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-- Original Authors: Shiv Nagarajan & Scott Francis
-- Accessed: March 12, 2018
-- Inspiration drawn from:
-- https://github.com/twitter/finagle/blob/1bc837c4feafc0096e43c0e98516a8e1c50c4421
-- /finagle-core/src/main/scala/com/twitter/finagle/loadbalancer/PeakEwma.scala
local util = require("util")
local split = require("util.split")
local DECAY_TIME = 10 -- this value is in seconds
local PICK_SET_SIZE = 2
local _M = { name = "ewma" }
local function decay_ewma(ewma, last_touched_at, rtt, now)
local td = now - last_touched_at
td = (td > 0) and td or 0
local weight = math.exp(-td/DECAY_TIME)
ewma = ewma * weight + rtt * (1.0 - weight)
return ewma
end
local function get_or_update_ewma(self, upstream, rtt, update)
local ewma = self.ewma[upstream] or 0
local now = ngx.now()
local last_touched_at = self.ewma_last_touched_at[upstream] or 0
ewma = decay_ewma(ewma, last_touched_at, rtt, now)
if not update then
return ewma, nil
end
self.ewma[upstream] = ewma
self.ewma_last_touched_at[upstream] = now
return ewma, nil
end
local function score(self, upstream)
-- Original implementation used names
-- Endpoints don't have names, so passing in IP:Port as key instead
local upstream_name = upstream.address .. ":" .. upstream.port
return get_or_update_ewma(self, upstream_name, 0, false)
end
-- implementation similar to https://en.wikipedia.org/wiki/Fisher%E2%80%93Yates_shuffle
-- or https://en.wikipedia.org/wiki/Random_permutation
-- loop from 1 .. k
-- pick a random value r from the remaining set of unpicked values (i .. n)
-- swap the value at position i with the value at position r
local function shuffle_peers(peers, k)
for i=1, k do
local rand_index = math.random(i,#peers)
peers[i], peers[rand_index] = peers[rand_index], peers[i]
end
-- peers[1 .. k] will now contain a randomly selected k from #peers
end
local function pick_and_score(self, peers, k)
shuffle_peers(peers, k)
local lowest_score_index = 1
local lowest_score = score(self, peers[lowest_score_index])
for i = 2, k do
local new_score = score(self, peers[i])
if new_score < lowest_score then
lowest_score_index, lowest_score = i, new_score
end
end
return peers[lowest_score_index]
end
function _M.balance(self)
local peers = self.peers
local endpoint = peers[1]
if #peers > 1 then
local k = (#peers < PICK_SET_SIZE) and #peers or PICK_SET_SIZE
local peer_copy = util.deepcopy(peers)
endpoint = pick_and_score(self, peer_copy, k)
end
-- TODO(elvinefendi) move this processing to _M.sync
return endpoint.address .. ":" .. endpoint.port
end
function _M.after_balance(self)
local response_time = tonumber(split.get_first_value(ngx.var.upstream_response_time)) or 0
local connect_time = tonumber(split.get_first_value(ngx.var.upstream_connect_time)) or 0
local rtt = connect_time + response_time
local upstream = split.get_first_value(ngx.var.upstream_addr)
if util.is_blank(upstream) then
return
end
get_or_update_ewma(self, upstream, rtt, true)
end
function _M.sync(self, backend)
self.traffic_shaping_policy = backend.trafficShapingPolicy
self.alternative_backends = backend.alternativeBackends
local changed = not util.deep_compare(self.peers, backend.endpoints)
if not changed then
return
end
self.peers = backend.endpoints
self.ewma = {}
self.ewma_last_touched_at = {}
end
function _M.new(self, backend)
local o = {
peers = backend.endpoints,
ewma = {},
ewma_last_touched_at = {},
traffic_shaping_policy = backend.trafficShapingPolicy,
alternative_backends = backend.alternativeBackends,
}
setmetatable(o, self)
self.__index = self
return o
end
return _M