[Opa] Db3 -> Mongo || Db3.fresh_key -> ?

[Rudy Sicard]

Hello,

If collision or compactness of the key range are a serious concern, you 
have to maintain a counter somewhere (with both intmap and set)
(see http://shiflett.org/blog/2010/jul/auto-increment-with-mongodb).
You can use findAndModify to increment and return a counter atomically 
from the db.
Or assuming only one server, you can use a cell (to ensure atomicity) 
that is saved in db (for server reload).

I am going to push code for both memory atomic operation, and 
findAndModify interface.
So if you go for seconds solutions, let me know, if you are lost, I can 
push it sooner.

If for you are using a integer key for bad reason and want to stick to 
it for good reason,
and you don't care about compactness of the key range,
and you don't intend to insert more than billions of entries (or 
thousands for 32bits machines)
and you are not writing code for a nuclear power plant (i.e. collisions 
are not lethal).
Just go with a proven solution, random key (=> e.g. freshkey() = 
Random.int(max_int)).
You can seriously reduce collision to a probability of almost zero just 
by checking the key is not already used,
so it is not as bad as you may think.
freshkey(check) =
     k = Random.int(max_int)
     if check(k) then k
     else freshkey(check)

Rudy

On 30/04/2012 18:38, Matthieu Guffroy wrote:
> Hello,
>
> How we get a fresh_key with mongo backend ?
> Is there a different way of using it ?
>
> I think the best would be to insert in a set instead of inserting in a 
> intmap with a freshkey. And let mongo autoincrement the primary key...
> Is it possible ? If yes how ?
>
>
> Thanks,
>
> --
> Matthieu GUFFROY
>
>
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