Module: Semian

Extended by:
Semian, Instrumentable
Included in:
Semian
Defined in:
lib/semian.rb,
lib/semian/adapter.rb,
lib/semian/version.rb,
lib/semian/platform.rb,
lib/semian/resource.rb,
lib/semian/instrumentable.rb,
lib/semian/circuit_breaker.rb,
lib/semian/protected_resource.rb,
lib/semian/unprotected_resource.rb,
lib/semian/mysql2.rb,
lib/semian/redis.rb

Overview

Overview

Semian is a library that can be used to control access to external services.

It’s desirable to control access to external services so that in the case that one is slow or not responding, the performance of an entire system is not compromised.

Semian uses the concept of a “resource” as an identifier that controls access to some external service. So for example, “mysql” or “redis” would be considered resources. If a system is sharded, like a database, you would typically create a resource for every shard.

Resources are visible across an IPC namespace. This means that you can register a resource in one process and access it from another. This is useful in application servers like Unicorn that are multi-process. A resource is persistent. It will continue to exist even after the application exits, and will only be destroyed by manually removing it with the ipcrm command, calling Resource.destroy, or rebooting the machine.

Each resource has a configurable number of tickets. Tickets are what controls access to the external service. If a client does not have a ticket, it cannot access a service. If there are no tickets available, the client will block for a configurable amount of time until a ticket is available. If there are no tickets available after the timeout period has elapsed, the client will be unable to access the service and an error will be raised.

Resources also integrate a circuit breaker in order to fail faster and to let the resource the time to recover. If ‘error_threshold` errors happen in the span of `error_timeout` then the circuit will be opened and every attempt to acquire the resource will immediately fail.

Once in open state, after ‘error_timeout` is elapsed, the ciruit will transition in the half-open state. In that state a single error will fully re-open the circuit, and the circuit will transition back to the closed state only after the resource is acquired `success_threshold` consecutive times.

A resource is registered by using the Semian.register method.

Examples

Registering a resource
Semian.register(:mysql_shard0, tickets: 10, timeout: 0.5, error_threshold: 3, error_timeout: 10, success_threshold: 2)

This registers a new resource called :mysql_shard0 that has 10 tickets andd a default timeout of 500 milliseconds.

After 3 failures in the span of 10 seconds the circuit will be open. After an additional 10 seconds it will transition to half-open. And finally after 2 successulf acquisitions of the resource it will transition back to the closed state.

Using a resource
Semian[:mysql_shard0].acquire do
  # Perform a MySQL query here
end

This acquires a ticket for the :mysql_shard0 resource. If we use the example above, the ticket count would be lowered to 9 when block is executed, then raised to 10 when the block completes.

Overriding the default timeout
Semian[:mysql_shard0].acquire(timeout: 1) do
  # Perform a MySQL query here
end

This is the same as the previous example, but overrides the timeout from the default value of 500 milliseconds to 1 second.

Defined Under Namespace

Modules: Adapter, AdapterError, Instrumentable, Mysql2, Redis Classes: CircuitBreaker, InternalError, ProtectedResource, Resource, SyscallError, TimeoutError, UnprotectedResource

Constant Summary collapse

BaseError = 
Class.new(StandardError)
OpenCircuitError = 
Class.new(BaseError)
MAX_TICKETS =

Maximum number of tickets available on this system.

INT2FIX(system_max_semaphore_count)
VERSION = 
'0.3.0'

Instance Attribute Summary collapse

Class Method Summary collapse

Instance Method Summary collapse

Methods included from Instrumentable

notify, subscribe, unsubscribe

Instance Attribute Details

#logger ⇒ Object

Returns the value of attribute logger.



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# File 'lib/semian.rb', line 96

def logger
  @logger
end

Class Method Details

.sysv_semaphores_supported? ⇒ Boolean

Determines if Semian supported on the current platform.

Returns:

  • (Boolean) 


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# File 'lib/semian/platform.rb', line 3

def self.sysv_semaphores_supported?
  /linux/.match(RUBY_PLATFORM)
end

Instance Method Details

#[](name) ⇒ Object

Retrieves a resource by name.



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# File 'lib/semian.rb', line 136

def [](name)
  resources[name]
end

#destroy(name) ⇒ Object 



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# File 'lib/semian.rb', line 140

def destroy(name)
  if resource = resources.delete(name)
    resource.destroy
  end
end

#register(name, tickets:, permissions: 0660, timeout: 0, error_threshold:, error_timeout:, success_threshold:, exceptions: []) ⇒ Object

Registers a resource.

name: Name of the resource - this can be either a string or symbol.

tickets: Number of tickets. If this value is 0, the ticket count will not be set, but the resource must have been previously registered otherwise an error will be raised.

permissions: Octal permissions of the resource.

timeout: Default timeout in seconds.

error_threshold: The number of errors that will trigger the circuit opening.

error_timeout: The duration in seconds since the last error after which the error count is reset to 0.

success_threshold: The number of consecutive success after which an half-open circuit will be fully closed.

exceptions: An array of exception classes that should be accounted as resource errors.

Returns the registered resource.



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# File 'lib/semian.rb', line 120

def register(name, tickets:, permissions: 0660, timeout: 0, error_threshold:, error_timeout:, success_threshold:, exceptions: [])
  circuit_breaker = CircuitBreaker.new(
    success_threshold: success_threshold,
    error_threshold: error_threshold,
    error_timeout: error_timeout,
    exceptions: Array(exceptions) + [::Semian::BaseError],
  )
  resource = Resource.new(name, tickets: tickets, permissions: permissions, timeout: timeout)
  resources[name] = ProtectedResource.new(resource, circuit_breaker)
end

#resources ⇒ Object

Retrieves a hash of all registered resources.



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# File 'lib/semian.rb', line 147

def resources
  @resources ||= {}
end

#retrieve_or_register(name, **args) ⇒ Object 



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# File 'lib/semian.rb', line 131

def retrieve_or_register(name, **args)
  self[name] || register(name, **args)
end

#semaphores_enabled? ⇒ Boolean

Returns:

  • (Boolean) 


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# File 'lib/semian.rb', line 80

def semaphores_enabled?
  !ENV['SEMIAN_SEMAPHORES_DISABLED']
end