turn 0.3.1

A shared-resource-locking queue system using python and redis.

Turn

Introduction

Turn is a shared-resource-locking queue system using python and Redis. Use it in separate programs that acess the same shared resource to make sure each program waits for its turn to handle the resource.

It is inspired on a the queueing system that is sometimes found in small shops, consisting of a serial number dispenser and a wall indicator.

Turn comes with a commandline tool for resetting and direct inspection of queues, and listening to message channels for one or more resources.

Installation

Install turn with pip:

$ pip install turn

Of course, you should also have a Redis server at hand.

Usage

Basic usage goes like this:

import turn

# a locker corresponds to a reusable Redis client
locker = turn.Locker(host='localhost', port=6379, db=0)

resource = 'my_valuable_resource'
label = 'This shows up in messages.'

with locker.lock(resource=resource, label=label):
    pass  # do your careful work on the resource here

lock() accepts two extra keyword arguments:

expire: maximum expire value for a users presence (default 60)

If a user crashes hard, its presence in the queue will be kept alive at most expire seconds. This value affects how often the EXPIRE command will be sent to redis to signal the continuing presence of a user in the queue.

patience: period of waiting before bumping the queue (default 60)

If a program waits longer than this value without receiving any progression messages on the queues pubsub channel, it will bump the queue to see if any users have left the queue in an unusual way.

Tools

The state of users and queues can be monitered by inspection of Redis values and subscription to Redis channels.

Inspection can be done using the console script requesting a snap-shot status report:

$ turn status my_valuable_resource --host localhost
my_valuable_resource                                       5
------------------------------------------------------------
This shows up in status reports and messages.              5

Running turn status without specifying any resources, produces a summary of all queues within the database.

Alternatively, one or more subscriptions to the Redis PubSub channels for a particular resource can be followed:

$ turn follow my_valuable_resource --port 6379
my_valuable_resource: 5 assigned to "This shows up in messages."
my_valuable_resource: 5 started
my_valuable_resource: 5 completed by "This shows up in messages."
my_valuable_resource: 6 granted

Similar to the status command, running turn follow without specifying any resources starts following the channels for any queue currently within the database. Note that new queues are not automatically added to the subscription.

Queues can also be reset (removed) from redis using turn reset optionally followed by resources queues to reset. Reset without resource names resets all available queues in the server. If a queue for a resource shows activity, it will not be reset and in addition a message will be produced. A reset command for a resource will also ‘bump’ the queue for that resource.

Implementation details

When a lock is requested, a unique serial number is obtained from Redis via INCR on a Redis value, called the dispenser. The lock is acquired only if another value, called the indicator, corresponds to the unique serial number.

There are two mechanisms that can change the indicator:

1. The user with the corresponding serial number is finished acting on the shared resource and increases the number, notifying any other subscribed waiting users. This is the preferred way to handle things.

2. Another user gets impatient and calls the bump procedure. This procedure checks if the user corresponding to the indicator is still active and if necessary sets the indicator to an appropriate value.

Activity is monitored via an expiring key-value pair in Redis. The turn library automatically arranges a thread that keeps updating the expiration time, to make sure the presence does not expire during waiting for, or handling of the resource.

Credits

• Written by Arjan Verkerk

Changes

0.3.1 (2015-04-30)

• Nothing changed yet.

0.3 (2015-04-30)

• All code covered by tests.

• Extend docs.

0.2.1 (2015-04-28)

• Patience adjusted to seconds

• PubSub connections closed automatically

• Make patience an argument of lock()

• Docs updated

0.2 (2015-04-28)

• Documentation adjustments.

• Move console stuff to separate module.

• Use select and not poll, to make increase platform independency.

• Use the name ‘Locker’ for the reusable object that locks things.

0.1.1 (2015-04-23)

• U can use pip now.

0.1 (2015-04-23)

• Initial project structure created with nensskel 1.36.dev0.

• First working version.