Likely happens when you're using a different session for fetching than for
saving. Use the same session instance for both actions (fetch and save). The
reason it deadlocks is that if you start a transaction on session A and
execute a fetch, it will place read locks on the table rows you touch.
Another session B with its own transaction will then wait till the read
locks are lifted.
E.g.: (pseudo code)
using(var s = GetSession())
{
using(var trans = s.StartTransaction())
{
// do some fetching here on s
DoSomeSaveWork();
}
}
and in DoSomeSaveWork() you start a NEW session and transaction -> this will
deadlock as it will wait for the readlocks of s' transaction, but these are
lifted AFTER DoSomeSaveWork() has been completed.
FB
> We have an Entity which has two fields (TargetType and TargetId).
> These are values which refer to a legacy data store and are immutable; the
> entity's constructor sets them. They are mapped as a composite natural ID,
> and the database has a UNIQUE constraint over them.
>
> The usual method by which the entity is fetched is to use CreateQuery with
> something like "from Entity e where e.TargetId = :id and e.TargetType =
> :type", and get the unique result. Application logic then checks if the
> result is null.
> * If it is null (no Entity found), a different instance is requested from
> the same table via a different query, possibly updated (if found), and a
new
> Entity created for the originally specified TargetId and TargetType
values.
> Stuff is done to it and it is saved.
> * If it is not null, stuff is done to it and it is saved.
>
> The problem is that threads trying to save new Entity instances to the
> database often deadlock. The failing thread always gives up during the
> initial INSERT statement.
>
> Setting isolation level to ReadUncommitted makes the problem go away, but
> we'd rather avoid using that. Any higher isolation level causes the
> deadlocks. Establishing an exclusive table lock during the first SELECT
> would probably fix it, but NHibernate appears to provide no means of doing
> so except raw SQL.
>
> The basic intent of the operation is to fetch-or-create an Entity with the
> specified TargetType and TargetId values, and possibly do something to
> another Entity if it was a 'create'. Does NHibernate provide an atomic
> 'fetch-or-create'?
>
>
> Thanks,
>
> Alex
>
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