On Tuesday, August 13, 2019 2:17 PM, George Neuner <gneun...@comcast.net> wrote:
> On 8/13/2019 11:47 AM, 'Wayne Harris' via Racket Users wrote:
>
> > I'd like to save database results in memory because my database only
> > changes between long time intervals. By building a minimum
> > application, I see my cache-strategy seems to work per servlet: by
> > opening a new browser, the first request yields a cache-miss.
> > Looking through the documentation, I got the idea that perhaps I
> > should serve/servlet using
> > #:servlet-namespace '("shared-model.rkt")
> > where shared-model.rkt is the module that talks to the database and
> > implements the caching-strategy. But adding this keyword to
> > serve/servlet did not make any perceived difference.
> > What should I do to save results in memory across all requests?
>
> AFAIK #:servlet-namespace isn't necessary - you can share data (via
> access functions) among different instances of request handlers just by
> requiring the modules (files) that define the common objects wherever
> you need them.
I think it's what I'm doing below --- I require shared-model.rkt and call
(get-in-memory-results)
when I need it.
> If I'm not mistaken about the serve/servlet call in your code below, you
> are relying on the application to open the new browser window ...
> terminating and restarting the application each time. That clears your
> "cache", guaranteeing that it misses the first time. You should set
> #:launch-browser? #f , start the application and connect to it from
> your browser with the URL http://localhost:1111.
The documentation doesn't say that #:launch-browser? would have this effect.
Nevertheless, I tried it with #:launch-browser? #f, but the behavior was the
same.
(define (main)
(displayln "Now serving...")
(serve/servlet dispatch
#:launch-browser? #f
#:stateless? #t
#:log-file (build-path "logs/httpd.log")
#:port 1111
#:listen-ip "127.0.0.1"
#:servlet-path "/"
#:servlet-regexp #rx""
#:extra-files-paths (list (build-path "pub/"))
#:server-root-path (build-path "/")
#:file-not-found-responder file-not-found))
A browser window didn't open. With Chrome, I visited localhost:1111/main a
first time and refreshed it. (So I got a cache miss followed by a cache hit.)
With Firefox, I did the same and got a cache miss on the first request. I
expected a cache hit.
$ racket share.rkt
Now serving...
Your Web application is running at http://localhost:1111.
Stop this program at any time to terminate the Web Server.
database: cache miss
database: cache hit
database: cache miss
database: cache hit
> That said:
>
> What DBMS are you using?
I'm using sqlite and in other applications I store lists on disk. These are
very small applications and it's useful not to depend on other software and
libraries.
> [...] Server based DBMS like Oracle, Postgresql,
> MySQL, SQLServer, etc. automatically cache query results in case the
> same query is run again. If the server is well provisioned memory-wise,
> it can take a long time for a popular query to age out the cache. If
> your application is co-resident (on the same machine) with the server,
> caching results yourself would be duplicating effort.
Thanks for the info!
> > --- server.rkt
> >
> > ---------------
> >
> > #lang racket/base
> > (require
> > web-server/servlet
> > web-server/servlet-env
> > (prefix-in model: "shared-model.rkt"))
> > (define-values (dispatch url)
> > (dispatch-rules
> > (("main") db->response)
> > (("update" (string-arg)) update->response)))
> > (define (update->response r s)
> > (define str (model:in-memory-db-set-and-get s))
> > (displayln str)
> > (string->response str))
> > (define (db->response r)
> > (string->response (model:get-in-memory-results)))
> > ;; no more api endpoints ==/==
> > (define (string->response s)
> > (response/full 200 #"Okay" (current-seconds)
> > TEXT/HTML-MIME-TYPE '() (list (string->bytes/utf-8 s))))
> > (define (file-not-found r)
> > (response/xexpr "File not found."))
> >
> > (module+ main
> > (file-stream-buffer-mode (current-output-port) 'line)
> > (define (main)
> > (displayln "Now serving...")
> > (serve/servlet dispatch
> > ;; #:servlet-namespace '("shared-model.rkt")
> > #:stateless? #t
> > #:log-file (build-path "logs/httpd.log")
> > #:port 1111
> > #:listen-ip "127.0.0.1"
> > #:servlet-path "/"
> > #:servlet-regexp #rx""
> > #:extra-files-paths (list (build-path "pub/"))
> > #:server-root-path (build-path "/")
> > #:file-not-found-responder file-not-found))
> > (main))
> >
> > --- shared-model.rkt
> >
> > ---------------------
> >
> > #lang racket/base
> > (define in-memory-database (make-parameter #f))
> > (define (in-memory-db-set-and-get x)
> > (in-memory-database (format "~a: data to be shared across servlets" x))
> > (in-memory-database))
> >
> > (define (get-in-memory-results [refresh? #f])
> > (if refresh?
> > (begin (displayln "database: force refresh")
> > (in-memory-db-set-and-get "forced"))
> > (let ([in-memory-db (in-memory-database)])
> > (if in-memory-db
> > (begin (displayln "database: cache hit")
> > in-memory-db)
> > (begin (displayln "database: cache miss")
> > (in-memory-db-set-and-get "miss"))))))
> > (provide (all-defined-out))
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