perf: cache the JSON schema validator

Creating a json schema validator is somewhat expensive, so we cache
this step with a local LRU cache

Author: tkan145

gateway/src/apicast/policy_config_validator.lua

@@ -3,8 +3,35 @@
 -- Validates a policy configuration against a policy config JSON schema.
 
 local jsonschema = require('jsonschema')
+local lrucache = require("resty.lrucache")
 
-local _M = { }
+local cached_validator = lrucache.new(100)
+
+local _M = {
+  _VERSION=0.1
+}
+
+local function create_validator(schema)
+  local ok, res = pcall(jsonschema.generate_validator, schema)
+  if ok then
+    return res
+  end
+
+  return nil, res
+end
+
+local function get_validator(schema)
+  local validator, err = cached_validator:get(schema)
+  if not validator then
+    validator, err = create_validator(schema)
+    if not validator then
+      return nil, err
+    end
+    cached_validator:set(schema, validator)
+  end
+
+  return validator, nil
+end
 
 --- Validate a policy configuration
 -- Checks if a policy configuration is valid according to the given schema.
@@ -13,7 +40,10 @@ local _M = { }
 -- @treturn boolean True if the policy configuration is valid. False otherwise.
 -- @treturn string Error message only when the policy config is invalid.
 function _M.validate_config(config, config_schema)
-  local validator = jsonschema.generate_validator(config_schema or {})
+  local validator, err = get_validator(config_schema or {})
+  if not validator then
+    return false, err
+  end
   return validator(config or {})
 end