Add EWMA as configurable load balancing algorithm (#2229)

docs/user-guide/configmap.md

@@ -519,6 +519,7 @@ The value can either be:
 - round_robin: to use the default round robin loadbalancer
 - least_conn: to use the least connected method
 - ip_hash: to use a hash of the server for routing.
+- ewma: to use the peak ewma method for routing (only available with `enable-dynamic-configuration` flag) 
 
 The default is least_conn.
 

rootfs/etc/nginx/lua/balancer.lua

@@ -4,6 +4,7 @@ local configuration = require("configuration")
 local util = require("util")
 local lrucache = require("resty.lrucache")
 local resty_lock = require("resty.lock")
+local ewma = require("balancer.ewma")
 
 -- measured in seconds
 -- for an Nginx worker to pick up the new list of upstream peers
@@ -11,6 +12,7 @@ local resty_lock = require("resty.lock")
 local BACKENDS_SYNC_INTERVAL = 1
 
 local ROUND_ROBIN_LOCK_KEY = "round_robin"
+local DEFAULT_LB_ALG = "round_robin"
 
 local round_robin_state = ngx.shared.round_robin_state
 
@@ -23,28 +25,49 @@ if not backends then
   return error("failed to create the cache for backends: " .. (err or "unknown"))
 end
 
-local function balance()
+local function get_current_backend()
   local backend_name = ngx.var.proxy_upstream_name
-  local backend = backends:get(backend_name)
-  -- lb_alg field does not exist for ingress.Backend struct for now, so lb_alg
-  -- will always be round_robin
-  local lb_alg = backend.lb_alg or "round_robin"
+  return backends:get(backend_name)
+end
+
+local function get_current_lb_alg()
+  local backend = get_current_backend()
+  return backend["load-balance"] or DEFAULT_LB_ALG
+end
+
+local function balance()
+  local backend = get_current_backend()
+  local lb_alg = get_current_lb_alg()
 
   if lb_alg == "ip_hash" then
     -- TODO(elvinefendi) implement me
     return backend.endpoints[0].address, backend.endpoints[0].port
+  elseif lb_alg == "ewma" then
+    local endpoint = ewma.balance(backend.endpoints)
+    return endpoint.address, endpoint.port
+  end
+
+  if lb_alg ~= DEFAULT_LB_ALG then
+    ngx.log(ngx.WARN, tostring(lb_alg) .. " is not supported, falling back to " .. DEFAULT_LB_ALG)
   end
 
   -- Round-Robin
-  round_robin_lock:lock(backend_name .. ROUND_ROBIN_LOCK_KEY)
-  local last_index = round_robin_state:get(backend_name)
+  round_robin_lock:lock(backend.name .. ROUND_ROBIN_LOCK_KEY)
+  local last_index = round_robin_state:get(backend.name)
   local index, endpoint = next(backend.endpoints, last_index)
   if not index then
     index = 1
     endpoint = backend.endpoints[index]
   end
-  round_robin_state:set(backend_name, index)
-  round_robin_lock:unlock(backend_name .. ROUND_ROBIN_LOCK_KEY)
+  local success, forcible
+  success, err, forcible = round_robin_state:set(backend.name, index)
+  if not success then
+    ngx.log(ngx.WARN, "round_robin_state:set failed " .. err)
+  end
+  if forcible then
+    ngx.log(ngx.WARN, "round_robin_state:set valid items forcibly overwritten")
+  end
+  round_robin_lock:unlock(backend.name .. ROUND_ROBIN_LOCK_KEY)
 
   return endpoint.address, endpoint.port
 end
@@ -55,6 +78,13 @@ local function sync_backend(backend)
   -- also reset the respective balancer state since backend has changed
   round_robin_state:delete(backend.name)
 
+  -- TODO: Reset state of EWMA per backend
+  local lb_alg = backend["load-balance"] or DEFAULT_LB_ALG
+  if lb_alg == "ewma" then
+    ngx.shared.balancer_ewma:flush_all()
+    ngx.shared.balancer_ewma_last_touched_at:flush_all()
+  end
+
   ngx.log(ngx.INFO, "syncronization completed for: " .. backend.name)
 end
 
@@ -84,6 +114,13 @@ local function sync_backends()
   end
 end
 
+local function after_balance()
+  local lb_alg = get_current_lb_alg()
+  if lb_alg == "ewma" then
+    ewma.after_balance()
+  end
+end
+
 function _M.init_worker()
   _, err = ngx.timer.every(BACKENDS_SYNC_INTERVAL, sync_backends)
   if err then
@@ -92,6 +129,15 @@ function _M.init_worker()
 end
 
 function _M.call()
+  local phase = ngx.get_phase()
+  if phase == "log" then
+    after_balance()
+    return
+  end
+  if phase ~= "balancer" then
+    return error("must be called in balancer or log, but was called in: " .. phase)
+  end
+
   ngx_balancer.set_more_tries(1)
 
   local host, port = balance()
@@ -99,7 +145,10 @@ function _M.call()
   local ok
   ok, err = ngx_balancer.set_current_peer(host, port)
   if ok then
-    ngx.log(ngx.INFO, "current peer is set to " .. host .. ":" .. port)
+    ngx.log(
+      ngx.INFO,
+      "current peer is set to " .. host .. ":" .. port .. " using lb_alg " .. tostring(get_current_lb_alg())
+    )
   else
     ngx.log(ngx.ERR, "error while setting current upstream peer to: " .. tostring(err))
   end

rootfs/etc/nginx/lua/balancer/ewma.lua

@@ -0,0 +1,141 @@
+-- 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 resty_lock = require("resty.lock")
+local util = require("util")
+
+local DECAY_TIME = 10 -- this value is in seconds
+local LOCK_KEY = ":ewma_key"
+local PICK_SET_SIZE = 2
+
+local ewma_lock = resty_lock:new("locks", {timeout = 0, exptime = 0.1})
+
+local _M = {}
+
+local function lock(upstream)
+  local _, err = ewma_lock:lock(upstream .. LOCK_KEY)
+  if err then
+    if err ~= "timeout" then
+      ngx.log(ngx.ERR, string.format("EWMA Balancer failed to lock: %s", tostring(err)))
+    end
+  end
+
+  return err
+end
+
+local function unlock()
+  local ok, err = ewma_lock:unlock()
+  if not ok then
+    ngx.log(ngx.ERR, string.format("EWMA Balancer failed to unlock: %s", tostring(err)))
+  end
+
+  return err
+end
+
+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(upstream, rtt, update)
+  local lock_err = nil
+  if update then
+    lock_err = lock(upstream)
+  end
+  local ewma = ngx.shared.balancer_ewma:get(upstream) or 0
+  if lock_err ~= nil then
+    return ewma, lock_err
+  end
+
+  local now = ngx.now()
+  local last_touched_at = ngx.shared.balancer_ewma_last_touched_at:get(upstream) or 0
+  ewma = decay_ewma(ewma, last_touched_at, rtt, now)
+
+  if not update then
+    return ewma, nil
+  end
+
+  local success, err, forcible = ngx.shared.balancer_ewma_last_touched_at:set(upstream, now)
+  if not success then
+    ngx.log(ngx.WARN, "balancer_ewma_last_touched_at:set failed " .. err)
+  end
+  if forcible then
+    ngx.log(ngx.WARN, "balancer_ewma_last_touched_at:set valid items forcibly overwritten")
+  end
+
+  success, err, forcible = ngx.shared.balancer_ewma:set(upstream, ewma)
+  if not success then
+    ngx.log(ngx.WARN, "balancer_ewma:set failed " .. err)
+  end
+  if forcible then
+    ngx.log(ngx.WARN, "balancer_ewma:set valid items forcibly overwritten")
+  end
+
+  unlock()
+  return ewma, nil
+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
+  -- peers[1 .. k] will now contain a randomly selected k from #peers
+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])
+  for i = 2, k do
+    local new_score = score(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(peers)
+  if #peers == 1 then
+    return peers[1]
+  end
+  local k = (#peers < PICK_SET_SIZE) and #peers or PICK_SET_SIZE
+  local peer_copy = util.deepcopy(peers)
+  return pick_and_score(peer_copy, k)
+end
+
+function _M.after_balance()
+  local response_time = tonumber(util.get_first_value(ngx.var.upstream_response_time)) or 0
+  local connect_time = tonumber(util.get_first_value(ngx.var.upstream_connect_time)) or 0
+  local rtt = connect_time + response_time
+  local upstream = util.get_first_value(ngx.var.upstream_addr)
+
+  if util.is_blank(upstream) then
+    return
+  end
+  get_or_update_ewma(upstream, rtt, true)
+end
+
+return _M

rootfs/etc/nginx/lua/util.lua

@@ -1,4 +1,5 @@
 local _M = {}
+local string_len = string.len
 
 -- this implementation is taken from
 -- https://web.archive.org/web/20131225070434/http://snippets.luacode.org/snippets/Deep_Comparison_of_Two_Values_3
@@ -24,4 +25,50 @@ local function deep_compare(t1, t2, ignore_mt)
 end
 _M.deep_compare = deep_compare
 
+function _M.is_blank(str)
+  return str == nil or string_len(str) == 0
+end
+
+-- http://nginx.org/en/docs/http/ngx_http_upstream_module.html#example
+-- CAVEAT: nginx is giving out : instead of , so the docs are wrong
+-- 127.0.0.1:26157 : 127.0.0.1:26157 , ngx.var.upstream_addr
+-- 200 : 200 , ngx.var.upstream_status
+-- 0.00 : 0.00, ngx.var.upstream_response_time
+function _M.split_upstream_var(var)
+  if not var then
+    return nil, nil
+  end
+  local t = {}
+  for v in var:gmatch("[^%s|,]+") do
+    if v ~= ":" then
+      t[#t+1] = v
+    end
+  end
+  return t
+end
+
+function _M.get_first_value(var)
+  local t = _M.split_upstream_var(var) or {}
+  if #t == 0 then return nil end
+  return t[1]
+end
+
+-- this implementation is taken from:
+-- https://github.com/luafun/luafun/blob/master/fun.lua#L33
+-- SHA: 04c99f9c393e54a604adde4b25b794f48104e0d0
+local function deepcopy(orig)
+  local orig_type = type(orig)
+  local copy
+  if orig_type == 'table' then
+      copy = {}
+      for orig_key, orig_value in next, orig, nil do
+          copy[deepcopy(orig_key)] = deepcopy(orig_value)
+      end
+  else
+      copy = orig
+  end
+  return copy
+end
+_M.deepcopy = deepcopy
+
 return _M

rootfs/etc/nginx/template/nginx.tmpl

@@ -42,6 +42,8 @@ http {
     lua_shared_dict configuration_data 5M;
     lua_shared_dict round_robin_state 1M;
     lua_shared_dict locks 512k;
+    lua_shared_dict balancer_ewma 1M;
+    lua_shared_dict balancer_ewma_last_touched_at 1M;
 
     init_by_lua_block {
         require("resty.core")
@@ -976,6 +978,11 @@ stream {
             {{ end }}
 
             {{ if not (empty $location.Backend) }}
+            {{ if $all.DynamicConfigurationEnabled}}
+            log_by_lua_block {
+                balancer.call()
+            }
+            {{ end }}
             {{ buildProxyPass $server.Hostname $all.Backends $location $all.DynamicConfigurationEnabled }}
             {{ if (or (eq $location.Proxy.ProxyRedirectFrom "default") (eq $location.Proxy.ProxyRedirectFrom "off")) }}
             proxy_redirect                          {{ $location.Proxy.ProxyRedirectFrom }};