Add Upstash Redis Adapter docs (nextauthjs#197)

docs/adapters/overview.md

@@ -20,7 +20,8 @@ We have a list of official adapters that are distributed as their own packages u
 - [`neo4j`](./neo4j)
 - [`typeorm-legacy`](./typeorm)
 - [`sequelize`](./sequelize)
-- [`dgraph`](./dgraph) 
+- [`dgraph`](./dgraph)
+- [`upstash-redis`](./upstash-redis)
 
 ## Custom Adapter
 

docs/adapters/upstash-redis.md

@@ -0,0 +1,69 @@
+---
+id: upstash-redis
+title: Upstash Redis
+---
+
+# Upstash Redis
+
+To use this Adapter, you need to install `@upstash/redis` and `@next-auth/upstash-redis-adapter` package:
+
+```bash npm2yarn
+npm install @upstash/redis @next-auth/upstash-redis-adapter
+```
+
+Configure your NextAuth.js to use the Upstash Redis Adapter:
+
+```javascript title="pages/api/auth/[...nextauth].js"
+import NextAuth from "next-auth"
+import GoogleProvider from "next-auth/providers/google"
+import { UpstashRedisAdapter } from "@next-auth/upstash-redis-adapter"
+import upstashRedisClient from "@upstash/redis"
+
+const redis = upstashRedisClient(
+  process.env.UPSTASH_REDIS_URL,
+  process.env.UPSTASH_REDIS_TOKEN
+)
+
+export default NextAuth({
+  adapter: UpstashRedisAdapter(redis),
+  providers: [
+    GoogleProvider({
+      clientId: process.env.GOOGLE_CLIENT_ID,
+      clientSecret: process.env.GOOGLE_CLIENT_SECRET,
+    }),
+  ],
+})
+```
+
+## Using Multiple Apps with a Single Upstash Redis Instance
+
+The Upstash free-tier allows for only one Redis instance. If you have multiple Next-Auth connected apps using this instance, you need different key prefixes for every app.
+
+You can change the prefixes by passing an `options` object as the second argument to the adapter factory function.
+
+The default values for this object are:
+
+```js
+const defaultOptions = {
+  baseKeyPrefix: "",
+  accountKeyPrefix: "user:account:",
+  accountByUserIdPrefix: "user:account:by-user-id:",
+  emailKeyPrefix: "user:email:",
+  sessionKeyPrefix: "user:session:",
+  sessionByUserIdKeyPrefix: "user:session:by-user-id:",
+  userKeyPrefix: "user:",
+  verificationTokenKeyPrefix: "user:token:",
+}
+```
+
+Usually changing the `baseKeyPrefix` should be enough for this scenario, but for more custom setups, you can also change the prefixes of every single key.
+
+Example:
+
+```js
+export default NextAuth({
+  ...
+  adapter: UpstashRedisAdapter(redis, {baseKeyPrefix: "app2:"})
+  ...
+})
+```