Is there a way to caching mechanism for Class::DBI?

Question

I have a set of rather complex ORM modules that inherit from Class::DBI. Since the data changes quite infrequently, I am considering using a Caching/Memoization layer on top of this to speed things up. I found a module: Class::DBI::Cacheable but no rating or any reviews on RT. I would appreciate hearing from people who have used this or any other Class::DBI caching scheme.

Thanks a ton.

Answer 1

I have used memcached before to cache objects, but not with Class::DBI (ORM makes me feel dirty).

Answer 2

On a few occasions we've rolled our own, but we limited it to special cases where profiling indicated we needed a boost (for example large joins). Since our applications typically use a custom abstraction layer (akin to a home-grown ORM) on top of the DB access, that's where we implemented the caching. We achieved good results that we were satisfied with and it didn't take a whole lot of effort. Of course, since we weren't using a CPAN ORM, we didn't really have any choice about using a CPAN caching module, either.

It was strictly case-by-case and opt-in. Whether you end up using a CPAN solution or rolling your own, it's probably a good idea to restrict it to cases where profiling indicates you need help, and make sure that it's opt-in so your caching doesn't undermine your application in subtle ways by being active when you didn't expect it.

Answer 3

I too have rolled my own ORM plenty of times I hate to say! Caching/Memoization is pretty easy if all your fetches happen through a single api (or subclasses thereof).

For any fetch based on a unique key you can just cache based on a concatenation of the keys. A naive approach might be:

my %_cache;

sub get_object_from_db {
    my ($self, $table, %table_lookup_key) = @_;

    # concatenate a unique key for this object
    my $cache_key = join('|', map { "$_|$table_lookup_key{$_}" }
                       sort keys %table_lookup_key

    return $_cache{$cache_key}
        if exists $_cache{$cache_key};

    # otherwise get the object from the db and cache it in the hash
    # before returning
}

Instead of a hash, you can use the Cache:: suite of modules on CPAN to implement time and memory limits in your cache.

If you're going to cache for some time you might want to think about a way to expire objects in the cache. If for instance all your updates also go through the ORM you can clear (or update) the cache entry in your update() ORM method.

A final point to think carefully about - you're returning the same object each time which has implications. If, eg., one piece of code retrieves an object and updates a value but doesn't commit that change to the db, all other code retrieving that object will see that change. This can be very useful if you're stringing together a series of operations - they can all update the object and then you can commit it at the end - but it may not be what you intend. I usually set a flag on the object when it is fresh from the database and then in your setter method invalidate that flag if the object is updated - that way you can always check that flag if you really want a fresh object.