How to fully utilise lwt_pool?

Question

I am struggling at the check and validate function for Lwt_pool.create and have some questions here.

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val create : 
  int ->

  ?check:('a -> (bool -> unit) -> unit) ->

  ?validate:('a -> bool Lwt.t) -> (unit -> 'a Lwt.t) -> 'a t

First of all, let me describe the background of my usage.

I wish to use Lwt_pool to manage a pool of database connections. The database is MongoDB and the driver was made by myself (Mongo.ml). The driver is actually simple that it is just a TCP (Unix.file_descr) connection to the MongoDB server and send requests / receive responses with the server.

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`create n ?check ?validate f` creates a new pool with at most n members. f is the function to use to create a new pool member.

An element of the pool is validated by the optional validate function before its Lwt_pool.use. Invalid elements are re-created.

The optional function check is called after a use of an element failed. It must call its argument excatly one with true if the pool member is still valid and false otherwise.

above is the documentation for create

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So here are my questions:

From the doc, I understand validate is to validate the connection before using it.

so my first question is How can I check the availability of a Unix.file_descr? I only know that in order to check it, I have send something through it, right? But if I send something through my connection in order to check, then I guess it would be urgly and I anyway want to send something out via Lwt_pool.use, why bother do similar things before use?

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My second question is about check.

So check will be used after use. From the doc, I really can't understand. check is a function which take a my_db_connection (in my case) and a (fun b -> unit) as parameter. Who will provide (fun b -> unit)? Does Lwt_pool itself has such a function? or I should provide it? To do what then?

thanks

Answer 1

I don't know anything about Lwt, but one thing to do is to wait to use a validate function until you see why you would need it. That's why it's an optional parameter (I suspect).

One thing you can do with a Unix file descriptor is to figure out whether it's attached to a network socket:

let is_a_socket fd = (Unix.fstat fd).st_kind = S_SOCK

Maybe this will be useful, though someone with Lwt experience can probably give a better answer.