AcidPtr & AcidCommitPtr

Synopsis

#include "ts/AcidPtr.h"

AcidPtr provides atomic access to a std::shared_ptr. AcidCommitPtr provides exclusive write access to data.

Description

Provides transparent interface for “copy on write” and “commit when done” in a familiar unique_ptr style.

Named after the desirable properties of a database, ACID acronym:

  • Atomic - reads and writes avoid skew, by using mutex locks.
  • Consistent - data can only be changed by a commit, and only one commit can exist at a time per data.
  • Isolated - commits of a single point of data are not concurrent. But commits of seperate data can be conncurrent.
  • Durable - shared_ptr is used to keep older versions of data in memory while references exist.

Performance

Note this code is currently implemented with mutex locks, it is expected to be fast due to the very brief duration of each lock. It would be plausible to change the implementation to use atomics or spin locks.

AcidCommitPtr

  • On construction, duplicate values from a shared_ptr to a unique_ptr. (aka copy read to write memory)
  • On destruction, move value ptr to shared_ptr. (aka move write ptr to read ptr)

The AcidCommitPtr executes this transparent to the writer code. It copies the data on construction, and finalizes on destruction. A MemLock is used to allow exclusive read and write access, however the access is made to as fast as possible.

Use Cases

Implemented for use AcidPtr interface in Extendible. But could be used elsewhere without modification.

title Read while Write

actor ExistingReaders
actor NewReaders
actor Writer
storage AcidPtr
card "x=foo" as Data
card "x=bar" as DataPrime

ExistingReaders --> Data : read only
AcidPtr -> Data : shared_ptr
NewReaders --> AcidPtr : read only
Writer --> DataPrime : read/write
Data .> DataPrime : copy

When the writer is done, ~AcidCommitPtr() is called and its AcidPtr is updated to point at the written copy, so that future read requests will use it. Existing reader will continue to use the old data.

title Write Finalize

actor ExistingReaders
actor NewReaders
storage AcidPtr
card "x=foo" as Data
card "x=bar" as DataPrime

ExistingReaders --> Data : read only
AcidPtr -> DataPrime : shared_ptr
NewReaders --> AcidPtr : read only

title AcidPtr Reader/Reader Contention
box "MemLock"
participant "Access" as AccessMutex
end box
participant AcidPtr
actor Reader_A #green
actor Reader_B #red
Reader_A -[#green]> AcidPtr: getPtr()
AcidPtr <[#green]- AccessMutex: lock
activate AccessMutex #green
Reader_B -> AcidPtr: getPtr()
AcidPtr -[#green]> Reader_A: copy const shared_ptr
activate Reader_A
note left
  Contention limited to
  duration of shared_ptr copy.
  (internally defined)
end note
AcidPtr -[#green]> AccessMutex: unlock
deactivate AccessMutex
AcidPtr <- AccessMutex: lock
activate AccessMutex #red
AcidPtr -> Reader_B: copy const shared_ptr
activate Reader_B
AcidPtr -> AccessMutex: unlock
deactivate AccessMutex

Title AcidPtr Writer/Reader Contention
box "MemLock"
participant "Access" as AccessMutex
participant "Write" as WriteMutex
end box
participant AcidPtr
actor Writer #red
actor Reader #green

Writer -> AcidPtr: startCommit()
AcidPtr <- WriteMutex: lock
activate WriteMutex #red
AcidPtr -> Writer: copy to AcidCommitPtr
activate Writer
hnote over Writer #pink
  update AcidCommitPtr
end hnote
Reader -[#green]> AcidPtr: getPtr()
AcidPtr <[#green]- AccessMutex: lock
activate AccessMutex #green

Writer -> AcidPtr: ~AcidCommitPtr()
deactivate Writer
AcidPtr -[#green]> Reader: copy const shared_ptr
activate Reader
note left
  Contention limited to duration
  of shared_ptr copy/reset.
  (internally defined)
end note
AcidPtr -[#green]> AccessMutex: unlock
deactivate AccessMutex

AcidPtr <- AccessMutex: lock
activate AccessMutex #red
hnote over Reader #lightgreen
  use shared copy
end hnote
AcidPtr -> AcidPtr: reset shared_ptr
AcidPtr -> AccessMutex: unlock
deactivate AccessMutex
AcidPtr -> WriteMutex: unlock
deactivate WriteMutex

Title AcidPtr Writer/Writer Contention
box "MemLock"
participant "Access" as AccessMutex
participant "Write" as WriteMutex
end box
participant AcidPtr
actor Writer_A #red
actor Writer_B #blue

Writer_A -> AcidPtr: startCommit
AcidPtr <- WriteMutex: lock
activate WriteMutex #red
AcidPtr -> Writer_A: copy to AcidCommitPtr
activate Writer_A
Writer_B -[#blue]> AcidPtr: startCommit
hnote over Writer_A #pink
  update AcidCommitPtr
end hnote
note over Writer_A
  Contention for duration
  of AcidCommitPtr scope.
  (externally defined)
end note
Writer_A -> AcidPtr: ~AcidCommitPtr()
deactivate Writer_A
AcidPtr <- AccessMutex: lock
activate AccessMutex #red
AcidPtr -> AcidPtr: reset shared_ptr
AcidPtr -> AccessMutex: unlock
deactivate AccessMutex
AcidPtr -> WriteMutex: unlock
deactivate WriteMutex

AcidPtr <[#blue]- WriteMutex: lock
activate WriteMutex #blue
AcidPtr -[#blue]> Writer_B: copy to AcidCommitPtr
activate Writer_B
hnote over Writer_B #lightblue
  update AcidCommitPtr
end hnote
Writer_B -[#blue]> AcidPtr: ~AcidCommitPtr()
deactivate Writer_B
deactivate AccessMutex

AcidPtr <[#blue]- AccessMutex: lock
activate AccessMutex #blue
AcidPtr -[#blue]> AcidPtr: reset shared_ptr
AcidPtr -[#blue]> AccessMutex: unlock
deactivate AccessMutex
AcidPtr -[#blue]> WriteMutex: unlock
deactivate WriteMutex

Reference

template<typename T>
class AcidPtr
template<typename T>
const std::shared_ptr<const T> getPtr() const
template<typename T>
class AcidCommitPtr
template<>
~AcidCommitPtr()
template<T>
AcidCommitPtr<T> startCommit()
template<>
void abort()