clientv3/concurrency/stm.go中
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")
}
}
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
}
重点在于冲突检查和数据读取
SerializableSnapshot
s.conflicts = func() []v3.Cmp {
return append(s.rset.cmps(), s.wset.cmps(s.rset.first()+1)...)
}
也就是不仅要确保读取的数据是最新的,还要确保第一个读之后没有新的写入
Serializable
s.conflicts = func() []v3.Cmp { return s.rset.cmps() }
也就是要确保读取的数据是最新的
RepeatableReads
s.conflicts = func() []v3.Cmp { return s.rset.cmps() }
也就是要确保读取的数据是最新的 和Serializable,SerializableSnapshot区别在于Serializable,SerializableSnapshot之后的读取是根据第一次读取的rev来获取的
Serializable
func (s *stmSerializable) Get(keys ...string) string {
...
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)
}
RepeatableReads
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) 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())
}
ReadCommitted
s.conflicts = func() []v3.Cmp { return nil }
也就是没有检查
