Additional "processing instruction" type

[coleifer]

In a project I've been hacking on, a redis-like frontend for LMDB, I implemented and extended the Redis protocol with a few additional types.

One of these I think may interest you I'm calling a "processing instruction". It allows the client to send additional metadata along with a single command. Currently I am only using this functionality to allow the client to specify a different database for a single command (LMDB, like Redis, supports multiple databases within a single environment). So rather than having the client do:

• SELECT 0
• do some operation on db 0
• SELECT 15
• do some operation on db 15
• SELECT 0 -- reset the client state.

I can just explicitly SELECT 0 once, and when I need to run a single command against DB 15, I send (along with the command) a processing instruction that indicates the DB for that command should be db 15:

• do some operation on db 0
• (include processing instruction to use db 15) + do some operation on db 15
• do another operation on db 0

Another usage for processing commands could be to set/modify the expiration time on the key being operated on.

[AngusP]

Would this not be possible by sending a attribute type with the request from the client? For example,

|1<CR><LF>
    %1<CR><LF>
        +use-database<CR><LF>
        :1<CR><LF>
*3<CR><LF>
    LPUSH<CR><LF>
    somekey<CR><LF>
    someval<CR><LF>

While Redis looks like it will only be allowing array-type requests from clients, the protocol can work in full-duplex. In this way you could put arbitrarily complex meta and state in the 'header' using attribute type data.

[coleifer]

Yeah exactly, it would just be a question of how the server supported and interpreted such.