Investigate how to support the use-cases from indexpiration

[akudiyar]

Currently, expirationd does not support iteration over a (secondary) index (since the expriration check callback must be applied to each tuple from a space). This creates a situation where a full scan is applied instead of selecting tuples from some start index value.

TODO:

• Investigate how indices are used in the current expirationd implementation (check how the primary index is used, is it possible to iterate over a complex primary index, simple or complex secondary index)
• Propose a RFC for changing the API to support iterating over a specified index or a field (the minimal satisfying index must be chosen in this case if it exists)
• Check how the module is initialized: is it possible to reload the configuration and any created fibers or the internal state at runtime, thus supporting the "hot reload" feature
• Propose a RFC for changes to support the "hot reload" feature

[ArtDu]

The result of analyzing the existing code:

1. Now there is a hardcode implementation of iterations by index zero, i.e. by primary.
   Secondary or complex indexes cannot be used at this time.
   

   expirationd/expirationd.lua

   Lines 91 to 100 in 29d1a25

   	local tuples = scan_space.index[0]:select({}, params)
   	while #tuples > 0 do
   	last_id = tuples[#tuples]
   	for _, tuple in ipairs(tuples) do
   	expiration_process(task, tuple)
   	end
   	suspend(scan_space, task)
   	local key = construct_key(scan_space.id, last_id)
   	tuples = scan_space.index[0]:select(key, params)
   	end

2. hot reload of the configuration is possible, since we have direct access to the variables that are used in the fibers.
   

   expirationd/expirationd.lua

   Line 30 in 29d1a25

   local task_list = {}

   
   and so we already have a function to reload the expirationd module - update ().
   

   expirationd/expirationd.lua

   Lines 515 to 519 in 29d1a25

   	-- Update expirationd version in running tarantool
   	-- * remove expirationd from package.loaded
   	-- * require new expirationd
   	-- * restart all tasks
   	local function expirationd_update()

• need to write a test for hot reload

[Mons]

We don't need this, since there is indexpiration already.
These modules have strongly different approaches: by index and by callback.
Mixing them up in one module will lead to many misuses.

I see no reason to implement it in expirationd. Different modules cause the user to choose between patterns: either full-scan with the arbitrary condition inside user function or very precise index scan by index with a timestamp.