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Re^9: method chaining fails where separate method calls succeed in DBIx::Simple (lifecycle)

by Juerd (Abbot)
on Aug 31, 2011 at 14:26 UTC ( #923441=note: print w/ replies, xml ) Need Help??


in reply to Re^8: method chaining fails where separate method calls succeed in DBIx::Simple (lifecycle)
in thread method chaining fails where separate method calls succeed in DBIx::Simple

DBIx::Simple goes to some significant lengths to make all DBIx::Simple::Statement objects suddenly become unusable as soon as their parent DBIx::Simple object is destroyed.

I don't pretend to know why this strange lifecycle interplay is implemented or even whether or not it is a good idea.

DBIx::Simple objects represent database connections, and when your connection to the database is gone, the corresponding statement handles will no longer function correctly. Because destruction/disconnection related bugs can be hard to find, DBIx::Simple actively shuts down the remaining active statements, and replaces them with objects that when used throw an error message that actually contains information about where the object was destroyed.

It is possible to build a DBIx::Simple object from an existing DBI connection, in which case destruction of the DBIx::Simple object does not cause disconnection. Whether statements should be kept around is debatable but I chose to keep it simple, and let DBIx::Simple clean its own mess regardless of how the database connection was originally made.

The option to pass an existing $dbh was added later and it appears that a part of the documentation was not updated accordingly:

Destroys (finishes) active statements and disconnects. Whenever the database object is destroyed, this happens automatically if DBIx::Simple handled the connection (i.e. you didn't use an existing DBI handle). After disconnecting, you can no longer use the database object or any of its result objects.
Destruction used to unconditionally also disconnect the $dbh; this was changed later, but that made that last sentence incomplete. It should instead read "After disconnecting or destroying the DBIx::Simple object, ..."

Although the documentation should be improved, DBIx::Simple is doing exactly what it was designed to do. Indeed, simply removing the quotes and making a real reference does not fix a bug, and it is certainly not a wise thing to do: it introduces new bugs, because users of DBIx::Simple depend on their objects being destroyed and their database connections disconnected when their $db goes out of scope.

The trick for wrappers like metaperl's Local::DBIx::Simple could be to somehow keep a reference around and do some of their own lifecycle management.


Comment on Re^9: method chaining fails where separate method calls succeed in DBIx::Simple (lifecycle)
Re^10: method chaining fails where separate method calls succeed in DBIx::Simple (lifecycle)
by tye (Cardinal) on Aug 31, 2011 at 15:02 UTC

    Thanks for the explanation. That is about what I expected, actually even better.

    The trick for wrappers like metaperl's Local::DBIx::Simple could be to somehow keep a reference around and do some of their own lifecycle management.

    Exactly.

    - tye        

Re^10: method chaining fails where separate method calls succeed in DBIx::Simple (lifecycle)
by metaperl (Curate) on Aug 31, 2011 at 15:18 UTC
    Because destruction/disconnection related bugs can be hard to find

    What is your opinion of DBIx::Connector? Do you think you should delegate the complexity of keeping database connections alive to it?

    Also, please note well that destruction bugs and disconnection bugs are two separate classes of problem. I dont know which the use of double quotes about $db was supposed to address. But I do know that common sense about reference counting between a DBIx::Simple database handle and DBIx::Simple::Statement which has-a database handle should not require any particular weakening like you are doing. Just think for a second:

    1. we create a database connection, D
    2. we create a statement instance, S, which refers to D. This makes the reference count for D== 2
    3. we create a another statement instance, S2, which also refers to D. This makes the reference count for D== 3
    4. S2 goes out of scope. Reference count for D drops to 2
    5. S goes out of scope. Reference count for D drops to 1
    6. D goes out of scope, reference count for D drops to 0 and D is extinguished
    I simply dont understand why we need to prevent the reference count from naturally increasing and decreasing as the DBIx::Simple instance becomes a compononent of DBIx::Simple::Statement instances.

    The trick for wrappers like metaperl's Local::DBIx::Simple could be to somehow keep a reference around and do some of their own lifecycle management.

    Without any concrete suggestions for modification, I dont know what to say or try. But I would say this. Local::DBIx::Simple is a very simple, clearly written wrapper and it isnt working.

      What is your opinion of DBIx::Connector? Do you think you should delegate the complexity of keeping database connections alive to it?

      It solves a different, unrelated problem. I'm not inclined to let DBIx::Simple depend on anything but DBI, except optionally.

      Also, please note well that destruction bugs and disconnection bugs are two separate classes of problem. I dont know which the use of double quotes about $db was supposed to address.

      I'm aware. Avoiding circular references has to do with avoiding problems regarding destruction; in particular, avoiding that destruction never happens during runtime. Because in DBIx::Simple, destruction causes disconnection (iff you let DBIx::Simple create the connection), I mentioned them together.

      But I do know that common sense about reference counting between a DBIx::Simple database handle and DBIx::Simple::Statement which has-a database handle should not require any particular weakening like you are doing.

      Common sense dictates that one does not use a circular reference if one can easily do without. There's no reason for ::Statement to have-a database handle, so it doesn't has-a one at all. All it needs is to refer to the object, as a hash key. That's a string. So all we need is that string. Et voila, we've avoided circular references without the complexity and risks of weakening. You're referring to the trick I'm using as "weakening" but it is very different.

      I simply dont understand why we need to prevent the reference count from naturally increasing and decreasing as the DBIx::Simple instance becomes a compononent of DBIx::Simple::Statement instances.

      tye has explained this thoroughly, with examples of what happens. I'm sorry, but if you don't understand his explanation, you don't understand destruction and you should really never use, or suggest that anyone use, a circular reference.

      But I would say this. Local::DBIx::Simple is a very simple, clearly written wrapper and it isnt working.

      It is working. I believe that the problem you encounter lies somewhere within the libraries you're using for object management, or, possibly, perl itself.

      In any case, the workaround is extremely simple and clear and you've already discovered it yourself: use a variable.

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