perf(dbless): load declarative schema during init() (#10932) [backport]

CHANGELOG.md

@@ -359,6 +359,13 @@ now encoded as `"[]"` to comply with standard.
 - Support for `upstream_status` field in log serializer.
   [#10296](https://github.com/Kong/kong/pull/10296)
 
+#### Performance
+
+- In dbless mode, the declarative schema is now fully initialized at startup
+  instead of on-demand in the request path. This is most evident in decreased
+  response latency when updating configuration via the `/config` API endpoint.
+  [#10932](https://github.com/Kong/kong/pull/10932)
+
 ### Fixes
 
 #### Core

kong/api/routes/config.lua

@@ -118,7 +118,13 @@ return {
       local opts = parse_config_post_opts(self.params)
 
       local old_hash = opts.check_hash and declarative.get_current_hash()
-      local dc = declarative.new_config(kong.configuration)
+
+      local dc = kong.db.declarative_config
+      if not dc then
+        kong.log.crit("received POST request to /config endpoint, but ",
+                      "kong.db.declarative_config was not initialized")
+        return kong.response.exit(500, { message = "An unexpected error occurred" })
+      end
 
       local dc_table, new_hash = hydrate_config_from_request(self.params, dc)
 

kong/clustering/data_plane.lua

@@ -59,8 +59,11 @@ function _M.new(clustering)
   assert(type(clustering.cert_key) == "cdata",
          "kong.clustering did not provide the cluster certificate private key")
 
+  assert(kong.db.declarative_config,
+         "kong.db.declarative_config was not initialized")
+
   local self = {
-    declarative_config = assert(declarative.new_config(clustering.conf)),
+    declarative_config = kong.db.declarative_config,
     conf = clustering.conf,
     cert = clustering.cert,
     cert_key = clustering.cert_key,

kong/init.lua

@@ -448,9 +448,7 @@ local function has_declarative_config(kong_config)
 end
 
 
-local function parse_declarative_config(kong_config)
-  local dc = declarative.new_config(kong_config)
-
+local function parse_declarative_config(kong_config, dc)
   local declarative_config, is_file, is_string = has_declarative_config(kong_config)
 
   local entities, err, _, meta, hash
@@ -625,13 +623,22 @@ function Kong.init()
   end
 
   if is_dbless(config) then
+    local dc, err = declarative.new_config(config)
+    if not dc then
+      error(err)
+    end
+
+    kong.db.declarative_config = dc
+
+
     if is_http_module or
        (#config.proxy_listeners == 0 and
         #config.admin_listeners == 0 and
         #config.status_listeners == 0)
     then
-      local err
-      declarative_entities, err, declarative_meta, declarative_hash = parse_declarative_config(kong.configuration)
+      declarative_entities, err, declarative_meta, declarative_hash =
+        parse_declarative_config(kong.configuration, dc)
+
       if not declarative_entities then
         error(err)
       end