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Subject: Re: [boost] [AFIO] Callback API
From: Bjorn Reese (breese_at_[hidden])
Date: 2013-12-28 06:12:29

Moving the discussion from boost-users to boost-dev, and thus the full
context is quoted.

On 12/21/2013 01:53 AM, Niall Douglas wrote:
> On 20 Dec 2013 at 19:16, Bjorn Reese wrote:
>>> Can you suggest something? I honestly can't think of anything simpler
>>> which also provides strong write ordering guarantees.
>> I have not given this much thought so consider the following a
>> brainstorm.
> We really ought to move this off boost-users ... but we'll see how it
> goes.
>> I am thinking about an API that uses handles that looks more like Asio
>> sockets. Write ordering can be handled, not by batching operations
>> together, but rather calling the next write operation from the callback
>> of the previous operation (Asio-style.) This will not always yield good
>> performance, but oftentimes that is less relevant. If you need
>> performance, then the "advanced" dispatcher API is available.
>> So there could be a file handle (and directory handle) class for file
>> (directory) manipulation calls, which hides all the details of the
>> dispatcher etc.
>> class file_handle
>> {
>> public:
>> void read(buffer, read_callback);
>> void write(buffer, write_callback);
>> // and so on
>> };
>> class directory_handle
>> {
>> public:
>> void create(name, create_callback); // uses file(single) or dir(single)
>> void remove(name, remove_callback); // uses rmdir(single)
>> void watch(name, watch_callback); // directory monitoring
>> // and so on
>> };
> I'm struggling to see the merit in such an approach - it would be
> incredibly verbose and complex to write even simple solutions,
> because i/o on files is not like sockets, *especially* that you
> almost never must strongly order i/o across a sequence of multiple
> sockets, but that is a very common case with files e.g. during ACID.

First, it follows a pattern that is familiar to Asio users. So rather
than you being the only one who can write applications on top of Afio,
you get a broader community who are already familiar with the callback

Second, you can more easily combine it with Asio socket code to create
compound services that mixes socket and file access. For instance, a
peer-to-peer file distribution mechanism.

> AFIO tries to help users to not write callbacks except when
> necessary, but if you do want a user defined callback you simply
> chain a call() or completion() operation to the item whose completion
> you want called back upon.
> The idea is, you see, that you subclass the async_io_dispatcher class
> with additional completion handlers, and use those as building blocks
> for further subclasses of async_io_dispatcher. That hopefully gets
> people to break up their callbacks into reusable completion handlers,
> and saves people writing and debugging code.

> Before you say "this should be in the documentation", yes it should
> and will be after Paul gets his directory monitoring implementation
> working. I'm thinking that will form the fourth section in the
> beginner's tutorial - how to modularise and make reusable the
> normally bespoke glue code which makes up traditional callbacks. It
> might actually be worth adding a section 3b which shows how the naïve
> approach in 3a is a very stupid idea :)
> As much as this sort of operation dependency graph based design works
> well for its niche, I agree there is a swathe of code which ends up
> looking like the find in files implementation, and for that hopefully
> fibers will make look much more sane.

I am not arguing against the current API. Instead, I am arguing for the
addition of a more familiar Asio-style callback API. I have no trouble
with a fiber-enchanced API (similar to Asio with coroutines.)

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