用django cache设置的redis key的名字前加的序号有什么意义？

Question

使用django自带的模块处理redis缓存时，设置好的key比如说时“banners”， 到了在redis的cli下看到name就变成了“:1:banners”，这是django的机制，还是redis的呢

from django.core.cache import cache

def get_queryset(self):
        key = "banners"
        queryset = cache.get(key)
        if not queryset:
            queryset = Banner.objects.filter(valid=True)
            cache.set(key, queryset, 3600)
        return queryset

Answer 1

这个是django cache的机制, key由前缀，版本号，真正的key组成。django在升级或者代码重构的时候有用，可以判断key是那个版本号，从而进行兼容（其它欢迎补充）

1.首先看django redis client api的实现:

# https://github.com/niwinz/django-redis/blob/master/django_redis/client/default.py
def get(self, key, default=None, version=None, client=None):
    """
    Retrieve a value from the cache.
    Returns decoded value if key is found, the default if not.
    """
    if client is None:
        client = self.get_client(write=False)

    key = self.make_key(key, version=version)

    try:
        value = client.get(key)
    except _main_exceptions as e:
        raise ConnectionInterrupted(connection=client, parent=e)

    if value is None:
        return default

    return self.decode(value)

... 中间省略代码 ...


def make_key(self, key, version=None, prefix=None):
    if isinstance(key, CacheKey):
        return key

    if prefix is None:
        prefix = self._backend.key_prefix

    if version is None:
        version = self._backend.version

    return CacheKey(self._backend.key_func(key, prefix, version))
  

它使用了make_key这个函数制作新的key,至于真正的key是从_backend.key_func来的。

2.其次查看django cache backend实现

# https://github.com/django/django/blob/master/django/core/cache/backends/base.py
def default_key_func(key, key_prefix, version):
    """
    Default function to generate keys.

    Constructs the key used by all other methods. By default it prepends
    the `key_prefix'. KEY_FUNCTION can be used to specify an alternate
    function with custom key making behavior.
    """
    return '%s:%s:%s' % (key_prefix, version, key)


def get_key_func(key_func):
    """
    Function to decide which key function to use.

    Defaults to ``default_key_func``.
    """
    if key_func is not None:
        if callable(key_func):
            return key_func
        else:
            return import_string(key_func)
    return default_key_func
    
class BaseCache(object):
    def __init__(self, params):
    
        ... 中间省略代码 ...
        
        self.key_prefix = params.get('KEY_PREFIX', '')
        self.version = params.get('VERSION', 1)
        self.key_func = get_key_func(params.get('KEY_FUNCTION'))
    

由此可见，django redis cache的key是有前缀，版本号，真正的key组成。

Answer 2

你自己在配置redis会选择redis的库，redis本身就分为几个库，看你配置是哪一个