Alex, MVCC concept and blocking/non-blocking behavior are not inter-related. You can have both blocking and non-blocking writes with MVCC and without it. The thing is that users *want* blocking behavior as it is easy to reason about. This is why even MVCC-based RDBMS vendors (Oracle, PostgreSQL) do block on writes.
OPTIMSITIC mode and non-blocking approaches are for rare enthusiasts. It is not in priority for us. On Thu, Sep 28, 2017 at 1:59 PM, Alexei Scherbakov < alexey.scherbak...@gmail.com> wrote: > Do we have a plan to support multi-version concurrency control ? > > Proper implementation allows to achieve better isolation without blocking > for the cost of additional resource consumption. > > The feature can be configurable per transaction. > > 6) What if I want to acquire write locks only for some specific reads > within same transaction ? > > > 2017-09-27 5:52 GMT+03:00 Dmitriy Setrakyan <dsetrak...@apache.org>: > > > OK, if we must change the current behavior, let's discuss the new design. > > My comments/questions are below... > > > > On Mon, Sep 25, 2017 at 12:09 PM, Vladimir Ozerov <voze...@gridgain.com> > > wrote: > > > > > Folks, > > > > > > Sorry for late reply. I had a chat with several Ignite veterans today. > We > > > tried to design transactional SQL for Ignite. One of our questions was > > how > > > to align SQL transactions with current Ignite transactions. And we > > failed. > > > And then we came to conclusion that current transaction API is > unusable. > > We > > > have 6 pairs of modes from API standpoint and 4 real modes. This is > very > > > counterintuitive and cannot be mapped to any transactional framework > our > > > users are familiar with. > > > > > > So we thought how new tx API might looks like, and here is the draft. > > > > > > 1) Define new enum *TransactionIsolationLevel *(to avoid clashes with > > > current enum) with three standard modes - READ_COMMITTED, > > REPEATABLE_READ, > > > SERIALIZABLE. > > > > > > > If it is the same values as we have today, why create a new enum? > > > > > > > 2) Define new enum *TransactionHint* - READ_ONLY, OPTIMISTIC_LOCKING > > > > > > > The word *hint* means no guarantee. When it comes to transactions, we > must > > have guarantees. Also, what happens to PESSIMISTIC locking? > > > > > > > 3) No more OPTIMISTIC and PESSIMISTIC. Seriously. > > > > > > > But you are still proposing OPTIMISTIC_LOCKING above. So, we have > > OPTIMISTIC without PESSIMISTIC? > > > > > > > 4) Reads never acuire locks > > > 5) Writes always acquire locks > > > > > > > Hm... what locks? I think we are getting to deep into the weeds here. > Isn't > > it our internal implementation detail to acquire locks or not? For > example, > > MVCC approach we are working on will be virtually lock-free for all > > scenarios. > > > > I would focus on transaction behavior, not locks. > > > > > > > > > 6) *IgniteCache.withReadForUpdate()* will return special facade which > > will > > > obtain locks on reads. This is analogue of SELECT FOR UPDATE in SQL. > > > > > > > This syntax is very ugly in databases and we do not need to carry this > over > > to Ignite. All it means is that a pessimistic lock is acquired. To be > > honest, I would rather start a pessimistic transaction to get the same > > behavior. > > > > > > > 7) *TransactionHint.READ_ONLY* - forces transaction to throw an > exception > > > on any update > > > > > > > I really like the READ_ONLY flag. Allows us to put many optimizations in > > place. > > > > > > > 8) *TransactionHint.OPTIMISTIC_LOCKING* - turns transaction into our > > > current deadlock-free OPTIMISTIC/SERIALIZABLE mode. Applicable only to > > > SERIALIZABLE isolation level. > > > > > > > Is there a PESSIMISTIC/SERIALIZABLE mode? > > > > > > > 9) Define new API methods: > > > - IgniteTransactions.txStart(TransactionIsolationLevel isolation) > > > - IgniteTransactions.txStart(TransactionIsolationLevel isolation, > > > TransactionHint... hints) > > > > 10) Deprecate old TX start methods > > > > > > As a result we will have simple, clean and extensible API. Which can be > > > explained to users in 5 minutes, instead of current half an hour. And > > which > > > is perfectly aligned with upcoming transactional SQL. > > > > > > > The API does not look clean yet. Still requires lots of work. > > > > > > > > > > Thoughts? > > > > > > > > > On Thu, Sep 7, 2017 at 6:48 AM, Dmitriy Setrakyan < > dsetrak...@apache.org > > > > > > wrote: > > > > > > > Vova, > > > > > > > > Thanks for doing the research. The changes you are suggesting are a > bit > > > too > > > > bold, so let's discuss them in some more detail... > > > > > > > > On Wed, Sep 6, 2017 at 4:51 AM, Vladimir Ozerov < > voze...@gridgain.com> > > > > wrote: > > > > > > > > > Igniters, > > > > > > > > > > We are moving towards DBMS system. None of them has a notion of > > > > > OPTIMISTIC/PESSIMISTIC transactions. Instead, they work as follows: > > > > > 1) Reads (SELECT) do not acquire exclusive row locks > > > > > 2) Exclusive lock on read could be forced explicitly (SELECT ... > FOR > > > > > UPDATE) > > > > > 3) Writes do acuire explicit row locks > > > > > 4) Locks are always acquired immediately once statement is executed > > > > > 5) The strictest concurrency level - typically SERIALIZABLE - rely > on > > > > > so-called *range locks* (or *predicate locks*) to track > dependencies > > > > > between transactions. Some vendors throw an exception in case of > > > > conflict - > > > > > these are ones where snapshot-based MVCC is used - PostgreSQL, > > Oracle. > > > > > Others do aggressive locking - ones where two-phase locking > algorithm > > > is > > > > > used - SQL Server, MySQL. > > > > > > > > > > As you see, there is no concept of PESSIMISTIC/OPTIMISTIC modes. > > > Instead, > > > > > all updates are "PESSIMISTIC", reads are "OPTIMISTIC" but could > > become > > > > > "PESSIMISTIC" if requested explicitly, and for snapshot-based > vendors > > > (we > > > > > are going in this direction) read-write conflicts are resolved in > > > manner > > > > > somewhat similar to our OPTIMISTIC/SERIALIZABLE. > > > > > > > > > > That said, I would propose to think on how transactions could look > > like > > > > in > > > > > future Ignite versions (say, 3.0). My rough vision: > > > > > > > > > > 1) No OPTIMISTIC mode at all - too counterintuitive and complex. > It's > > > > only > > > > > advantage is deadlock-freedom when combined with SERIALIZABLE. If > we > > > have > > > > > good deadlock detector and nice administrative capabilities, this > > would > > > > not > > > > > be a problem for us. > > > > > > > > > > > > Hm... The advantage of Optimistic Serialiazable mode is actually > > > lock-free > > > > transactions. The deadlock is impossible in this case. I doubt any > > > deadlock > > > > detector would match the performance advantage we get from lock-free > > > > transactions. > > > > > > > > > > > > > > > > > > 2) Behavior of reads could be controlled through "with" facade: > > > > > V val1 = cache.get(key1); // Shared lock or no lock > > > > > V val2 = cache.withForUpdate().get(key2); // Exclusive lock > > > > > > > > > > > > > Don't like the API. We are not trying to abandon the data grid > use-case > > > or > > > > API, we are trying to add the database use case. > > > > > > > > > > > > > 3) REPEATABLE_READ - throw exception in case of write-write > conflict > > > > > > > > > > > > > Well, I would like to preserve the PESSIMISTIC mode. I find it more > > > > convenient than the "withForUpdate" API. It almost seems like you are > > > > trying to force the pendulum too far in the opposite direction. > > > > > > > > > > > > > 4) SERIALIZABLE - throw exception in case of write-write and > > write-read > > > > > confilct (this is how our OPTIMISTIC/SERIALZABLE works now, but it > > > > doesn't > > > > > support predicates) > > > > > > > > > > > > > So, no change here? Good :) > > > > > > > > > > > > > 5) Add READ_ONLY isolation mode where updates will not be allowed > at > > > all. > > > > > Such transacrtons would be able to bypass some Ignite internals to > > > > achieve > > > > > greater performance, what could be valuable for mostly-read use > cases > > > > (e.g. > > > > > OLAP). > > > > > > > > > > > > > Love the idea. We have already seen many use cases that could benefit > > > from > > > > it. > > > > How hard is it to implement? > > > > > > > > > > > > > > > > > > Thoughts? > > > > > > > > > > Vladimir. > > > > > > > > > > > > > > > > > > -- > > Best regards, > Alexei Scherbakov >
