apollo13/asgi_redis

Redis-py backend for ASGI, with built-in sharding

asgi_redis

An ASGI channel layer that uses Redis as its backing store, and supports both a single-server and sharded configurations, as well as group support.

Usage

You'll need to instantiate the channel layer with at least hosts, and other options if you need them.

Example:

channel_layer = RedisChannelLayer(
    host="redis",
    db=4,
    channel_capacity={
        "http.request": 200,
        "http.response*": 10,
    }
)

hosts

The server(s) to connect to, as either URIs or (host, port) tuples. Defaults to ['localhost', 6379]. Pass multiple hosts to enable sharding, but note that changing the host list will lose some sharded data.

prefix

Prefix to add to all Redis keys. Defaults to asgi:. If you're running two or more entirely separate channel layers through the same Redis instance, make sure they have different prefixes. All servers talking to the same layer should have the same prefix, though.

expiry

Message expiry in seconds. Defaults to 60. You generally shouldn't need to change this, but you may want to turn it down if you have peaky traffic you wish to drop, or up if you have peaky traffic you want to backlog until you get to it.

group_expiry

Group expiry in seconds. Defaults to 86400. Interface servers will drop connections after this amount of time; it's recommended you reduce it for a healthier system that encourages disconnections.

capacity

Default channel capacity. Defaults to 100. Once a channel is at capacity, it will refuse more messages. How this affects different parts of the system varies; a HTTP server will refuse connections, for example, while Django sending a response will just wait until there's space.

channel_capacity

Per-channel capacity configuration. This lets you tweak the channel capacity based on the channel name, and supports both globbing and regular expressions.

It should be a dict mapping channel name pattern to desired capacity; if the dict key is a string, it's intepreted as a glob, while if it's a compiled re object, it's treated as a regular expression.

This example sets http.request to 200, all http.response! channels to 10, and all websocket.send! channels to 20:

channel_capacity={
    "http.request": 200,
    "http.response!*": 10,
    re.compile(r"^websocket.send\!.+"): 20,
}

If you want to enforce a matching order, use an OrderedDict as the argument; channels will then be matched in the order the dict provides them.

symmetric_encryption_keys

Pass this to enable the optional symmetric encryption mode of the backend. To use it, make sure you have the cryptography package installed, or specify the cryptography extra when you install asgi_redis:

pip install asgi_redis[cryptography]

symmetric_encryption_keys should be a list of strings, with each string being an encryption key. The first key is always used for encryption; all are considered for decryption, so you can rotate keys without downtime - just add a new key at the start and move the old one down, then remove the old one after the message expiry time has passed.

Data is encrypted both on the wire and at rest in Redis, though we advise you also route your Redis connections over TLS for higher security; the Redis protocol is still unencrypted, and the channel and group key names could potentially contain metadata patterns of use to attackers.

Keys should have at least 32 bytes of entropy - they are passed through the SHA256 hash function before being used as an encryption key. Any string will work, but the shorter the string, the easier the encryption is to break.

If you're using Django, you may also wish to set this to your site's SECRET_KEY setting via the CHANNEL_LAYERS setting.

TODO

• Efficient send_group implementation with Lua