Re: [Monotone-devel] mtn: error: sqlite error: database is locked

[Alvaro Herrera]

Peter Stirling wrote:
> Normally write transactions are exclusive because of the infamous 'two 
> withdrawals from your bank account' example.
>
> When you change a value you need to be able to stop others from changing it 
> again when you don't expect them to.

Sure.  That's the database system's job, not the application
developer's.  The transactions need not actually be what you call
"exclusive" (which I take to mean "no two such transactions can be
running at any given instant", though on SQLite the constraint is even
stricter) -- they need to be what the SQL standard calls "serializable".
The simplest serialization failure occurs when two concurrent
transactions want to modify the same tuple at the same time.  However,
if two concurrent transactions modify different tuples, then they can
continue in parallel and no harm is done to the database integrity.

I will note that the SQL standard mentions no such thing as an exclusive
transaction.

(Of course, other constraints can be imposed on the system so that it
fulfills higher-level integrity constraints.  In Monotone's case, my
guess is that the ancestor tree needs several additional checks, but
they are probably implementable by using triggers or other mechanisms).

-- 
Alvaro Herrera                 http://www.amazon.com/gp/registry/DXLWNGRJD34J
"El hombre nunca sabe de lo que es capaz hasta que lo intenta" (C. Dickens)