docs: RFC for keyspace

docs/design/2022-12-06-Keyspace.md

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+# Proposal: Keyspace
+* Authors: [ystaticy](https://github.com/ystaticy), [iosmanthus](https://github.com/iosmanthus), [AmoebaProtozoa](https://github.com/AmoebaProtozoa)
+
+
+## Motivation
+We hope to store data of multiple applications in a TiKV cluster.
+For example in multi-tenant scenario.A common solution is to add a prefix in front of the key to distinguish the data of multiple applications.
+So we introduce a new concept 'Keyspace' to describe the logical of the isolation with different business scenarios in a TiKV cluster.
+
+
+## Keyspace key prefix
+Keyspace can only be used when TiKV enabled api v2.
+### Key encoding:
+We use different key prefix to distinguish different Keyspace.
+[Keyspace key encoding](https://github.com/tikv/rfcs/blob/master/text/0069-api-v2.md#key-encoding)
+
+'x', 'r' are key mode prefixes that indicates which mode the key is belonging to.
+After mode prefix is 3 bytes for keyspace.
+1. `x`: It represent TxnKV key.
+2. `r`: It represent RawKV key.
+
+So when TiDB enabled Keyspace, there are 4 bytes in key more than the scenario which does not use Keyspace.
+It will start with 'x' + keyspace Id (3 bytes) ,because TiDB uses TxnKV client.
+
+### Keyspace ID
+Behind key mode, it take 3 bytes to store `keyspaceID`, the default keyspaceID is `[0, 0, 0]`.
+The max keyspace id is 16777216; Keyspace ID is allocated by PD createKeyspace interface.
+
+## Architecture
+![group and expression](./imgs/keyspace-arch.png)
+### PD
+PD is responsible for Keyspace Meta managment.
+It provide http interface and RPC interface to create, load, change state or configuration Keyspace Meta.
+Keyspace Meta store in etcd data in PD.
+
+### Client-go
+Compare with API V1. API V2 add 1 byte Keymode and 3 bytes keyspace Id in front of userkey.
+
+### TiDB
+A tenant can have multiple TiDB nodes. But a TiDB node can only serve one tenant.
+It can enabled by tidb configuration by `keyspace-name`.
+
+### TiKV
+TiKV should enable API V2, and different keyspace will distinguish by different key prefix.
+
+## Implementation
+
+### PD
+To be supplemented in another PR
+
+### TiDB
+#### Using API V2 and etcd namespace to distinguish TiKV data and etcd path by Keyspace.
+1. Configure Keyspace name by setting `keyspace-name` in the configuration file.
+   * When TiDB server start, it will open a TiKV driver. The driver have a TxnKV client object `tikv.KVStore`. If `keyspace-name` has been set, it will start to use `tikv.KVStore` by API V2 to access TiKV.
+2. The etcd path should has different prefix by etcd namespace.
+   * There is a etcd client be created when domain init. If `keyspace-name` has been set, it will add a etcd namespace when create etcd client.
+   * The format of etcd namespace is `/keyspaces/tidb/$keyspaceId`.
+
+#### TiDB BR support Keyspace
+To be supplemented in another PR
+
+### TiKV
+#### client-go
+To be supplemented in another PR
+#### Coprocessor
+To be supplemented in another PR
+
+
+