On Sat, Nov 14, 2009 at 11:42 AM, André Thieme
<splendidl...@googlemail.com>wrote:
> Dereferencing *persons* will result in:
> {"Tina" #<r...@7ae6d: {:name "Tina", :age 19, :friends []}>,
> "Jeff" #<r...@125d92c: {:name "Jeff", :age 22, :friends []}>,
> "Karl" #<r...@14fa0ef: {:name "Karl", :age 20, :friends []}>}
> Great so far.
>
> People can become friends, so we need
> (defn add-friend [#^String person #^String friend]
> (dosync
> (let [p (get @*persons* person)
> f (get @*persons* friend)]
> (alter p update-in [:friends] conj f))))
>
> So, Tina can get the friends Karl and Jeff:
> (add-friend "Tina" "Karl")
> (add-friend "Tina" "Jeff")
>
> Now the problem is: *persons* acts as our data base, and
> when a request comes in (to our web application, hosted by,
> say, Compojure) we want to see a consistent snapshot of the
> DB.
> But derefing *persons* won't do this for us.
> Even when we deref Tina, we can not be sure who her friends
> are, as those live in their own refs.
> They have to. If Karl gets one year older then we only want to
> change his date, and not traverse all persons first and see if
> they have Karl as a friend and if yes updating his age there too.
>
> How can we handle this situation?
I think the design in this sort of case needs changing.
Repeated structure references also pose a problem for storage and retrieval
(at least if not using a real DB) as Clojure's prn and read will turn shared
substructures into separate copies.
What's needed here is to make all person lookups go through one point, the
*persons* table, so the :friends key would store a vector not of person
struct references but simply of names.
When you start using a real DB this won't change; only the name of the
concept will, to a "key column". Name will be a key column in the *persons*
table and will be how you get at particular rows (persons) in the table.
--
You received this message because you are subscribed to the Google
Groups "Clojure" group.
To post to this group, send email to clojure@googlegroups.com
Note that posts from new members are moderated - please be patient with your
first post.
To unsubscribe from this group, send email to
clojure+unsubscr...@googlegroups.com
For more options, visit this group at
http://groups.google.com/group/clojure?hl=en
