Rotate slaves

[TimWeils]

While using this connector in my code I noticed that the operations towards slaves are not balanced between all slave instances returned by Sentinel. Connector just tries to connect to slaves in the order returned by Sentinel and uses the first one that succeeds. I checked how other Redis Clients solve this problem and for example client for Python rotates the slaves in round-robin manner (as you can see here). I implemented similar rotation for this connector so that all slave instances are utilized equally.

[stepulak]

This is really helpful, thank you.

Could you please @pintsized or @piotrp look at this pull request?

[pintsized]

LGTM. Could you please add a test case to cover the change? You can find examples in sentinel.t.

[piotrp]

You shouldn't keep rotation state in a global variable, this should be kept in connector instance. And, preferably, be a opt-in behavior (or at least opt-out?).

A few technical remarks:

• you call #slaves a lot of times, this should be kept in a variable
• a new array should be created by tab_new

A generalized rotation function could look like this:

local function rotate_slaves_2(slaves, rotate_by)
    local slave_count = #slaves
    local rotated_slaves = tbl_new(#slaves, 0)

    for i, slave in ipairs(slaves) do
        local new_index = (i - rotate_by - 1) % slave_count + 1
        rotated_slaves[new_index] = slave
    end

    return rotated_slaves
end