diff --git a/src/shardctrler/client.go b/src/shardctrler/client.go
new file mode 100644
index 0000000..1a90443
--- /dev/null
+++ b/src/shardctrler/client.go
@@ -0,0 +1,101 @@
+package shardctrler
+
+//
+// Shardctrler clerk.
+//
+
+import "6.824/labrpc"
+import "time"
+import "crypto/rand"
+import "math/big"
+
+type Clerk struct {
+	servers []*labrpc.ClientEnd
+	// Your data here.
+}
+
+func nrand() int64 {
+	max := big.NewInt(int64(1) << 62)
+	bigx, _ := rand.Int(rand.Reader, max)
+	x := bigx.Int64()
+	return x
+}
+
+func MakeClerk(servers []*labrpc.ClientEnd) *Clerk {
+	ck := new(Clerk)
+	ck.servers = servers
+	// Your code here.
+	return ck
+}
+
+func (ck *Clerk) Query(num int) Config {
+	args := &QueryArgs{}
+	// Your code here.
+	args.Num = num
+	for {
+		// try each known server.
+		for _, srv := range ck.servers {
+			var reply QueryReply
+			ok := srv.Call("ShardCtrler.Query", args, &reply)
+			if ok && reply.WrongLeader == false {
+				return reply.Config
+			}
+		}
+		time.Sleep(100 * time.Millisecond)
+	}
+}
+
+func (ck *Clerk) Join(servers map[int][]string) {
+	args := &JoinArgs{}
+	// Your code here.
+	args.Servers = servers
+
+	for {
+		// try each known server.
+		for _, srv := range ck.servers {
+			var reply JoinReply
+			ok := srv.Call("ShardCtrler.Join", args, &reply)
+			if ok && reply.WrongLeader == false {
+				return
+			}
+		}
+		time.Sleep(100 * time.Millisecond)
+	}
+}
+
+func (ck *Clerk) Leave(gids []int) {
+	args := &LeaveArgs{}
+	// Your code here.
+	args.GIDs = gids
+
+	for {
+		// try each known server.
+		for _, srv := range ck.servers {
+			var reply LeaveReply
+			ok := srv.Call("ShardCtrler.Leave", args, &reply)
+			if ok && reply.WrongLeader == false {
+				return
+			}
+		}
+		time.Sleep(100 * time.Millisecond)
+	}
+}
+
+func (ck *Clerk) Move(shard int, gid int) {
+	args := &MoveArgs{}
+	// Your code here.
+	args.Shard = shard
+	args.GID = gid
+
+	for {
+		// try each known server.
+		for _, srv := range ck.servers {
+			var reply MoveReply
+			ok := srv.Call("ShardCtrler.Move", args, &reply)
+			if ok && reply.WrongLeader == false {
+				return
+			}
+		}
+		time.Sleep(100 * time.Millisecond)
+	}
+}
diff --git a/src/shardctrler/common.go b/src/shardctrler/common.go
new file mode 100644
index 0000000..031806a
--- /dev/null
+++ b/src/shardctrler/common.go
@@ -0,0 +1,73 @@
+package shardctrler
+
+//
+// Shard controler: assigns shards to replication groups.
+//
+// RPC interface:
+// Join(servers) -- add a set of groups (gid -> server-list mapping).
+// Leave(gids) -- delete a set of groups.
+// Move(shard, gid) -- hand off one shard from current owner to gid.
+// Query(num) -> fetch Config # num, or latest config if num==-1.
+//
+// A Config (configuration) describes a set of replica groups, and the
+// replica group responsible for each shard. Configs are numbered. Config
+// #0 is the initial configuration, with no groups and all shards
+// assigned to group 0 (the invalid group).
+//
+// You will need to add fields to the RPC argument structs.
+//
+
+// The number of shards.
+const NShards = 10
+
+// A configuration -- an assignment of shards to groups.
+// Please don't change this.
+type Config struct {
+	Num    int              // config number
+	Shards [NShards]int     // shard -> gid
+	Groups map[int][]string // gid -> servers[]
+}
+
+const (
+	OK = "OK"
+)
+
+type Err string
+
+type JoinArgs struct {
+	Servers map[int][]string // new GID -> servers mappings
+}
+
+type JoinReply struct {
+	WrongLeader bool
+	Err         Err
+}
+
+type LeaveArgs struct {
+	GIDs []int
+}
+
+type LeaveReply struct {
+	WrongLeader bool
+	Err         Err
+}
+
+type MoveArgs struct {
+	Shard int
+	GID   int
+}
+
+type MoveReply struct {
+	WrongLeader bool
+	Err         Err
+}
+
+type QueryArgs struct {
+	Num int // desired config number
+}
+
+type QueryReply struct {
+	WrongLeader bool
+	Err         Err
+	Config      Config
+}
diff --git a/src/shardctrler/config.go b/src/shardctrler/config.go
new file mode 100644
index 0000000..ee984ec
--- /dev/null
+++ b/src/shardctrler/config.go
@@ -0,0 +1,355 @@
+package shardctrler
+
+import "6.824/labrpc"
+import "6.824/raft"
+import "testing"
+import "os"
+
+// import "log"
+import crand "crypto/rand"
+import "math/rand"
+import "encoding/base64"
+import "sync"
+import "runtime"
+import "time"
+
+func randstring(n int) string {
+	b := make([]byte, 2*n)
+	crand.Read(b)
+	s := base64.URLEncoding.EncodeToString(b)
+	return s[0:n]
+}
+
+// Randomize server handles
+func random_handles(kvh []*labrpc.ClientEnd) []*labrpc.ClientEnd {
+	sa := make([]*labrpc.ClientEnd, len(kvh))
+	copy(sa, kvh)
+	for i := range sa {
+		j := rand.Intn(i + 1)
+		sa[i], sa[j] = sa[j], sa[i]
+	}
+	return sa
+}
+
+type config struct {
+	mu           sync.Mutex
+	t            *testing.T
+	net          *labrpc.Network
+	n            int
+	servers      []*ShardCtrler
+	saved        []*raft.Persister
+	endnames     [][]string // names of each server's sending ClientEnds
+	clerks       map[*Clerk][]string
+	nextClientId int
+	start        time.Time // time at which make_config() was called
+}
+
+func (cfg *config) checkTimeout() {
+	// enforce a two minute real-time limit on each test
+	if !cfg.t.Failed() && time.Since(cfg.start) > 120*time.Second {
+		cfg.t.Fatal("test took longer than 120 seconds")
+	}
+}
+
+func (cfg *config) cleanup() {
+	cfg.mu.Lock()
+	defer cfg.mu.Unlock()
+	for i := 0; i < len(cfg.servers); i++ {
+		if cfg.servers[i] != nil {
+			cfg.servers[i].Kill()
+		}
+	}
+	cfg.net.Cleanup()
+	cfg.checkTimeout()
+}
+
+// Maximum log size across all servers
+func (cfg *config) LogSize() int {
+	logsize := 0
+	for i := 0; i < cfg.n; i++ {
+		n := cfg.saved[i].RaftStateSize()
+		if n > logsize {
+			logsize = n
+		}
+	}
+	return logsize
+}
+
+// attach server i to servers listed in to
+// caller must hold cfg.mu
+func (cfg *config) connectUnlocked(i int, to []int) {
+	// log.Printf("connect peer %d to %v\n", i, to)
+
+	// outgoing socket files
+	for j := 0; j < len(to); j++ {
+		endname := cfg.endnames[i][to[j]]
+		cfg.net.Enable(endname, true)
+	}
+
+	// incoming socket files
+	for j := 0; j < len(to); j++ {
+		endname := cfg.endnames[to[j]][i]
+		cfg.net.Enable(endname, true)
+	}
+}
+
+func (cfg *config) connect(i int, to []int) {
+	cfg.mu.Lock()
+	defer cfg.mu.Unlock()
+	cfg.connectUnlocked(i, to)
+}
+
+// detach server i from the servers listed in from
+// caller must hold cfg.mu
+func (cfg *config) disconnectUnlocked(i int, from []int) {
+	// log.Printf("disconnect peer %d from %v\n", i, from)
+
+	// outgoing socket files
+	for j := 0; j < len(from); j++ {
+		if cfg.endnames[i] != nil {
+			endname := cfg.endnames[i][from[j]]
+			cfg.net.Enable(endname, false)
+		}
+	}
+
+	// incoming socket files
+	for j := 0; j < len(from); j++ {
+		if cfg.endnames[j] != nil {
+			endname := cfg.endnames[from[j]][i]
+			cfg.net.Enable(endname, false)
+		}
+	}
+}
+
+func (cfg *config) disconnect(i int, from []int) {
+	cfg.mu.Lock()
+	defer cfg.mu.Unlock()
+	cfg.disconnectUnlocked(i, from)
+}
+
+func (cfg *config) All() []int {
+	all := make([]int, cfg.n)
+	for i := 0; i < cfg.n; i++ {
+		all[i] = i
+	}
+	return all
+}
+
+func (cfg *config) ConnectAll() {
+	cfg.mu.Lock()
+	defer cfg.mu.Unlock()
+	for i := 0; i < cfg.n; i++ {
+		cfg.connectUnlocked(i, cfg.All())
+	}
+}
+
+// Sets up 2 partitions with connectivity between servers in each  partition.
+func (cfg *config) partition(p1 []int, p2 []int) {
+	cfg.mu.Lock()
+	defer cfg.mu.Unlock()
+	// log.Printf("partition servers into: %v %v\n", p1, p2)
+	for i := 0; i < len(p1); i++ {
+		cfg.disconnectUnlocked(p1[i], p2)
+		cfg.connectUnlocked(p1[i], p1)
+	}
+	for i := 0; i < len(p2); i++ {
+		cfg.disconnectUnlocked(p2[i], p1)
+		cfg.connectUnlocked(p2[i], p2)
+	}
+}
+
+// Create a clerk with clerk specific server names.
+// Give it connections to all of the servers, but for
+// now enable only connections to servers in to[].
+func (cfg *config) makeClient(to []int) *Clerk {
+	cfg.mu.Lock()
+	defer cfg.mu.Unlock()
+
+	// a fresh set of ClientEnds.
+	ends := make([]*labrpc.ClientEnd, cfg.n)
+	endnames := make([]string, cfg.n)
+	for j := 0; j < cfg.n; j++ {
+		endnames[j] = randstring(20)
+		ends[j] = cfg.net.MakeEnd(endnames[j])
+		cfg.net.Connect(endnames[j], j)
+	}
+
+	ck := MakeClerk(random_handles(ends))
+	cfg.clerks[ck] = endnames
+	cfg.nextClientId++
+	cfg.ConnectClientUnlocked(ck, to)
+	return ck
+}
+
+func (cfg *config) deleteClient(ck *Clerk) {
+	cfg.mu.Lock()
+	defer cfg.mu.Unlock()
+
+	v := cfg.clerks[ck]
+	for i := 0; i < len(v); i++ {
+		os.Remove(v[i])
+	}
+	delete(cfg.clerks, ck)
+}
+
+// caller should hold cfg.mu
+func (cfg *config) ConnectClientUnlocked(ck *Clerk, to []int) {
+	// log.Printf("ConnectClient %v to %v\n", ck, to)
+	endnames := cfg.clerks[ck]
+	for j := 0; j < len(to); j++ {
+		s := endnames[to[j]]
+		cfg.net.Enable(s, true)
+	}
+}
+
+func (cfg *config) ConnectClient(ck *Clerk, to []int) {
+	cfg.mu.Lock()
+	defer cfg.mu.Unlock()
+	cfg.ConnectClientUnlocked(ck, to)
+}
+
+// caller should hold cfg.mu
+func (cfg *config) DisconnectClientUnlocked(ck *Clerk, from []int) {
+	// log.Printf("DisconnectClient %v from %v\n", ck, from)
+	endnames := cfg.clerks[ck]
+	for j := 0; j < len(from); j++ {
+		s := endnames[from[j]]
+		cfg.net.Enable(s, false)
+	}
+}
+
+func (cfg *config) DisconnectClient(ck *Clerk, from []int) {
+	cfg.mu.Lock()
+	defer cfg.mu.Unlock()
+	cfg.DisconnectClientUnlocked(ck, from)
+}
+
+// Shutdown a server by isolating it
+func (cfg *config) ShutdownServer(i int) {
+	cfg.mu.Lock()
+	defer cfg.mu.Unlock()
+
+	cfg.disconnectUnlocked(i, cfg.All())
+
+	// disable client connections to the server.
+	// it's important to do this before creating
+	// the new Persister in saved[i], to avoid
+	// the possibility of the server returning a
+	// positive reply to an Append but persisting
+	// the result in the superseded Persister.
+	cfg.net.DeleteServer(i)
+
+	// a fresh persister, in case old instance
+	// continues to update the Persister.
+	// but copy old persister's content so that we always
+	// pass Make() the last persisted state.
+	if cfg.saved[i] != nil {
+		cfg.saved[i] = cfg.saved[i].Copy()
+	}
+
+	kv := cfg.servers[i]
+	if kv != nil {
+		cfg.mu.Unlock()
+		kv.Kill()
+		cfg.mu.Lock()
+		cfg.servers[i] = nil
+	}
+}
+
+// If restart servers, first call ShutdownServer
+func (cfg *config) StartServer(i int) {
+	cfg.mu.Lock()
+
+	// a fresh set of outgoing ClientEnd names.
+	cfg.endnames[i] = make([]string, cfg.n)
+	for j := 0; j < cfg.n; j++ {
+		cfg.endnames[i][j] = randstring(20)
+	}
+
+	// a fresh set of ClientEnds.
+	ends := make([]*labrpc.ClientEnd, cfg.n)
+	for j := 0; j < cfg.n; j++ {
+		ends[j] = cfg.net.MakeEnd(cfg.endnames[i][j])
+		cfg.net.Connect(cfg.endnames[i][j], j)
+	}
+
+	// a fresh persister, so old instance doesn't overwrite
+	// new instance's persisted state.
+	// give the fresh persister a copy of the old persister's
+	// state, so that the spec is that we pass StartKVServer()
+	// the last persisted state.
+	if cfg.saved[i] != nil {
+		cfg.saved[i] = cfg.saved[i].Copy()
+	} else {
+		cfg.saved[i] = raft.MakePersister()
+	}
+
+	cfg.mu.Unlock()
+
+	cfg.servers[i] = StartServer(ends, i, cfg.saved[i])
+
+	kvsvc := labrpc.MakeService(cfg.servers[i])
+	rfsvc := labrpc.MakeService(cfg.servers[i].rf)
+	srv := labrpc.MakeServer()
+	srv.AddService(kvsvc)
+	srv.AddService(rfsvc)
+	cfg.net.AddServer(i, srv)
+}
+
+func (cfg *config) Leader() (bool, int) {
+	cfg.mu.Lock()
+	defer cfg.mu.Unlock()
+
+	for i := 0; i < cfg.n; i++ {
+		_, is_leader := cfg.servers[i].rf.GetState()
+		if is_leader {
+			return true, i
+		}
+	}
+	return false, 0
+}
+
+// Partition servers into 2 groups and put current leader in minority
+func (cfg *config) make_partition() ([]int, []int) {
+	_, l := cfg.Leader()
+	p1 := make([]int, cfg.n/2+1)
+	p2 := make([]int, cfg.n/2)
+	j := 0
+	for i := 0; i < cfg.n; i++ {
+		if i != l {
+			if j < len(p1) {
+				p1[j] = i
+			} else {
+				p2[j-len(p1)] = i
+			}
+			j++
+		}
+	}
+	p2[len(p2)-1] = l
+	return p1, p2
+}
+
+func make_config(t *testing.T, n int, unreliable bool) *config {
+	runtime.GOMAXPROCS(4)
+	cfg := &config{}
+	cfg.t = t
+	cfg.net = labrpc.MakeNetwork()
+	cfg.n = n
+	cfg.servers = make([]*ShardCtrler, cfg.n)
+	cfg.saved = make([]*raft.Persister, cfg.n)
+	cfg.endnames = make([][]string, cfg.n)
+	cfg.clerks = make(map[*Clerk][]string)
+	cfg.nextClientId = cfg.n + 1000 // client ids start 1000 above the highest serverid
+	cfg.start = time.Now()
+
+	// create a full set of KV servers.
+	for i := 0; i < cfg.n; i++ {
+		cfg.StartServer(i)
+	}
+
+	cfg.ConnectAll()
+
+	cfg.net.Reliable(!unreliable)
+
+	return cfg
+}
diff --git a/src/shardctrler/server.go b/src/shardctrler/server.go
new file mode 100644
index 0000000..87a8e98
--- /dev/null
+++ b/src/shardctrler/server.go
@@ -0,0 +1,80 @@
+package shardctrler
+
+
+import "6.824/raft"
+import "6.824/labrpc"
+import "sync"
+import "6.824/labgob"
+
+
+type ShardCtrler struct {
+	mu      sync.Mutex
+	me      int
+	rf      *raft.Raft
+	applyCh chan raft.ApplyMsg
+
+	// Your data here.
+
+	configs []Config // indexed by config num
+}
+
+
+type Op struct {
+	// Your data here.
+}
+
+
+func (sc *ShardCtrler) Join(args *JoinArgs, reply *JoinReply) {
+	// Your code here.
+}
+
+func (sc *ShardCtrler) Leave(args *LeaveArgs, reply *LeaveReply) {
+	// Your code here.
+}
+
+func (sc *ShardCtrler) Move(args *MoveArgs, reply *MoveReply) {
+	// Your code here.
+}
+
+func (sc *ShardCtrler) Query(args *QueryArgs, reply *QueryReply) {
+	// Your code here.
+}
+
+
+//
+// the tester calls Kill() when a ShardCtrler instance won't
+// be needed again. you are not required to do anything
+// in Kill(), but it might be convenient to (for example)
+// turn off debug output from this instance.
+//
+func (sc *ShardCtrler) Kill() {
+	sc.rf.Kill()
+	// Your code here, if desired.
+}
+
+// needed by shardkv tester
+func (sc *ShardCtrler) Raft() *raft.Raft {
+	return sc.rf
+}
+
+//
+// servers[] contains the ports of the set of
+// servers that will cooperate via Paxos to
+// form the fault-tolerant shardctrler service.
+// me is the index of the current server in servers[].
+//
+func StartServer(servers []*labrpc.ClientEnd, me int, persister *raft.Persister) *ShardCtrler {
+	sc := new(ShardCtrler)
+	sc.me = me
+
+	sc.configs = make([]Config, 1)
+	sc.configs[0].Groups = map[int][]string{}
+
+	labgob.Register(Op{})
+	sc.applyCh = make(chan raft.ApplyMsg)
+	sc.rf = raft.Make(servers, me, persister, sc.applyCh)
+
+	// Your code here.
+
+	return sc
+}
diff --git a/src/shardctrler/test_test.go b/src/shardctrler/test_test.go
new file mode 100644
index 0000000..5c95d56
--- /dev/null
+++ b/src/shardctrler/test_test.go
@@ -0,0 +1,380 @@
+package shardctrler
+
+import (
+	"sync"
+	"testing"
+)
+
+// import "time"
+import "fmt"
+
+func check(t *testing.T, groups []int, ck *Clerk) {
+	c := ck.Query(-1)
+	if len(c.Groups) != len(groups) {
+		t.Fatalf("wanted %v groups, got %v", len(groups), len(c.Groups))
+	}
+
+	// are the groups as expected?
+	for _, g := range groups {
+		_, ok := c.Groups[g]
+		if ok != true {
+			t.Fatalf("missing group %v", g)
+		}
+	}
+
+	// any un-allocated shards?
+	if len(groups) > 0 {
+		for s, g := range c.Shards {
+			_, ok := c.Groups[g]
+			if ok == false {
+				t.Fatalf("shard %v -> invalid group %v", s, g)
+			}
+		}
+	}
+
+	// more or less balanced sharding?
+	counts := map[int]int{}
+	for _, g := range c.Shards {
+		counts[g] += 1
+	}
+	min := 257
+	max := 0
+	for g, _ := range c.Groups {
+		if counts[g] > max {
+			max = counts[g]
+		}
+		if counts[g] < min {
+			min = counts[g]
+		}
+	}
+	if max > min+1 {
+		t.Fatalf("max %v too much larger than min %v", max, min)
+	}
+}
+
+func check_same_config(t *testing.T, c1 Config, c2 Config) {
+	if c1.Num != c2.Num {
+		t.Fatalf("Num wrong")
+	}
+	if c1.Shards != c2.Shards {
+		t.Fatalf("Shards wrong")
+	}
+	if len(c1.Groups) != len(c2.Groups) {
+		t.Fatalf("number of Groups is wrong")
+	}
+	for gid, sa := range c1.Groups {
+		sa1, ok := c2.Groups[gid]
+		if ok == false || len(sa1) != len(sa) {
+			t.Fatalf("len(Groups) wrong")
+		}
+		if ok && len(sa1) == len(sa) {
+			for j := 0; j < len(sa); j++ {
+				if sa[j] != sa1[j] {
+					t.Fatalf("Groups wrong")
+				}
+			}
+		}
+	}
+}
+
+func TestBasic(t *testing.T) {
+	const nservers = 3
+	cfg := make_config(t, nservers, false)
+	defer cfg.cleanup()
+
+	ck := cfg.makeClient(cfg.All())
+
+	fmt.Printf("Test: Basic leave/join ...\n")
+
+	cfa := make([]Config, 6)
+	cfa[0] = ck.Query(-1)
+
+	check(t, []int{}, ck)
+
+	var gid1 int = 1
+	ck.Join(map[int][]string{gid1: []string{"x", "y", "z"}})
+	check(t, []int{gid1}, ck)
+	cfa[1] = ck.Query(-1)
+
+	var gid2 int = 2
+	ck.Join(map[int][]string{gid2: []string{"a", "b", "c"}})
+	check(t, []int{gid1, gid2}, ck)
+	cfa[2] = ck.Query(-1)
+
+	cfx := ck.Query(-1)
+	sa1 := cfx.Groups[gid1]
+	if len(sa1) != 3 || sa1[0] != "x" || sa1[1] != "y" || sa1[2] != "z" {
+		t.Fatalf("wrong servers for gid %v: %v\n", gid1, sa1)
+	}
+	sa2 := cfx.Groups[gid2]
+	if len(sa2) != 3 || sa2[0] != "a" || sa2[1] != "b" || sa2[2] != "c" {
+		t.Fatalf("wrong servers for gid %v: %v\n", gid2, sa2)
+	}
+
+	ck.Leave([]int{gid1})
+	check(t, []int{gid2}, ck)
+	cfa[4] = ck.Query(-1)
+
+	ck.Leave([]int{gid2})
+	cfa[5] = ck.Query(-1)
+
+	fmt.Printf("  ... Passed\n")
+
+	fmt.Printf("Test: Historical queries ...\n")
+
+	for s := 0; s < nservers; s++ {
+		cfg.ShutdownServer(s)
+		for i := 0; i < len(cfa); i++ {
+			c := ck.Query(cfa[i].Num)
+			check_same_config(t, c, cfa[i])
+		}
+		cfg.StartServer(s)
+		cfg.ConnectAll()
+	}
+
+	fmt.Printf("  ... Passed\n")
+
+	fmt.Printf("Test: Move ...\n")
+	{
+		var gid3 int = 503
+		ck.Join(map[int][]string{gid3: []string{"3a", "3b", "3c"}})
+		var gid4 int = 504
+		ck.Join(map[int][]string{gid4: []string{"4a", "4b", "4c"}})
+		for i := 0; i < NShards; i++ {
+			cf := ck.Query(-1)
+			if i < NShards/2 {
+				ck.Move(i, gid3)
+				if cf.Shards[i] != gid3 {
+					cf1 := ck.Query(-1)
+					if cf1.Num <= cf.Num {
+						t.Fatalf("Move should increase Config.Num")
+					}
+				}
+			} else {
+				ck.Move(i, gid4)
+				if cf.Shards[i] != gid4 {
+					cf1 := ck.Query(-1)
+					if cf1.Num <= cf.Num {
+						t.Fatalf("Move should increase Config.Num")
+					}
+				}
+			}
+		}
+		cf2 := ck.Query(-1)
+		for i := 0; i < NShards; i++ {
+			if i < NShards/2 {
+				if cf2.Shards[i] != gid3 {
+					t.Fatalf("expected shard %v on gid %v actually %v",
+						i, gid3, cf2.Shards[i])
+				}
+			} else {
+				if cf2.Shards[i] != gid4 {
+					t.Fatalf("expected shard %v on gid %v actually %v",
+						i, gid4, cf2.Shards[i])
+				}
+			}
+		}
+		ck.Leave([]int{gid3})
+		ck.Leave([]int{gid4})
+	}
+	fmt.Printf("  ... Passed\n")
+
+	fmt.Printf("Test: Concurrent leave/join ...\n")
+
+	const npara = 10
+	var cka [npara]*Clerk
+	for i := 0; i < len(cka); i++ {
+		cka[i] = cfg.makeClient(cfg.All())
+	}
+	gids := make([]int, npara)
+	ch := make(chan bool)
+	for xi := 0; xi < npara; xi++ {
+		gids[xi] = int((xi * 10) + 100)
+		go func(i int) {
+			defer func() { ch <- true }()
+			var gid int = gids[i]
+			var sid1 = fmt.Sprintf("s%da", gid)
+			var sid2 = fmt.Sprintf("s%db", gid)
+			cka[i].Join(map[int][]string{gid + 1000: []string{sid1}})
+			cka[i].Join(map[int][]string{gid: []string{sid2}})
+			cka[i].Leave([]int{gid + 1000})
+		}(xi)
+	}
+	for i := 0; i < npara; i++ {
+		<-ch
+	}
+	check(t, gids, ck)
+
+	fmt.Printf("  ... Passed\n")
+
+	fmt.Printf("Test: Minimal transfers after joins ...\n")
+
+	c1 := ck.Query(-1)
+	for i := 0; i < 5; i++ {
+		var gid = int(npara + 1 + i)
+		ck.Join(map[int][]string{gid: []string{
+			fmt.Sprintf("%da", gid),
+			fmt.Sprintf("%db", gid),
+			fmt.Sprintf("%db", gid)}})
+	}
+	c2 := ck.Query(-1)
+	for i := int(1); i <= npara; i++ {
+		for j := 0; j < len(c1.Shards); j++ {
+			if c2.Shards[j] == i {
+				if c1.Shards[j] != i {
+					t.Fatalf("non-minimal transfer after Join()s")
+				}
+			}
+		}
+	}
+
+	fmt.Printf("  ... Passed\n")
+
+	fmt.Printf("Test: Minimal transfers after leaves ...\n")
+
+	for i := 0; i < 5; i++ {
+		ck.Leave([]int{int(npara + 1 + i)})
+	}
+	c3 := ck.Query(-1)
+	for i := int(1); i <= npara; i++ {
+		for j := 0; j < len(c1.Shards); j++ {
+			if c2.Shards[j] == i {
+				if c3.Shards[j] != i {
+					t.Fatalf("non-minimal transfer after Leave()s")
+				}
+			}
+		}
+	}
+
+	fmt.Printf("  ... Passed\n")
+}
+
+func TestMulti(t *testing.T) {
+	const nservers = 3
+	cfg := make_config(t, nservers, false)
+	defer cfg.cleanup()
+
+	ck := cfg.makeClient(cfg.All())
+
+	fmt.Printf("Test: Multi-group join/leave ...\n")
+
+	cfa := make([]Config, 6)
+	cfa[0] = ck.Query(-1)
+
+	check(t, []int{}, ck)
+
+	var gid1 int = 1
+	var gid2 int = 2
+	ck.Join(map[int][]string{
+		gid1: []string{"x", "y", "z"},
+		gid2: []string{"a", "b", "c"},
+	})
+	check(t, []int{gid1, gid2}, ck)
+	cfa[1] = ck.Query(-1)
+
+	var gid3 int = 3
+	ck.Join(map[int][]string{gid3: []string{"j", "k", "l"}})
+	check(t, []int{gid1, gid2, gid3}, ck)
+	cfa[2] = ck.Query(-1)
+
+	cfx := ck.Query(-1)
+	sa1 := cfx.Groups[gid1]
+	if len(sa1) != 3 || sa1[0] != "x" || sa1[1] != "y" || sa1[2] != "z" {
+		t.Fatalf("wrong servers for gid %v: %v\n", gid1, sa1)
+	}
+	sa2 := cfx.Groups[gid2]
+	if len(sa2) != 3 || sa2[0] != "a" || sa2[1] != "b" || sa2[2] != "c" {
+		t.Fatalf("wrong servers for gid %v: %v\n", gid2, sa2)
+	}
+	sa3 := cfx.Groups[gid3]
+	if len(sa3) != 3 || sa3[0] != "j" || sa3[1] != "k" || sa3[2] != "l" {
+		t.Fatalf("wrong servers for gid %v: %v\n", gid3, sa3)
+	}
+
+	ck.Leave([]int{gid1, gid3})
+	check(t, []int{gid2}, ck)
+	cfa[3] = ck.Query(-1)
+
+	cfx = ck.Query(-1)
+	sa2 = cfx.Groups[gid2]
+	if len(sa2) != 3 || sa2[0] != "a" || sa2[1] != "b" || sa2[2] != "c" {
+		t.Fatalf("wrong servers for gid %v: %v\n", gid2, sa2)
+	}
+
+	ck.Leave([]int{gid2})
+
+	fmt.Printf("  ... Passed\n")
+
+	fmt.Printf("Test: Concurrent multi leave/join ...\n")
+
+	const npara = 10
+	var cka [npara]*Clerk
+	for i := 0; i < len(cka); i++ {
+		cka[i] = cfg.makeClient(cfg.All())
+	}
+	gids := make([]int, npara)
+	var wg sync.WaitGroup
+	for xi := 0; xi < npara; xi++ {
+		wg.Add(1)
+		gids[xi] = int(xi + 1000)
+		go func(i int) {
+			defer wg.Done()
+			var gid int = gids[i]
+			cka[i].Join(map[int][]string{
+				gid: []string{
+					fmt.Sprintf("%da", gid),
+					fmt.Sprintf("%db", gid),
+					fmt.Sprintf("%dc", gid)},
+				gid + 1000: []string{fmt.Sprintf("%da", gid+1000)},
+				gid + 2000: []string{fmt.Sprintf("%da", gid+2000)},
+			})
+			cka[i].Leave([]int{gid + 1000, gid + 2000})
+		}(xi)
+	}
+	wg.Wait()
+	check(t, gids, ck)
+
+	fmt.Printf("  ... Passed\n")
+
+	fmt.Printf("Test: Minimal transfers after multijoins ...\n")
+
+	c1 := ck.Query(-1)
+	m := make(map[int][]string)
+	for i := 0; i < 5; i++ {
+		var gid = npara + 1 + i
+		m[gid] = []string{fmt.Sprintf("%da", gid), fmt.Sprintf("%db", gid)}
+	}
+	ck.Join(m)
+	c2 := ck.Query(-1)
+	for i := int(1); i <= npara; i++ {
+		for j := 0; j < len(c1.Shards); j++ {
+			if c2.Shards[j] == i {
+				if c1.Shards[j] != i {
+					t.Fatalf("non-minimal transfer after Join()s")
+				}
+			}
+		}
+	}
+
+	fmt.Printf("  ... Passed\n")
+
+	fmt.Printf("Test: Minimal transfers after multileaves ...\n")
+
+	var l []int
+	for i := 0; i < 5; i++ {
+		l = append(l, npara+1+i)
+	}
+	ck.Leave(l)
+	c3 := ck.Query(-1)
+	for i := int(1); i <= npara; i++ {
+		for j := 0; j < len(c1.Shards); j++ {
+			if c2.Shards[j] == i {
+				if c3.Shards[j] != i {
+					t.Fatalf("non-minimal transfer after Leave()s")
+				}
+			}
+		}
+	}
+
+	fmt.Printf("  ... Passed\n")
+}
diff --git a/src/shardmaster/mkclass.sh b/src/shardmaster/mkclass.sh
deleted file mode 100644
index 6bf5f4e..0000000
--- a/src/shardmaster/mkclass.sh
+++ /dev/null
@@ -1,82 +0,0 @@
-#!/bin/sh
-
-#
-# create or update 6.824-golabs-YEAR
-#
-# if a directory has a file mkclass.ignore, ignore the files listed in that file
-
-umask 2
-
-# update this repository:
-CLASSREPO=git@g.csail.mit.edu:6.824-golabs-2021
-
-# include only these src/ directories (e.g. not paxos)
-SRCS="mr mrapps main labrpc raft kvraft shardkv shardmaster labgob porcupine models"
-
-SD=$(cd $(dirname $0)/.. && /bin/pwd)
-CD=/tmp/golabs.$$
-git clone $CLASSREPO $CD || exit 1
-
-mkdir -p $CD/src/
-
-cp $SD/mygo/Makefile $CD/Makefile
-(cd $CD/ && git add Makefile 2> /dev/null )
-
-cp $SD/mygo/.check-build $CD/.check-build
-(cd $CD/ && git add .check-build 2> /dev/null )
-
-cp $SD/mygo/.gitignore $CD/.gitignore
-(cd $CD/ && git add .gitignore 2> /dev/null )
-
-cp $SD/mygo/src/.gitignore $CD/src/.gitignore || exit 1
-(cd $CD/src && git add .gitignore)
-
-cp $SD/mygo/src/go.mod $CD/src/go.mod
-(cd $CD/ && git add src/go.mod 2> /dev/null )
-
-cp $SD/mygo/src/go.sum $CD/src/go.sum
-(cd $CD/ && git add src/go.sum 2> /dev/null )
-
-# for D in `(cd $SD/mygo/src ; ls)`
-for D in `echo $SRCS`
-do
-  mkdir -p $CD/src/$D || exit 1
-  (cd $SD/mygo/src/$D
-  for F in `ls`
-  do
-    if [ -s mkclass.ignore ]
-    then
-      if grep -q $F mkclass.ignore
-      then
-        I=1
-      else
-        I=0
-      fi
-    else
-      I=0
-    fi
-    if [ "$F" = "out" ]
-    then
-      I=1
-    fi
-    if [ "$F" = "mkclass.ignore" ]
-    then
-      I=1
-    fi
-    if echo $F | grep -q '~$'
-    then
-      I=1
-    fi
-    if [ $I -eq 1 ]
-    then
-      echo "ignore $F"
-    else
-      $SD/bin/mklab.pl $CD/src/$D $F
-      (cd $CD/src/$D && git add $F 2> /dev/null )
-    fi
-  done)
-done
-
-(cd $CD ; git commit -am 'update')
-
-echo "Now, examine and push the repo in $CD"
diff --git a/src/shardmaster/mklab.pl b/src/shardmaster/mklab.pl
deleted file mode 100644
index 7e74792..0000000
--- a/src/shardmaster/mklab.pl
+++ /dev/null
@@ -1,89 +0,0 @@
-#!/usr/bin/perl -w
-
-#
-# ./mklab output-directory files...
-#
-# strips out lines from
-#   // SOL
-# to
-#   // END
-#
-# eliminates DELETE
-# e.g. func DELETELock(...)
-#
-
-use strict;
-use File::Copy qw(copy);
-
-sub usage {
-    print STDERR "Usage: mklab.pl output-directory files...\n";
-    exit(1);
-}
-
-sub onefile {
-    my($infile, $outfile) = @_;
-
-    if ($infile !~ /\.go$/){
-        print STDERR "copying non-Go $infile without SOL parsing\n";
-        die "cannot copy $infile > $outfile" if !copy($infile, $outfile);
-        return;
-    }
-
-    die "cannot read $infile" if !open(IF, $infile);
-    my $out = "";
-
-    my $omit = 0;
-    while(<IF>){
-        s/DELETE//g;
-        if(/ SOL$/){
-            if($omit == 0){
-                $omit = 1;
-            } else {
-                die "nested SOL";
-            }
-        } elsif(/SOL/){
-            die "malformed SOL";
-        } elsif(/ END$/){
-            if($omit == 1){
-                $omit = 0;
-            } else {
-                die "extra END in $infile";
-            }
-        } elsif(/END/){
-            die "malformed END";
-        } elsif($omit == 0){
-            $out .= $_;
-        }
-    }
-    close(IF);
-
-    if($omit){
-        die "SOL without END";
-    }
-
-    die "cannot write $outfile" if !open(OF, ">$outfile");
-    print OF $out;
-    close(OF);
-}
-
-sub main {
-    if($#ARGV < 1){
-        usage();
-    }
-    
-    my $dir = $ARGV[0];
-
-    system("mkdir -p $dir");
-    if(! -d $dir){
-        print STDERR "cannot create directory $dir\n";
-        exit(1)
-    }
-
-    for(my $i = 1; $i <= $#ARGV; $i++){
-        my $f = $ARGV[$i];
-        onefile($f, $dir . "/" . $f);
-    }
-}
-
-main();
-exit(0);