Pottery: Redis for Humans

Redis is awesome, but Redis commands are not always fun. Pottery is a Pythonic way to access Redis. If you know how to use Python dicts, then you already know how to use Pottery.

Installation

$ pip3 install pottery

Usage

First, set up your Redis client:

>>> from redis import Redis
>>> redis = Redis.from_url('redis://localhost:6379/')
>>>

Dicts

RedisDict is a Redis-backed container compatible with Python’s dict.

Here is a small example using a RedisDict:

>>> from pottery import RedisDict
>>> tel = RedisDict({'jack': 4098, 'sape': 4139}, redis=redis, key='tel')
>>> tel['guido'] = 4127
>>> tel
RedisDict{'jack': 4098, 'sape': 4139, 'guido': 4127}
>>> tel['jack']
4098
>>> del tel['sape']
>>> tel['irv'] = 4127
>>> tel
RedisDict{'jack': 4098, 'guido': 4127, 'irv': 4127}
>>> list(tel)
['jack', 'guido', 'irv']
>>> sorted(tel)
['guido', 'irv', 'jack']
>>> 'guido' in tel
True
>>> 'jack' not in tel
False
>>>

Notice the first two keyword arguments to RedisDict(): The first is your Redis client. The second is the Redis key name for your dict. Other than that, you can use your RedisDict the same way that you use any other Python dict.

Limitations:

1. Keys and values must be JSON serializable.

Sets

RedisSet is a Redis-backed container compatible with Python’s set.

Here is a brief demonstration:

>>> from pottery import RedisSet
>>> basket = RedisSet({'apple', 'orange', 'apple', 'pear', 'orange', 'banana'}, redis=redis, key='basket')
>>> sorted(basket)
['apple', 'banana', 'orange', 'pear']
>>> 'orange' in basket
True
>>> 'crabgrass' in basket
False

>>> a = RedisSet('abracadabra', redis=redis, key='magic')
>>> b = set('alacazam')
>>> sorted(a)
['a', 'b', 'c', 'd', 'r']
>>> sorted(a - b)
['b', 'd', 'r']
>>> sorted(a | b)
['a', 'b', 'c', 'd', 'l', 'm', 'r', 'z']
>>> sorted(a & b)
['a', 'c']
>>> sorted(a ^ b)
['b', 'd', 'l', 'm', 'r', 'z']
>>>

Notice the two keyword arguments to RedisSet(): The first is your Redis client. The second is the Redis key name for your set. Other than that, you can use your RedisSet the same way that you use any other Python set.

Limitations:

1. Elements must be JSON serializable.

Lists

RedisList is a Redis-backed container compatible with Python’s list.

>>> from pottery import RedisList
>>> squares = RedisList([1, 4, 9, 16, 25], redis=redis, key='squares')
>>> squares
RedisList[1, 4, 9, 16, 25]
>>> squares[0]
1
>>> squares[-1]
25
>>> squares[-3:]
[9, 16, 25]
>>> squares[:]
[1, 4, 9, 16, 25]
>>> squares + [36, 49, 64, 81, 100]
RedisList[1, 4, 9, 16, 25, 36, 49, 64, 81, 100]
>>>

Notice the two keyword arguments to RedisList(): The first is your Redis client. The second is the Redis key name for your list. Other than that, you can use your RedisList the same way that you use any other Python list.

Limitations:

1. Elements must be JSON serializable.

Counters

RedisCounter is a Redis-backed container compatible with Python’s collections.Counter.

>>> from pottery import RedisCounter
>>> c = RedisCounter(redis=redis, key='my-counter')
>>> c = RedisCounter('gallahad', redis=redis, key='my-counter')
>>> c.clear()
>>> c = RedisCounter({'red': 4, 'blue': 2}, redis=redis, key='my-counter')
>>> c.clear()
>>> c = RedisCounter(redis=redis, key='my-counter', cats=4, dogs=8)
>>> c.clear()

>>> c = RedisCounter(['eggs', 'ham'], redis=redis, key='my-counter')
>>> c['bacon']
0
>>> c['sausage'] = 0
>>> del c['sausage']
>>> c.clear()

>>> c = RedisCounter(redis=redis, key='my-counter', a=4, b=2, c=0, d=-2)
>>> sorted(c.elements())
['a', 'a', 'a', 'a', 'b', 'b']
>>> c.clear()

>>> RedisCounter('abracadabra', redis=redis, key='my-counter').most_common(3)
[('a', 5), ('b', 2), ('r', 2)]
>>> c.clear()

>>> c = RedisCounter(redis=redis, key='my-counter', a=4, b=2, c=0, d=-2)
>>> from collections import Counter
>>> d = Counter(a=1, b=2, c=3, d=4)
>>> c.subtract(d)
>>> c
RedisCounter{'a': 3, 'b': 0, 'c': -3, 'd': -6}
>>>

Notice the first two keyword arguments to RedisCounter(): The first is your Redis client. The second is the Redis key name for your counter. Other than that, you can use your RedisCounter the same way that you use any other Python Counter.

Limitations:

1. Keys must be JSON serializable.

Deques

RedisDeque is a Redis-backed container compatible with Python’s collections.deque.

Example:

>>> from pottery import RedisDeque
>>> d = RedisDeque('ghi', redis=redis, key='letters')
>>> for elem in d:
...     print(elem.upper())
G
H
I

>>> d.append('j')
>>> d.appendleft('f')
>>> d
RedisDeque(['f', 'g', 'h', 'i', 'j'])

>>> d.pop()
'j'
>>> d.popleft()
'f'
>>> list(d)
['g', 'h', 'i']
>>> d[0]
'g'
>>> d[-1]
'i'

>>> list(reversed(d))
['i', 'h', 'g']
>>> 'h' in d
True
>>> d.extend('jkl')
>>> d
RedisDeque(['g', 'h', 'i', 'j', 'k', 'l'])
>>> d.rotate(1)
>>> d
RedisDeque(['l', 'g', 'h', 'i', 'j', 'k'])
>>> d.rotate(-1)
>>> d
RedisDeque(['g', 'h', 'i', 'j', 'k', 'l'])

>>> RedisDeque(reversed(d), redis=redis)
RedisDeque(['l', 'k', 'j', 'i', 'h', 'g'])
>>> d.clear()

>>> d.extendleft('abc')
>>> d
RedisDeque(['c', 'b', 'a'])
>>>

Notice the two keyword arguments to RedisDeque(): The first is your Redis client. The second is the Redis key name for your deque. Other than that, you can use your RedisDeque the same way that you use any other Python deque.

Limitations:

1. Elements must be JSON serializable.

Redlock

Redlock is a safe and reliable lock to coordinate access to a resource shared across threads, processes, and even machines, without a single point of failure. Rationale and algorithm description.

Redlock implements Python’s excellent threading.Lock API as closely as is feasible. In other words, you can use Redlock the same way that you use threading.Lock.

Instantiate a Redlock:

>>> from pottery import Redlock
>>> lock = Redlock(key='printer', masters={redis})
>>>

The key argument represents the resource, and the masters argument specifies your Redis masters across which to distribute the lock (in production, you should have 5 Redis masters). Now you can protect access to your resource:

>>> lock.acquire()
True
>>> # Critical section - print stuff here.
>>> lock.release()
>>>

Or you can protect access to your resource inside a context manager:

>>> with lock:
...     # Critical section - print stuff here.
...     pass
>>>

Redlocks time out (by default, after 10 seconds). You should take care to ensure that your critical section completes well within the timeout. The reasons that Redlocks time out are to preserve “liveness” and to avoid deadlocks (in the event that a process dies inside a critical section before it releases its lock).

>>> import time
>>> lock.acquire()
True
>>> bool(lock.locked())
True
>>> # Critical section - print stuff here.
>>> time.sleep(10)
>>> bool(lock.locked())
False
>>>

If 10 seconds isn’t enough to complete executing your critical section, then you can specify your own timeout:

>>> lock = Redlock(key='printer', auto_release_time=15*1000)
>>> lock.acquire()
True
>>> bool(lock.locked())
True
>>> # Critical section - print stuff here.
>>> time.sleep(10)
>>> bool(lock.locked())
True
>>> time.sleep(5)
>>> bool(lock.locked())
False
>>>

NextId

NextId safely and reliably produces increasing IDs across threads, processes, and even machines, without a single point of failure. Rationale and algorithm description.

Instantiate an ID generator:

>>> from pottery import NextId
>>> user_ids = NextId(key='user-ids', masters={redis})
>>>

The key argument represents the sequence (so that you can have different sequences for user IDs, comment IDs, etc.), and the masters argument specifies your Redis masters across which to distribute ID generation (in production, you should have 5 Redis masters). Now, whenever you need a user ID, call next() on the ID generator:

>>> next(user_ids)
1
>>> next(user_ids)
2
>>> next(user_ids)
3
>>>

Two caveats:

1. If many clients are generating IDs concurrently, then there may be “holes” in the sequence of IDs (e.g.: 1, 2, 6, 10, 11, 21, …).
2. This algorithm scales to about 5,000 IDs per second (with 5 Redis masters). If you need IDs faster than that, then you may want to consider other techniques.

redis_cache()

redis_cache() is a simple function return value cache, sometimes called “memoize”. redis_cache() implements Python’s excellent functools.lru_cache() API as closely as is feasible. In other words, you can use redis_cache() the same way that you use functools.lru_cache().

Limitations:

1. Arguments to the function must be hashable.
2. Return values from the function must be JSON serializable.

In general, you should only use redis_cache() when you want to reuse previously computed values. Accordingly, it doesn’t make sense to cache functions with side-effects or impure functions such as time() or random().

Decorate a function:

>>> import time
>>> from pottery import redis_cache
>>> @redis_cache(redis=redis, key='expensive-function-cache')
... def expensive_function(n):
...     time.sleep(1)  # Simulate an expensive computation or database lookup.
...     return n
...
>>>

Notice the two keyword arguments to redis_cache(): The first is your Redis client. The second is the Redis key name for your function’s return value cache.

Call your function and observe the cache hit/miss rates:

>>> expensive_function(5)
5
>>> expensive_function.cache_info()
CacheInfo(hits=0, misses=1, maxsize=None, currsize=1)
>>> expensive_function(5)
5
>>> expensive_function.cache_info()
CacheInfo(hits=1, misses=1, maxsize=None, currsize=1)
>>> expensive_function(6)
6
>>> expensive_function.cache_info()
CacheInfo(hits=1, misses=2, maxsize=None, currsize=2)
>>>

Notice that the first call to expensive_function() takes 1 second and results in a cache miss; but the second call returns almost immediately and results in a cache hit. This is because after the first call, redis_cache() cached the return value for the call when n == 5.

You can access your original undecorated underlying expensive_function() as expensive_function.__wrapped__. This is useful for introspection, for bypassing the cache, or for rewrapping the original function with a different cache.

You can force a cache reset for a particular combination of args/kwargs with expensive_function.__bypass__. A call to expensive_function.__bypass__(*args, **kwargs) bypasses the cache lookup, calls the original underlying function, then caches the results for future calls to expensive_function(*args, **kwargs). Note that a call to expensive_function.__bypass__(*args, **kwargs) results in neither a cache hit nor a cache miss.

Finally, clear/invalidate your function’s entire return value cache with expensive_function.cache_clear():

>>> expensive_function.cache_info()
CacheInfo(hits=1, misses=2, maxsize=None, currsize=2)
>>> expensive_function.cache_clear()
>>> expensive_function.cache_info()
CacheInfo(hits=0, misses=0, maxsize=None, currsize=0)
>>>

CachedOrderedDict

The best way that I can explain CachedOrderedDict is through an example use-case. Imagine that your search engine returns document IDs, which then you have to hydrate into full documents via the database to return to the client. The data structure used to represent such search results must have the following properties:

1. It must preserve the order of the document IDs returned by the search engine.
2. It must map document IDs to hydrated documents.
3. It must cache previously hydrated documents.

Properties 1 and 2 are satisfied by Python’s collections.OrderedDict. However, CachedOrderedDict extends Python’s OrderedDict to also satisfy property 3.

The most common usage pattern for CachedOrderedDict is as follows:

1. Instantiate CachedOrderedDict with the IDs that you must look up or compute passed in as the dict_keys argument to the initializer.
2. Compute and store the cache misses for future lookups.
3. Return some representation of your CachedOrderedDict to the client.

Instantiate a CachedOrderedDict:

>>> from pottery import CachedOrderedDict
>>> search_results_1 = CachedOrderedDict(
...     redis_client=redis,
...     redis_key='search-results',
...     dict_keys=(1, 2, 3, 4, 5),
... )
>>>

The redis_client argument to the initializer is your Redis client, and the redis_key argument is the Redis key for the Redis Hash backing your cache. The dict_keys argument represents an ordered iterable of keys to be looked up and automatically populated in your CachedOrderedDict (on cache hits), or that you’ll have to compute and populate for future lookups (on cache misses). Regardless of whether keys are cache hits or misses, CachedOrderedDict preserves the order of dict_keys (like a list), maps those keys to values (like a dict), and maintains an underlying cache for future key lookups.

In the beginning, the cache is empty, so let’s populate it:

>>> sorted(search_results_1.misses())
[1, 2, 3, 4, 5]
>>> search_results_1[1] = 'one'
>>> search_results_1[2] = 'two'
>>> search_results_1[3] = 'three'
>>> search_results_1[4] = 'four'
>>> search_results_1[5] = 'five'
>>> sorted(search_results_1.misses())
[]
>>>

Note that CachedOrderedDict preserves the order of dict_keys:

>>> for key, value in search_results_1.items():
...     print(f'{key}: {value}')
1: one
2: two
3: three
4: four
5: five
>>>

Now, let’s look at a combination of cache hits and misses:

>>> search_results_2 = CachedOrderedDict(
...     redis_client=redis,
...     redis_key='search-results',
...     dict_keys=(2, 4, 6, 8, 10),
... )
>>> sorted(search_results_2.misses())
[6, 8, 10]
>>> search_results_2[2]
'two'
>>> search_results_2[6] = 'six'
>>> search_results_2[8] = 'eight'
>>> search_results_2[10] = 'ten'
>>> sorted(search_results_2.misses())
[]
>>> for key, value in search_results_2.items():
...     print(f'{key}: {value}')
2: two
4: four
6: six
8: eight
10: ten
>>>

Limitations:

1. Keys and values must be JSON serializable.

Bloom filters

Bloom filters are a powerful data structure that help you to answer the questions, “Have I seen this element before?” and “How many distinct elements have I seen?”; but not the question, “What are all of the elements that I’ve seen before?” So think of Bloom filters as Python sets that you can add elements to, use to test element membership, and get the length of; but that you can’t iterate through or get elements back out of.

Bloom filters are probabilistic, which means that they can sometimes generate false positives (as in, they may report that you’ve seen a particular element before even though you haven’t). But they will never generate false negatives (so every time that they report that you haven’t seen a particular element before, you really must never have seen it). You can tune your acceptable false positive probability, though at the expense of the storage size and the element insertion/lookup time of your Bloom filter.

Create a BloomFilter:

>>> from pottery import BloomFilter
>>> dilberts = BloomFilter(
...     num_elements=100,
...     false_positives=0.01,
...     redis=redis,
...     key='dilberts',
... )
>>>

Here, num_elements represents the number of elements that you expect to insert into your BloomFilter, and false_positives represents your acceptable false positive probability. Using these two parameters, BloomFilter automatically computes its own storage size and number of times to run its hash functions on element insertion/lookup such that it can guarantee a false positive rate at or below what you can tolerate, given that you’re going to insert your specified number of elements.

Insert an element into the BloomFilter:

>>> dilberts.add('rajiv')
>>>

Test for membership in the BloomFilter:

>>> 'rajiv' in dilberts
True
>>> 'raj' in dilberts
False
>>> 'dan' in dilberts
False
>>>

See how many elements we’ve inserted into the BloomFilter:

>>> len(dilberts)
1
>>>

Note that BloomFilter.__len__() is an approximation, not an exact value, though it’s quite accurate.

Insert multiple elements into the BloomFilter:

>>> dilberts.update({'raj', 'dan'})
>>>

Remove all of the elements from the BloomFilter:

>>> dilberts.clear()
>>> len(dilberts)
0
>>>

Limitations:

1. Elements must be JSON serializable.

HyperLogLogs

HyperLogLogs are an interesting data structure that allow you to answer the question, “How many distinct elements have I seen?”; but not the questions, “Have I seen this element before?” or “What are all of the elements that I’ve seen before?” So think of HyperLogLogs as Python sets that you can add elements to and get the length of; but that you can’t use to test element membership, iterate through, or get elements back out of.

HyperLogLogs are probabilistic, which means that they’re accurate within a margin of error up to 2%. However, they can reasonably accurately estimate the cardinality (size) of vast datasets (like the number of unique Google searches issued in a day) with a tiny amount of storage (1.5 KB).

Create a HyperLogLog:

>>> from pottery import HyperLogLog
>>> google_searches = HyperLogLog(redis=redis, key='google-searches')
>>>

Insert an element into the HyperLogLog:

>>> google_searches.add('sonic the hedgehog video game')
>>>

See how many elements we’ve inserted into the HyperLogLog:

>>> len(google_searches)
1
>>>

Insert multiple elements into the HyperLogLog:

>>> google_searches.update({
...     'google in 1998',
...     'minesweeper',
...     'joey tribbiani',
...     'wizard of oz',
...     'rgb to hex',
...     'pac-man',
...     'breathing exercise',
...     'do a barrel roll',
...     'snake',
... })
>>> len(google_searches)
10
>>>

Remove all of the elements from the HyperLogLog:

>>> google_searches.clear()
>>> len(google_searches)
0
>>>

Limitations:

1. Elements must be JSON serializable.

ContextTimer

ContextTimer helps you easily and accurately measure elapsed time. Note that ContextTimer measures wall (real-world) time, not CPU time; and that elapsed() returns time in milliseconds.

You can use ContextTimer stand-alone…

>>> import time
>>> from pottery import ContextTimer
>>> timer = ContextTimer()
>>> timer.start()
>>> time.sleep(0.1)
>>> 100 <= timer.elapsed() < 200
True
>>> timer.stop()
>>> time.sleep(0.1)
>>> 100 <= timer.elapsed() < 200
True
>>>

…or as a context manager:

>>> tests = []
>>> with ContextTimer() as timer:
...     time.sleep(0.1)
...     tests.append(100 <= timer.elapsed() < 200)
>>> time.sleep(0.1)
>>> tests.append(100 <= timer.elapsed() < 200)
>>> tests
[True, True]
>>>

Contributing

Install prerequisites

1. Install Xcode.

Obtain source code

1. Clone the git repo:
2. $ git clone git@github.com:brainix/pottery.git
3. $ cd pottery/
4. Install project-level dependencies:
5. $ make install

Run tests

1. In one Terminal session:
2. $ cd pottery/
3. $ redis-server
4. In a second Terminal session:
5. $ cd pottery/
6. $ make test

make test runs all of the unit tests as well as the coverage test. However, sometimes, when debugging, it can be useful to run an individual test module, class, or method:

1. In one Terminal session:
2. $ cd pottery/
3. $ redis-server
4. In a second Terminal session:
5. Run a test module with $ make test tests=tests.test_dict
6. Run a test class with: $ make test tests=tests.test_dict.DictTests
7. Run a test method with: $ make test tests=tests.test_dict.DictTests.test_keyexistserror