grafana/pkg/apiserver/rest/dualwriter.go

package rest

import (
	"context"
	"errors"
	"fmt"

	"github.com/grafana/grafana/pkg/infra/kvstore"
	"github.com/prometheus/client_golang/prometheus"
	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
	"k8s.io/apimachinery/pkg/runtime"
	"k8s.io/apiserver/pkg/registry/rest"
	"k8s.io/klog/v2"
)

var (
	_ rest.Storage              = (DualWriter)(nil)
	_ rest.Scoper               = (DualWriter)(nil)
	_ rest.TableConvertor       = (DualWriter)(nil)
	_ rest.CreaterUpdater       = (DualWriter)(nil)
	_ rest.CollectionDeleter    = (DualWriter)(nil)
	_ rest.GracefulDeleter      = (DualWriter)(nil)
	_ rest.SingularNameProvider = (DualWriter)(nil)
)

// Storage is a storage implementation that satisfies the same interfaces as genericregistry.Store.
type Storage interface {
	rest.Storage
	rest.Scoper
	rest.TableConvertor
	rest.SingularNameProvider
	rest.Getter
	// TODO: when watch is implemented, we can replace all the below with rest.StandardStorage
	rest.Lister
	rest.CreaterUpdater
	rest.GracefulDeleter
	rest.CollectionDeleter
	// Compare asserts on the equality of objects returned from both stores	(object storage and legacy storage)
	Compare(storageObj, legacyObj runtime.Object) bool
}

// LegacyStorage is a storage implementation that writes to the Grafana SQL database.
type LegacyStorage interface {
	rest.Storage
	rest.Scoper
	rest.SingularNameProvider
	rest.CreaterUpdater
	rest.Lister
	rest.GracefulDeleter
	rest.CollectionDeleter
	rest.TableConvertor
	rest.Getter
}

// DualWriter is a storage implementation that writes first to LegacyStorage and then to Storage.
// If writing to LegacyStorage fails, the write to Storage is skipped and the error is returned.
// Storage is used for all read operations.  This is useful as a migration step from SQL based
// legacy storage to a more standard kubernetes backed storage interface.
//
// NOTE: Only values supported by legacy storage will be preserved in the CREATE/UPDATE commands.
// For example, annotations, labels, and managed fields may not be preserved.  Everything in upstream
// storage can be recrated from the data in legacy storage.
//
// The LegacyStorage implementation must implement the following interfaces:
// - rest.Storage
// - rest.TableConvertor
// - rest.Scoper
// - rest.SingularNameProvider
//
// These interfaces are optional, but they all should be implemented to fully support dual writes:
// - rest.Creater
// - rest.Updater
// - rest.GracefulDeleter
// - rest.CollectionDeleter

type DualWriter interface {
	Storage
	LegacyStorage
	Mode() DualWriterMode
}

type DualWriterMode int

const (
	// Mode0 represents writing to and reading from solely LegacyStorage. This mode is enabled when the
	// `unifiedStorage` feature flag is not set. All reads and writes are made to LegacyStorage. None are made to Storage.
	Mode0 DualWriterMode = iota
	// Mode1 represents writing to and reading from LegacyStorage for all primary functionality while additionally
	// reading and writing to Storage on a best effort basis for the sake of collecting metrics.
	Mode1
	// Mode2 is the dual writing mode that represents writing to LegacyStorage and Storage and reading from LegacyStorage.
	Mode2
	// Mode3 represents writing to LegacyStorage and Storage and reading from Storage.
	Mode3
	// Mode4 represents writing and reading from Storage.
	Mode4
)

// TODO: make this function private as there should only be one public way of setting the dual writing mode
// NewDualWriter returns a new DualWriter.
func NewDualWriter(mode DualWriterMode, legacy LegacyStorage, storage Storage, reg prometheus.Registerer) DualWriter {
	metrics := &dualWriterMetrics{}
	metrics.init(reg)
	switch mode {
	// It is not possible to initialize a mode 0 dual writer. Mode 0 represents
	// writing to legacy storage without `unifiedStorage` enabled.
	case Mode1:
		// read and write only from legacy storage
		return newDualWriterMode1(legacy, storage, metrics)
	case Mode2:
		// write to both, read from storage but use legacy as backup
		return newDualWriterMode2(legacy, storage, metrics)
	case Mode3:
		// write to both, read from storage only
		return newDualWriterMode3(legacy, storage, metrics)
	case Mode4:
		// read and write only from storage
		return newDualWriterMode4(legacy, storage, metrics)
	default:
		return newDualWriterMode1(legacy, storage, metrics)
	}
}

type updateWrapper struct {
	upstream rest.UpdatedObjectInfo
	updated  runtime.Object
}

// Returns preconditions built from the updated object, if applicable.
// May return nil, or a preconditions object containing nil fields,
// if no preconditions can be determined from the updated object.
func (u *updateWrapper) Preconditions() *metav1.Preconditions {
	return u.upstream.Preconditions()
}

// UpdatedObject returns the updated object, given a context and old object.
// The only time an empty oldObj should be passed in is if a "create on update" is occurring (there is no oldObj).
func (u *updateWrapper) UpdatedObject(ctx context.Context, oldObj runtime.Object) (newObj runtime.Object, err error) {
	return u.updated, nil
}

func SetDualWritingMode(
	ctx context.Context,
	kvs *kvstore.NamespacedKVStore,
	legacy LegacyStorage,
	storage Storage,
	entity string,
	desiredMode DualWriterMode,
	reg prometheus.Registerer,
) (DualWriter, error) {
	toMode := map[string]DualWriterMode{
		// It is not possible to initialize a mode 0 dual writer. Mode 0 represents
		// writing to legacy storage without `unifiedStorage` enabled.
		"1": Mode1,
		"2": Mode2,
		"3": Mode3,
		"4": Mode4,
	}
	errDualWriterSetCurrentMode := errors.New("failed to set current dual writing mode")

	// Use entity name as key
	m, ok, err := kvs.Get(ctx, entity)
	if err != nil {
		return nil, errors.New("failed to fetch current dual writing mode")
	}

	currentMode, valid := toMode[m]

	if !valid && ok {
		// Only log if "ok" because initially all instances will have mode unset for playlists.
		klog.Info("invalid dual writing mode for playlists mode:", m)
	}

	if !valid || !ok {
		// Default to mode 1
		currentMode = Mode1

		err := kvs.Set(ctx, entity, fmt.Sprint(currentMode))
		if err != nil {
			return nil, errDualWriterSetCurrentMode
		}
	}

	// Desired mode is 2 and current mode is 1
	if (desiredMode == Mode2) && (currentMode == Mode1) {
		// This is where we go through the different gates to allow the instance to migrate from mode 1 to mode 2.
		// There are none between mode 1 and mode 2
		currentMode = Mode2

		err := kvs.Set(ctx, entity, fmt.Sprint(currentMode))
		if err != nil {
			return nil, errDualWriterSetCurrentMode
		}
	}
	if (desiredMode == Mode1) && (currentMode == Mode2) {
		// This is where we go through the different gates to allow the instance to migrate from mode 2 to mode 1.
		// There are none between mode 1 and mode 2
		currentMode = Mode1

		err := kvs.Set(ctx, entity, fmt.Sprint(currentMode))
		if err != nil {
			return nil, errDualWriterSetCurrentMode
		}
	}

	// 	#TODO add support for other combinations of desired and current modes

	return NewDualWriter(currentMode, legacy, storage, reg), nil
}