hot-redis 0.3

Higher Order Types for Redis

Created by Stephen McDonald

Introduction

HOT Redis is a wrapper library for the redis-py client. Rather than calling the Redis commands directly from a client library, HOT Redis provides a wide range of data types that mimic many of the built-in data types provided by Python, such as lists, dicts, sets, and more, as well as many of the classes found throughout the standard library, such as those found in the Queue, threading, and collections modules.

These types are then backed by Redis, allowing objects to be manipulated atomically over the network - the atomic nature of the methods implemented on objects in HOT Redis is one of its core features, and many of these are backed by Lua code executed within Redis, which ensures atomic operations where applicable.

The name HOT Redis originally stood for “Higher Order Types for Redis”, but since the implementation doesn’t strictly fit the definition, the recursive acronym “HOT Object Toolkit for Redis” should appease the most luscious of bearded necks.

HOT Redis was drawn from the infrastructure behind the Kouio RSS reader, a popular alternative to Google Reader.

Installation

The easiest way to install hot-redis is directly from PyPi using pip by running the following command:

$ pip install -U hot-redis

Otherwise you can download and install it directly from source:

$ python setup.py install

Usage

Each of the types provided by HOT Redis strive to implement the same method signatures and return values as their Python built-in and standard library counterparts. The main difference is each type’s __init__ method. Every HOT Redis type’s __init__ method will optionally accept initial and key keyword arguments, which are used for defining an initial value to be stored in Redis for the object, and the key that should be used, respectively. If no key is provided, a key will be generated, which can then be accessed via the key attribute:

>>> from hot_redis import List
>>> my_list = List()
>>> my_list.key
'93366bdb-90b2-4226-a52a-556f678af40e'
>>> my_list_with_key = List(key="foo")
>>> my_list_with_key.key
'foo'

Once you’ve determined a strategy for naming keys, you can then create HOT Redis objects and interact with them over the network, for example here is a List created on a computer we’ll refer to as computer A:

>>> list_on_computer_a = List(key="foo", initial=["a", "b", "c"])

then on another computer we’ll creatively refer to as computer B:

>>> list_on_computer_b = List(key="foo")
>>> list_on_computer_b[:]  # Performs: LRANGE foo 0 -1
['a', 'b', 'c']
>>> list_on_computer_b += ['d', 'e', 'f']  # Performs: RPUSH foo d e f

and back to computer A:

>>> list_on_computer_a[:]  # Performs: LRANGE foo 0 -1
['a', 'b', 'c', 'd', 'e', 'f']
>>> 'c' in list_on_computer_a  # Works like Python lists where expected
True
>>> list_on_computer_a.reverse()
>>> list_on_computer_a[:]
['f', 'e', 'd', 'c', 'b', 'a']

The last interaction here is an interesting one. Python’s list.reverse() is an in-place reversal of the list, that is, it modifies the existing list, rather than returning a reversed copy. If we were to implement this naively, we would first read the list from Redis, reverse it locally, then store the reversed list back in Redis again. But what if another client were to modify the list at approximately the same time? One computer’s modification to the list would certainly overwrite the other’s. In this scenario, and many others, HOT Redis provides its own Lua routine specifically for reversing the list in-place, within Redis atomically. I wrote in more detail about this in a blog post, Bitwise Lua Operations in Redis.

Configuration

By default, HOT Redis attempts to connect to a Redis instance running locally on the default port 6379. You can configure the default client by calling the hot_redis.configure function, prior to instantiating any HOT Redis objects. The arguments given to configure are passed onto the underlying redis-py client:

>>> from hot_redis import configure
configure(host='myremotehost', port=6380)

Alternatively, if you wish to use a different client per object, you can explicitly create a HotClient instance, and pass it to each object:

>>> from hot_redis import HotClient, Queue
>>> client = HotClient(host="myremotehost", port=6380)
>>> my_queue = Queue(client=client)

Transactions

Basic support for thread-safe transactions are provided using the Redis MULTI and EXEC commands:

>>> from hot_redis import List, Queue, transaction
>>> my_list = List(key="foo")
>>> my_queue = Queue(key="bar")
>>> with transaction():
...     for i in range(20):
...         my_list.append(i)
...         my_queue.put(i)

In the above example, all of the append and put calls are batched together into a single transaction, that is executed once the transaction() context is exited.

Data Types

The following table is the complete list of types provided by HOT Redis, mapped to their Python counterparts and underlying Redis types, along with any special considerations worth noting.

HOT Redis	Python	Redis	Notes
List	list	list
Set	set	set
Dict	dict	hash
String	string	string	Mutable - string methods that normally create a new string object in Python will mutate the string stored in Redis
ImmutableString	string	string	Immutable - behaves like a regular Python string
Int	int	int
Float	float	float
Queue	Queue.Queue	list
LifoQueue	Queue.LifoQueue	list
SetQueue	N/A	list + set	Extension of Queue with unique members
LifoSetQueue	N/A	list + set	Extension of LifoQueue with unique members
BoundedSemaphore	threading.BoundedSemaphore	list	Extension of Queue leveraging Redis’ blocking list pop operations with timeouts, while using Queue’s maxsize arg to provide BoundedSemaphore’s value arg
Semaphore	threading.Semaphore	list	Extension of BoundedSemaphore without a queue size
Lock	threading.Lock	list	Extension of BoundedSemaphore with a queue size of 1
RLock	threading.RLock	list	Extension of Lock allowing multiple acquire calls
DefaultDict	collections.DefaultDict	hash
MultiSet	collections.Counter	hash