Support watch for get

Signed-off-by: Shiming Zhang <[email protected]>

augerctl/command/get_command.go

@@ -31,6 +31,9 @@ type getFlagpole struct {
 	Output    string
 	ChunkSize int64
 	Prefix    string
+
+	Watch     bool
+	WatchOnly bool
 }
 
 var (
@@ -89,6 +92,8 @@ func newCtlGetCommand(f *flagpole) *cobra.Command {
 	cmd.Flags().Int64Var(&flags.ChunkSize, "chunk-size", 500, "chunk size of the list pager")
 	cmd.Flags().StringVar(&flags.Prefix, "prefix", "/registry", "prefix to prepend to the resource")
 
+	cmd.Flags().BoolVarP(&flags.Watch, "watch", "w", false, "after listing/getting the requested object, watch for changes")
+	cmd.Flags().BoolVar(&flags.WatchOnly, "watch-only", false, "watch for changes to the requested object(s), without listing/getting first")
 	return cmd
 }
 
@@ -124,13 +129,30 @@ func getCommand(ctx context.Context, etcdclient client.Client, flags *getFlagpol
 		client.WithResponse(printer.Print),
 	}
 
-	// TODO: Support watch
+	if flags.Watch {
+		if !flags.WatchOnly {
+			rev, err := etcdclient.Get(ctx, flags.Prefix,
+				opOpts...,
+			)
+			if err != nil {
+				return err
+			}
+			opOpts = append(opOpts, client.WithRevision(rev))
+		}
 
-	_, err := etcdclient.Get(ctx, flags.Prefix,
-		opOpts...,
-	)
-	if err != nil {
-		return err
+		err := etcdclient.Watch(ctx, flags.Prefix,
+			opOpts...,
+		)
+		if err != nil {
+			return err
+		}
+	} else {
+		_, err := etcdclient.Get(ctx, flags.Prefix,
+			opOpts...,
+		)
+		if err != nil {
+			return err
+		}
 	}
 
 	return nil

augerctl/command/printer.go

@@ -40,6 +40,9 @@ func NewPrinter(w io.Writer, printerType string) Printer {
 
 func formatResponse(w io.Writer, outMediaType string, kv *client.KeyValue) error {
 	value := kv.Value
+	if kv.PrevValue != nil {
+		value = kv.PrevValue
+	}
 	inMediaType, _, err := encoding.DetectAndExtract(value)
 	if err != nil {
 		_, err0 := fmt.Fprintf(w, "---\n# %s | raw | %v\n# %s\n", kv.Key, err, value)

pkg/client/client.go

@@ -31,6 +31,9 @@ type Client interface {
 	// Get is a method that retrieves a key-value pair from the etcd server.
 	// It returns the revision of the key-value pair
 	Get(ctx context.Context, prefix string, opOpts ...OpOption) (rev int64, err error)
+
+	// Watch is a method that watches for changes to a key-value pair on the etcd server.
+	Watch(ctx context.Context, prefix string, opOpts ...OpOption) error
 }
 
 // client is the etcd client.
@@ -89,6 +92,13 @@ func WithChunkSize(chunkSize int64) OpOption {
 	}
 }
 
+// WithRevision sets the revision for the target.
+func WithRevision(revision int64) OpOption {
+	return func(o *op) {
+		o.revision = revision
+	}
+}
+
 func opOption(opts []OpOption) op {
 	var opt op
 	for _, o := range opts {
@@ -101,6 +111,9 @@ func opOption(opts []OpOption) op {
 type KeyValue struct {
 	Key   []byte
 	Value []byte
+
+	// For delete event
+	PrevValue []byte
 }
 
 func iterateList(kvs []*mvccpb.KeyValue, callback func(kv *KeyValue) error) error {

pkg/client/client_watch.go

@@ -0,0 +1,66 @@
+/*
+Copyright 2024 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package client
+
+import (
+	"context"
+	"fmt"
+
+	clientv3 "go.etcd.io/etcd/client/v3"
+)
+
+func (c *client) Watch(ctx context.Context, prefix string, opOpts ...OpOption) error {
+	opt := opOption(opOpts)
+	if opt.response == nil {
+		return fmt.Errorf("response is required")
+	}
+
+	path, single, err := getPrefix(prefix, opt.gr, opt.name, opt.namespace)
+	if err != nil {
+		return err
+	}
+
+	opts := make([]clientv3.OpOption, 0, 3)
+
+	if !single {
+		opts = append(opts, clientv3.WithPrefix())
+	}
+
+	if opt.revision != 0 {
+		opts = append(opts, clientv3.WithRev(opt.revision))
+	}
+
+	opts = append(opts, clientv3.WithPrevKV())
+
+	watchChan := c.client.Watch(ctx, path, opts...)
+	for watchResp := range watchChan {
+		for _, event := range watchResp.Events {
+			r := &KeyValue{
+				Key:   event.Kv.Key,
+				Value: event.Kv.Value,
+			}
+			if event.PrevKv != nil {
+				r.PrevValue = event.PrevKv.Value
+			}
+			err := opt.response(r)
+			if err != nil {
+				return err
+			}
+		}
+	}
+	return nil
+}