Closes #264

kong/templates/nginx_kong.lua

@@ -34,6 +34,7 @@ lua_shared_dict cache ${{MEM_CACHE_SIZE}};
 lua_shared_dict reports_locks 100k;
 lua_shared_dict cluster_locks 100k;
 lua_shared_dict cluster_autojoin_locks 100k;
+lua_shared_dict cache_locks 100k;
 lua_shared_dict cassandra 1m;
 lua_shared_dict cassandra_prepared 5m;
 lua_socket_log_errors off;

kong/tools/database_cache.lua

@@ -1,3 +1,4 @@
+local resty_lock = require "resty.lock"
 local cjson = require "cjson"
 local cache = ngx.shared.cache
 
@@ -120,21 +121,42 @@ function _M.all_apis_by_dict_key()
 end
 
 function _M.get_or_set(key, cb)
+  local lock = resty_lock:new("cache_locks", {
+    exptime = 10,
+    timeout = 10
+  })
+
   local value, err
-  -- Try to get
+
+  -- Try to get the value from the cache
+  value = _M.get(key)
+  if value then return value end
+
+  -- The value is missing, aquire a lock
+  local elapsed, err = lock:lock(key)
+  if not elapsed then
+    ngx.log(ngx.ERR, "failed to acquire cache lock: "..err)
+  end
+
+  -- Lock acquired. Since in the meanwhile another worker may have 
+  -- populated the value we have to check again
   value = _M.get(key)
   if not value then
     -- Get from closure
     value, err = cb()
-    if err then
-      return nil, err
-    elseif value then
+    if value then
       local ok, err = _M.set(key, value)
-      if not ok then
+      if err then
         ngx.log(ngx.ERR, err)
       end
     end
   end
+
+  local ok, err = lock:unlock()
+  if not ok then
+    ngx.log(ngx.ERR, "failed to unlock: "..err)
+  end
+
   return value
 end
 

spec/02-integration/07-cache/database_cache_spec.lua

@@ -0,0 +1,84 @@
+local helpers = require "spec.helpers"
+local cjson = require "cjson"
+local meta = require "kong.meta"
+
+describe("Resolver", function()
+  local client
+  setup(function()
+    helpers.kill_all()
+
+    assert(helpers.dao.apis:insert {
+      request_host = "mockbin.com",
+      upstream_url = "http://mockbin.com"
+    })
+
+    assert(helpers.start_kong({
+      ["custom_plugins"] = "database-cache",
+      lua_package_path = "?/init.lua;./kong/?.lua;./spec/fixtures/?.lua"
+    }))
+
+    -- Add the plugin
+    local admin_client = helpers.admin_client()
+    local res = assert(admin_client:send {
+      method = "POST",
+      path = "/apis/mockbin.com/plugins/",
+      body = {
+        name = "database-cache"
+      },
+      headers = {
+        ["Content-Type"] = "application/json"
+      }
+    })
+    assert.res_status(201, res)
+    admin_client:close()
+  end)
+
+  teardown(function()
+    helpers.stop_kong()
+  end)
+
+  it("avoids dog-pile effect", function()
+    local function make_request(premature, sleep_time)
+      local client = helpers.proxy_client()
+      local res = assert(client:send {
+        method = "GET",
+        path = "/status/200?sleep="..sleep_time,
+        headers = {
+          ["Host"] = "mockbin.com"
+        }
+      })
+      res:read_body()
+      client:close()
+    end
+
+    local ok, err = ngx.timer.at(0, make_request, 2)
+    assert.truthy(ok)
+
+    local ok, err = ngx.timer.at(0, make_request, 5)
+    assert.truthy(ok)
+
+    local ok, err = ngx.timer.at(0, make_request, 1)
+    assert.truthy(ok)
+
+    helpers.wait_until(function()
+      local admin_client = helpers.admin_client()
+      local res = assert(admin_client:send {
+        method = "GET",
+        path = "/cache/invocations"
+      })
+      local body = res:read_body()
+      admin_client:close()
+      return cjson.decode(body).message == 3
+    end, 10)
+
+    -- Invocation are 3, but lookups should be 1
+    local admin_client = helpers.admin_client()
+    local res = assert(admin_client:send {
+      method = "GET",
+      path = "/cache/lookups"
+    })
+    local body = res:read_body()
+    admin_client:close()
+    assert.equal(1, cjson.decode(body).message)
+  end)
+end)

spec/fixtures/kong/plugins/database-cache/handler.lua

@@ -0,0 +1,35 @@
+local BasePlugin = require "kong.plugins.base_plugin"
+local cache = require "kong.tools.database_cache"
+
+local INVOCATIONS = "invocations"
+local LOOKUPS = "lookups"
+
+local DatabaseCacheHandler = BasePlugin:extend()
+
+DatabaseCacheHandler.PRIORITY = 1000
+
+function DatabaseCacheHandler:new()
+  DatabaseCacheHandler.super.new(self, "database-cache")
+end
+
+function DatabaseCacheHandler:init_worker()
+  DatabaseCacheHandler.super.init_worker(self)
+
+  cache.rawset(INVOCATIONS, 0)
+  cache.rawset(LOOKUPS, 0)
+end
+
+function DatabaseCacheHandler:access(conf)
+  DatabaseCacheHandler.super.access(self)
+
+  cache.get_or_set("pile_effect", function()
+    cache.incr(LOOKUPS, 1)
+    -- Adds some delay
+    ngx.sleep(tonumber(ngx.req.get_uri_args().sleep))
+    return true
+  end)
+
+  cache.incr(INVOCATIONS, 1)
+end
+
+return DatabaseCacheHandler

spec/fixtures/kong/plugins/database-cache/schema.lua

@@ -0,0 +1,3 @@
+return { 
+  fields = {}
+}