fix ewma.balance and add unit tests for it

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

@@ -119,12 +119,14 @@ end
 
 function _M.balance(self)
   local peers = self.peers
-  if #peers == 1 then
+  local endpoint = peers[1]
-    return 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(peer_copy, k)
   end
-  local k = (#peers < PICK_SET_SIZE) and #peers or PICK_SET_SIZE
+
-  local peer_copy = util.deepcopy(peers)
-  local endpoint = pick_and_score(peer_copy, k)
   return endpoint.address, endpoint.port
 end
 
@@ -145,6 +147,9 @@ function _M.sync(self, backend)
   if not changed then
     return
   end
+
+  self.peers = backend.endpoints
+
   -- TODO: Reset state of EWMA per backend
   ngx.shared.balancer_ewma:flush_all()
   ngx.shared.balancer_ewma_last_touched_at:flush_all()

rootfs/etc/nginx/lua/test/balancer/ewma_test.lua

@@ -0,0 +1,103 @@
+package.path = "./rootfs/etc/nginx/lua/?.lua;./rootfs/etc/nginx/lua/test/mocks/?.lua;" .. package.path
+
+local util = require("util")
+
+local _ngx = {
+  shared = {
+    balancer_ewma = { flush_all = function() end },
+    balancer_ewma_last_touched_at = { flush_all = function() end }
+  },
+  log = function(...) end,
+  now = function() return os.time() end,
+}
+_G.ngx = _ngx
+
+describe("Balancer ewma", function()
+  local balancer_ewma = require("balancer.ewma")
+
+  describe("balance()", function()
+    it("returns single endpoint when the given backend has only one endpoint", function()
+      local backend = {
+        name = "my-dummy-backend", ["load-balance"] = "ewma",
+        endpoints = { { address = "10.184.7.40", port = "8080", maxFails = 0, failTimeout = 0 } }
+      }
+      local instance = balancer_ewma:new(backend)
+
+      local host, port = instance:balance()
+      assert.equal("10.184.7.40", host)
+      assert.equal("8080", port)
+    end)
+
+    it("picks the endpoint with lowest score when there two of them", function()
+      local backend = {
+        name = "my-dummy-backend", ["load-balance"] = "ewma",
+        endpoints = {
+          { address = "10.184.7.40", port = "8080", maxFails = 0, failTimeout = 0 },
+          { address = "10.184.97.100", port = "8080", maxFails = 0, failTimeout = 0 },
+        }
+      }
+      local instance = balancer_ewma:new(backend)
+      
+      local stats = { ["10.184.7.40:8080"] = 0.5, ["10.184.97.100:8080"] = 0.3 }
+      ngx.shared.balancer_ewma.get = function(self, key) return stats[key] end
+      local t = ngx.now()-10
+      ngx.shared.balancer_ewma_last_touched_at.get = function(self, key) return t end
+
+
+      local host, port = instance:balance()
+      assert.equal("10.184.97.100", host)
+      assert.equal("8080", port)
+    end)
+  end)
+
+  describe("sync()", function()
+    local backend, instance
+
+    before_each(function()
+      backend = {
+        name = "my-dummy-backend", ["load-balance"] = "ewma",
+        endpoints = { { address = "10.184.7.40", port = "8080", maxFails = 0, failTimeout = 0 } }
+      }
+      instance = balancer_ewma:new(backend)
+    end)
+
+    it("does nothing when endpoints do not change", function()
+      local new_backend = {
+        endpoints = { { address = "10.184.7.40", port = "8080", maxFails = 0, failTimeout = 0 } }
+      }
+
+      local s1 = spy.on(ngx.shared.balancer_ewma, "flush_all")
+      local s2 = spy.on(ngx.shared.balancer_ewma_last_touched_at, "flush_all")
+
+      instance:sync(new_backend)
+
+      assert.spy(s1).was_not_called()
+      assert.spy(s2).was_not_called()
+    end)
+
+    it("updates endpoints", function()
+      local new_backend = {
+        endpoints = {
+          { address = "10.184.7.40", port = "8080", maxFails = 0, failTimeout = 0 },
+          { address = "10.184.97.100", port = "8080", maxFails = 0, failTimeout = 0 },
+        }
+      }
+
+      instance:sync(new_backend)
+      assert.are.same(new_backend.endpoints, instance.peers)
+    end)
+
+    it("resets stats", function()
+      local new_backend = util.deepcopy(backend)
+      new_backend.endpoints[1].maxFails = 3
+
+      local s1 = spy.on(ngx.shared.balancer_ewma, "flush_all")
+      local s2 = spy.on(ngx.shared.balancer_ewma_last_touched_at, "flush_all")
+
+      instance:sync(new_backend)
+
+      assert.spy(s1).was_called()
+      assert.spy(s2).was_called()
+    end)
+  end)
+end)