#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:

Collaboration diagram for storages::postgres::DistLockComponentBase:

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

dist_lock::DistLockedWorker &	GetWorker ()

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 ()
	Override this function to provide custom testsuite handler.

void	AutostartDistLock ()
	Must be called in ctr.

void	StopDistLock ()
	Must be called in dtr.

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.

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

userver/storages/postgres/dist_lock_component_base.hpp