Consistent hashing to a subset of nodes. It works like consistent hash,

but instead of mapping to a single node, we map to a subset of nodes.

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

@@ -16,7 +16,7 @@ describe("Balancer chash", function()
       end
 
       local backend = {
-        name = "my-dummy-backend", ["upstream-hash-by"] = "$request_uri",
+        name = "my-dummy-backend", upstreamHashByConfig = { ["upstream-hash-by"] = "$request_uri" },
         endpoints = { { address = "10.184.7.40", port = "8080", maxFails = 0, failTimeout = 0 } }
       }
       local instance = balancer_chash:new(backend)

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

@@ -0,0 +1,82 @@
+
+local function get_test_backend(n_endpoints) 
+   local backend = {
+    name = "my-dummy-backend", 
+    ["upstreamHashByConfig"] = {
+      ["upstream-hash-by"] = "$request_uri",
+      ["upstream-hash-by-subset"] = true,
+      ["upstream-hash-by-subset-size"] = 3
+    },
+    endpoints = {}
+  }
+
+  for i = 1, n_endpoints do
+    backend.endpoints[i] = { address = "10.184.7." .. tostring(i), port = "8080", maxFails = 0, failTimeout = 0 }
+  end
+
+  return backend
+end
+ 
+describe("Balancer chash subset", function()
+  local balancer_chashsubset = require("balancer.chashsubset")
+
+  describe("balance()", function()
+    it("returns peers from the same subset", function()
+      _G.ngx = { var = { request_uri = "/alma/armud" }}
+
+      local backend = get_test_backend(9)
+      
+      local instance = balancer_chashsubset:new(backend)
+
+      instance:sync(backend)
+
+      local first_node = instance:balance()
+      local subset_id
+      local endpoint_strings
+
+      local function has_value (tab, val)
+        for _, value in ipairs(tab) do
+          if value == val then
+            return true
+           end
+        end
+
+        return false
+      end
+
+      for id, endpoints in pairs(instance["subsets"]) do
+        endpoint_strings = {}
+        for _, endpoint in pairs(endpoints) do
+          local endpoint_string = endpoint.address .. ":" .. endpoint.port
+          table.insert(endpoint_strings, endpoint_string)
+          if first_node == endpoint_string then
+            -- found the set of first_node
+            subset_id = id
+          end
+        end
+        if subset_id then
+          break
+        end
+      end
+
+      -- multiple calls to balance must return nodes from the same subset
+      for i = 0, 10 do
+        assert.True(has_value(endpoint_strings, instance:balance()))
+      end
+    end)
+  end)
+  describe("new(backend)", function()
+    it("fills last subset correctly", function()
+      _G.ngx = { var = { request_uri = "/alma/armud" }}
+
+      local backend = get_test_backend(7)
+      
+      local instance = balancer_chashsubset:new(backend)
+
+      instance:sync(backend)
+      for id, endpoints in pairs(instance["subsets"]) do
+        assert.are.equal(#endpoints, 3)
+      end
+    end)
+  end)
+end)

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

@@ -32,7 +32,10 @@ local function reset_backends()
       sessionAffinityConfig = { name = "", cookieSessionAffinity = { name = "", hash = "" } },
     },
     { name = "my-dummy-app-1", ["load-balance"] = "round_robin", },
-    { name = "my-dummy-app-2", ["load-balance"] = "round_robin", ["upstream-hash-by"] = "$request_uri", },
+    { 
+      name = "my-dummy-app-2", ["load-balance"] = "chash",
+      upstreamHashByConfig = { ["upstream-hash-by"] = "$request_uri", },
+    },
     {
       name = "my-dummy-app-3", ["load-balance"] = "ewma",
       sessionAffinityConfig = { name = "cookie", cookieSessionAffinity = { name = "route", hash = "sha1" } }