storages::postgres::DistLockComponentBase Class Reference

#include <userver/storages/postgres/dist_lock_component_base.hpp>

Detailed Description

Base class for postgres-based distlock worker components.

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

The class must be used for infinite loop jobs. If you want a distributed periodic, you should look at locked_periodiccomponents::PgLockedPeriodic.

See also

dist_lock::DistLockedTask

locked_periodiccomponents::PgLockedPeriodic

Static configuration example:

example-distlock:
    cluster: postgresql-service
    table: service.distlocks
    lockname: master
    pg-timeout: 1s
    lock-ttl: 10s
    autostart: true

Static options:

name	Description	Default value
cluster	postgres cluster name	–
table	table name to store distlocks	–
lockname	name of the lock	–
lock-ttl	TTL of the lock; must be at least as long as the duration between subsequent cancellation checks, otherwise brain split is possible	–
pg-timeout	timeout, must be less than lock-ttl/2	–
restart-delay	how much time to wait after failed task restart	100ms
autostart	if true, start automatically after component load	false
task-processor	the name of the TaskProcessor for running DoWork	main-task-processor
testsuite-support	Enable testsuite support	false

Migration example

You have to create a SQL table for distlocks. An example of the migration script is as following:

CREATE TABLE service.distlocks
(
    key             TEXT PRIMARY KEY,
    owner           TEXT,
    expiration_time TIMESTAMPTZ
);

See also

Periodics and DistLocks

Definition at line 74 of file dist_lock_component_base.hpp.

Inheritance diagram for storages::postgres::DistLockComponentBase:

Public Member Functions
	DistLockComponentBase (const components::ComponentConfig &, const components::ComponentContext &)
dist_lock::DistLockedWorker &	GetWorker ()
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 ()
	Override this function to provide custom testsuite handler.
void	AutostartDistLock ()
	Must be called in ctr.
void	StopDistLock ()
	Must be called in dtr.

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 storages::postgres::DistLockComponentBase::DoWork ( )

protectedpure virtual

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

Example implementation

void MyDistLockComponent::DoWork()
{
    while (!engine::ShouldCancel())
    {
        // Start a new trace_id
        auto span = tracing::Span::MakeRootSpan("my-dist-lock");
 
        // If Foo() or other function 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 `lock-ttl`
        // seconds to have time to notice lock prolongation failure.
        if (engine::ShouldCancel()) break;
 
        Bar();
    }
}

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).

DoWorkTestsuite()

virtual void storages::postgres::DistLockComponentBase::DoWorkTestsuite ( )

inlineprotectedvirtual

Override this function to provide custom testsuite handler.

Definition at line 118 of file dist_lock_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.

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

• userver/storages/postgres/dist_lock_component_base.hpp