Merge pull request #5500 from ethereum-optimism/felipe/proxyd-consensus

proxyd: add consensus poller

proxyd/backend.go

@@ -365,6 +365,36 @@ func (b *Backend) setOffline() {
 	}
 }
 
+// ForwardRPC makes a call directly to a backend and populate the response into `res`
+func (b *Backend) ForwardRPC(ctx context.Context, res *RPCRes, id string, method string, params ...any) error {
+	jsonParams, err := json.Marshal(params)
+	if err != nil {
+		return err
+	}
+
+	rpcReq := RPCReq{
+		JSONRPC: JSONRPCVersion,
+		Method:  method,
+		Params:  jsonParams,
+		ID:      []byte(id),
+	}
+
+	slicedRes, err := b.doForward(ctx, []*RPCReq{&rpcReq}, false)
+	if err != nil {
+		return err
+	}
+
+	if len(slicedRes) != 1 {
+		return fmt.Errorf("unexpected response len for non-batched request (len != 1)")
+	}
+	if slicedRes[0].IsError() {
+		return fmt.Errorf(slicedRes[0].Error.Error())
+	}
+
+	*res = *(slicedRes[0])
+	return nil
+}
+
 func (b *Backend) doForward(ctx context.Context, rpcReqs []*RPCReq, isBatch bool) ([]*RPCRes, error) {
 	isSingleElementBatch := len(rpcReqs) == 1
 
@@ -484,8 +514,9 @@ func sortBatchRPCResponse(req []*RPCReq, res []*RPCRes) {
 }
 
 type BackendGroup struct {
-	Name     string
-	Backends []*Backend
+	Name      string
+	Backends  []*Backend
+	Consensus *ConsensusPoller
 }
 
 func (b *BackendGroup) Forward(ctx context.Context, rpcReqs []*RPCReq, isBatch bool) ([]*RPCRes, error) {

proxyd/cmd/proxyd/main.go

@@ -52,7 +52,7 @@ func main() {
 		),
 	)
 
-	shutdown, err := proxyd.Start(config)
+	_, shutdown, err := proxyd.Start(config)
 	if err != nil {
 		log.Crit("error starting proxyd", "err", err)
 	}

proxyd/config.go

@@ -82,6 +82,7 @@ type BackendConfig struct {
 	Password         string `toml:"password"`
 	RPCURL           string `toml:"rpc_url"`
 	WSURL            string `toml:"ws_url"`
+	WSPort           int    `toml:"ws_port"`
 	MaxRPS           int    `toml:"max_rps"`
 	MaxWSConns       int    `toml:"max_ws_conns"`
 	CAFile           string `toml:"ca_file"`
@@ -93,7 +94,9 @@ type BackendConfig struct {
 type BackendsConfig map[string]*BackendConfig
 
 type BackendGroupConfig struct {
-	Backends []string `toml:"backends"`
+	Backends              []string `toml:"backends"`
+	ConsensusAware        bool     `toml:"consensus_aware"`
+	ConsensusAsyncHandler string   `toml:"consensus_handler"`
 }
 
 type BackendGroupsConfig map[string]*BackendGroupConfig

proxyd/consensus_poller.go

+package proxyd
+
+import (
+	"context"
+	"fmt"
+	"strings"
+	"sync"
+	"time"
+
+	"github.com/ethereum/go-ethereum/common/hexutil"
+
+	"github.com/ethereum/go-ethereum/log"
+)
+
+const (
+	PollerInterval = 1 * time.Second
+)
+
+// ConsensusPoller checks the consensus state for each member of a BackendGroup
+// resolves the highest common block for multiple nodes, and reconciles the consensus
+// in case of block hash divergence to minimize re-orgs
+type ConsensusPoller struct {
+	cancelFunc context.CancelFunc
+
+	backendGroup      *BackendGroup
+	backendState      map[*Backend]*backendState
+	consensusGroupMux sync.Mutex
+	consensusGroup    []*Backend
+
+	tracker      ConsensusTracker
+	asyncHandler ConsensusAsyncHandler
+}
+
+type backendState struct {
+	backendStateMux sync.Mutex
+
+	latestBlockNumber hexutil.Uint64
+	latestBlockHash   string
+
+	lastUpdate time.Time
+
+	bannedUntil time.Time
+}
+
+// GetConsensusGroup returns the backend members that are agreeing in a consensus
+func (cp *ConsensusPoller) GetConsensusGroup() []*Backend {
+	defer cp.consensusGroupMux.Unlock()
+	cp.consensusGroupMux.Lock()
+
+	g := make([]*Backend, len(cp.backendGroup.Backends))
+	copy(g, cp.consensusGroup)
+
+	return g
+}
+
+// GetConsensusBlockNumber returns the agreed block number in a consensus
+func (ct *ConsensusPoller) GetConsensusBlockNumber() hexutil.Uint64 {
+	return ct.tracker.GetConsensusBlockNumber()
+}
+
+func (cp *ConsensusPoller) Shutdown() {
+	cp.asyncHandler.Shutdown()
+}
+
+// ConsensusAsyncHandler controls the asynchronous polling mechanism, interval and shutdown
+type ConsensusAsyncHandler interface {
+	Init()
+	Shutdown()
+}
+
+// NoopAsyncHandler allows fine control updating the consensus
+type NoopAsyncHandler struct{}
+
+func NewNoopAsyncHandler() ConsensusAsyncHandler {
+	log.Warn("using NewNoopAsyncHandler")
+	return &NoopAsyncHandler{}
+}
+func (ah *NoopAsyncHandler) Init()     {}
+func (ah *NoopAsyncHandler) Shutdown() {}
+
+// PollerAsyncHandler asynchronously updates each individual backend and the group consensus
+type PollerAsyncHandler struct {
+	ctx context.Context
+	cp  *ConsensusPoller
+}
+
+func NewPollerAsyncHandler(ctx context.Context, cp *ConsensusPoller) ConsensusAsyncHandler {
+	return &PollerAsyncHandler{
+		ctx: ctx,
+		cp:  cp,
+	}
+}
+func (ah *PollerAsyncHandler) Init() {
+	// create the individual backend pollers
+	for _, be := range ah.cp.backendGroup.Backends {
+		go func(be *Backend) {
+			for {
+				timer := time.NewTimer(PollerInterval)
+				ah.cp.UpdateBackend(ah.ctx, be)
+
+				select {
+				case <-timer.C:
+				case <-ah.ctx.Done():
+					timer.Stop()
+					return
+				}
+			}
+		}(be)
+	}
+
+	// create the group consensus poller
+	go func() {
+		for {
+			timer := time.NewTimer(PollerInterval)
+			ah.cp.UpdateBackendGroupConsensus(ah.ctx)
+
+			select {
+			case <-timer.C:
+			case <-ah.ctx.Done():
+				timer.Stop()
+				return
+			}
+		}
+	}()
+}
+func (ah *PollerAsyncHandler) Shutdown() {
+	ah.cp.cancelFunc()
+}
+
+type ConsensusOpt func(cp *ConsensusPoller)
+
+func WithTracker(tracker ConsensusTracker) ConsensusOpt {
+	return func(cp *ConsensusPoller) {
+		cp.tracker = tracker
+	}
+}
+
+func WithAsyncHandler(asyncHandler ConsensusAsyncHandler) ConsensusOpt {
+	return func(cp *ConsensusPoller) {
+		cp.asyncHandler = asyncHandler
+	}
+}
+
+func NewConsensusPoller(bg *BackendGroup, opts ...ConsensusOpt) *ConsensusPoller {
+	ctx, cancelFunc := context.WithCancel(context.Background())
+
+	state := make(map[*Backend]*backendState, len(bg.Backends))
+	for _, be := range bg.Backends {
+		state[be] = &backendState{}
+	}
+
+	cp := &ConsensusPoller{
+		cancelFunc:   cancelFunc,
+		backendGroup: bg,
+		backendState: state,
+	}
+
+	for _, opt := range opts {
+		opt(cp)
+	}
+
+	if cp.tracker == nil {
+		cp.tracker = NewInMemoryConsensusTracker()
+	}
+
+	if cp.asyncHandler == nil {
+		cp.asyncHandler = NewPollerAsyncHandler(ctx, cp)
+	}
+
+	cp.asyncHandler.Init()
+
+	return cp
+}
+
+// UpdateBackend refreshes the consensus state of a single backend
+func (cp *ConsensusPoller) UpdateBackend(ctx context.Context, be *Backend) {
+	bs := cp.backendState[be]
+	if time.Now().Before(bs.bannedUntil) {
+		log.Warn("skipping backend banned", "backend", be.Name, "bannedUntil", bs.bannedUntil)
+		return
+	}
+
+	if be.IsRateLimited() || !be.Online() {
+		return
+	}
+
+	// we'll introduce here checks to ban the backend
+	// i.e. node is syncing the chain
+
+	// then update backend consensus
+
+	latestBlockNumber, latestBlockHash, err := cp.fetchBlock(ctx, be, "latest")
+	if err != nil {
+		log.Warn("error updating backend", "name", be.Name, "err", err)
+		return
+	}
+
+	changed := cp.setBackendState(be, latestBlockNumber, latestBlockHash)
+
+	if changed {
+		RecordBackendLatestBlock(be, latestBlockNumber)
+		log.Info("backend state updated", "name", be.Name, "state", bs)
+	}
+}
+
+// UpdateBackendGroupConsensus resolves the current group consensus based on the state of the backends
+func (cp *ConsensusPoller) UpdateBackendGroupConsensus(ctx context.Context) {
+	var lowestBlock hexutil.Uint64
+	var lowestBlockHash string
+
+	currentConsensusBlockNumber := cp.GetConsensusBlockNumber()
+
+	for _, be := range cp.backendGroup.Backends {
+		backendLatestBlockNumber, backendLatestBlockHash := cp.getBackendState(be)
+		if lowestBlock == 0 || backendLatestBlockNumber < lowestBlock {
+			lowestBlock = backendLatestBlockNumber
+			lowestBlockHash = backendLatestBlockHash
+		}
+	}
+
+	// no block to propose (i.e. initializing consensus)
+	if lowestBlock == 0 {
+		return
+	}
+
+	proposedBlock := lowestBlock
+	proposedBlockHash := lowestBlockHash
+	hasConsensus := false
+
+	// check if everybody agrees on the same block hash
+	consensusBackends := make([]*Backend, 0, len(cp.backendGroup.Backends))
+	consensusBackendsNames := make([]string, 0, len(cp.backendGroup.Backends))
+	filteredBackendsNames := make([]string, 0, len(cp.backendGroup.Backends))
+
+	if lowestBlock > currentConsensusBlockNumber {
+		log.Info("validating consensus on block", lowestBlock)
+	}
+
+	broken := false
+	for !hasConsensus {
+		allAgreed := true
+		consensusBackends = consensusBackends[:0]
+		filteredBackendsNames = filteredBackendsNames[:0]
+		for _, be := range cp.backendGroup.Backends {
+			if be.IsRateLimited() || !be.Online() || time.Now().Before(cp.backendState[be].bannedUntil) {
+				filteredBackendsNames = append(filteredBackendsNames, be.Name)
+				continue
+			}
+
+			actualBlockNumber, actualBlockHash, err := cp.fetchBlock(ctx, be, proposedBlock.String())
+			if err != nil {
+				log.Warn("error updating backend", "name", be.Name, "err", err)
+				continue
+			}
+			if proposedBlockHash == "" {
+				proposedBlockHash = actualBlockHash
+			}
+			blocksDontMatch := (actualBlockNumber != proposedBlock) || (actualBlockHash != proposedBlockHash)
+			if blocksDontMatch {
+				if currentConsensusBlockNumber >= actualBlockNumber {
+					log.Warn("backend broke consensus", "name", be.Name, "blockNum", actualBlockNumber, "proposedBlockNum", proposedBlock, "blockHash", actualBlockHash, "proposedBlockHash", proposedBlockHash)
+					broken = true
+				}
+				allAgreed = false
+				break
+			}
+			consensusBackends = append(consensusBackends, be)
+			consensusBackendsNames = append(consensusBackendsNames, be.Name)
+		}
+		if allAgreed {
+			hasConsensus = true
+		} else {
+			// walk one block behind and try again
+			proposedBlock -= 1
+			proposedBlockHash = ""
+			log.Info("no consensus, now trying", "block:", proposedBlock)
+		}
+	}
+
+	if broken {
+		// propagate event to other interested parts, such as cache invalidator
+		log.Info("consensus broken", "currentConsensusBlockNumber", currentConsensusBlockNumber, "proposedBlock", proposedBlock, "proposedBlockHash", proposedBlockHash)
+	}
+
+	cp.tracker.SetConsensusBlockNumber(proposedBlock)
+	RecordGroupConsensusLatestBlock(cp.backendGroup, proposedBlock)
+	cp.consensusGroupMux.Lock()
+	cp.consensusGroup = consensusBackends
+	cp.consensusGroupMux.Unlock()
+
+	log.Info("group state", "proposedBlock", proposedBlock, "consensusBackends", strings.Join(consensusBackendsNames, ", "), "filteredBackends", strings.Join(filteredBackendsNames, ", "))
+}
+
+// fetchBlock Convenient wrapper to make a request to get a block directly from the backend
+func (cp *ConsensusPoller) fetchBlock(ctx context.Context, be *Backend, block string) (blockNumber hexutil.Uint64, blockHash string, err error) {
+	var rpcRes RPCRes
+	err = be.ForwardRPC(ctx, &rpcRes, "67", "eth_getBlockByNumber", block, false)
+	if err != nil {
+		return 0, "", err
+	}
+
+	jsonMap, ok := rpcRes.Result.(map[string]interface{})
+	if !ok {
+		return 0, "", fmt.Errorf("unexpected response type checking consensus on backend %s", be.Name)
+	}
+	blockNumber = hexutil.Uint64(hexutil.MustDecodeUint64(jsonMap["number"].(string)))
+	blockHash = jsonMap["hash"].(string)
+
+	return
+}
+
+func (cp *ConsensusPoller) getBackendState(be *Backend) (blockNumber hexutil.Uint64, blockHash string) {
+	bs := cp.backendState[be]
+	bs.backendStateMux.Lock()
+	blockNumber = bs.latestBlockNumber
+	blockHash = bs.latestBlockHash
+	bs.backendStateMux.Unlock()
+	return
+}
+
+func (cp *ConsensusPoller) setBackendState(be *Backend, blockNumber hexutil.Uint64, blockHash string) (changed bool) {
+	bs := cp.backendState[be]
+	bs.backendStateMux.Lock()
+	changed = bs.latestBlockHash != blockHash
+	bs.latestBlockNumber = blockNumber
+	bs.latestBlockHash = blockHash
+	bs.lastUpdate = time.Now()
+	bs.backendStateMux.Unlock()
+	return
+}

proxyd/consensus_tracker.go

+package proxyd
+
+import (
+	"context"
+	"fmt"
+	"sync"
+
+	"github.com/ethereum/go-ethereum/common/hexutil"
+
+	"github.com/go-redis/redis/v8"
+)
+
+// ConsensusTracker abstracts how we store and retrieve the current consensus
+// allowing it to be stored locally in-memory or in a shared Redis cluster
+type ConsensusTracker interface {
+	GetConsensusBlockNumber() hexutil.Uint64
+	SetConsensusBlockNumber(blockNumber hexutil.Uint64)
+}
+
+// InMemoryConsensusTracker store and retrieve in memory, async-safe
+type InMemoryConsensusTracker struct {
+	consensusBlockNumber hexutil.Uint64
+	mutex                sync.Mutex
+}
+
+func NewInMemoryConsensusTracker() ConsensusTracker {
+	return &InMemoryConsensusTracker{
+		consensusBlockNumber: 0,
+		mutex:                sync.Mutex{},
+	}
+}
+
+func (ct *InMemoryConsensusTracker) GetConsensusBlockNumber() hexutil.Uint64 {
+	defer ct.mutex.Unlock()
+	ct.mutex.Lock()
+
+	return ct.consensusBlockNumber
+}
+
+func (ct *InMemoryConsensusTracker) SetConsensusBlockNumber(blockNumber hexutil.Uint64) {
+	defer ct.mutex.Unlock()
+	ct.mutex.Lock()
+
+	ct.consensusBlockNumber = blockNumber
+}
+
+// RedisConsensusTracker uses a Redis `client` to store and retrieve consensus, async-safe
+type RedisConsensusTracker struct {
+	ctx          context.Context
+	client       *redis.Client
+	backendGroup string
+}
+
+func NewRedisConsensusTracker(ctx context.Context, r *redis.Client, namespace string) ConsensusTracker {
+	return &RedisConsensusTracker{
+		ctx:          ctx,
+		client:       r,
+		backendGroup: namespace,
+	}
+}
+
+func (ct *RedisConsensusTracker) key() string {
+	return fmt.Sprintf("consensus_latest_block:%s", ct.backendGroup)
+}
+
+func (ct *RedisConsensusTracker) GetConsensusBlockNumber() hexutil.Uint64 {
+	return hexutil.Uint64(hexutil.MustDecodeUint64(ct.client.Get(ct.ctx, ct.key()).Val()))
+}
+
+func (ct *RedisConsensusTracker) SetConsensusBlockNumber(blockNumber hexutil.Uint64) {
+	ct.client.Set(ct.ctx, ct.key(), blockNumber, 0)
+}

proxyd/example.config.toml

@@ -15,6 +15,7 @@ rpc_port = 8080
 # Host for the proxyd WS server to listen on.
 ws_host = "0.0.0.0"
 # Port for the above
+# Set the ws_port to 0 to disable WS
 ws_port = 8085
 # Maximum client body size, in bytes, that the server will accept.
 max_body_size_bytes = 10485760

proxyd/go.mod

@@ -11,10 +11,12 @@ require (
 	github.com/gorilla/mux v1.8.0
 	github.com/gorilla/websocket v1.5.0
 	github.com/hashicorp/golang-lru v0.5.5-0.20210104140557-80c98217689d
+	github.com/pkg/errors v0.9.1
 	github.com/prometheus/client_golang v1.11.1
 	github.com/rs/cors v1.8.2
 	github.com/stretchr/testify v1.7.0
 	golang.org/x/sync v0.0.0-20210220032951-036812b2e83c
+	gopkg.in/yaml.v2 v2.4.0
 )
 
 require (

proxyd/integration_tests/batch_timeout_test.go

@@ -22,7 +22,7 @@ func TestBatchTimeout(t *testing.T) {
 
 	config := ReadConfig("batch_timeout")
 	client := NewProxydClient("http://127.0.0.1:8545")
-	shutdown, err := proxyd.Start(config)
+	_, shutdown, err := proxyd.Start(config)
 	require.NoError(t, err)
 	defer shutdown()
 

proxyd/integration_tests/batching_test.go

@@ -148,7 +148,7 @@ func TestBatching(t *testing.T) {
 			require.NoError(t, os.Setenv("GOOD_BACKEND_RPC_URL", goodBackend.URL()))
 
 			client := NewProxydClient("http://127.0.0.1:8545")
-			shutdown, err := proxyd.Start(config)
+			_, shutdown, err := proxyd.Start(config)
 			require.NoError(t, err)
 			defer shutdown()
 

proxyd/integration_tests/caching_test.go

@@ -35,7 +35,7 @@ func TestCaching(t *testing.T) {
 	require.NoError(t, os.Setenv("REDIS_URL", fmt.Sprintf("redis://127.0.0.1:%s", redis.Port())))
 	config := ReadConfig("caching")
 	client := NewProxydClient("http://127.0.0.1:8545")
-	shutdown, err := proxyd.Start(config)
+	_, shutdown, err := proxyd.Start(config)
 	require.NoError(t, err)
 	defer shutdown()
 
@@ -171,7 +171,7 @@ func TestBatchCaching(t *testing.T) {
 
 	config := ReadConfig("caching")
 	client := NewProxydClient("http://127.0.0.1:8545")
-	shutdown, err := proxyd.Start(config)
+	_, shutdown, err := proxyd.Start(config)
 	require.NoError(t, err)
 	defer shutdown()
 

proxyd/integration_tests/consensus_test.go

+package integration_tests
+
+import (
+	"context"
+	"fmt"
+	"net/http"
+	"os"
+	"path"
+	"testing"
+
+	"github.com/ethereum-optimism/optimism/proxyd"
+	ms "github.com/ethereum-optimism/optimism/proxyd/tools/mockserver/handler"
+	"github.com/stretchr/testify/require"
+)
+
+func TestConsensus(t *testing.T) {
+	node1 := NewMockBackend(nil)
+	defer node1.Close()
+	node2 := NewMockBackend(nil)
+	defer node2.Close()
+
+	dir, err := os.Getwd()
+	require.NoError(t, err)
+
+	responses := path.Join(dir, "testdata/consensus_responses.yml")
+
+	h1 := ms.MockedHandler{
+		Overrides:    []*ms.MethodTemplate{},
+		Autoload:     true,
+		AutoloadFile: responses,
+	}
+	h2 := ms.MockedHandler{
+		Overrides:    []*ms.MethodTemplate{},
+		Autoload:     true,
+		AutoloadFile: responses,
+	}
+
+	require.NoError(t, os.Setenv("NODE1_URL", node1.URL()))
+	require.NoError(t, os.Setenv("NODE2_URL", node2.URL()))
+
+	node1.SetHandler(http.HandlerFunc(h1.Handler))
+	node2.SetHandler(http.HandlerFunc(h2.Handler))
+
+	config := ReadConfig("consensus")
+	ctx := context.Background()
+	svr, shutdown, err := proxyd.Start(config)
+	require.NoError(t, err)
+	defer shutdown()
+
+	bg := svr.BackendGroups["node"]
+	require.NotNil(t, bg)
+	require.NotNil(t, bg.Consensus)
+
+	t.Run("initial consensus", func(t *testing.T) {
+		h1.ResetOverrides()
+		h2.ResetOverrides()
+
+		// unknown consensus at init
+		require.Equal(t, "0x0", bg.Consensus.GetConsensusBlockNumber().String())
+
+		// first poll
+		for _, be := range bg.Backends {
+			bg.Consensus.UpdateBackend(ctx, be)
+		}
+		bg.Consensus.UpdateBackendGroupConsensus(ctx)
+
+		// consensus at block 0x1
+		require.Equal(t, "0x1", bg.Consensus.GetConsensusBlockNumber().String())
+	})
+
+	t.Run("advance consensus", func(t *testing.T) {
+		h1.ResetOverrides()
+		h2.ResetOverrides()
+
+		for _, be := range bg.Backends {
+			bg.Consensus.UpdateBackend(ctx, be)
+		}
+		bg.Consensus.UpdateBackendGroupConsensus(ctx)
+
+		// all nodes start at block 0x1
+		require.Equal(t, "0x1", bg.Consensus.GetConsensusBlockNumber().String())
+
+		// advance latest on node2 to 0x2
+		h2.AddOverride(&ms.MethodTemplate{
+			Method:   "eth_getBlockByNumber",
+			Block:    "latest",
+			Response: buildResponse("0x2", "hash2"),
+		})
+
+		// poll for group consensus
+		for _, be := range bg.Backends {
+			bg.Consensus.UpdateBackend(ctx, be)
+		}
+		bg.Consensus.UpdateBackendGroupConsensus(ctx)
+
+		// consensus should stick to 0x1, since node1 is still lagging there
+		bg.Consensus.UpdateBackendGroupConsensus(ctx)
+		require.Equal(t, "0x1", bg.Consensus.GetConsensusBlockNumber().String())
+
+		// advance latest on node1 to 0x2
+		h1.AddOverride(&ms.MethodTemplate{
+			Method:   "eth_getBlockByNumber",
+			Block:    "latest",
+			Response: buildResponse("0x2", "hash2"),
+		})
+
+		// poll for group consensus
+		for _, be := range bg.Backends {
+			bg.Consensus.UpdateBackend(ctx, be)
+		}
+		bg.Consensus.UpdateBackendGroupConsensus(ctx)
+
+		// should stick to 0x2, since now all nodes are at 0x2
+		require.Equal(t, "0x2", bg.Consensus.GetConsensusBlockNumber().String())
+	})
+
+	t.Run("broken consensus", func(t *testing.T) {
+		h1.ResetOverrides()
+		h2.ResetOverrides()
+
+		for _, be := range bg.Backends {
+			bg.Consensus.UpdateBackend(ctx, be)
+		}
+		bg.Consensus.UpdateBackendGroupConsensus(ctx)
+
+		// all nodes start at block 0x1
+		require.Equal(t, "0x1", bg.Consensus.GetConsensusBlockNumber().String())
+
+		// advance latest on both nodes to 0x2
+		h1.AddOverride(&ms.MethodTemplate{
+			Method:   "eth_getBlockByNumber",
+			Block:    "latest",
+			Response: buildResponse("0x2", "hash2"),
+		})
+		h2.AddOverride(&ms.MethodTemplate{
+			Method:   "eth_getBlockByNumber",
+			Block:    "latest",
+			Response: buildResponse("0x2", "hash2"),
+		})
+
+		// poll for group consensus
+		for _, be := range bg.Backends {
+			bg.Consensus.UpdateBackend(ctx, be)
+		}
+		bg.Consensus.UpdateBackendGroupConsensus(ctx)
+
+		// at 0x2
+		require.Equal(t, "0x2", bg.Consensus.GetConsensusBlockNumber().String())
+
+		// make node2 diverge on hash
+		h2.AddOverride(&ms.MethodTemplate{
+			Method:   "eth_getBlockByNumber",
+			Block:    "0x2",
+			Response: buildResponse("0x2", "wrong_hash"),
+		})
+
+		// poll for group consensus
+		for _, be := range bg.Backends {
+			bg.Consensus.UpdateBackend(ctx, be)
+		}
+		bg.Consensus.UpdateBackendGroupConsensus(ctx)
+
+		// should resolve to 0x1, since 0x2 is out of consensus at the moment
+		require.Equal(t, "0x1", bg.Consensus.GetConsensusBlockNumber().String())
+
+		// later, when impl events, listen to broken consensus event
+	})
+
+	t.Run("broken consensus with depth 2", func(t *testing.T) {
+		h1.ResetOverrides()
+		h2.ResetOverrides()
+
+		for _, be := range bg.Backends {
+			bg.Consensus.UpdateBackend(ctx, be)
+		}
+		bg.Consensus.UpdateBackendGroupConsensus(ctx)
+
+		// all nodes start at block 0x1
+		require.Equal(t, "0x1", bg.Consensus.GetConsensusBlockNumber().String())
+
+		// advance latest on both nodes to 0x2
+		h1.AddOverride(&ms.MethodTemplate{
+			Method:   "eth_getBlockByNumber",
+			Block:    "latest",
+			Response: buildResponse("0x2", "hash2"),
+		})
+		h2.AddOverride(&ms.MethodTemplate{
+			Method:   "eth_getBlockByNumber",
+			Block:    "latest",
+			Response: buildResponse("0x2", "hash2"),
+		})
+
+		// poll for group consensus
+		for _, be := range bg.Backends {
+			bg.Consensus.UpdateBackend(ctx, be)
+		}
+		bg.Consensus.UpdateBackendGroupConsensus(ctx)
+
+		// at 0x2
+		require.Equal(t, "0x2", bg.Consensus.GetConsensusBlockNumber().String())
+
+		// advance latest on both nodes to 0x3
+		h1.AddOverride(&ms.MethodTemplate{
+			Method:   "eth_getBlockByNumber",
+			Block:    "latest",
+			Response: buildResponse("0x3", "hash3"),
+		})
+		h2.AddOverride(&ms.MethodTemplate{
+			Method:   "eth_getBlockByNumber",
+			Block:    "latest",
+			Response: buildResponse("0x3", "hash3"),
+		})
+
+		// poll for group consensus
+		for _, be := range bg.Backends {
+			bg.Consensus.UpdateBackend(ctx, be)
+		}
+		bg.Consensus.UpdateBackendGroupConsensus(ctx)
+
+		// at 0x3
+		require.Equal(t, "0x3", bg.Consensus.GetConsensusBlockNumber().String())
+
+		// make node2 diverge on hash for blocks 0x2 and 0x3
+		h2.AddOverride(&ms.MethodTemplate{
+			Method:   "eth_getBlockByNumber",
+			Block:    "0x2",
+			Response: buildResponse("0x2", "wrong_hash2"),
+		})
+		h2.AddOverride(&ms.MethodTemplate{
+			Method:   "eth_getBlockByNumber",
+			Block:    "0x3",
+			Response: buildResponse("0x3", "wrong_hash3"),
+		})
+
+		// poll for group consensus
+		for _, be := range bg.Backends {
+			bg.Consensus.UpdateBackend(ctx, be)
+		}
+		bg.Consensus.UpdateBackendGroupConsensus(ctx)
+
+		// should resolve to 0x1
+		require.Equal(t, "0x1", bg.Consensus.GetConsensusBlockNumber().String())
+	})
+
+	t.Run("fork in advanced block", func(t *testing.T) {
+		h1.ResetOverrides()
+		h2.ResetOverrides()
+
+		for _, be := range bg.Backends {
+			bg.Consensus.UpdateBackend(ctx, be)
+		}
+		bg.Consensus.UpdateBackendGroupConsensus(ctx)
+
+		// all nodes start at block 0x1
+		require.Equal(t, "0x1", bg.Consensus.GetConsensusBlockNumber().String())
+
+		// make nodes 1 and 2 advance in forks
+		h1.AddOverride(&ms.MethodTemplate{
+			Method:   "eth_getBlockByNumber",
+			Block:    "0x2",
+			Response: buildResponse("0x2", "node1_0x2"),
+		})
+		h2.AddOverride(&ms.MethodTemplate{
+			Method:   "eth_getBlockByNumber",
+			Block:    "0x2",
+			Response: buildResponse("0x2", "node2_0x2"),
+		})
+		h1.AddOverride(&ms.MethodTemplate{
+			Method:   "eth_getBlockByNumber",
+			Block:    "0x3",
+			Response: buildResponse("0x3", "node1_0x3"),
+		})
+		h2.AddOverride(&ms.MethodTemplate{
+			Method:   "eth_getBlockByNumber",
+			Block:    "0x3",
+			Response: buildResponse("0x3", "node2_0x3"),
+		})
+		h1.AddOverride(&ms.MethodTemplate{
+			Method:   "eth_getBlockByNumber",
+			Block:    "latest",
+			Response: buildResponse("0x3", "node1_0x3"),
+		})
+		h2.AddOverride(&ms.MethodTemplate{
+			Method:   "eth_getBlockByNumber",
+			Block:    "latest",
+			Response: buildResponse("0x3", "node2_0x3"),
+		})
+
+		// poll for group consensus
+		for _, be := range bg.Backends {
+			bg.Consensus.UpdateBackend(ctx, be)
+		}
+		bg.Consensus.UpdateBackendGroupConsensus(ctx)
+
+		// should resolve to 0x1, the highest common ancestor
+		require.Equal(t, "0x1", bg.Consensus.GetConsensusBlockNumber().String())
+	})
+}
+
+func buildResponse(number string, hash string) string {
+	return fmt.Sprintf(`{
+      "jsonrpc": "2.0",
+      "id": 67,
+      "result": {
+        "number": "%s",
+		"hash": "%s"
+      }
+    }`, number, hash)
+}

proxyd/integration_tests/failover_test.go

@@ -30,7 +30,7 @@ func TestFailover(t *testing.T) {
 
 	config := ReadConfig("failover")
 	client := NewProxydClient("http://127.0.0.1:8545")
-	shutdown, err := proxyd.Start(config)
+	_, shutdown, err := proxyd.Start(config)
 	require.NoError(t, err)
 	defer shutdown()
 
@@ -128,7 +128,7 @@ func TestRetries(t *testing.T) {
 	require.NoError(t, os.Setenv("GOOD_BACKEND_RPC_URL", backend.URL()))
 	config := ReadConfig("retries")
 	client := NewProxydClient("http://127.0.0.1:8545")
-	shutdown, err := proxyd.Start(config)
+	_, shutdown, err := proxyd.Start(config)
 	require.NoError(t, err)
 	defer shutdown()
 
@@ -171,7 +171,7 @@ func TestOutOfServiceInterval(t *testing.T) {
 
 	config := ReadConfig("out_of_service_interval")
 	client := NewProxydClient("http://127.0.0.1:8545")
-	shutdown, err := proxyd.Start(config)
+	_, shutdown, err := proxyd.Start(config)
 	require.NoError(t, err)
 	defer shutdown()
 
@@ -226,7 +226,7 @@ func TestBatchWithPartialFailover(t *testing.T) {
 	require.NoError(t, os.Setenv("BAD_BACKEND_RPC_URL", badBackend.URL()))
 
 	client := NewProxydClient("http://127.0.0.1:8545")
-	shutdown, err := proxyd.Start(config)
+	_, shutdown, err := proxyd.Start(config)
 	require.NoError(t, err)
 	defer shutdown()
 
@@ -273,7 +273,7 @@ func TestInfuraFailoverOnUnexpectedResponse(t *testing.T) {
 	require.NoError(t, os.Setenv("BAD_BACKEND_RPC_URL", badBackend.URL()))
 
 	client := NewProxydClient("http://127.0.0.1:8545")
-	shutdown, err := proxyd.Start(config)
+	_, shutdown, err := proxyd.Start(config)
 	require.NoError(t, err)
 	defer shutdown()
 

proxyd/integration_tests/max_rpc_conns_test.go

@@ -41,7 +41,7 @@ func TestMaxConcurrentRPCs(t *testing.T) {
 
 	config := ReadConfig("max_rpc_conns")
 	client := NewProxydClient("http://127.0.0.1:8545")
-	shutdown, err := proxyd.Start(config)
+	_, shutdown, err := proxyd.Start(config)
 	require.NoError(t, err)
 	defer shutdown()
 

proxyd/integration_tests/rate_limit_test.go

@@ -29,7 +29,7 @@ func TestBackendMaxRPSLimit(t *testing.T) {
 
 	config := ReadConfig("backend_rate_limit")
 	client := NewProxydClient("http://127.0.0.1:8545")
-	shutdown, err := proxyd.Start(config)
+	_, shutdown, err := proxyd.Start(config)
 	require.NoError(t, err)
 	defer shutdown()
 	limitedRes, codes := spamReqs(t, client, ethChainID, 503, 3)
@@ -45,7 +45,7 @@ func TestFrontendMaxRPSLimit(t *testing.T) {
 	require.NoError(t, os.Setenv("GOOD_BACKEND_RPC_URL", goodBackend.URL()))
 
 	config := ReadConfig("frontend_rate_limit")
-	shutdown, err := proxyd.Start(config)
+	_, shutdown, err := proxyd.Start(config)
 	require.NoError(t, err)
 	defer shutdown()
 

proxyd/integration_tests/sender_rate_limit_test.go

@@ -43,7 +43,7 @@ func TestSenderRateLimitValidation(t *testing.T) {
 	// validation.
 	config.SenderRateLimit.Limit = math.MaxInt
 	client := NewProxydClient("http://127.0.0.1:8545")
-	shutdown, err := proxyd.Start(config)
+	_, shutdown, err := proxyd.Start(config)
 	require.NoError(t, err)
 	defer shutdown()
 
@@ -73,7 +73,7 @@ func TestSenderRateLimitLimiting(t *testing.T) {
 
 	config := ReadConfig("sender_rate_limit")
 	client := NewProxydClient("http://127.0.0.1:8545")
-	shutdown, err := proxyd.Start(config)
+	_, shutdown, err := proxyd.Start(config)
 	require.NoError(t, err)
 	defer shutdown()
 

proxyd/integration_tests/testdata/consensus.toml

+[server]
+rpc_port = 8080
+
+[backend]
+response_timeout_seconds = 1
+
+[backends]
+[backends.node1]
+rpc_url = "$NODE1_URL"
+
+[backends.node2]
+rpc_url = "$NODE2_URL"
+
+[backend_groups]
+[backend_groups.node]
+backends = ["node1", "node2"]
+consensus_aware = true
+consensus_handler = "noop" # allow more control over the consensus poller for tests
+
+[rpc_method_mappings]
+eth_call = "node"
+eth_chainId = "node"
+eth_blockNumber = "node"
+eth_getBlockByNumber = "node"

proxyd/integration_tests/testdata/consensus_responses.yml

+- method: eth_getBlockByNumber
+  block: latest
+  response: >
+    {
+      "jsonrpc": "2.0",
+      "id": 67,
+      "result": {
+        "hash": "hash1",
+        "number": "0x1"
+      }
+    }
+- method: eth_getBlockByNumber
+  block: 0x1
+  response: >
+    {
+      "jsonrpc": "2.0",
+      "id": 67,
+      "result": {
+        "hash": "hash1",
+        "number": "0x1"
+      }
+    }
+- method: eth_getBlockByNumber
+  block: 0x2
+  response: >
+    {
+      "jsonrpc": "2.0",
+      "id": 67,
+      "result": {
+        "hash": "hash2",
+        "number": "0x2"
+      }
+    }
+- method: eth_getBlockByNumber
+  block: 0x3
+  response: >
+    {
+      "jsonrpc": "2.0",
+      "id": 67,
+      "result": {
+        "hash": "hash3",
+        "number": "0x3"
+      }
+    }

proxyd/integration_tests/validation_test.go

@@ -26,7 +26,7 @@ func TestSingleRPCValidation(t *testing.T) {
 
 	config := ReadConfig("whitelist")
 	client := NewProxydClient("http://127.0.0.1:8545")
-	shutdown, err := proxyd.Start(config)
+	_, shutdown, err := proxyd.Start(config)
 	require.NoError(t, err)
 	defer shutdown()
 
@@ -110,7 +110,7 @@ func TestBatchRPCValidation(t *testing.T) {
 
 	config := ReadConfig("whitelist")
 	client := NewProxydClient("http://127.0.0.1:8545")
-	shutdown, err := proxyd.Start(config)
+	_, shutdown, err := proxyd.Start(config)
 	require.NoError(t, err)
 	defer shutdown()
 

proxyd/integration_tests/ws_test.go

@@ -38,7 +38,7 @@ func TestConcurrentWSPanic(t *testing.T) {
 	require.NoError(t, os.Setenv("GOOD_BACKEND_RPC_URL", backend.URL()))
 
 	config := ReadConfig("ws")
-	shutdown, err := proxyd.Start(config)
+	_, shutdown, err := proxyd.Start(config)
 	require.NoError(t, err)
 	client, err := NewProxydWSClient("ws://127.0.0.1:8546", nil, nil)
 	require.NoError(t, err)
@@ -147,7 +147,7 @@ func TestWS(t *testing.T) {
 	require.NoError(t, os.Setenv("GOOD_BACKEND_RPC_URL", backend.URL()))
 
 	config := ReadConfig("ws")
-	shutdown, err := proxyd.Start(config)
+	_, shutdown, err := proxyd.Start(config)
 	require.NoError(t, err)
 	client, err := NewProxydWSClient("ws://127.0.0.1:8546", func(msgType int, data []byte) {
 		clientHdlr.MsgCB(msgType, data)
@@ -238,7 +238,7 @@ func TestWSClientClosure(t *testing.T) {
 	require.NoError(t, os.Setenv("GOOD_BACKEND_RPC_URL", backend.URL()))
 
 	config := ReadConfig("ws")
-	shutdown, err := proxyd.Start(config)
+	_, shutdown, err := proxyd.Start(config)
 	require.NoError(t, err)
 	defer shutdown()
 
@@ -278,7 +278,7 @@ func TestWSClientMaxConns(t *testing.T) {
 	require.NoError(t, os.Setenv("GOOD_BACKEND_RPC_URL", backend.URL()))
 
 	config := ReadConfig("ws")
-	shutdown, err := proxyd.Start(config)
+	_, shutdown, err := proxyd.Start(config)
 	require.NoError(t, err)
 	defer shutdown()
 

proxyd/metrics.go

@@ -5,6 +5,8 @@ import (
 	"strconv"
 	"strings"
 
+	"github.com/ethereum/go-ethereum/common/hexutil"
+
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/client_golang/prometheus/promauto"
 )
@@ -242,6 +244,22 @@ var (
 		Name:      "rate_limit_take_errors",
 		Help:      "Count of errors taking frontend rate limits",
 	})
+
+	consensusLatestBlock = promauto.NewGaugeVec(prometheus.GaugeOpts{
+		Namespace: MetricsNamespace,
+		Name:      "group_consensus_latest_block",
+		Help:      "Consensus latest block",
+	}, []string{
+		"backend_group_name",
+	})
+
+	backendLatestBlockBackend = promauto.NewGaugeVec(prometheus.GaugeOpts{
+		Namespace: MetricsNamespace,
+		Name:      "backend_latest_block",
+		Help:      "Current latest block observed per backend",
+	}, []string{
+		"backend_name",
+	})
 )
 
 func RecordRedisError(source string) {
@@ -302,3 +320,11 @@ func RecordCacheMiss(method string) {
 func RecordBatchSize(size int) {
 	batchSizeHistogram.Observe(float64(size))
 }
+
+func RecordBackendLatestBlock(be *Backend, blockNumber hexutil.Uint64) {
+	backendLatestBlockBackend.WithLabelValues(be.Name).Set(float64(blockNumber))
+}
+
+func RecordGroupConsensusLatestBlock(group *BackendGroup, blockNumber hexutil.Uint64) {
+	consensusLatestBlock.WithLabelValues(group.Name).Set(float64(blockNumber))
+}

proxyd/proxyd.go

@@ -18,20 +18,20 @@ import (
 	"golang.org/x/sync/semaphore"
 )
 
-func Start(config *Config) (func(), error) {
+func Start(config *Config) (*Server, func(), error) {
 	if len(config.Backends) == 0 {
-		return nil, errors.New("must define at least one backend")
+		return nil, nil, errors.New("must define at least one backend")
 	}
 	if len(config.BackendGroups) == 0 {
-		return nil, errors.New("must define at least one backend group")
+		return nil, nil, errors.New("must define at least one backend group")
 	}
 	if len(config.RPCMethodMappings) == 0 {
-		return nil, errors.New("must define at least one RPC method mapping")
+		return nil, nil, errors.New("must define at least one RPC method mapping")
 	}
 
 	for authKey := range config.Authentication {
 		if authKey == "none" {
-			return nil, errors.New("cannot use none as an auth key")
+			return nil, nil, errors.New("cannot use none as an auth key")
 		}
 	}
 
@@ -39,16 +39,16 @@ func Start(config *Config) (func(), error) {
 	if config.Redis.URL != "" {
 		rURL, err := ReadFromEnvOrConfig(config.Redis.URL)
 		if err != nil {
-			return nil, err
+			return nil, nil, err
 		}
 		redisClient, err = NewRedisClient(rURL)
 		if err != nil {
-			return nil, err
+			return nil, nil, err
 		}
 	}
 
 	if redisClient == nil && config.RateLimit.UseRedis {
-		return nil, errors.New("must specify a Redis URL if UseRedis is true in rate limit config")
+		return nil, nil, errors.New("must specify a Redis URL if UseRedis is true in rate limit config")
 	}
 
 	var lim BackendRateLimiter
@@ -80,10 +80,10 @@ func Start(config *Config) (func(), error) {
 
 	if config.SenderRateLimit.Enabled {
 		if config.SenderRateLimit.Limit <= 0 {
-			return nil, errors.New("limit in sender_rate_limit must be > 0")
+			return nil, nil, errors.New("limit in sender_rate_limit must be > 0")
 		}
 		if time.Duration(config.SenderRateLimit.Interval) < time.Second {
-			return nil, errors.New("interval in sender_rate_limit must be >= 1s")
+			return nil, nil, errors.New("interval in sender_rate_limit must be >= 1s")
 		}
 	}
 
@@ -100,17 +100,14 @@ func Start(config *Config) (func(), error) {
 
 		rpcURL, err := ReadFromEnvOrConfig(cfg.RPCURL)
 		if err != nil {
-			return nil, err
+			return nil, nil, err
 		}
 		wsURL, err := ReadFromEnvOrConfig(cfg.WSURL)
 		if err != nil {
-			return nil, err
+			return nil, nil, err
 		}
 		if rpcURL == "" {
-			return nil, fmt.Errorf("must define an RPC URL for backend %s", name)
-		}
-		if wsURL == "" {
-			return nil, fmt.Errorf("must define a WS URL for backend %s", name)
+			return nil, nil, fmt.Errorf("must define an RPC URL for backend %s", name)
 		}
 
 		if config.BackendOptions.ResponseTimeoutSeconds != 0 {
@@ -135,13 +132,13 @@ func Start(config *Config) (func(), error) {
 		if cfg.Password != "" {
 			passwordVal, err := ReadFromEnvOrConfig(cfg.Password)
 			if err != nil {
-				return nil, err
+				return nil, nil, err
 			}
 			opts = append(opts, WithBasicAuth(cfg.Username, passwordVal))
 		}
 		tlsConfig, err := configureBackendTLS(cfg)
 		if err != nil {
-			return nil, err
+			return nil, nil, err
 		}
 		if tlsConfig != nil {
 			log.Info("using custom TLS config for backend", "name", name)
@@ -162,7 +159,7 @@ func Start(config *Config) (func(), error) {
 		backends := make([]*Backend, 0)
 		for _, bName := range bg.Backends {
 			if backendsByName[bName] == nil {
-				return nil, fmt.Errorf("backend %s is not defined", bName)
+				return nil, nil, fmt.Errorf("backend %s is not defined", bName)
 			}
 			backends = append(backends, backendsByName[bName])
 		}
@@ -177,17 +174,17 @@ func Start(config *Config) (func(), error) {
 	if config.WSBackendGroup != "" {
 		wsBackendGroup = backendGroups[config.WSBackendGroup]
 		if wsBackendGroup == nil {
-			return nil, fmt.Errorf("ws backend group %s does not exist", config.WSBackendGroup)
+			return nil, nil, fmt.Errorf("ws backend group %s does not exist", config.WSBackendGroup)
 		}
 	}
 
 	if wsBackendGroup == nil && config.Server.WSPort != 0 {
-		return nil, fmt.Errorf("a ws port was defined, but no ws group was defined")
+		return nil, nil, fmt.Errorf("a ws port was defined, but no ws group was defined")
 	}
 
 	for _, bg := range config.RPCMethodMappings {
 		if backendGroups[bg] == nil {
-			return nil, fmt.Errorf("undefined backend group %s", bg)
+			return nil, nil, fmt.Errorf("undefined backend group %s", bg)
 		}
 	}
 
@@ -198,7 +195,7 @@ func Start(config *Config) (func(), error) {
 		for secret, alias := range config.Authentication {
 			resolvedSecret, err := ReadFromEnvOrConfig(secret)
 			if err != nil {
-				return nil, err
+				return nil, nil, err
 			}
 			resolvedAuth[resolvedSecret] = alias
 		}
@@ -217,11 +214,11 @@ func Start(config *Config) (func(), error) {
 		)
 
 		if config.Cache.BlockSyncRPCURL == "" {
-			return nil, fmt.Errorf("block sync node required for caching")
+			return nil, nil, fmt.Errorf("block sync node required for caching")
 		}
 		blockSyncRPCURL, err := ReadFromEnvOrConfig(config.Cache.BlockSyncRPCURL)
 		if err != nil {
-			return nil, err
+			return nil, nil, err
 		}
 
 		if redisClient == nil {
@@ -233,7 +230,7 @@ func Start(config *Config) (func(), error) {
 		// Ideally, the BlocKSyncRPCURL should be the sequencer or a HA replica that's not far behind
 		ethClient, err := ethclient.Dial(blockSyncRPCURL)
 		if err != nil {
-			return nil, err
+			return nil, nil, err
 		}
 		defer ethClient.Close()
 
@@ -260,7 +257,7 @@ func Start(config *Config) (func(), error) {
 		redisClient,
 	)
 	if err != nil {
-		return nil, fmt.Errorf("error creating server: %w", err)
+		return nil, nil, fmt.Errorf("error creating server: %w", err)
 	}
 
 	if config.Metrics.Enabled {
@@ -300,12 +297,28 @@ func Start(config *Config) (func(), error) {
 				log.Crit("error starting WS server", "err", err)
 			}
 		}()
+	} else {
+		log.Info("WS server not enabled (ws_port is set to 0)")
+	}
+
+	for bgName, bg := range backendGroups {
+		if config.BackendGroups[bgName].ConsensusAware {
+			log.Info("creating poller for consensus aware backend_group", "name", bgName)
+
+			copts := make([]ConsensusOpt, 0)
+
+			if config.BackendGroups[bgName].ConsensusAsyncHandler == "noop" {
+				copts = append(copts, WithAsyncHandler(NewNoopAsyncHandler()))
+			}
+			cp := NewConsensusPoller(bg, copts...)
+			bg.Consensus = cp
+		}
 	}
 
 	<-errTimer.C
 	log.Info("started proxyd")
 
-	return func() {
+	shutdownFunc := func() {
 		log.Info("shutting down proxyd")
 		if blockNumLVC != nil {
 			blockNumLVC.Stop()
@@ -318,7 +331,9 @@ func Start(config *Config) (func(), error) {
 			log.Error("error flushing backend ws conns", "err", err)
 		}
 		log.Info("goodbye")
-	}, nil
+	}
+
+	return srv, shutdownFunc, nil
 }
 
 func secondsToDuration(seconds int) time.Duration {

proxyd/server.go

@@ -39,7 +39,7 @@ const (
 var emptyArrayResponse = json.RawMessage("[]")
 
 type Server struct {
-	backendGroups          map[string]*BackendGroup
+	BackendGroups          map[string]*BackendGroup
 	wsBackendGroup         *BackendGroup
 	wsMethodWhitelist      *StringSet
 	rpcMethodMappings      map[string]string
@@ -152,7 +152,7 @@ func NewServer(
 	}
 
 	return &Server{
-		backendGroups:        backendGroups,
+		BackendGroups:        backendGroups,
 		wsBackendGroup:       wsBackendGroup,
 		wsMethodWhitelist:    wsMethodWhitelist,
 		rpcMethodMappings:    rpcMethodMappings,
@@ -476,7 +476,7 @@ func (s *Server) handleBatchRPC(ctx context.Context, reqs []json.RawMessage, isL
 			start := i * s.maxUpstreamBatchSize
 			end := int(math.Min(float64(start+s.maxUpstreamBatchSize), float64(len(cacheMisses))))
 			elems := cacheMisses[start:end]
-			res, err := s.backendGroups[group.backendGroup].Forward(ctx, createBatchRequest(elems), isBatch)
+			res, err := s.BackendGroups[group.backendGroup].Forward(ctx, createBatchRequest(elems), isBatch)
 			if err != nil {
 				log.Error(
 					"error forwarding RPC batch",
@@ -559,7 +559,7 @@ func (s *Server) populateContext(w http.ResponseWriter, r *http.Request) context
 	}
 	ctx := context.WithValue(r.Context(), ContextKeyXForwardedFor, xff) // nolint:staticcheck
 
-	if s.authenticatedPaths == nil {
+	if len(s.authenticatedPaths) == 0 {
 		// handle the edge case where auth is disabled
 		// but someone sends in an auth key anyway
 		if authorization != "" {

proxyd/tools/mockserver/handler/handler.go

+package handler
+
+import (
+	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
+	"os"
+
+	"github.com/gorilla/mux"
+	"github.com/pkg/errors"
+	"gopkg.in/yaml.v3"
+)
+
+type MethodTemplate struct {
+	Method   string `yaml:"method"`
+	Block    string `yaml:"block"`
+	Response string `yaml:"response"`
+}
+
+type MockedHandler struct {
+	Overrides    []*MethodTemplate
+	Autoload     bool
+	AutoloadFile string
+}
+
+func (mh *MockedHandler) Serve(port int) error {
+	r := mux.NewRouter()
+	r.HandleFunc("/", mh.Handler)
+	http.Handle("/", r)
+	fmt.Printf("starting server up on :%d serving MockedResponsesFile %s\n", port, mh.AutoloadFile)
+	err := http.ListenAndServe(fmt.Sprintf(":%d", port), nil)
+
+	if errors.Is(err, http.ErrServerClosed) {
+		fmt.Printf("server closed\n")
+	} else if err != nil {
+		fmt.Printf("error starting server: %s\n", err)
+		return err
+	}
+	return nil
+}
+
+func (mh *MockedHandler) Handler(w http.ResponseWriter, req *http.Request) {
+	body, err := io.ReadAll(req.Body)
+	if err != nil {
+		fmt.Printf("error reading request: %v\n", err)
+	}
+
+	var j map[string]interface{}
+	err = json.Unmarshal(body, &j)
+	if err != nil {
+		fmt.Printf("error reading request: %v\n", err)
+	}
+
+	var template []*MethodTemplate
+	if mh.Autoload {
+		template = append(template, mh.LoadFromFile(mh.AutoloadFile)...)
+	}
+	if mh.Overrides != nil {
+		template = append(template, mh.Overrides...)
+	}
+
+	method := j["method"]
+	block := ""
+	if method == "eth_getBlockByNumber" {
+		block = (j["params"].([]interface{})[0]).(string)
+	}
+
+	var selectedResponse *string
+	for _, r := range template {
+		if r.Method == method && r.Block == block {
+			selectedResponse = &r.Response
+		}
+	}
+	if selectedResponse != nil {
+		_, err := fmt.Fprintf(w, *selectedResponse)
+		if err != nil {
+			fmt.Printf("error writing response: %v\n", err)
+		}
+	}
+}
+
+func (mh *MockedHandler) LoadFromFile(file string) []*MethodTemplate {
+	contents, err := os.ReadFile(file)
+	if err != nil {
+		fmt.Printf("error reading MockedResponsesFile: %v\n", err)
+	}
+	var template []*MethodTemplate
+	err = yaml.Unmarshal(contents, &template)
+	if err != nil {
+		fmt.Printf("error reading MockedResponsesFile: %v\n", err)
+	}
+	return template
+}
+
+func (mh *MockedHandler) AddOverride(template *MethodTemplate) {
+	mh.Overrides = append(mh.Overrides, template)
+}
+
+func (mh *MockedHandler) ResetOverrides() {
+	mh.Overrides = make([]*MethodTemplate, 0)
+}

proxyd/tools/mockserver/main.go

+package main
+
+import (
+	"fmt"
+	"os"
+	"path"
+	"strconv"
+
+	"github.com/ethereum-optimism/optimism/proxyd/tools/mockserver/handler"
+)
+
+func main() {
+	if len(os.Args) < 3 {
+		fmt.Printf("simply mock a response based on an external text MockedResponsesFile\n")
+		fmt.Printf("usage: mockserver <port> <MockedResponsesFile.yml>\n")
+		os.Exit(1)
+	}
+	port, _ := strconv.ParseInt(os.Args[1], 10, 32)
+	dir, _ := os.Getwd()
+
+	h := handler.MockedHandler{
+		Autoload:     true,
+		AutoloadFile: path.Join(dir, os.Args[2]),
+	}
+
+	err := h.Serve(int(port))
+	if err != nil {
+		fmt.Printf("error starting mockserver: %v\n", err)
+	}
+}

proxyd/tools/mockserver/node1.yml

+- method: eth_getBlockByNumber
+  block: latest
+  response: >
+    {
+      "jsonrpc": "2.0",
+      "id": 67,
+      "result": {
+        "hash": "hash2",
+        "number": "0x2"
+      }
+    }
+- method: eth_getBlockByNumber
+  block: 0x1
+  response: >
+    {
+      "jsonrpc": "2.0",
+      "id": 67,
+      "result": {
+        "hash": "hash1",
+        "number": "0x1"
+      }
+    }
+- method: eth_getBlockByNumber
+  block: 0x2
+  response: >
+    {
+      "jsonrpc": "2.0",
+      "id": 67,
+      "result": {
+        "hash": "hash2",
+        "number": "0x2"
+      }
+    }
+- method: eth_getBlockByNumber
+  block: 0x3
+  response: >
+    {
+      "jsonrpc": "2.0",
+      "id": 67,
+      "result": {
+        "hash": "hash34",
+        "number": "0x3"
+      }
+    }
\ No newline at end of file

proxyd/tools/mockserver/node2.yml

+- method: eth_getBlockByNumber
+  block: latest
+  response: >
+    {
+      "jsonrpc": "2.0",
+      "id": 67,
+      "result": {
+        "hash": "hash2",
+        "number": "0x2"
+      }
+    }
+- method: eth_getBlockByNumber
+  block: 0x1
+  response: >
+    {
+      "jsonrpc": "2.0",
+      "id": 67,
+      "result": {
+        "hash": "hash1",
+        "number": "0x1"
+      }
+    }
+- method: eth_getBlockByNumber
+  block: 0x2
+  response: >
+    {
+      "jsonrpc": "2.0",
+      "id": 67,
+      "result": {
+        "hash": "hash2",
+        "number": "0x2"
+      }
+    }
+- method: eth_getBlockByNumber
+  block: 0x3
+  response: >
+    {
+      "jsonrpc": "2.0",
+      "id": 67,
+      "result": {
+        "hash": "hash3",
+        "number": "0x3"
+      }
+    }
\ No newline at end of file