Re: I am confused by Mirage_kv.S.batch

[Mindy <mindy@xxxxxxxxxxxxxxxxxxx>]

Er, s/Mirage_kv\.S/Mirage_kv.RW/g below. :sweat_smile:

On 1/2/22 16:13, Mindy wrote:
> Hi all,
>
> I've been working on a block-backed Mirage_kv.S implementation 
> inspired by (and largely interoperable with) littlefs[1] . I'm 
> approaching readiness for release, but there remains one large 
> unimplemented piece of Mirage_kv.S : `batch`. The relevant excerpt of 
> mirage_kv.mli is reproduced below:
> ```
>
>   val batch: t -> ?retries:int -> (t -> 'a Lwt.t) -> 'a Lwt.t
>   (** [batch t f] run [f] in batch. Ensure the durability of
>      operations.
>
>       Since a batch is applied at once, the readings inside a batch
>      will return the state before the entire batch. Concurrent
>      operations will not affect other ones executed during the batch.
>
>       Batch applications can fail to apply if other operations are
>      happening concurrently. In case of failure, [f] will run again
>      with the most recent version of [t]. The result is
>      [Error `Too_many_retries] if [f] is run for more then [retries] 
> attemps
>      (default is [13]). *)
> ```
>
> I confess to being uncertain how to implement the "readings inside a 
> batch" portion of this. Given an arbitrary function `t -> 'a Lwt.t`, 
> how is the implementer expected to differentiate between operations 
> within such a function, possibly implemented as a series of sequential 
> binds?
> Currently my implementation simply runs the given function against the 
> filesystem, but this is clearly semantically wrong given the 
> explanation in the docstring.
> I see `wodan` implements this by differentiating between a `t` used 
> for read operations and one used for write operations[2] , but I lack 
> the understanding of `wodan` necessary to understand how this can work 
> for a `t -> 'a Lwt.t` that mixes read and write operations, which I 
> think the docstring implies to be an expected use of `batch`.
> Thanks for your help,
>
> Mindy
>
> [1] https://github.com/littlefs-project/littlefs
>
> [2] 
> https://github.com/mirage/wodan/blob/main/src/wodan-irmin/wodan_irmin.ml#L345