On Mon, Apr 24, 2017 at 9:46 PM, Philip McGrath <phi...@philipmcgrath.com> wrote:
> Another thing that might be relevant: >> >> In contrast, direct assignment to a parameter (by calling the parameter >> procedure with a value) changes the value in a thread cell, and therefore >> changes the setting only for the current thread. http://docs.racket- >> lang.org/reference/parameters.html > > > Also, do you want multiple underlying connections of the same virtual > connection to potentially use different values for the > user/database/password/port? > Nope, a given dbh should only be connected to one DB. > > If not, the most recent way I've dealt with this sort of thing is to come > up with some type of "database connection spec" — for a quick example, I'll > use a hash table — and then do some things with parameter guards, opaque > structures, and a tiny macro to keep the details away from higher-level > code. > > I think code might be clearer than a description, so here's a minimal > sketch: > > I admit that I had to stare at this for a few minutes and look up some details (e.g. how hash/dc works), but when it clicked my reaction was "Racket. Is. Epic." Thank you for this; it's exactly what I needed and I'll definitely use it. Two questions: 1) This line was intended to reference (current-database-handle), not (current-database), right? [_ (force (database-handle-promise (current-database)))])) 2) We're already using hashes for data passing in a lot of areas because (a) they are really convenient and (b) my co-founder and I both come from Perl backgrounds. In this particular case, though, it seems like maybe I should define a struct for the database spec instead of a hash, as hash/dc validates the keys and values that happen to exist in the hash, as opposed to validating that the hash has a given set of keys/values. Does this make sense or am I overcomplicating things? Dave NB: I'm glad you used hashes for the demo, though, as it was much easier to follow and introduced me to the hash/dc contract. #lang racket > > (require db > ) > > (struct database-handle (promise)) > > (define database-spec/c > (or/c database-handle? > (hash/dc [k (or/c 'user 'database 'password 'port)] > [v (k) > (case k > [(user database password) > string?] > [(port) > natural-number/c] > [else none/c])]))) > > (define default-database-spec > #hasheq([user . "postgres"] > [database . "test-db"] > [password . "mellon"] > [port . 5433])) > > > (define/contract current-database-handle > (parameter/c database-spec/c database-handle?) > (let ([initialize > (λ (spec) > (if (database-handle? spec) > spec > (let ([u (hash-ref spec 'user)] > [d (hash-ref spec 'database)] > [p (hash-ref spec 'password)] > [prt (hash-ref spec 'port)]) > (database-handle > (delay (virtual-connection > (connection-pool > (λ () > (postgresql-connect > #:user u > #:database d > #:password p > #:port prt > )))))))))]) > (make-parameter (initialize default-database-spec) > initialize))) > > (define-syntax db > (syntax-id-rules () > [_ (force (database-handle-promise (current-database)))])) > > (provide database-handle? ;keep constructor & accessor private > database-spec/c > current-database-handle > db > ) > > > > On Mon, Apr 24, 2017 at 7:19 PM, David Storrs <david.sto...@gmail.com> > wrote: > >> >> >> On Mon, Apr 24, 2017 at 4:43 PM, Scott Moore <sc...@thinkmoore.net> >> wrote: >> >>> Parameters are thread local, and from reading the docs of >>> virtual-connection and connection-pool, I think they create new threads to >>> handle the connections. These threads are probably created before you >>> parameterize, and thus see the original values. >>> >>> Try replacing the virtual-connection stuff with a single call to >>> do-connect, and see if that works. (Obviously not the real fix, but may >>> help identify the error). >>> >>> >> This was exactly it. Thanks, Scott. >> >> Now I just need to figure out how to work around it. >> >> >>> On Apr 24, 2017, 4:35 PM -0400, David Storrs <david.sto...@gmail.com>, >>> wrote: >>> >>> I'm finding parameters confusing and was hoping someone could explain >>> them for me. The following is a simplified version of our production code; >>> I'm expecting it to fail but it does not. What am I not understanding? >>> >>> The sequence goes: >>> >>> *) db.conf creates and provides some parameters (db username, db >>> password, db name, db port) and a function for creating database >>> connections using those parameters >>> >>> *) dbstuff.rkt requires db.conf, re-provides the parameters from >>> db.conf, and also provides a wrapper around the db connector from db.conf >>> >>> *) bar.rkt parses the command line, uses that command line data to set >>> the parameters that were imported from db.conf by way of dbstuff.rkt and >>> makes a DB call >>> >>> *) I run bar.rkt from the command line with an invalid username for the >>> database, expecting it to fail to connect, but it works fine??? >>> >>> >>> >>> My first guess was that since I was using the parameters as default >>> values perhaps they were being evaluated at compile time instead of >>> runtime. I tried changing the default value to #f and then doing (or u >>> (db-user)) in the body of the function, but that had no effect. >>> >>> I am utterly baffled at this point. Any help would be much appreciated. >>> >>> >>> >>> ################################# file: "db.conf" >>> #lang racket >>> >>> (require db) >>> >>> (define db-user (make-parameter "postgres")) >>> (define db-name (make-parameter "test-db")) >>> (define db-password (make-parameter "mellon")) >>> (define db-port-num (make-parameter 5433)) >>> >>> (define (do-connect #:u [u (db-user)] >>> #:d [d (db-name)] >>> #:p [p (db-password)] >>> #:prt [prt (db-port-num)]) >>> (println (~a "In db.conf db-user: " (db-user))) >>> (println (~a "db-name: " (db-name))) >>> (println (~a "db-password: " (db-password))) >>> (println (~a "db-port: " (db-port-num))) >>> >>> (postgresql-connect #:user u >>> #:database d >>> #:password p >>> #:port prt >>> )) >>> >>> (define dbh (virtual-connection >>> (connection-pool >>> do-connect))) >>> >>> (define (make-connect) >>> dbh) >>> >>> (provide (all-defined-out)) >>> >>> >>> ################################# file: "dbstuff.rkt" >>> #lang racket >>> >>> (require db) >>> (require "db.conf") >>> >>> (define/contract (dbh) >>> (-> connection?) >>> (make-connect)) >>> >>> (provide (all-defined-out) >>> (all-from-out db) >>> (all-from-out "db.conf")) >>> >>> >>> ################################# file: "bar.rkt" >>> #lang racket >>> >>> (require "dbstuff.rkt") >>> >>> (command-line >>> #:program "db demo" >>> #:once-each >>> [("--db-user") dbu "Username for database handles" >>> (db-user dbu)] >>> ) >>> >>> (println (~a "In bar: db user: " (db-user))) >>> (query-value (dbh) "select 'this should not have worked'") >>> >>> >>> ################################# command line: >>> >>> $ racket bar.rkt --db-user invalid_user >>> "In bar: db user: invalid_user" >>> "In db.conf db-user: postgres" >>> "db-name: test-db" >>> "db-password: mellon" >>> "db-port: 5433" >>> "this should not have worked" >>> >>> >>> >>> >>> -- >>> You received this message because you are subscribed to the Google >>> Groups "Racket Users" group. >>> To unsubscribe from this group and stop receiving emails from it, send >>> an email to racket-users+unsubscr...@googlegroups.com. >>> For more options, visit https://groups.google.com/d/optout >>> <https://urldefense.proofpoint.com/v2/url?u=https-3A__groups.google.com_d_optout&d=CwMFaQ&c=WO-RGvefibhHBZq3fL85hQ&r=OPR-Xys5wfSBIeTkWaH0D_htBR-X7qY24pTHU6ib2iM&m=Lcp1Jw9QrhH7FcWlq8qeLkEBKViDQXsoSPmFvPX80kw&s=VCtNBj2gcTR_1WATmbFxyRwK11FRxpWqOxTfpfzQYNA&e=> >>> . >>> >>> >> -- >> You received this message because you are subscribed to the Google Groups >> "Racket Users" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to racket-users+unsubscr...@googlegroups.com. >> For more options, visit https://groups.google.com/d/optout. >> > > -- You received this message because you are subscribed to the Google Groups "Racket Users" group. 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