memoize balancer for a request

rootfs/etc/nginx/lua/balancer.lua

@@ -190,6 +190,10 @@ local function route_to_alternative_balancer(balancer)
 end
 
 local function get_balancer()
+  if ngx.ctx.balancer then
+    return ngx.ctx.balancer
+  end
+
   local backend_name = ngx.var.proxy_upstream_name
 
   local balancer = balancers[backend_name]
@@ -201,9 +205,11 @@ local function get_balancer()
     local alternative_backend_name = balancer.alternative_backends[1]
     ngx.var.proxy_alternative_upstream_name = alternative_backend_name
 
-    return balancers[alternative_backend_name]
+    balancer = balancers[alternative_backend_name]
   end
 
+  ngx.ctx.balancer = balancer
+
   return balancer
 end
 
@@ -260,6 +266,7 @@ if _TEST then
   _M.get_implementation = get_implementation
   _M.sync_backend = sync_backend
   _M.route_to_alternative_balancer = route_to_alternative_balancer
+  _M.get_balancer = get_balancer
 end
 
 return _M

rootfs/etc/nginx/lua/test/balancer_test.lua

@@ -86,6 +86,44 @@ describe("Balancer", function()
     end)
   end)
 
+  describe("get_balancer()", function()
+    it("always returns the same balancer for given request context", function()
+      local backend = {
+        name = "my-dummy-app-6", ["load-balance"] = "ewma",
+        alternativeBackends = { "my-dummy-canary-app-6" },
+        endpoints = { { address = "10.184.7.40", port = "8080", maxFails = 0, failTimeout = 0 } },
+        trafficShapingPolicy = {
+          weight = 0,
+          header = "",
+          headerValue = "",
+          cookie = ""
+        },
+      }
+      local canary_backend = {
+        name = "my-dummy-canary-app-6", ["load-balance"] = "ewma",
+        alternativeBackends = { "my-dummy-canary-app-6" },
+        endpoints = { { address = "11.184.7.40", port = "8080", maxFails = 0, failTimeout = 0 } },
+        trafficShapingPolicy = {
+          weight = 5,
+          header = "",
+          headerValue = "",
+          cookie = ""
+        },
+      }
+
+      balancer.sync_backend(backend)
+      balancer.sync_backend(canary_backend)
+
+      mock_ngx({ var = { proxy_upstream_name = backend.name } })
+
+      local expected = balancer.get_balancer()
+
+      for i = 1,50,1 do
+        assert.are.same(expected, balancer.get_balancer())
+      end
+    end)
+  end)
+
   describe("route_to_alternative_balancer()", function()
     local backend, _balancer