[WIP] Store Sync (#1365)

* Initial cache implementation

* Write queue implementation

* Accidentally started writing the storage sync service

sync/store/cache.go

@@ -0,0 +1,110 @@
+// Package store syncs multiple go-micro stores
+package store
+
+import (
+	"fmt"
+	"sync"
+	"time"
+
+	"github.com/ef-ds/deque"
+	"github.com/micro/go-micro/v2/store"
+	"github.com/pkg/errors"
+)
+
+// Cache implements a cache in front of go-micro Stores
+type Cache interface {
+	store.Store
+
+	// Force a full sync
+	Sync() error
+}
+type cache struct {
+	sOptions            store.Options
+	cOptions            Options
+	pendingWrites       []*deque.Deque
+	pendingWriteTickers []*time.Ticker
+	sync.RWMutex
+}
+
+// NewCache returns a new Cache
+func NewCache(opts ...Option) Cache {
+	c := &cache{}
+	for _, o := range opts {
+		o(&c.cOptions)
+	}
+	if c.cOptions.SyncInterval == 0 {
+		c.cOptions.SyncInterval = 1 * time.Minute
+	}
+	if c.cOptions.SyncMultiplier == 0 {
+		c.cOptions.SyncMultiplier = 5
+	}
+	return c
+}
+
+// Init initialises the storeOptions
+func (c *cache) Init(opts ...store.Option) error {
+	for _, o := range opts {
+		o(&c.sOptions)
+	}
+	if len(c.cOptions.Stores) == 0 {
+		return errors.New("the cache has no stores")
+	}
+	if c.sOptions.Context == nil {
+		return errors.New("please provide a context to the cache. Cancelling the context signals that the cache is being disposed and syncs the cache")
+	}
+	for _, s := range c.cOptions.Stores {
+		if err := s.Init(); err != nil {
+			return errors.Wrapf(err, "Store %s failed to Init()", s.String())
+		}
+	}
+	c.pendingWrites = make([]*deque.Deque, len(c.cOptions.Stores)-1)
+	c.pendingWriteTickers = make([]*time.Ticker, len(c.cOptions.Stores)-1)
+	for i := 0; i < len(c.pendingWrites); i++ {
+		c.pendingWrites[i] = deque.New()
+		c.pendingWrites[i].Init()
+		c.pendingWriteTickers[i] = time.NewTicker(c.cOptions.SyncInterval * time.Duration(intpow(c.cOptions.SyncMultiplier, int64(i))))
+	}
+	go c.cacheManager()
+	return nil
+}
+
+// Options returns the cache's store options
+func (c *cache) Options() store.Options {
+	return c.sOptions
+}
+
+// String returns a printable string describing the cache
+func (c *cache) String() string {
+	backends := make([]string, len(c.cOptions.Stores))
+	for i, s := range c.cOptions.Stores {
+		backends[i] = s.String()
+	}
+	return fmt.Sprintf("cache %v", backends)
+}
+
+func (c *cache) List(opts ...store.ListOption) ([]string, error) {
+	return c.cOptions.Stores[0].List(opts...)
+}
+
+func (c *cache) Read(key string, opts ...store.ReadOption) ([]*store.Record, error) {
+	return c.cOptions.Stores[0].Read(key, opts...)
+}
+
+func (c *cache) Write(r *store.Record, opts ...store.WriteOption) error {
+	return c.cOptions.Stores[0].Write(r, opts...)
+}
+
+// Delete removes a key from the cache
+func (c *cache) Delete(key string, opts ...store.DeleteOption) error {
+	return c.cOptions.Stores[0].Delete(key, opts...)
+}
+
+func (c *cache) Sync() error {
+	return nil
+}
+
+type internalRecord struct {
+	key       string
+	value     []byte
+	expiresAt time.Time
+}