HermesCache 0.3.0

Usage

The following demonstrates all end-user API.

import hermes.backend.redis

cache = hermes.Hermes(hermes.backend.redis.Backend, ttl = 600, host = 'localhost', db = 1)


@cache
def foo(a, b):
  return a * b

class Example:

  @cache(tags = ('math', 'power'), ttl = 1200)
  def bar(self, a, b):
    return a ** b

  @cache(tags = ('math', 'avg'), key = lambda fn, *args, **kwargs: 'avg:{0}:{1}'.format(*args))
  def baz(self, a, b):
    return (a + b) / 2.0


print foo(2, 333)

example = Example()
print example.bar(2, 10)
print example.baz(2, 10)

foo.invalidate(2, 333)
example.bar.invalidate(2, 10)
example.baz.invalidate(2, 10)

cache.clean(['math']) # invalidate entries tagged 'math'
cache.clean()         # flush cache

For advanced examples look in test suite.

Tagging cache entries

First let’s look how basic caching works.

import hermes.backend.dict

cache = hermes.Hermes(hermes.backend.dict.Backend)

@cache
def foo(a, b):
  return a * b

foo(2, 2)
foo(2, 4)

print cache.backend.dump()
#  {
#    'cache:entry:foo:515d5cb1a98de31d': 8,
#    'cache:entry:foo:a1c97600eac6febb': 4
#                            ↓
#                      argument hash
#  }

Basically we have a key-value storage with O(1) complexity for set, get and delete. This means that the speed of operation is constant and irrelevant of number of items already stored. When a callable (function or method) is cached, the key is calculated per invocation from callable itself and passed arguments. Callable’s return value is saved to the key. Next invocation we can use the value from cache.

“There are only two hard problems in Computer Science: cache invalidation and naming things.” — Phil Karlton

So it comes in a complex application. There’s a case that certain group of methods operate the same data and it’s impractical to invalidate individual entries. In particular, it often happens when method returns complex values, spanning multiple entities. Cache tagging makes it possible to mark this group of method results with a tag and invalidate them all at once.

Here’s article by Eric Florenzano which explains the idea. Let’s look the code.

import hermes.backend.dict

cache = hermes.Hermes(hermes.backend.dict.Backend)

@cache(tags = ('tag1', 'tag2'))
def foo(a, b):
  return a * b

foo(2, 2)

print cache.backend.dump()
#  {
#    u'cache:tag:tag1': '0674536f9eb4eb19',
#    u'cache:tag:tag2': 'db22b5ab2e504895',
#    'cache:entry:foo:a1c97600eac6febb:c1da510b3d42bad6': 4
#                                              ↓
#                                           tag hash
#  }

When we want to tag a cache entry, first we need to create the tag entries. Each tag is represented by its own entry. Value of tag entry is set to random value each time tag is created. Once all tags values exist, they are joined and hashed. Tag hash is added to cache entry key.

Once we want to invalidate tagged entries we just need to remove the tag entry. Without any of tag values tag hash was created with, it is impossible to construct the entry key so the tagged cache entries become inaccessible thus invalidated. As usually a feature built on-top of another feature adds complexity.

Speed. All operations become O(n) where n is number of entry tags. However since we can rarely need more than a few dozens of tags, practically it is still O(1). Tag entry operations are batched so the implications on number of network operations go as follow:

• set – 3x backend calls (get + 2 * set) in worst case. Average is expected to be 2x when all used tag entries are created.

• get – 2x backend calls.

• delete – 2x backend calls.

Memory overhead consists of tag entries and stale cache entries. Demonstrated below.

import hermes.backend.dict

cache = hermes.Hermes(hermes.backend.dict.Backend)

@cache(tags = ('tag1', 'tag2'))
def foo(a, b):
  return a * b

foo(2, 2)

print cache.backend.dump()
#  {
#    u'cache:tag:tag1': '047820ac777abe8a',
#    u'cache:tag:tag2': '126365ec7175e851',
#    'cache:entry:foo:a1c97600eac6febb:5cae80f5e7d58329': 4
#  }

cache.clean(['tag1'])
foo(2, 2)

print cache.backend.dump()
#  {
#    u'cache:tag:tag1': '66336fec212def16',  ← recreated tag entry
#    u'cache:tag:tag2': '126365ec7175e851',
#    'cache:entry:foo:a1c97600eac6febb:8e7e24cf70c1f0ab': 4,
#    'cache:entry:foo:a1c97600eac6febb:5cae80f5e7d58329': 4  ← garbage
#  }

So the TTLs should be chosen elaborately. With Redis backend it’s also recommended to set maxmemory-policy to volatile-lru.

Reviewed implementations

Before I wrote the library I looked through the Cheese Shop for one that fits my needs. Unfortunately there was none, however some matched partially and were the inspiration in certain aspects:

• cache

  Pro:

  • clean end-user API

  • straightforward design

  Con:

  • no auto cache key calculation

  • no dogpile effect prevention

  • no cache entry tagging

  • fail with instance methods

• dogpile.cache

  Pro:

  • mature

  • very well documented

  • prevents dogpile effect

  Con:

  • no cache entry tagging

  • complicated code-base

  • not concise end-user API

• cache-tagging

  Pro:

  • cache entry tagging

  Con:

  • designed for the news website scaffolding framework

  • thus bloat is all around