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@heyitsanthony @gyuho @shrajfr12 @huikang @derekchiang @ajityagaty
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// Copyright 2016 The etcd 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 concurrency
import (
"context"
"math"
v3 "go.etcd.io/etcd/clientv3"
)
// STM is an interface for software transactional memory.
type STM interface {
// Get returns the value for a key and inserts the key in the txn's read set.
// If Get fails, it aborts the transaction with an error, never returning.
Get(key ...string) string
// Put adds a value for a key to the write set.
Put(key, val string, opts ...v3.OpOption)
// Rev returns the revision of a key in the read set.
Rev(key string) int64
// Del deletes a key.
Del(key string)
// commit attempts to apply the txn's changes to the server.
commit() *v3.TxnResponse
reset()
}
// Isolation is an enumeration of transactional isolation levels which
// describes how transactions should interfere and conflict.
type Isolation int
const (
// SerializableSnapshot provides serializable isolation and also checks
// for write conflicts.
SerializableSnapshot Isolation = iota
// Serializable reads within the same transaction attempt return data
// from the at the revision of the first read.
Serializable
// RepeatableReads reads within the same transaction attempt always
// return the same data.
RepeatableReads
// ReadCommitted reads keys from any committed revision.
ReadCommitted
)
// stmError safely passes STM errors through panic to the STM error channel.
type stmError struct{ err error }
type stmOptions struct {
iso Isolation
ctx context.Context
prefetch []string
}
type stmOption func(*stmOptions)
// WithIsolation specifies the transaction isolation level.
func WithIsolation(lvl Isolation) stmOption {
return func(so *stmOptions) { so.iso = lvl }
}
// WithAbortContext specifies the context for permanently aborting the transaction.
func WithAbortContext(ctx context.Context) stmOption {
return func(so *stmOptions) { so.ctx = ctx }
}
// WithPrefetch is a hint to prefetch a list of keys before trying to apply.
// If an STM transaction will unconditionally fetch a set of keys, prefetching
// those keys will save the round-trip cost from requesting each key one by one
// with Get().
func WithPrefetch(keys ...string) stmOption {
return func(so *stmOptions) { so.prefetch = append(so.prefetch, keys...) }
}
// NewSTM initiates a new STM instance, using serializable snapshot isolation by default.
func NewSTM(c *v3.Client, apply func(STM) error, so ...stmOption) (*v3.TxnResponse, error) {
opts := &stmOptions{ctx: c.Ctx()}
for _, f := range so {
f(opts)
}
if len(opts.prefetch) != 0 {
f := apply
apply = func(s STM) error {
s.Get(opts.prefetch...)
return f(s)
}
}
return runSTM(mkSTM(c, opts), apply)
}
func mkSTM(c *v3.Client, opts *stmOptions) STM {
switch opts.iso {
case SerializableSnapshot:
s := &stmSerializable{
stm: stm{client: c, ctx: opts.ctx},
prefetch: make(map[string]*v3.GetResponse),
}
s.conflicts = func() []v3.Cmp {
return append(s.rset.cmps(), s.wset.cmps(s.rset.first()+1)...)
}
return s
case Serializable:
s := &stmSerializable{
stm: stm{client: c, ctx: opts.ctx},
prefetch: make(map[string]*v3.GetResponse),
}
s.conflicts = func() []v3.Cmp { return s.rset.cmps() }
return s
case RepeatableReads:
s := &stm{client: c, ctx: opts.ctx, getOpts: []v3.OpOption{v3.WithSerializable()}}
s.conflicts = func() []v3.Cmp { return s.rset.cmps() }
return s
case ReadCommitted:
s := &stm{client: c, ctx: opts.ctx, getOpts: []v3.OpOption{v3.WithSerializable()}}
s.conflicts = func() []v3.Cmp { return nil }
return s
default:
panic("unsupported stm")
}
}
type stmResponse struct {
resp *v3.TxnResponse
err error
}
func runSTM(s STM, apply func(STM) error) (*v3.TxnResponse, error) {
outc := make(chan stmResponse, 1)
go func() {
defer func() {
if r := recover(); r != nil {
e, ok := r.(stmError)
if !ok {
// client apply panicked
panic(r)
}
outc <- stmResponse{nil, e.err}
}
}()
var out stmResponse
for {
s.reset()
if out.err = apply(s); out.err != nil {
break
}
if out.resp = s.commit(); out.resp != nil {
break
}
}
outc <- out
}()
r := <-outc
return r.resp, r.err
}
// stm implements repeatable-read software transactional memory over etcd
type stm struct {
client *v3.Client
ctx context.Context
// rset holds read key values and revisions
rset readSet
// wset holds overwritten keys and their values
wset writeSet
// getOpts are the opts used for gets
getOpts []v3.OpOption
// conflicts computes the current conflicts on the txn
conflicts func() []v3.Cmp
}
type stmPut struct {
val string
op v3.Op
}
type readSet map[string]*v3.GetResponse
func (rs readSet) add(keys []string, txnresp *v3.TxnResponse) {
for i, resp := range txnresp.Responses {
rs[keys[i]] = (*v3.GetResponse)(resp.GetResponseRange())
}
}
// first returns the store revision from the first fetch
func (rs readSet) first() int64 {
ret := int64(math.MaxInt64 - 1)
for _, resp := range rs {
if rev := resp.Header.Revision; rev < ret {
ret = rev
}
}
return ret
}
// cmps guards the txn from updates to read set
func (rs readSet) cmps() []v3.Cmp {
cmps := make([]v3.Cmp, 0, len(rs))
for k, rk := range rs {
cmps = append(cmps, isKeyCurrent(k, rk))
}
return cmps
}
type writeSet map[string]stmPut
func (ws writeSet) get(keys ...string) *stmPut {
for _, key := range keys {
if wv, ok := ws[key]; ok {
return &wv
}
}
return nil
}
// cmps returns a cmp list testing no writes have happened past rev
func (ws writeSet) cmps(rev int64) []v3.Cmp {
cmps := make([]v3.Cmp, 0, len(ws))
for key := range ws {
cmps = append(cmps, v3.Compare(v3.ModRevision(key), "<", rev))
}
return cmps
}
// puts is the list of ops for all pending writes
func (ws writeSet) puts() []v3.Op {
puts := make([]v3.Op, 0, len(ws))
for _, v := range ws {
puts = append(puts, v.op)
}
return puts
}
func (s *stm) Get(keys ...string) string {
if wv := s.wset.get(keys...); wv != nil {
return wv.val
}
return respToValue(s.fetch(keys...))
}
func (s *stm) Put(key, val string, opts ...v3.OpOption) {
s.wset[key] = stmPut{val, v3.OpPut(key, val, opts...)}
}
func (s *stm) Del(key string) { s.wset[key] = stmPut{"", v3.OpDelete(key)} }
func (s *stm) Rev(key string) int64 {
if resp := s.fetch(key); resp != nil && len(resp.Kvs) != 0 {
return resp.Kvs[0].ModRevision
}
return 0
}
func (s *stm) commit() *v3.TxnResponse {
txnresp, err := s.client.Txn(s.ctx).If(s.conflicts()...).Then(s.wset.puts()...).Commit()
if err != nil {
panic(stmError{err})
}
if txnresp.Succeeded {
return txnresp
}
return nil
}
func (s *stm) fetch(keys ...string) *v3.GetResponse {
if len(keys) == 0 {
return nil
}
ops := make([]v3.Op, len(keys))
for i, key := range keys {
if resp, ok := s.rset[key]; ok {
return resp
}
ops[i] = v3.OpGet(key, s.getOpts...)
}
txnresp, err := s.client.Txn(s.ctx).Then(ops...).Commit()
if err != nil {
panic(stmError{err})
}
s.rset.add(keys, txnresp)
return (*v3.GetResponse)(txnresp.Responses[0].GetResponseRange())
}
func (s *stm) reset() {
s.rset = make(map[string]*v3.GetResponse)
s.wset = make(map[string]stmPut)
}
type stmSerializable struct {
stm
prefetch map[string]*v3.GetResponse
}
func (s *stmSerializable) Get(keys ...string) string {
if wv := s.wset.get(keys...); wv != nil {
return wv.val
}
firstRead := len(s.rset) == 0
for _, key := range keys {
if resp, ok := s.prefetch[key]; ok {
delete(s.prefetch, key)
s.rset[key] = resp
}
}
resp := s.stm.fetch(keys...)
if firstRead {
// txn's base revision is defined by the first read
s.getOpts = []v3.OpOption{
v3.WithRev(resp.Header.Revision),
v3.WithSerializable(),
}
}
return respToValue(resp)
}
func (s *stmSerializable) Rev(key string) int64 {
s.Get(key)
return s.stm.Rev(key)
}
func (s *stmSerializable) gets() ([]string, []v3.Op) {
keys := make([]string, 0, len(s.rset))
ops := make([]v3.Op, 0, len(s.rset))
for k := range s.rset {
keys = append(keys, k)
ops = append(ops, v3.OpGet(k))
}
return keys, ops
}
func (s *stmSerializable) commit() *v3.TxnResponse {
keys, getops := s.gets()
txn := s.client.Txn(s.ctx).If(s.conflicts()...).Then(s.wset.puts()...)
// use Else to prefetch keys in case of conflict to save a round trip
txnresp, err := txn.Else(getops...).Commit()
if err != nil {
panic(stmError{err})
}
if txnresp.Succeeded {
return txnresp
}
// load prefetch with Else data
s.rset.add(keys, txnresp)
s.prefetch = s.rset
s.getOpts = nil
return nil
}
func isKeyCurrent(k string, r *v3.GetResponse) v3.Cmp {
if len(r.Kvs) != 0 {
return v3.Compare(v3.ModRevision(k), "=", r.Kvs[0].ModRevision)
}
return v3.Compare(v3.ModRevision(k), "=", 0)
}
func respToValue(resp *v3.GetResponse) string {
if resp == nil || len(resp.Kvs) == 0 {
return ""
}
return string(resp.Kvs[0].Value)
}
// NewSTMRepeatable is deprecated.
func NewSTMRepeatable(ctx context.Context, c *v3.Client, apply func(STM) error) (*v3.TxnResponse, error) {
return NewSTM(c, apply, WithAbortContext(ctx), WithIsolation(RepeatableReads))
}
// NewSTMSerializable is deprecated.
func NewSTMSerializable(ctx context.Context, c *v3.Client, apply func(STM) error) (*v3.TxnResponse, error) {
return NewSTM(c, apply, WithAbortContext(ctx), WithIsolation(Serializable))
}
// NewSTMReadCommitted is deprecated.
func NewSTMReadCommitted(ctx context.Context, c *v3.Client, apply func(STM) error) (*v3.TxnResponse, error) {
return NewSTM(c, apply, WithAbortContext(ctx), WithIsolation(ReadCommitted))
}
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