james anderson wrote:
On 2020-12-04, at 03:07:19, Howard Chu hyc@symas.com wrote:
james anderson wrote:
On 2020-12-03, at 23:40:18, Howard Chu hyc@symas.com wrote:
james anderson wrote:
On 2020-12-03, at 19:17:46, Howard Chu hyc@symas.com wrote:
Gábor Melis wrote: > On Wed, 2 Dec 2020 at 22:50, james anderson > anderson.james.1955@gmail.com wrote: >> >> >>> On 2020-12-02, at 22:53:58, Howard Chu hyc@symas.com wrote: >>> >>> James Anderson wrote: >>>> the mdb_env_open documentation includes in its note about NOTLS, that >>>> >>>> A read-only transaction may span threads if the user synchronizes its use. >>>> >>>> to which read-only operations would this constraint apply? >>> >>> It depends. >>> >>> The only safe approach is to ensure that a txn is not active simultaneously >>> in multiple threads. >> >> where “active” includes read-only cursors? >> >> does mean, either one constrains the threads such that there can be no parallel access to the database, or each thread must establish its own transaction, in which case there is no guarantee that they operate om the same database state? > > Chiming in here, a cleaner api could be to allow starting a > transaction with a given txn id. That way one would have separate > transaction objects, but consistent state. The client code would need > to synchronize threads a bit to guarantee that the txn id is still > valid, but this would be more lightweight and easier to reason about.
In an actively written database there is no legitimate use case for opening a new transaction on anything but the newest version of the data. Reading or depending on stale data would be an application bug.
without considering the relation between that notion and the management of data in a bitemporal store, the question remains, how are two independent threads to ensure that they are reading the same “newest” version when some other, likewise independent, process may commit a write transaction in the time interval between the instants of the respective read transaction begins?
How would you do this in any other database system?
i would expect it to permit one of the alternatives which has been mentioned:
- allow multiple threads to perform read operations in the context of a single transaction
No. A transaction is a single unit for concurrency control. Allowing multiple threads to operate within a single transaction means you have no control, and thus invites memory corruption.
you note in your paper on lmdb that, “because of [mvcc] isolation read accesses ... always have a self-consistent view of the database." how are operations performed by threads within a transaction to corrupt this view when their access is through read-only memory and they do nothing to change it?
You assume that the read-only on-disk state is the only state maintained within a transaction structure, which is not necessarily true. Anyway, it's not your place to make any assumptions about the internal state of the library beyond what the API docs guarantee.
Relying on internal implementation details like that is how you write broken software.