dynamic configuration change proposal

Signed-off-by: Ryan Leung <rleungx@gmail.com>

docs/design/2019-11-21-dynamic-configuration-change.md

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+# Proposal: Dynamic Configuration Change
+
+- Author(s):     [Yutong Liang](https://github.com/rleungx)
+- Last updated:  2019-11-21
+- Discussion at:
+
+## Abstract
+
+This proposal proposes a unified way to manage the configuration options of TiDB
+, TiKV and PD by storing them in PD. So that we change the configuration options
+dynamically through SQL or `pd-ctl`.
+
+## Background
+
+Although some configuration options of TiDB, TiKV, and PD support modification,
+the way to change them is different in different components, which increases the
+burden of maintenance for users and DBAs, resulting in poor usability.
+Currently, the way to change the configuration of each component is as follows:
+
+- PD: via HTTP interface
+- TiKV: debug service through gRPC
+- TiDB: modify configuration table through SQL
+
+For better usability, we need a unified way to manage the configuration options.
+
+## Proposal
+
+Since PD integrates etcd itself, and etcd is good at managing cluster
+configuration, we propose centralized management which the configuration of
+the whole cluster, including TiDB, TiKV, and PD components, can be handed over to
+PD leader. The main things the PD should do are as follows:
+
+- Persistence of configuration
+- Configuration updates
+- Distribution of configuration
+
+## Rationale
+
+The new way of managing configuration has better usability. The configuration
+files of those components can be removed and don't need to learn how those
+tools works. It reduces administrative costs. For configuration options that
+cannot be modified dynamically, we still can change it using this unified way,
+but we need to wait for the next restart after modification to take effect.
+
+## Implemetation
+
+### Interface
+
+The communication related to the configuration among components will use gRPC.
+The following interfaces need to be added:
+
+```proto
+service Config {
+    rpc Create(CreateRequest) returns (CreateResponse) {}
+    rpc Get(GetRequest) returns (GetResponse) {}
+    rpc Update(UpdateRequest) returns (UpdateResponse) {}
+}
+```
+
+The functions of each interface are as follows:
+
+*Create* is used to register a configuration to PD when the components start.
+*Get* is used to get the complete configuration of the component periodically
+from PD and decide if the component configuration need to update. *Update* is
+used to update some configuration of some components in PD.
+
+### WorkfLow
+
+Before TiDB and TiKV start, the first thing need to do is sending a *Create*
+request to register themselves in PD. And then the response of *Create* will
+carry all configuration options which the component needs to start the process.
+Later, TiDB and TiKV need to create an independent thread to call *Get*
+periodically to get configuration from PD and decide whether to update after
+comparing it with the local configuration. TiDB can update the configuration of
+each component on PD through SQL at any time by calling *Update*.
+
+![Figure 1: WorkfLow](./imgs/dynamic-config-workflow.png)
+
+For the users who only use TiKV, the *Update* method can be added to the
+implementation of existing clients, such as rust client or go client.
+
+### Configuration Item
+
+The protocol definition of a specific configuration option is as follows:
+
+```proto
+message ConfigEntry {
+    Oneof type {
+        Local local = 1;
+        Global global = 2;
+    }
+    string name = 3;
+    string value = 4;
+}
+
+
+Each *ConfigEntry* represents a configuration option. The *name* represents a
+specific configuration item name. All configuration items use a unified naming
+standard. For example, for the following configuration item, the name should be
+*readpool.storage.high-concurrency*.
+
+```toml
+[readpool.storage]
+high-concurrency = 4
+```
+
+There are two types for a configuration item: local and global. It is defined
+as follows:
+
+```proto
+message Local {
+    bytes component_id = 1;
+}
+
+message Global {
+    Component component = 1;
+}
+
+These two types are used to distinguish whether the configuration is shared by
+components. For example, the label configuration of TiKV is individual for each
+TiKV instance. So the type should be local. Each instance here is uniquely identified
+by the *component_id*, which can be obtained by hashing *IP: port*.
+Another example is the *raft-base-tick-interval* which is the configuration
+shared by all TiKV instances, so the type should be global. The definition of
+component type is as follows:
+
+```proto
+enum Component {
+    UNKNOWN = 0;
+    PD = 1;
+    TiKV = 2;
+    TiDB = 3;
+}
+```
+
+### Version Control
+
+To determine whether the corresponding component needs to update the
+configuration, the version field needs to be added to the request and response.
+The protocol definition is as follows:
+
+```proto
+message Version {
+    uint64 local = 1;
+    uint64 global = 2;
+}
+```
+
+For global configuration items, PD only maintains a set of global configuration
+items and a version variable, while for local, it needs to maintain a mapping
+from component ID to a set of local configuration items, and each set of local
+configuration items need to have an independent version variable.
+
+The following lists how version works In common scenarios:
+
+- New cluster. Both TiDB and TiKV use the default configuration and send it to
+PD to complete the registration. The registration needs to establish the mapping
+relationship between the component ID, version and local configuration. For
+customized requirements, such as modifying the size of block cache. It needs
+external tools to help modify them. After that, the global and local versions
+also need to be modified by adding one respectively according to the
+configuration option type. TiDB or TiKV obtains the configuration and version
+through *Get* method and decides whether to modify them by comparing with the
+local version. If the local version is smaller than the version carried by the
+response, the corresponding configuration options will be overwritten or merged
+and the version will be updated at the same time.
+
+- Add a new component or restart the component. The initialization of
+configuration calls the *Create* method. After receiving the request, PD first
+registers or queries the component ID. By comparing the version carried by the
+request with the version stored in PD, it determines whether to return the
+configuration of the component in the response. After receiving the reply, TiDB
+or TiKV decides whether to update the configuration or not after comparing with
+the version stored in the component.
+
+- Delete the node. PD can directly delete the corresponding component ID, the
+associated set of local configuration items and the version.
+
+### Appendix
+
+The entire agreement is as follows:
+
+```proto
+service Config {
+    rpc Create(CreateRequest) returns (CreateResponse) {}
+    rpc Get(GetRequest) returns (GetResponse) {}
+    rpc Update(UpdateRequest) returns (UpdateResponse) {}
+}
+
+message Status {
+    enum StatusCode {
+        UNKNOWN = 0;
+        FAILED = 1;
+        STALE_VERSION = 3;
+    }
+    StatusCode code = 1;
+    string message = 2;
+    Version version = 3;
+}
+
+message Version {
+    uint64 local = 1;
+    uint64 global = 2;
+}
+
+enum Component {
+    UNKNOWN = 0;
+    PD = 1;
+    TiKV = 2;
+    TiDB = 3;
+}
+
+message Local {
+    bytes component_id = 1;
+}
+
+message Global {
+    Component component = 1;
+}
+
+message ConfigEntry {
+    oneof type {
+        Local local = 1;
+        Global global = 2;
+    }
+
+    string name = 3;
+    string value = 4;
+}
+
+message CreateRequest {
+    uint64 cluster_id = 1;
+    Version version = 2;
+    bytes component_id = 3;
+    string config = 4;
+}
+
+message CreateResponse {
+    uint64 cluster_id = 1;
+    Status status = 2;
+    Version version = 3;
+    string config = 4;
+}
+
+message GetRequest {
+    uint64 cluster_id = 1;
+    Version version = 2;
+    Component component = 3;
+    bytes component_id = 4;
+ }
+
+message GetResponse {
+    uint64 cluster_id = 1;
+    Status status = 2;
+    Version version = 3;
+    string config = 4;
+}
+
+message UpdateRequest {
+    uint64 cluster_id = 1;
+    Version version = 2;
+    repeated ConfigEntry entries = 3;
+}
+
+message UpdateResponse {
+    uint64 cluster_id = 1;
+    Status status = 2;
+    Version version = 3;
+    string config = 4;
+}
+```