feature: Round Robin with optional random start

README.markdown

@@ -126,7 +126,7 @@ Both `resty.chash` and `resty.roundrobin` have the same apis.
 
 new
 ---
-**syntax:** `obj, err = class.new(nodes)`
+**syntax:** `obj, err = class.new(nodes, random_start?)`
 
 Instantiates an object of this class. The `class` value is returned by the call `require "resty.chash"`.
 
@@ -135,6 +135,9 @@ when we need to keep consistency with nginx chash.
 
 The `id` can be any string value when we do not need to keep consistency with nginx chash.
 
+`random_start` can optionally be set when initializing `resty.roundrobin`. If it is set then
+the algorithm will pick first node uniformly at random instead of starting with the first one.
+
 ```lua
 local nodes = {
     -- id => weight

lib/resty/roundrobin.lua

@@ -3,6 +3,7 @@ local pairs = pairs
 local next = next
 local tonumber = tonumber
 local setmetatable = setmetatable
+local math_random = math.random
 
 
 local _M = {}
@@ -46,18 +47,40 @@ local function get_gcd(nodes)
     return only_key, gcd, max_weight
 end
 
+local function get_random_node_id(nodes)
+  local count = 0
+  for _, _ in pairs(nodes) do
+    count = count + 1
+  end
 
-function _M.new(_, nodes)
+  local id = nil
+  local random_index = math_random(count)
+
+  for _ = 1, random_index do
+    id = next(nodes, id)
+  end
+
+  return id
+end
+
+
+function _M.new(_, nodes, random_start)
     local newnodes = copy(nodes)
     local only_key, gcd, max_weight = get_gcd(newnodes)
+    local last_id = nil
+
+    if random_start then
+      last_id = get_random_node_id(nodes)
+    end
 
     local self = {
         nodes = newnodes,  -- it's safer to copy one
         only_key = only_key,
         max_weight = max_weight,
         gcd = gcd,
         cw = max_weight,
-        last_id = nil,
+        last_id = last_id,
+        random_start = random_start,
     }
     return setmetatable(self, mt)
 end
@@ -70,6 +93,10 @@ function _M.reinit(self, nodes)
     self.nodes = newnodes
     self.last_id = nil
     self.cw = self.max_weight
+
+    if self.random_start then
+      self.last_id = get_random_node_id(nodes)
+    end
 end
 
 

t/roundrobin.t

@@ -109,3 +109,76 @@ server3: 16666
 server2: 33333
 --- no_error_log
 [error]
+
+
+
+=== TEST 3: new with random start
+--- http_config eval: $::HttpConfig
+--- config
+    location /t {
+        content_by_lua_block {
+            math.randomseed(75098)
+
+            local roundrobin = require "resty.roundrobin"
+
+            local servers = {
+                ["server1"] = 1,
+                ["server2"] = 1,
+                ["server3"] = 1,
+            }
+
+            local rr = roundrobin:new(servers, true)
+            local id = rr:find()
+            ngx.say(id)
+        }
+    }
+--- request
+GET /t
+--- response_body
+server3
+--- no_error_log
+[error]
+
+
+
+=== TEST 4: reinit with random start
+--- http_config eval: $::HttpConfig
+--- config
+    location /t {
+        content_by_lua_block {
+            math.randomseed(75098)
+
+            local roundrobin = require "resty.roundrobin"
+
+            local servers = {
+                ["server1"] = 1,
+                ["server2"] = 1,
+                ["server3"] = 1,
+            }
+
+            local rr = roundrobin:new(servers, true)
+            local id = rr:find()
+            ngx.say(id)
+
+            math.randomseed(99111)
+
+            local new_servers = {
+                ["server1"] = 1,
+                ["server2"] = 1,
+                ["server3"] = 1,
+                ["server4"] = 1,
+                ["server5"] = 1,
+            }
+
+            rr:reinit(new_servers)
+            id = rr:find()
+            ngx.say(id)
+        }
+    }
+--- request
+GET /t
+--- response_body
+server3
+server5
+--- no_error_log
+[error]