On 12/08/14 08:58, Claude Warren wrote:
On Mon, Aug 11, 2014 at 6:19 PM, Andy Seaborne <[email protected]> wrote:
Transactions:
The text around transactions does not distinguish being inside or outside
a transaction.
There are 2 base kinds of graphs - ones in datasets (views) and standalone
ones, then things like InfGraph and other added functionality. Transactions
on view graphs need to be defined in the context of the dataset because
transactions are connected.
The point here is that several graphs may be in one transaction but
other graphs (other datasets) may not.
Using databases as a transaction example. There are multiple types of
transactions -- I don't know if we want to get into supporting or
identifying the type of transaction supported by a graph, but...
In the current case (Jena 2.12.x) whith 2 threads T1 and T2.
T1 begins a write transaction
T1 add to graph
T2 begins a read transaction (is this possible -- I think so)
Can T2 find the triples written by T1 (note that the transaction is not yet
committed)?
No
T1 commits the transaction
Can T2 now find the triples written by T1?
No
The isolation level in TDB transactions is serializable, not read
committed. TDB transaction are not lock based nor do they ever cause
system aborts due to unresolvable contention [*].
T2's view of the database is all commits up until then (so not T1) and
does not change. That would be read committed.
T2 ends transaction (just for completeness)
And if T3 starts, it sees T1 updates and T2 still does not. see them.
Given the nature of storing triples/quads and not entities (meaningful
rows in the data abstraction which is what SQL tends to do), the lower
isolation levels can have rather strange effects. And phantom reads are
horrendous.
Andy
PS 4.2 does not differentiate between inside and outside the transaction
so the isolation level is not relevant.
[*] That would change if we have begin() with no read/write indicator at
the point of transaction promotion from read to write, an abort would be
possible if the DB is inconsistent with that happening.
Claude
