Forum OpenACS Development: connection-global cache: namespace problem?

Hi,

I've run into a little problem with the snippet of code below. It is not working in the sense it won't cache values in the global namespace as intended. Please help. (Relevant parts of the code is highlighted in bold)

I call (a variant of) this proc from my scripts like this:

set name [curriculum::conn name] and expect it to return the value from a query (or whatever) the first time I call it, and that it returns a cached value from the global variable cu_conn(name) on the following calls.

It turns out, however, that it always returns the uncached name; "Name from an expensive query, say - NOT CACHED" ...

This technique has more or less (with emphasis on more) been lifted from bug_tracker::conn and, of course, from ad_conn, so, if what I've found is an actual bug and not just a result of my working to hard on a saturday evening 😊, it _may_ also be a problem in Bug Tracker (Probably not with ad_conn since it doesn'n utilize namespaces). I'm not saying Bug Tracker suffers from this, but it could explain the (ahem) relative sluggishness of the bug tracker here on openacs.org 😉.

namespace eval curriculum {

    ad_proc -public conn {
        args
    } {
        global cu_conn

        set flag [lindex $args 0]
        if { [string index $flag 0] != "-" } {
            set var $flag
            set flag "-get"
        } else {
            set var [lindex $args 1]
        }

        switch -- $flag {
            -set {
                set cu_conn($var) [lindex $args 2]
            }

            -get {
                if { [info exists cu_conn($var)] } {
                    return "WOO HOO! $cu_conn($var) - CACHED"
		}
                switch -- $var {
                    name {
                        set cu_conn($var) "Name from an expensive query, say"

                        return "$cu_conn($var) - NOT CACHED"
                    }
                    default {
                        error "Unknown variable $var"
                    }
                }
            }

            default {
                error "cu_conn: unknown flag $flag"
            }
        }
    }



}

This kind of variable caching only works for variables that are filled at startup and never changes.  Once the server is running it has a number of distinct interpreters that do not share global variables.

Try to use "nsv_set collection_name var_name var_value" and "nsv_get collection_name var_name" instead.

Another important benefit from Roberto's method is that it allows Emacs to index the proc ::foo::bar with the fully qualified name rather than bar.

When bar is called outside the namespace foo the procedure is called with the fully qualified name ::foo::bar. Looking up the proc's definition with Emacs's tag search (M-.) only works with Roberto's method.

/Bart

Thanks David,

I will try something like the following and see how it works (it seems to work fairly well so far) ...

ad_proc -public ::curriculum::conn {
    args
} {
    set flag [lindex $args 0]
    if { [string index $flag 0] != "-" } {
        set var $flag
        set flag "-get"
    } else {
        set var [lindex $args 1]
    }

    set my_array MyArray

    switch -- $flag {
        -set {
            # Set cache key $var to some value and return it
            return [nsv_set $my_array $var [lindex $args 2]]
        }
        -flush {
            # Unset key $var - effectively flushes the cache
            nsv_unset $my_array $var
        }
        -nocache {
            # Unset key $var - effectively flushes the cache
            nsv_unset $my_array $var

            # Call ourself with the -get flag and return fresh data
            ::curriculum::conn -get $var
        }
        -get {
            # Get value of key $var - from query or cache
            if { [nsv_exists $my_array $var] } {
                return "[nsv_get $my_array $var] - CACHED"
            }
            switch -- $var {
                name -
                shoesize -
		weight -
                whatever {
                    return "[nsv_set $my_array $var "$var from an expensive query, say"] - NOT CACHED"
		}
                default {
                    error "Unknown variable $var"
                }
            }
        }
        default {
            error "::curriculum::conn: unknown flag $flag"
        }
    }
}

Roberto: That's a couple of really good points, I think. We should stick to this advice unless we come up with something better still.

Getting rid of an unnessecary level of indentation is worth gold!

Don: I have checked out ns_cache two weeks ago and a thread-private cache does not work since there are ~5 threads in each connection, I believe. The Global ns_cache is accessible to all threads, which is not what I want (I guess).

I want to cache across interpreters but not _all_ interpreters - only the five or so per connection, no?

I hope there is a smooth way to make caches accessible on a per user basis.

I feel as though I've probably misunderstood some of the interpreter/thread/connection mess (nothing the community couldn't help straighten out, I'm sure) and it's too late in the evening now, anyway, so I'll have to look at this and eventual extra answers in the morning...

/Ola

If you want to cache something per session (user) you might also want to look into ad_set_client_property and ad_get_client_property, with '-persistent f' set.