[tarantool-patches] Re: [PATCH 2/2] swim: introduce generation

[Konstantin Osipov]

* Vladislav Shpilevoy <v.shpilevoy at tarantool.org> [19/06/22 01:03]:

Yup.

> We have decided in a chat, that a user should be
> able to get separately 'generation', 'version', and
> accumulated 'incarnation' as a binary merge of these
> two values. It allows in simple cases do not care
> about generation/version, and stick to the incarnation
> only.
> 
> Concerning API. I suggest the following C API changes:
> 
> -	uint64_t
> -	swim_member_incarnation(const struct swim_member *member);
> 
> +	struct swim_incarnation {
> +		uint64_t generation;
> +		uint64_t version;
> +	};
> +
> +	int
> +	swim_incarnation_cmp(const struct swim_incarnation *l,
> +			     const struct swim_incarnation *r);
> +
> +	struct swim_incarnation
> +	swim_member_incarnation(const struct swim_member *member);
> 
> struct swim_incarnation will be a public structure in order to
> simplify working with C API. I am trying to avoid returning some
> const char * or similarly abstract thing.
> 
> 
> 
> Lua API changes are different. I know, we've talked about
> returning a binary string as an incarnation, but then we need
> separate methods to take version and generation, and waste
> memory on these temporary binary strings, useless for anything
> but comparison. Worse, we will desync with C API.
> 
> 
> I've found a revolutionary feature of Lua FFI which can help us
> very much. It combines your 'merged' incarnation, and my 'totally
> different numbers' incarnation. In Lua FFI you can define your
> own comparison operators for C structs (just learned about that)!
> 
> Method member:incarnation() will return cdata struct swim_incarnation,
> with ffi.metatype containing comparators. struct swim_incarnation
> will be initialized in Lua like this:
> 
> 
> 	ffi = require('ffi')
> 	ffi.cdef[[
> 		struct swim_incarnation {
> 			uint64_t generation;
> 			uint64_t version;
> 		};
> 	]]
> 	inc_t = ffi.typeof('struct swim_incarnation')
> 	inc_mt = {
> 		__eq = function(l, r)
> 			return l.generation == r.generation and
> 			       l.version == r.version
> 		end,
> 		__lt = function(l, r)
> 			return l.generation < r.generation or
> 			       l.generation == r.generation and l.version < r.version
> 		end,
> 		__le = function(l, r)
> 			return l.generation < r.generation or
> 			       l.generation == r.generation and l.version <= r.version
> 		end,
> 	}
> 	ffi.metatype(inc_t, inc_mt)
> 	ia = ffi.new('struct swim_incarnation')
> 	ib = ffi.new('struct swim_incarnation')
> 
> ia and ib can be compared using just '< > <= >= == ~=' operators,
> and at the same time you can access individual members!!!
> 
> What do you think? Both about C and Lua API changes.

-- 
Konstantin Osipov, Moscow, Russia