#include <userver/ydb/dist_lock/component_base.hpp>

Detailed Description

Base class for YDB-based distlock worker components.

A component that implements a distlock with lock in YDB. Inherit from DistLockComponentBase and implement DoWork(). Lock options are configured in static config.

The class must be used for infinite loop jobs.

Static configuration example:

        # yaml
        sample-dist-lock:
            # Settings that might be used for a group of related distlock instances
            database-settings:
                # The key of the database within ydb component (NOT the actual database path)
                dbname: sampledb
                # Name of the coordination node within the database
                coordination-node: sample-node
            # Name of the semaphore within the coordination node
            semaphore-name: sample-lock

Static options:

name	Description	Default value
semaphore-name	name of the semaphore within the coordination node	–
database-settings.dbname	the key of the database within ydb component (NOT the actual database path)	–
database-settings.coordination-node	name of the coordination node within the database	–
initial-setup	if true, then create the coordination node and the semaphore unless they already exist	true
task-processor	the name of the TaskProcessor for running DoWork	main-task-processor
node-settings.session-grace-period	the time after which the lock will be given to another host after a network failure	10s
session-timeout	for how long we will try to restore session after a network failure before dropping it	5s
restart-session-delay	backoff before attempting to reconnect session after it returns "permanent failure"	1s
acquire-interval	backoff before repeating a failed Acquire call	100ms
restart-delay	backoff before calling DoWork again after it returns or throws	100ms
cancel-task-time-limit	time, within which a cancelled DoWork is expected to finish	5s

See also: Periodics and DistLocks

Definition at line 55 of file component_base.hpp.

Inheritance diagram for ydb::DistLockComponentBase:

Collaboration diagram for ydb::DistLockComponentBase:

Public Member Functions
	DistLockComponentBase (const components::ComponentConfig &, const components::ComponentContext &)

bool	OwnsLock () const noexcept
	Checks if the the current service instance owns the lock.

Public Member Functions inherited from components::LoggableComponentBase
	LoggableComponentBase (const ComponentConfig &, const ComponentContext &)

	LoggableComponentBase (LoggableComponentBase &&)=delete

	LoggableComponentBase (const LoggableComponentBase &)=delete

	~LoggableComponentBase () override=default

ComponentHealth	GetComponentHealth () const override

void	OnLoadingCancelled () override

void	OnAllComponentsLoaded () override

void	OnAllComponentsAreStopping () override

Static Public Member Functions
static yaml_config::Schema	GetStaticConfigSchema ()

Static Public Member Functions inherited from components::LoggableComponentBase
static yaml_config::Schema	GetStaticConfigSchema ()

Protected Member Functions
virtual void	DoWork ()=0

virtual void	DoWorkTestsuite ()

void	Start ()
	Must be called at the end of the constructor of the derived component.

void	Stop () noexcept
	Must be called in the destructor of the derived component.

Member Function Documentation

◆ DoWork()

virtual void ydb::DistLockComponentBase::DoWork ( )

protectedpure virtual

Override this function with anything that must be done under the lock.

Note: DoWork 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).; When DoWork exits for any reason, the lock is dropped, then after restart-delay the lock is attempted to be re-acquired (but by that time another host likely acquires the lock).

Example implementation

void SampleDistLock::DoWork() {
  while (!engine::current_task::ShouldCancel()) {
    // Start a new trace_id.
    auto span = tracing::Span::MakeRootSpan("sample-dist-lock");
 
    // If Foo() or anything in DoWork() throws an exception,
    // DoWork() will be restarted in `restart-delay` seconds.
    Foo();
 
    // Check for cancellation after cpu-intensive Foo().
    // You must check for cancellation at least every
    // `cancel-task-time-limit`
    // seconds to have time to notice lock prolongation failure.
    if (engine::current_task::ShouldCancel()) break;
 
    Bar();
 
    engine::InterruptibleSleepFor(
        config_source_.GetCopy(kSampleDistLockIntervalMs));
  }
}

◆ DoWorkTestsuite()

virtual void ydb::DistLockComponentBase::DoWorkTestsuite ( )

inlineprotectedvirtual

Override this function to provide a custom testsuite handler, e.g. one that does not contain a "while not cancelled" loop. Calls DoWork by default.

In testsuite runs, the normal DoWork execution disabled by default. There is an API to call DoWorkTestsuite from testsuite, imitating waiting until DoWork runs:

Definition at line 100 of file component_base.hpp.

◆ OwnsLock()

bool ydb::DistLockComponentBase::OwnsLock ( ) const

noexcept

Checks if the the current service instance owns the lock.

Useful for:

Writing metrics only on the primary (for the given distlock) host.
Running on-demand work, e.g. in handlers, only on the primary host. The work must be short-lived (single requests with small timeout), otherwise brain split (where multiple hosts consider themselves primary) is possible.

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

userver/ydb/dist_lock/component_base.hpp