Gerbil Scheme

Connection Pools

(import :std/db/conpool)

A connection pool is simply a pool of database connections that use the same connect function. Beyond that there’s a max parameter where one can specify the max connections.

make-conpool

We need a pool to grab db connections from. The init for a conpool is simple enough.

(lambda (self connect (max #f)) ... )

So let’s make a pool.

(def (make-pgpool)
  (make-conpool
   (cut sql-connect postgresql-connect
        host: "localhost" user: "foo" passwd: "bar" db: "baz")
   2))

(def pgpool (make-pgpool))

We now have a pool of two connections to our database. Yay!

conpool?

A predicate. I’ll leave you to guess what it does by looking at the code.

(conpool? pgpool) ;; => #t
(conpool? 'not-pgpool) ;; => #f

conpool-get

A way to get a connection is somewhat important. (conpool-get pool timeout-in-seconds) is that way.

(def (conpool-get cp (timeo absent-obj)) ...)

That timeout is somewhat important was well. After all, we only have two connections and once they are used, once, for now ....

(def (my-first-query q . args)
  (apply sql-eval-query (conpool-get pgpool 5) q args))

(my-first-query "select 42") ;; => (42)
(my-first-query "select 42") ;; => (42)

;; But!!

(my-first-query "select 42")

; Evaluation aborted on conpool-get: [timeout-error] Error getting connection; timeout

Because of that we need to close and reset the conpool for now using conpool-close.

(conpool-close pgpool)
(set! pgpool (make-pgpool))

conpool-put

When we’re done with a connection that’s still working we need to put it back.

(import :std/sugar)

(def (my-sql-e sql-fn . args)
  (def con (conpool-get pgpool 5))
  (try
   (apply sql-fn con args)
   (finally (conpool-put pgpool con))))

It works.

(my-sql-e sql-eval-query "select 42") ;; => (42)
(my-sql-e sql-eval-query "select 42") ;; => (42)
(my-sql-e sql-eval-query "select 42") ;; => (42)
(my-sql-e sql-eval-query "select 42") ;; => (42)
;;; etc ...

But there’s still a problem of course. Connections themselves may have errors so they do not work.

Like, for example, erroring out in the middle of a transaction.

(my-sql-e sql-eval-query "begin;") ;; => ()
(my-sql-e sql-eval-query "select foo")
; Evaluation aborted on postgresql-prepare!: [sql-error] column "foo" does not
; exist --- irritants: (S . ERROR) (V . ERROR) (C . 42703) (M . column "foo"
; does not exist) (P . 8) (F . parse_relation.c) (L . 3349) (R .
; errorMissingColumn)

(my-sql-e sql-eval-query "select 1")
; Evaluation aborted on postgresql-prepare!: [sql-error] current transaction is
; aborted, commands ignored until end of transaction block --- irritants: (S .
; ERROR) (V . ERROR) (C . 25P02) (M . current transaction is aborted, commands
; ignored until end of transaction block) (F . postgres.c) (L . 1424) (R .
; exec_parse_message)

We can still use conpool-close, of course, but closing the entire conpool because one connection is in error ruins the point of a pool, more or less.

(conpool-close pgpool)
(set! pgpool (make-pgpool))

To get around that, conpool-release does exactly what we want.

conpool-release

In the case of an error the easy way is to release the connection. This basically says to the pool maintenance guy that there’s been some usage and we need to refill it a little bit.

(import :std/sugar)

(def (my-sql-ee sql-fn . args)
  (def err #f)
  (def con (conpool-get pgpool 5))
  (try
   (apply sql-fn con args)
   (catch (e) (set! err e))
   (finally
    (if err (begin (conpool-release pgpool con) (raise err))
        (conpool-put pgpool con)))))

Now it always has the pool ready for swimmers.

(my-sql-e sql-eval-query "begin;") ;; => ()
(my-sql-e sql-eval-query "select foo")
; Evaluation aborted on postgresql-prepare!: [sql-error] column "foo" does not
; exist --- irritants: (S . ERROR) (V . ERROR) (C . 42703) (M . column "foo"
; does not exist) (P . 8) (F . parse_relation.c) (L . 3349) (R .
; errorMissingColumn)

(my-sql-e sql-eval-query "select 1") ;; => (1)

conpool-close

If you’ve read this far you already know: conpool-close is for closing a conpool. After all, one does need to shut things down and often database servers prefer a clean shutdown of an open connection.

(conpool-close pgpool)

(my-sql-ee sql-eval-query "select 1")
; Evaluation aborted on Connection pool is closed

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