Re: Error handling strategy for Redis multi-bulk replies, in context of Lua bindings

["Robert G. Jakabosky" <bobby@...>]

On Wednesday 23, Alexander Gladysh wrote:
> Hi, all!
> 
> I'm working on bindings for hiredis Redis client library.
> 
> Here is the library: https://github.com/antirez/hiredis
> 
> Here is my module: https://github.com/agladysh/lua-hiredis (WARNING:
> work in progress.)
> 
> I have question about error handling. (Sorry, I have to give some
> context first.)
> 
> 1. In Redis one can do a transaction — atomically execute multiple
> commands, using MULTI/EXEC commands.
> 
> More info here: http://redis.io/topics/transactions
> 
> 2. If a command inside a transaction fails — all other commands are
> still executed.
> 
> 3. On transport protocol level, MULTI/EXEC transaction results are
> transmitted to client as a reply to EXEC command.
> 
> 4. This reply is in "multibulk" format (REDIS_REPLY_ARRAY) — an array
> of arrays (nesting level is not limited).
> 
> 5. Command execution errors are encoded as a distinct transport
> protocol type. In hiredis — REDIS_REPLY_ERR.
> 
> More info on hiredis API here:
> https://github.com/antirez/hiredis/blob/master/README.md
> 
> My question: How to expose command execution errors to Lua code?
> 
> See my current prototype implementation here: http://bit.ly/fDE5mc
> 
> Now, whenever I see REDIS_REPLY_ERR redisResponse, I abort everything
> and just return nil, error_message.
> 
> For multibulk replies it is plain wrong — user does want to see
> results of execution of other commands in MULTI/EXEC.
> 
> To give some concrete example:
> 
> MULTI
> SET a 3
> LPOP a
> INCR a
> EXEC
> 
> Returns:
> 
> 1. OK
> 2. (error) ERR Operation against a key holding the wrong kind of value
> 3. (integer) 4
> 
> How should this result be returned to Lua?
> 
> {
>   "OK",
>   -- how to express error message here?
>   4
> }
> 
> Any hints?
> 
> Thanks,
> Alexander.
> 
> P.S. Note that "OK" is a status reply (REDIS_REPLY_STATUS), not a
> string (REDIS_REPLY_STRING). I'd like to separate those two as well,
> but this is of much less priority (I think).

I would wrap status/error replies in a status userdata object with a is_error 
field/method.  For string/number reply values convert them to basic lua 
string/numbers.

-- 
Robert G. Jakabosky