> >> >> tx caches are able to update the L1 with the value.  Writes would do
> >> >> the
> >> >> same cancellation/wait method as non-tx.
> >> >>
> >
> >
> > Hmm, I don't think your current approach for L1 invalidations would work
> for
> > L1 writes, because the actual write to the data container is not
> > synchronized (well, technically you still have the 0-timeout locking for
> > invalidation commands, but I think you're planning to remove that). So
> it's
> > possible for an L1 write and an L1 invalidation to wait for the same
> remote
> > get and then to get executed in the wrong order.
> >
>
> For non-tx that doesn't matter as a write and L1 invalidation both do
> invalidation.
>
> For tx it should be fine as well since it will hold the primary owner
> lock for the write duration, which includes L1 writes (so only that tx
> is writing it's value).  Tx caches don't process the L1 invalidation
> message when it comes back since they were the originator also.
>
>
True, in most cases different writers would be synchronized via the lock on
the primary owner. There are 2 situations where that doesn't happen, though:
* When syncCommitPhase = false, the primary lock may be released before the
entry is actually committed to L1.
* During state transfer, the received state is applied without holding a
lock on the primary, or even a local lock. State transfer only uses
StateConsumerImpl.updatedKeys for synchronization, which turns out to be a
problem with regular entries as well:
https://issues.jboss.org/browse/ISPN-3287


> >
> >>
> >> >> 3. Writes that require the previous value and don't have it in the L1
> >> >> would also do it's get operations using the same method.
> >> >
> >> >
> >> > Just like for non-tx caches, I don't think these remote gets have to
> be
> >> > stored in L1.
> >>
> >> Tx caches do the remote get and could cache the L1 value immediately.
> >> This would help if the transaction is rolled back or a conditional
> >> operation failed etc.  There are some locking concerns, here but I
> >> will leave that the other post.
> >>
> >
> > Right, the L1 entry could be immediately written to the data container,
> but
> > not if it complicates things too much: most writes should be successful
> > anyway.
> >
>
> I agree the writes should be successful, but there is no telling if
> the transaction will be committed.  Something else outside of
> infinispan could error causing a rollback or at worst case 2PC could
> fail prepare (not touching heuristic failures).
>
> Also this would help plug up this hole:
>
> https://issues.jboss.org/browse/ISPN-2965?focusedCommentId=12779780&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-12779780
>
>
Sorry, I wasn't clear... I meant if 99.9% of conditional writes write the
initial value to L1 and then immediately write the new value, perhaps the
additional overhead of an extra write to the data container is not worth
the benefit of having the value in L1 in the 0.1% of cases where the write
is not successful or the tx fails.

Of course, we don't have proper statistics about the percentage of
conditional writes that succeed in real-life applications, so in the end
it's your decision.


>  >> >
> >> >>
> >> >> 4. For tx cache [2] would also have to be done.
> >> >>
> >> >> [1] -
> >> >>
> >> >>
> https://issues.jboss.org/browse/ISPN-2965?focusedCommentId=12779780&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-12779780
> >> >> [2] - https://issues.jboss.org/browse/ISPN-1540
> >> >>
> >> >> Also rehashing is another issue, but we should be able to acquire the
> >> >> state transfer lock before updating the L1 on a get, just like when
> an
> >> >> entry
> >> >> is committed to the data container.
> >> >>
> >> >
> >> > The same for L1 invalidations - we don't want to remove real entries
> >> > from
> >> > the data container after the local node became an owner.
> >>
> >> Yeah actually as you mentioned this, it sounds like a hole currently
> >> even.  I don't know if this case can happen, but what if you received
> >> a L1 invalidation and then got a rehash event right before it was
> >> committing it to the container?  It seems the L1 commit to the
> >> container would block until it could get the shared topology lock and
> >> after it could then removes the value.  I probably need to dig into
> >> the state transfer stuff deeper to know myself.
> >>
> >
> > I think that's ok, because the value is stale, and any future
> > get/conditional write command will request the new value from the
> previous
> > owners.
> >
> > I don't think the new value can arrive via a StateResponseCommand and be
> > applied to the data container before the invalidation command manages to
> > commit, because the put command sends the invalidations first
> > (synchronously) and only then commits on the owner.
> >
>
> I defer to your judgement here ;-)
>
>
On a second read, that second paragraph doesn't make a lot of sense...

To rephrase: the InvalidateL1Command could remove an entry after it was
written via state transfer. Since the invalidation is synchronous, that
means the commit command will be forwarded to the new owner, and it will
write the new value gain. If any tx tries to read the key in the meantime,
it will re-read it from the old owners, so that shouldn't be a problem.

But if there are 2 old owners (let's call them A and B), the commit command
forwarded from A could be executed on the new owner just before the
invalidation command from B. And once the forwarded commit from A is
executed, the forwarded commit from B for the same tx won't do anything,
and k will be lost. Even if state transfer receives k after that, it won't
overwrite it because it's in the StateConsumerImpl.updatedKeys set.

There is another hitch with async commit - it's possible that the new owner
didn't receive the transaction data via state transfer (because it
requested transaction data from the B, and the transaction had already been
committed there). Then any forwarded commit won't do anything and the entry
will be lost.


Going back to my proposal of adding an ownership check before actually
removing the L1 entry (and while holding the shared topology lock)...

Say nodes [A, B] were the previous owners of key k, C has k in its L1
cache, and [B, C] are the new owners (so the owners in the write CH are [A,
B, C]). If there is a transaction modifying key k, the timing could be
something like this:

1. The commit command finishes on A and B before they install the new
topology. Because StateRequestCommands block until the state provider
installs the new topology, that means node C didn't request the state for k
yet, so the invalidateL1(k) command runs on C before the state transfer
put(k, v) command.

However, there is a chance that C already installed the new topology before
executing the invalidate(k) command, so if we do just the ownership check
in the L1 interceptor, the key won't be deleted. That's not ok, because
we'll have a stale value until state transfer overwrites it. And even
worse, if we had a remove(k) instead of a put(k, v), C would only remove
the stale value for k when it expired.

We may need to clear all the keys for which C is becoming an owner after
installing the new topology, but before requesting transaction data (which
means before processing transactions with the new topology id as well),
when L1 is enabled.

2. The commit starts on A and B before the new topology is installed, but
by the time it finishes on A, A installs the new topology.
StateTransferInterceptor then forwards the commit command to C, and on C it
writes the new value (eventually waiting for C to receive the transaction
data from A). With sync commit + tx completion notification, C is
guaranteed to receive the tx whether it requests it from A or B.

If StateConsumer receives a value for k later, it won't overwrite it,
except for this bug: https://issues.jboss.org/browse/ISPN-3287

There is a chance that the forwarded commit command from A reaches C around
the same time with the invalidateL1(k) command from B. But the commit
command is sent with the new topology id, so it will only execute after C
installed the new topology as well. If the invalidateL1(k) command executes
at the same time, the ownership check will prevent it from doing anything.
(Without the ownership check we could remove key k just after writing it.)

With async commit it's more complicated: if C requested the transaction
data from B, and B had already finished committing by the time it sent
transaction data, the forwarded commit on C won't do anything. Instead, C
should receive the new value via state transfer, going back to case 1.

3. The commit is initiated after the new topology is installed. The commit
command is sent to C directly, it waits there for C to receive transaction
data from A or B, and it will write the new value. Since the commit was
sent with the new topology id, it will block on A and B until they install
the new topology, and won't trigger any invalidateL1(k) command to C.

With async commit there is still a chance that A and/or B already finish
the commit by the time C requests the transaction data, and C will only
receive the new value via state transfer - just like in case 1.


> >>
> >> >
> >> >>
> >> >> Any comments/concerns would be appreciated.
> >> >>
> >> >> Thanks,
> >> >>
> >> >>  - Will
> >> >>
> >> >> _______________________________________________
> >> >> infinispan-dev mailing list
> >> >> infinispan-dev at lists.jboss.org
> >> >> https://lists.jboss.org/mailman/listinfo/infinispan-dev
> >> >
> >> >
> >> >
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> >
> >
> >
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