Explain server vs database commands; recurring FAQ

Docs/Docs.csproj

@@ -45,6 +45,7 @@
       <Link>README.md</Link>
     </None>
     <None Include="Basics.md" />
+    <None Include="KeysScan.md" />
     <None Include="Events.md" />
     <None Include="Configuration.md" />
     <None Include="ExecSync.md" />

Docs/KeysScan.md

@@ -0,0 +1,53 @@
+Where are `KEYS`, `SCAN`, `FLUSHDB`, `FLUSHALL` ?
+===
+
+Some very common recurring questions are:
+
+> There doesn't seem to be a `Keys(...)` or `Scan(...)` method? How can I query which keys exist in the database?
+
+or
+
+> There doesn't seem to be a `Flush(...)` method? How can I remove all the keys in the database?
+
+The key word here, oddly enough, is the last one: database. Because StackExchange.Redis aims to target scenarios such as cluster, it is important to know which commands target the *database* (the logical database that could be distributed over multiple nodes), and which commands target the *server*. These commands:
+
+- `KEYS` / `SCAN`
+- `FLUSHDB` / `FLUSHALL`
+- `RANDOMKEY`
+- `CLIENT *`
+- `CLUSTER *`
+- `CONFIG` / `INFO` / `TIME`
+- `SLAVEOF`
+- `SAVE` / `BGSAVE` / `LASTSAVE`
+- `SCRIPT` (not to be confused with `EVAL` / `EVALSHA`)
+- `SHUTDOWN`
+- `SLOWLOG`
+- `PUBSUB *` (not to be confused with `PUBLISH` / `SUBSCRIBE` / etc)
+
+(I've probably missed at least one) Most of these will seem pretty obvious, but the first 3 rows are not so obvious:
+
+- `KEYS` / `SCAN` only list keys that are on the current server; not the wider logical database
+- `FLUSHDB` / `FLUSHALL` only remove keys that are on the current server; not the wider logical database
+- `RANDOMKEY` only selects a key that is on the current server; not the wider logical database
+
+Actually, StackExchange.Redis spoofs the `RANDOMKEY` one on the `IDatabase` API by simply selecting a target server at random, but this is not possible for the others.
+
+So how do I use them?
+===
+
+Simple: start from a server, not a database.
+
+    var server = conn.GetServer(someServer);
+    foreach(var key in server.Keys(pattern: "*foo*")) {
+        Console.WriteLine(key);
+    }
+    server.FlushDatabase();
+
+Note that unlike the `IDatabase` API (where the target database has already been selected in the `GetDatabase()` call), these methods take an optional parameter for the database, or it defaults to `0`.
+
+The `Keys(...)` method deserves special mention: it is unusual in that it does not have an `*Async` counterpart. The reason for this is that behind the scenes, the system will determine the most appropriate method to use (`KEYS` vs `SCAN`, based on the server version), and if possible will use the `SCAN` approach to hand you back an `IEnumerable<RedisKey>` that does all the paging internally - so you never need to see the implementation details of the cursor operations. If `SCAN` is not available, it will use `KEYS`, which can cause blockages at the server. Either way, both `SCAN` and `KEYS` will need to sweep the entire keyspace, so should be avoided on production servers - or at least, targeted at slaves.
+
+So I need to remember which server I connected to? That sucks!
+===
+
+No, not quite. You can use `conn.GetEndPoints()` to list the endpoints (either all known, or the ones specified in the original configuration - these are not necessarily the same thing), and iterate with `GetServer()` to find the server you want (for example, selecting a slave).

README.md

@@ -33,6 +33,7 @@ Documentation
 - [Transactions](https://github.com/StackExchange/StackExchange.Redis/blob/master/Docs/Transactions.md) - how atomic transactions work in redis
 - [Events](https://github.com/StackExchange/StackExchange.Redis/blob/master/Docs/Events.md) - the events available for logging / information purposes
 - [Pub/Sub Message Order](https://github.com/StackExchange/StackExchange.Redis/blob/master/Docs/PubSubOrder.md) - advice on sequential and concurrent processing
+- [Where are `KEYS` / `SCAN` / `FLUSH*`?](https://github.com/StackExchange/StackExchange.Redis/blob/master/Docs/KeysScan.md) - how to use server-based commands
 
 Questions and Contributions
 ---