feat: fetch and store last good config

CHANGELOG.md

@@ -111,6 +111,10 @@ Adding a new version? You'll need three changes:
   Gateways using `/status/ready` as their health check ready after receiving the
   initial configuration (even if it's empty).
   [#4316](https://github.com/Kong/kubernetes-ingress-controller/pull/4316)
+- Fetch the last known good configuration from existing proxy instances. If
+  KIC restarts, it is now able to fetch the last good configuration from a running
+  proxy instance and store it in its internal cache.
+  [#4265](https://github.com/Kong/kubernetes-ingress-controller/pull/4265)
 
 ### Changed
 

internal/dataplane/configfetcher/config_fetcher.go

@@ -0,0 +1,95 @@
+package configfetcher
+
+import (
+	"context"
+	"errors"
+
+	"github.com/kong/deck/dump"
+	"github.com/kong/deck/utils"
+	"github.com/kong/go-kong/kong"
+	"github.com/kong/kubernetes-ingress-controller/v2/internal/adminapi"
+	"github.com/kong/kubernetes-ingress-controller/v2/internal/dataplane/kongstate"
+	"github.com/kong/kubernetes-ingress-controller/v2/internal/dataplane/sendconfig"
+	"github.com/sirupsen/logrus"
+)
+
+type LastValidConfigFetcher interface {
+	// TryFetchingValidConfigFromGateways tries to fetch a valid configuration from all gateways and persists it if found.
+	TryFetchingValidConfigFromGateways(ctx context.Context, logger logrus.FieldLogger, gatewayClients []*adminapi.Client) error
+
+	// LastValidConfig returns the last valid config and true if there's one available. Otherwise, second return value is false.
+	LastValidConfig() (*kongstate.KongState, bool)
+
+	// StoreLastValidConfig stores a given configuration as the last valid config. Should be used when the configuration was successfully accepted by a gateway.
+	StoreLastValidConfig(s *kongstate.KongState)
+}
+
+type DefaultKongLastGoodConfigFetcher struct {
+	config         dump.Config
+	lastValidState *kongstate.KongState
+	// fillIDs enables the last valid kongState to be filled in the IDs fields of Kong entities
+	// - Services, Routes, and Consumers - based on their names. It ensures that IDs remain
+	// stable across restarts of the controller.
+	fillIDs bool
+}
+
+func NewDefaultKongLastGoodConfigFetcher(fillIDs bool) *DefaultKongLastGoodConfigFetcher {
+	return &DefaultKongLastGoodConfigFetcher{
+		config:  dump.Config{},
+		fillIDs: fillIDs,
+	}
+}
+
+func (cf *DefaultKongLastGoodConfigFetcher) LastValidConfig() (*kongstate.KongState, bool) {
+	if cf.lastValidState != nil {
+		return cf.lastValidState, true
+	}
+	return nil, false
+}
+
+func (cf *DefaultKongLastGoodConfigFetcher) StoreLastValidConfig(s *kongstate.KongState) {
+	cf.lastValidState = s
+}
+
+func (cf *DefaultKongLastGoodConfigFetcher) TryFetchingValidConfigFromGateways(
+	ctx context.Context,
+	logger logrus.FieldLogger,
+	gatewayClients []*adminapi.Client,
+) error {
+	logger.Debugf("fetching last good configuration from %d gateway clients", len(gatewayClients))
+
+	var goodKongState *kongstate.KongState
+	var errs error
+	for _, client := range gatewayClients {
+		rs, err := cf.getKongRawState(ctx, client.AdminAPIClient())
+		if err != nil {
+			errs = errors.Join(errs, err)
+		}
+		status, err := cf.getKongStatus(ctx, client.AdminAPIClient())
+		if err != nil {
+			errs = errors.Join(errs, err)
+		}
+		ks := KongRawStateToKongState(rs)
+		if status.ConfigurationHash != sendconfig.WellKnownInitialHash {
+			// Get the first good one as the one to be used.
+			goodKongState = ks
+			break
+		}
+	}
+	if goodKongState != nil {
+		if cf.fillIDs {
+			goodKongState.FillIDs(logger)
+		}
+		cf.lastValidState = goodKongState
+		logger.Debug("last good configuration fetched from a Kong node")
+	}
+	return errs
+}
+
+func (cf *DefaultKongLastGoodConfigFetcher) getKongRawState(ctx context.Context, client *kong.Client) (*utils.KongRawState, error) {
+	return dump.Get(ctx, client, cf.config)
+}
+
+func (cf *DefaultKongLastGoodConfigFetcher) getKongStatus(ctx context.Context, client *kong.Client) (*kong.Status, error) {
+	return client.Status(ctx)
+}