[GitHub] [apisix] redynasc commented on a change in pull request #2001: add ewma balancer

[GitBox <gi...@apache.org>]

redynasc commented on a change in pull request #2001:
URL: https://github.com/apache/apisix/pull/2001#discussion_r466926550



##########
File path: apisix/balancer/ewma.lua
##########
@@ -0,0 +1,240 @@
+--
+-- Licensed to the Apache Software Foundation (ASF) under one or more
+-- contributor license agreements.  See the NOTICE file distributed with
+-- this work for additional information regarding copyright ownership.
+-- The ASF licenses this file to You under the Apache License, Version 2.0
+-- (the "License"); you may not use this file except in compliance with
+-- the License.  You may obtain a copy of the License at
+--
+--     http://www.apache.org/licenses/LICENSE-2.0
+--
+-- Unless required by applicable law or agreed to in writing, software
+-- distributed under the License is distributed on an "AS IS" BASIS,
+-- WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+-- See the License for the specific language governing permissions and
+-- limitations under the License.
+--
+
+local resty_lock = require("resty.lock")
+local core = require("apisix.core")
+local ngx = ngx
+local ngx_shared = ngx.shared
+local ngx_now = ngx.now
+local tostring = tostring
+local math = math
+local pairs = pairs
+local next = next
+local tonumber = tonumber
+
+local _M = {}
+local DECAY_TIME = 10 -- this value is in seconds
+local LOCK_KEY = ":ewma_key"
+local PICK_SET_SIZE = 2
+local ewma_lock, ewma_lock_err
+local shm_ewma = ngx_shared.balancer_ewma
+local shm_last_touched_at= ngx_shared.balancer_ewma_last_touched_at
+
+
+local function lock(upstream)
+    local _, err = ewma_lock:lock(upstream .. LOCK_KEY)
+    if err then
+        if err ~= "timeout" then
+            core.log.error("EWMA Balancer failed to lock:", err)
+        end
+        return false, err
+    end
+
+    return true
+end
+
+
+local function unlock()
+    local ok, err = ewma_lock:unlock()
+    if not ok then
+        core.log.error("EWMA Balancer failed to unlock:", err)
+        return false, err
+    end
+
+    return true
+end
+
+
+local function decay_ewma(ewma, last_touched_at, rtt, now)
+    local td = now - last_touched_at
+    td =  math.max(td, 0)
+    local weight = math.exp(-td / DECAY_TIME)
+
+    ewma = ewma * weight + rtt * (1.0 - weight)
+    return ewma
+end
+
+
+local function store_stats(upstream, ewma, now)
+    local success, err, forcible = shm_last_touched_at:set(upstream, now)
+    if not success then
+        core.log.error("balancer_ewma_last_touched_at:set failed ", err)
+    end
+    if forcible then
+        core.log.warn("balancer_ewma_last_touched_at:set valid items forcibly overwritten")
+    end
+
+    success, err, forcible = shm_ewma:set(upstream, ewma)
+    if not success then
+        core.log.error("balancer_ewma:set failed ", err)
+    end
+    if forcible then
+        core.log.warn("balancer_ewma:set valid items forcibly overwritten")
+    end
+end
+
+
+local function get_or_update_ewma(upstream, rtt, update)
+    local lock_ok, err = nil
+    if update then
+        lock_ok, err = lock(upstream)
+    end
+    local ewma = shm_ewma:get(upstream) or 0
+    if not lock_ok then
+        return ewma, err
+    end
+
+    local now = ngx_now()
+    local last_touched_at = shm_last_touched_at:get(upstream) or 0
+    ewma = decay_ewma(ewma, last_touched_at, rtt, now)
+
+    if not update then
+        return ewma
+    end
+
+    store_stats(upstream, ewma, now)
+
+    unlock()
+
+    return ewma
+end
+
+
+local function score(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(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
+end
+
+
+local function pick_and_score(peers, k)
+    shuffle_peers(peers, k)
+    local lowest_score_index = 1
+    local lowest_score = score(peers[lowest_score_index])

Review comment:
       this may return 0, err, do not return  nil, err




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