#include <userver/dist_lock/dist_locked_task.hpp>

Detailed Description

A task that tries to acquire a distributed lock and runs user callback once while the lock is held.

When dist lock starts, the lock worker tries to take a lock in the loop. If succeeded, a task is launched that executes the user code. In the background, dist lock tries to extend the lock. In case of loss of the lock, the user task is canceled.

Example with retrying

  auto strategy = GetSomeDistLockStrategyForTheSample();
  std::atomic<std::size_t> counter = 0;
  dist_lock::DistLockedTask locked_task(
      "example",
      [&]() {
        counter++;
        if (counter < 5) {
          throw std::runtime_error("");
        }
      },
      strategy);
 
  UEXPECT_NO_THROW(locked_task.Get());
  EXPECT_EQ(counter, 5);

Example without retrying

  auto strategy = GetSomeDistLockStrategyForTheSample();
  std::atomic<std::size_t> counter = 0;
  dist_lock::DistLockedTask locked_task(
      "example",
      [&]() {
        counter++;
        throw std::runtime_error("123");
      },
      strategy, /*default settings*/ {}, dist_lock::DistLockWaitingMode::kWait,
      dist_lock::DistLockRetryMode::kSingleAttempt);
 
  try {
    locked_task.Get();
    FAIL() << "Should have thrown";
  } catch (const std::runtime_error& exception) {
    EXPECT_EQ(exception.what(), std::string("123"));
  }
  EXPECT_EQ(counter, 1);

See also: Periodics and DistLocks; AlwaysBusyDistLockStrategy

Definition at line 48 of file dist_locked_task.hpp.

Inheritance diagram for dist_lock::DistLockedTask:

Collaboration diagram for dist_lock::DistLockedTask:

Public Types
using	WorkerFunc = std::function<void()>

Public Types inherited from engine::TaskBase
enum class	Importance { kNormal , kCritical }
	Task importance. More...

enum class	State { kInvalid , kNew , kQueued , kRunning , kSuspended , kCancelled , kCompleted }
	Task state. More...

enum class	WaitMode { kSingleWaiter , kMultipleWaiters }
	Task wait mode. More...

Public Member Functions
	DistLockedTask ()=default

	DistLockedTask (DistLockedTask &&)=delete

DistLockedTask &	operator= (DistLockedTask &&)=delete

	DistLockedTask (const DistLockedTask &)=delete

DistLockedTask &	operator= (const DistLockedTask &&)=delete

	DistLockedTask (std::string name, WorkerFunc worker_func, std::shared_ptr< DistLockStrategyBase > strategy, const DistLockSettings &settings={}, DistLockWaitingMode mode=DistLockWaitingMode::kWait, DistLockRetryMode retry_mode=DistLockRetryMode::kRetry)

	DistLockedTask (engine::TaskProcessor &task_processor, std::string name, WorkerFunc worker_func, std::shared_ptr< DistLockStrategyBase > strategy, const DistLockSettings &settings={}, DistLockWaitingMode mode=DistLockWaitingMode::kWait, DistLockRetryMode retry_mode=DistLockRetryMode::kRetry)
	Creates a DistLockedTask to be run in a specific engine::TaskProcessor.

std::optional< std::chrono::steady_clock::duration >	GetLockedDuration () const

void	Get () noexcept(false)

Public Member Functions inherited from engine::TaskBase
bool	IsValid () const
	Checks whether this object owns an actual task (not `State::kInvalid`)

State	GetState () const
	Gets the task State.

bool	IsFinished () const
	Returns whether the task finished execution.

void	Wait () const noexcept(false)
	Suspends execution until the task finishes or caller is cancelled. Can be called from coroutine context only. For non-coroutine context use BlockingWait().

template<typename Rep , typename Period >
void	WaitFor (const std::chrono::duration< Rep, Period > &) const noexcept(false)
	Suspends execution until the task finishes or after the specified timeout or until caller is cancelled.

template<typename Clock , typename Duration >
void	WaitUntil (const std::chrono::time_point< Clock, Duration > &) const noexcept(false)
	Suspends execution until the task finishes or until the specified time point is reached or until caller is cancelled.

void	WaitUntil (Deadline) const
	Suspends execution until the task finishes or until the specified deadline is reached or until caller is cancelled.

void	RequestCancel ()
	Queues task cancellation request.

void	SyncCancel () noexcept
	Cancels the task and suspends execution until it is finished. Can be called from coroutine context only. For non-coroutine context use RequestCancel() + BlockingWait().

TaskCancellationReason	CancellationReason () const
	Gets task cancellation reason.

void	BlockingWait () const

Additional Inherited Members
Static Public Member Functions inherited from engine::TaskBase
static const std::string &	GetStateName (State state)

Member Typedef Documentation

◆ WorkerFunc

using dist_lock::DistLockedTask::WorkerFunc = std::function<void()>

Definition at line 50 of file dist_locked_task.hpp.

Constructor & Destructor Documentation

◆ DistLockedTask() [1/2]

dist_lock::DistLockedTask::DistLockedTask ( )

default

Default constructor. Creates an invalid task.

◆ DistLockedTask() [2/2]

dist_lock::DistLockedTask::DistLockedTask	(	std::string	name,
		WorkerFunc	worker_func,
		std::shared_ptr< DistLockStrategyBase >	strategy,
		const DistLockSettings &	settings = {},
		DistLockWaitingMode	mode = DistLockWaitingMode::kWait,
		DistLockRetryMode	retry_mode = DistLockRetryMode::kRetry )

Creates a DistLockedTask.

Parameters

name	name of the task
worker_func	a callback that is started once we've acquired the lock and is cancelled when the lock is lost.
settings	distributed lock settings
strategy	distributed locking strategy
mode	distributed lock waiting mode

Note: worker_func must honour task cancellation and stop ASAP when it is cancelled, otherwise brain split is possible (IOW, two different users do work assuming both of them hold the lock, which is not true).

Member Function Documentation

◆ GetLockedDuration()

std::optional< std::chrono::steady_clock::duration > dist_lock::DistLockedTask::GetLockedDuration ( ) const

Returns for how long the lock is held (if held at all). Returned value may be less than the real duration.

The documentation for this class was generated from the following file:

userver/dist_lock/dist_locked_task.hpp