ydb::DistLockComponentBase Class Reference

#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:

Public Member Functions
	DistLockComponentBase (const components::ComponentConfig &, const components::ComponentContext &)
bool	OwnsLock () const noexcept
	Checks if the the current service instance owns the lock.
ComponentHealth	GetComponentHealth () const override
void	OnLoadingCancelled () override
void	OnAllComponentsLoaded () override
void	OnAllComponentsAreStopping () override

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

Protected Types
using	LoggableComponentBase = ComponentBase
	Legacy alias, use ComponentBase instead.

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 Typedef Documentation

LoggableComponentBase

using components::ComponentBase::LoggableComponentBase = ComponentBase

protectedinherited

Legacy alias, use ComponentBase instead.

Definition at line 69 of file component_base.hpp.

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.

GetComponentHealth()

ComponentHealth components::ComponentBase::GetComponentHealth ( ) const

inlineoverridevirtualinherited

Override this function to inform the world of the state of your component.

Warning

The function is called concurrently from multiple threads.

Reimplemented from components::RawComponentBase.

Definition at line 35 of file component_base.hpp.

OnAllComponentsAreStopping()

void components::ComponentBase::OnAllComponentsAreStopping ( )

inlineoverridevirtualinherited

Component may use this function to stop doing work before the stop of the components that depend on it.

Base components may override it and make final to do some work before the derived object constructor is called. Don't use it otherwise.

Reimplemented from components::RawComponentBase.

Reimplemented in urabbitmq::ConsumerComponentBase, and components::Server.

Definition at line 60 of file component_base.hpp.

OnAllComponentsLoaded()

void components::ComponentBase::OnAllComponentsLoaded ( )

inlineoverridevirtualinherited

Component may use this function to finalize registration of other components that depend on it (for example, handler components register in server component, and the latter uses OnAllComponentsLoaded() to start processing requests).

Base components may override it and make final to do some work after the derived object constructor is called. Don't use it otherwise.

Reimplemented from components::RawComponentBase.

Reimplemented in urabbitmq::ConsumerComponentBase, components::Server, and server::handlers::Ping.

Definition at line 53 of file component_base.hpp.

OnLoadingCancelled()

void components::ComponentBase::OnLoadingCancelled ( )

inlineoverridevirtualinherited

Called once if the creation of any other component failed. If the current component expects some other component to take any action with the current component, this call is a signal that such action may never happen due to components loading was cancelled. Application components might not want to override it.

Reimplemented from components::RawComponentBase.

Definition at line 44 of file component_base.hpp.

OwnsLock()

bool ydb::DistLockComponentBase::OwnsLock ( ) const

noexcept

Checks if the the current service instance owns the lock.

Useful for:

1. Writing metrics only on the primary (for the given distlock) host.
2. 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.

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

• userver/ydb/dist_lock/component_base.hpp