Hi, I have implemented the simple journal-based persistence library for Clojure programs and now making it public.
It follows "Prevalent system" design pattern.
I consider it a good fit for the prototyping stage of a project development.
When you don't want to pay the price of impedance mismatch between the
language data structures and your database because your database schema is
not stable yet.
However this pattern is used in production by some teams, see Prevayler mail
list for details. Of course this implementation can contain bugs and must be
tested well before doing that. Though it is very simple and I made some
tests.
The disadvantage of the pattern is that your data structures must fit in
memory (unless you implement ad-hoc paging solution). However the journaling
nature lets you easily switch to other databases. For that you just
re-implement your transaction functions (subjects of apply-transaction* ) to
write/read from another DB and replay transactions one time (init-db).
Snapshotting is not yet implemented. It can solve another pattern problem -
growing startup time.
Usage examples:
1. first run
(use 'persister)
(def refx (ref 0))
(def refy (ref 0))
(defn tr-fn [x y]
(do
(alter refx + x)
(alter refy + y) ))
(defn tr-fn-swap []
(let [tmp @refx]
(ref-set refx @refy)
(ref-set refy tmp)))
(defn tr-inc []
(ref-set refx (inc @refx))
(ref-set refy (inc @refy)) )
(init-db)
(apply-transaction tr-fn 1 2)
(apply-transaction tr-fn 10 20)
(apply-transaction tr-fn-swap)
(apply-transaction tr-inc)
[refx refy]
[#<r...@5bb966: 23> #<r...@1e903d5: 12>]
2. the second run
(use 'persister)
(def refx (ref 0))
(def refy (ref 0))
(defn tr-fn [x y]
(do
(alter refx + x)
(alter refy + y) ))
(defn tr-fn-swap []
(let [tmp @refx]
(ref-set refx @refy)
(ref-set refy tmp)))
(defn tr-inc []
(ref-set refx (inc @refx))
(ref-set refy (inc @refy)) )
(init-db)
[refx refy]
[#<r...@5bb966: 23> #<r...@1e903d5: 12>]
Note that journaled functions must be accessible in the current namespace
when you replay transactions.
See inline doc for details.
Looking for your feedback!
Regards, Sergey.
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