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@snej
Created July 12, 2013 20:43
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New implementation of memcached.CAS that doesn't reserve the client connection for the duration of the transaction.
type CASState struct {
initialized bool // false on the first call to CASNext, then true
Value []byte // Current value of key; update in place to new value
Cas uint64 // Current CAS value of key
Exists bool // Does a value exist for the key? (If not, Value will be nil)
Err error // Error, if any, after CASNext returns false
resp *gomemcached.MCResponse
}
// Non-callback, loop-based version of CAS method. Usage is like this:
//
// var state memcached.CASState
// for client.CASNext(vb, key, exp, &state) {
// state.Value = some_mutation(state.Value)
// }
// if state.Err != nil { ... }
func (client *Client) CASNext(vb uint16, k string, exp int, state *CASState) bool {
if state.initialized {
if !state.Exists {
// Adding a new key:
if state.Value == nil {
state.Cas = 0
return false // no-op (delete of non-existent value)
}
state.resp, state.Err = UnwrapMemcachedError(client.Add(vb, k, 0, exp, state.Value))
if state.Err != nil {
return false
} else if state.resp.Status == gomemcached.SUCCESS {
return false // Successful add
}
} else {
// Updating / deleting a key:
req := &gomemcached.MCRequest{
Opcode: gomemcached.DELETE,
VBucket: vb,
Key: []byte(k),
Cas: state.Cas}
if state.Value != nil {
req.Opcode = gomemcached.SET
req.Opaque = 0
req.Extras = []byte{0, 0, 0, 0, 0, 0, 0, 0}
req.Body = state.Value
flags := 0
exp := 0 // ??? Should we use initialexp here instead?
binary.BigEndian.PutUint64(req.Extras, uint64(flags)<<32|uint64(exp))
}
if state.resp, state.Err = client.Send(req); state.Err == nil {
return false // Successful update or delete!
}
}
}
// Initial call, or after a conflict: GET the current value and CAS and return them:
state.initialized = true
if state.resp, state.Err = client.Get(vb, k); state.Err == nil {
state.Exists = true
state.Value = state.resp.Body
state.Cas = state.resp.Cas
} else if state.resp != nil && state.resp.Status == gomemcached.KEY_ENOENT {
state.Err = nil
state.Exists = false
state.Value = nil
state.Cas = 0
} else {
return false
}
return true // keep going...
}
// A function to perform a CAS transform.
// Input is the current value, or nil if no value exists.
// The function should return the new value (if any) to set, and the store/quit/delete operation.
type CasFunc func(current []byte) ([]byte, CasOp)
// Perform a CAS transform with the given function.
//
// If the value does not exist, a nil current value will be sent to f.
func (client *Client) CAS(vb uint16, k string, f CasFunc,
initexp int) (*gomemcached.MCResponse, error) {
var state CASState
for client.CASNext(vb, k, initexp, &state) {
newValue, operation := f(state.Value)
if operation == CASQuit || (operation == CASDelete && state.Value == nil) {
return nil, operation
}
state.Value = newValue
}
return state.resp, state.Err
}
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