Hello, everybody. Let me offer a bunch of ideas for further improvements in replication, and the parallel one specifically. Your comments, thoughts and critical notes are most welcome! Thank you for your time. Andrei Slave balanced parallel applier ================================ There are a few motivations for this project which include the following reflections: - the slower binlog group transactions applying rate by slave the more transaction sizes differ (must've been few bugs reported): the speed of a caravan is determined by the pace of the slowest camel, or more formally as specified by Amdahl's law (KrinstianN pointed that out to me) - while ordered committing of grouped transactions on slave presents some virtues (like guaranteed consistency with master, monotonic changes to slave execution status as measured by GTID) it still must be taxing on performance. Observe that co-prepared transactions of a master binlog group represent a sort of a transaction on its own. Call it epoch [which must revive memories of NDB cluster], and the original transactions as its sub-transactions or branches. Notice that there is no meaning to sub-transaction internal ordering within the epoch. They can be logged to the binary log in any mutual order, still without exposing any inconsistencies to the user. Specifically, say there are two prepared transactions BEGIN /* A */ BEGIN /* B */ INSERT... INSERT... UPDATE... UPDATE... DELETE... DELETE... COMMIT COMMIT entering the binlog group (ordered) commit module. They can end up into binlog as A,B or B,A, and regardless of that (Gtid actually) order also be acknowledged to the user in either way. The latter fact infers that slave does not have to commit the group sub-transaction/branches in the GTID order. It could be done in any order including no order at all like to do so as one epoch commit. Naturally the latter option is impractical to replication. The current conservative scheduler offers a decent parallelization method still suffering from the aforementioned uneven branch's sizes and inter-thread communications caused by ordering. The optimistic scheduler remedies to some level but not fully (an optimistically executed group may be unbalanced too) and even conceding some risk of performance degradation (rollback is costly). There is an idea reminiscent to pipelining to handle the epoch (the master binlog group) as a single transaction but in parallel. That is to execute its statements by multiple workers, this time scheduling them fairly, and "atomically" commit their works. To the fairness part, workers get assigned by a statement at a time from the epoch transaction(sub-transaction BEGIN, COMMIT removed). Say 'E' an epoch consists of 's' of sub-transaction E := { T_1, T_2, ... T_s } For each m from 1 to s sub-transactions T_m gets scheduled to some worker W_1 W_2 ... W_k | | | V V V T_m := { e_1, e_2, ... e_l } Hereinafter I use the TeX math notation, '^' - a superscript attached to the event to designate its parent transaction sequence number (Gtid seq_no), '_' - a subscript to enumerate an object within its compound object. e_i stands for T-ransation's statements, W_t:s are workers. Effectively the epoch breaks into modified branches executed by workers on one to one basic: E := { T'_1, T'_2, ... T'_k } here 'k' is the number of workers engaged, T' indicates the modified transaction. The branches are executed until they are ready to prepare which event is triggered by scheduling of the last statement of the last original sub-transaction. It's clear that sizes of the modified branches are even this time. The last statement worker coordinates 2pc. Thinking more technically workers can consume/pull from the current epoch presented as a queue which is pushed into by statement producer. Here is a possible state of the queue when m:th statement of T_n is about to be pushed: e^n_k -> [ e^n_m-1, ..., e^n_2, e^n_1; ...; e^1_l1, ... e^1_2, e^1_1 ] ...----- T_n ------------| T_n-1,...T_2 |---------- T_1 ---------| A pull by consumer at this point would return e^1_1. Very likely that by the epoch prepare event *all* the branches have been already ready for prepare(). This pipelining parallelization method can work for the single "large" transaction in the binlog group and also could drag into the parallelizable input transactions from later groups if we additionally create master side dependency tracking (think of mysql logical timestamp method). Also notice that the optimisitic scheduler is orthogonal to this method so the two can be combined. Consistency concern ------------------- The epoch parallel execution (scheduling) must respect intra-transaction statement dependencies (e.g FIFO execution order of operations over the same object [record, table]). There is no inter-transaction dependencies, at least in theory. The notion of a leader, the two-phase-commits leader, remains roughly the same as in the existing conservative scheduler. This time more than initiating commits of the epoch branches it also takes care to advance the slave gtid execution status accordingly. It could be implemented as the very last statement of its own T' branch. It's clear that causal dependencies of an user write and a read (to find the effect of that write) can be tracked by WAIT_FOR_GTID() as currently. A plain two-phase commit does admit possibility to catch on slave combinations of the old (pre-epoch) and new version (post-epoch) or some objects of the same transaction if the user hits with non-locking SELECT in a "tiny" time window between commit():s of two branches like in the following example. Let t1,t2 some tables and the binlog group consists of just one transaction T_1 := { update(t1), update(t2) } Let on slave it's modified into two parallel ones: T'_1 := { update(t1) } || T'2 := { update(t2) } After both respond OK to 2pc prepare() and then the leader initiates commit() where could be 4 possible result sets to the contemporary SELECT SET @@session.tx_isolation='REPEATABLE-READ'; SELECT * from t1 as t_1, t2 as t_2; => ... While this is perhaps more than imperfect, I believe it may be relevant only to remote corner cases, but more importantly, we still can refine that doing some *adjustments* in the Engine studied briefly myself of what would be a sub-transaction in Innodb, "ideologically" endorsed and guided by Marko Makela. The idea is to hold the branch's locks even after the branch commits, and delegate their release to the leader. Recoverability -------------- When slave is configured to log in the binary log we must make sure to rollback any prepared epoch branches at the server recovery unless the last branch (of the leader) is also there, prepared. When it does not binlog the problem is that the modified branch's binlog images can't be just flushed one after another especially if the user specifies to log on the slave uniformly with the master that is to preserve the original sub-transaction structures and their boundaries. In such case there are several multiple option, here are some: - use relay-logged image of original sub-transactions (may be limited to the master and slave version combination though;) - reconstruct original sub-transaction's binlog images. That is binlog events "belonging" to an original sub-transaction will be cached cooperatively by workers that took share of the original sub-transaction execution. Optionally at least the caching could be set to allow out of order while the statements are parallelizable, like in the above T1(t1,t2) example. Otherwise the ordering will be preserved through implementing the shared cache as as a queue with an interface to insert an item into the middle. - exotics like to create the slave side "merge" GTID which would embed the original "commit". (The quotes here to allude to the Git's way). While it's a serious subject I am leaving out mariabackup for this moment. I believe it will be feasible to adjust its logics to account as prepared only such transactions that that are prepared in all their branches. More about pipeline style optimization -------------------------------------- As mentioned workers can be made self-serving to assign events for execution at once when become available. That must turn to think of producer-consumer model, potentially lock-free queues, dynamical # of workers to stay in balance with the receiver's pace etc. I think *eventually* we will need to assign the producer role to the IO (receiver) thread, and convert the current SQL (driver) thread into (relay-) logger (we still may have to have such logger for the semisync replication, the relay logger can be opted out otherwise). Here is a picture IO_thread.push() | +---- worker.pull() V V e^s_i -> [ e^s_i-1, ... e^s_1 }, T_s-1, ... ] ^ | SQL.pull() where on the right hand side the two consumers handle a transaction and when both of them have done with it (a worker has read its last event and the logger written it to the relay-log) the query element becomes garbage-collectable.