Add kaya-session-redis package for Redis-backed session storage

packages/kaya-session-redis/README.md

@@ -0,0 +1,50 @@
+# kaya-session-redis
+
+Redis-backed session storage for the Kaya web framework.
+
+Provides `RedisSessionStore`, a `SessionStore` implementation (from
+`kaya-session`) that persists session data in Redis, so sessions are shared
+across processes and hosts.
+
+## Usage
+
+```python
+from redis.asyncio import Redis
+
+from kaya.core import KayaApp, HttpContext
+from kaya.session import SessionMixin
+from kaya.session_redis import RedisSessionStore
+
+client = Redis(host='localhost', port=6379)
+session = SessionMixin(RedisSessionStore(client))
+app = KayaApp(mixins=[session])
+
+@app.GET('/')
+async def home(ctx: HttpContext):
+    n = ctx.session.get('visits', 0) + 1
+    ctx.session['visits'] = n
+    await ctx.send_str(200, f'visits: {n}')
+```
+
+Sessions are stored under keys with the prefix `kaya:session:` (configurable
+via the `prefix` argument). Server-side expiry uses Redis key TTLs and slides
+on each access when the session mixin passes a `max_age`.
+
+## Serialization
+
+Session data is serialized with `pickle` by default, so arbitrary Python
+objects can be stored. A different serializer can be plugged in via the
+`dumps`/`loads` arguments:
+
+```python
+import json
+
+store = RedisSessionStore(
+    client,
+    dumps=lambda d: json.dumps(d).encode('utf-8'),
+    loads=lambda b: json.loads(b.decode('utf-8')),
+)
+```
+
+**Warning:** pickle deserialization of untrusted data is unsafe. Only use the
+default serializer with a trusted Redis server.