[query] Noop storage backend to serve admin APIs (#2236)

- Some use cases require the coordinator for serving m3admin APIs but
  have no need to communicate with M3DB or serve query data.
- Currently need a live M3DB cluster in order to serve m3admin APIs.
- This PR adds a `noop-etcd` storage backend for serving m3admin APIs.

Example of querying:
```
$ curl -v "localhost:7201/api/v1/query_range?start=$(date '+%s')&end=$(date '+%s')&step=10&query=foo"
*   Trying ::1...
* TCP_NODELAY set
* Connected to localhost (::1) port 7201 (#0)
> GET /api/v1/query_range?start=1585603982&end=1585603982&step=10&query=foo HTTP/1.1
> Host: localhost:7201
> User-Agent: curl/7.54.0
> Accept: */*
>
< HTTP/1.1 500 Internal Server Error
< Access-Control-Allow-Headers: accept, content-type, authorization
< Access-Control-Allow-Methods: POST, GET, OPTIONS, PUT, DELETE
< Access-Control-Allow-Origin: *
< Date: Mon, 30 Mar 2020 21:33:02 GMT
< Content-Length: 48
< Content-Type: text/plain; charset=utf-8
<
{"error":"operation not valid for noop client"}
```

scripts/docker-integration-tests/coordinator_noop/docker-compose.yml

@@ -0,0 +1,42 @@
+version: "3.5"
+services:
+  coordinator01:
+    expose:
+      - "7201"
+    ports:
+      - "0.0.0.0:7201:7201"
+    networks:
+      - backend
+    image: "m3coordinator_integration:${REVISION}"
+    volumes:
+      - "./m3coordinator.yml:/etc/m3coordinator/m3coordinator.yml"
+  etcd01:
+    expose:
+      - "2379-2380"
+    ports:
+      - "0.0.0.0:2379-2380:2379-2380"
+    networks:
+      - backend
+    image: quay.io/coreos/etcd:v3.4.3
+    command:
+      - "etcd"
+      - "--name"
+      - "etcd01"
+      - "--listen-peer-urls"
+      - "http://0.0.0.0:2380"
+      - "--listen-client-urls"
+      - "http://0.0.0.0:2379"
+      - "--advertise-client-urls"
+      - "http://etcd01:2379"
+      - "--initial-cluster-token"
+      - "etcd-cluster-1"
+      - "--initial-advertise-peer-urls"
+      - "http://etcd01:2380"
+      - "--initial-cluster"
+      - "etcd01=http://etcd01:2380"
+      - "--initial-cluster-state"
+      - "new"
+      - "--data-dir"
+      - "/var/lib/etcd"
+networks:
+  backend:

scripts/docker-integration-tests/coordinator_noop/m3coordinator.yml

@@ -0,0 +1,30 @@
+listenAddress:
+  value: "0.0.0.0:7201"
+
+logging:
+  level: info
+
+metrics:
+  scope:
+    prefix: "coordinator"
+  prometheus:
+    handlerPath: /metrics
+    listenAddress: 0.0.0.0:7203 # until https://github.com/m3db/m3/issues/682 is resolved
+  sanitization: prometheus
+  samplingRate: 1.0
+  extended: none
+
+backend: noop-etcd
+clusterManagement:
+  etcd:
+    env: default_env
+    zone: embedded
+    service: m3db
+    cacheDir: /var/lib/m3kv
+    etcdClusters:
+    - zone: embedded
+      endpoints:
+      - etcd01:2379
+
+tagOptions:
+  idScheme: quoted

scripts/docker-integration-tests/coordinator_noop/test.sh

@@ -0,0 +1,51 @@
+#!/usr/bin/env bash
+
+set -xe
+
+source $GOPATH/src/github.com/m3db/m3/scripts/docker-integration-tests/common.sh
+REVISION=$(git rev-parse HEAD)
+SCRIPT_PATH=$GOPATH/src/github.com/m3db/m3/scripts/docker-integration-tests/coordinator_noop
+COMPOSE_FILE=$SCRIPT_PATH/docker-compose.yml
+export REVISION
+
+echo "Run coordinator with no etcd"
+docker-compose -f ${COMPOSE_FILE} up -d --renew-anon-volumes coordinator01
+docker-compose -f ${COMPOSE_FILE} up -d --renew-anon-volumes etcd01
+
+function defer {
+  docker-compose -f ${COMPOSE_FILE} down || echo "unable to shutdown containers" # CI fails to stop all containers sometimes
+}
+trap defer EXIT
+
+I=0
+RES=""
+while [[ "$I" -le 5 ]]; do
+  if curl -vvvsSf -X POST localhost:7201/api/v1/services/m3coordinator/placement/init -d '{
+    "instances": [
+        {
+            "id": "m3coordinator01",
+            "zone": "embedded",
+            "endpoint": "m3coordinator01:7507",
+            "hostname": "m3coordinator01",
+            "port": 7507
+        }
+    ]
+  }'; then
+    break
+  fi
+  # Need some time for coordinators to come up.
+  sleep 2
+  I=$((I+1))
+done
+
+if ! curl -vvvsSf localhost:7201/api/v1/services/m3coordinator/placement; then
+  echo "could not fetch existing placement"
+  exit 1
+fi
+
+QUERY_EXP='{"error":"operation not valid for noop client"}'
+RES=$(curl "localhost:7201/api/v1/query_range?start=$(date '+%s')&end=$(date '+%s')&step=10&query=foo")
+if [[ "$RES" != "$QUERY_EXP" ]]; then
+  echo "Expected resp '$QUERY_EXP', GOT '$RES'"
+  exit 1
+fi

scripts/docker-integration-tests/run.sh

@@ -15,6 +15,7 @@ TESTS=(
 	scripts/docker-integration-tests/repair_and_replication/test.sh
 	scripts/docker-integration-tests/multi_cluster_write/test.sh
 	scripts/docker-integration-tests/coordinator_config_rules/test.sh
+	scripts/docker-integration-tests/coordinator_noop/test.sh
 )
 
 # Some systems, including our default Buildkite hosts, don't come with netcat

src/cmd/services/m3query/config/config.go

@@ -52,6 +52,11 @@ const (
 	GRPCStorageType BackendStorageType = "grpc"
 	// M3DBStorageType is for m3db backend.
 	M3DBStorageType BackendStorageType = "m3db"
+	// NoopEtcdStorageType is for a noop backend which returns empty results for
+	// any query and blackholes any writes, but requires that a valid etcd cluster
+	// is defined and can be connected to. Primarily used for standalone
+	// coordinators used only to serve m3admin APIs.
+	NoopEtcdStorageType BackendStorageType = "noop-etcd"
 
 	defaultCarbonIngesterListenAddress = "0.0.0.0:7204"
 	errNoIDGenerationScheme            = "error: a recent breaking change means that an ID " +

src/query/server/query.go

@@ -257,7 +257,8 @@ func Run(runOpts RunOptions) {
 		tsdbOpts = runOpts.ApplyCustomTSDBOptions(tsdbOpts)
 	}
 
-	if cfg.Backend == config.GRPCStorageType {
+	switch cfg.Backend {
+	case config.GRPCStorageType:
 		// For grpc backend, we need to setup only the grpc client and a storage
 		// accompanying that client.
 		poolWrapper := pools.NewPoolsWrapper(
@@ -281,7 +282,24 @@ func Run(runOpts RunOptions) {
 		backendStorage = fanout.NewStorage(remotes, r, w, c,
 			instrumentOptions)
 		logger.Info("setup grpc backend")
-	} else {
+
+	case config.NoopEtcdStorageType:
+		backendStorage = storage.NewNoopStorage()
+		mgmt := cfg.ClusterManagement
+
+		if mgmt == nil || len(mgmt.Etcd.ETCDClusters) == 0 {
+			logger.Fatal("must specify cluster management config and at least one etcd cluster")
+		}
+
+		opts := mgmt.Etcd.NewOptions()
+		clusterClient, err = etcdclient.NewConfigServiceClient(opts)
+		if err != nil {
+			logger.Fatal("error constructing etcd client", zap.Error(err))
+		}
+		logger.Info("setup noop storage backend with etcd")
+
+	// Empty backend defaults to M3DB.
+	case "":
 		// For m3db backend, we need to make connections to the m3db cluster
 		// which generates a session and use the storage with the session.
 		m3dbClusters, m3dbPoolWrapper, err = initClusters(cfg,
@@ -300,6 +318,9 @@ func Run(runOpts RunOptions) {
 			logger.Fatal("unable to setup m3db backend", zap.Error(err))
 		}
 		defer cleanup()
+
+	default:
+		logger.Fatal("unrecognized backend", zap.String("backend", string(cfg.Backend)))
 	}
 
 	chainedEnforcer, chainedEnforceCloser, err := newConfiguredChainedEnforcer(&cfg,

src/query/storage/noop_storage.go

@@ -0,0 +1,88 @@
+// Copyright (c) 2020 Uber Technologies, Inc.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+
+package storage
+
+import (
+	"context"
+	"errors"
+
+	"github.com/m3db/m3/src/query/block"
+)
+
+var noopClientError = errors.New("operation not valid for noop client")
+
+// NewNoopStorage returns a fake implementation of Storage that rejects all
+// writes and returns errors for all queries.
+func NewNoopStorage() Storage {
+	return noopStorage{}
+}
+
+type noopStorage struct{}
+
+func (noopStorage) Fetch(ctx context.Context, query *FetchQuery, options *FetchOptions) (*FetchResult, error) {
+	return nil, noopClientError
+}
+
+func (noopStorage) FetchProm(ctx context.Context, query *FetchQuery, options *FetchOptions) (PromResult, error) {
+	return PromResult{}, noopClientError
+}
+
+// FetchBlocks fetches timeseries as blocks based on a query.
+func (noopStorage) FetchBlocks(ctx context.Context, query *FetchQuery, options *FetchOptions) (block.Result, error) {
+	return block.Result{}, noopClientError
+}
+
+// SearchSeries returns series IDs matching the current query.
+func (noopStorage) SearchSeries(ctx context.Context, query *FetchQuery, options *FetchOptions) (*SearchResults, error) {
+	return nil, noopClientError
+}
+
+// CompleteTags returns autocompleted tag results.
+func (noopStorage) CompleteTags(ctx context.Context, query *CompleteTagsQuery, options *FetchOptions) (*CompleteTagsResult, error) {
+	return nil, noopClientError
+}
+
+// Write writes a batched set of datapoints to storage based on the provided
+// query.
+func (noopStorage) Write(ctx context.Context, query *WriteQuery) error {
+	return noopClientError
+}
+
+// Type identifies the type of the underlying
+func (noopStorage) Type() Type {
+	return TypeLocalDC
+}
+
+// Close is used to close the underlying storage and free up resources.
+func (noopStorage) Close() error {
+	return noopClientError
+}
+
+// ErrorBehavior dictates what fanout storage should do when this storage
+// encounters an error.
+func (noopStorage) ErrorBehavior() ErrorBehavior {
+	return BehaviorWarn
+}
+
+// Name gives the plaintext name for this storage, used for logging purposes.
+func (noopStorage) Name() string {
+	return "noopStorage"
+}