how-to: add placement rule guide (#2171)

TOC.md

@@ -63,6 +63,7 @@
   + Configure
     - [Time Zone](/how-to/configure/time-zone.md)
     - [Memory Control](/how-to/configure/memory-control.md)
+    - [Placement Rules](/how-to/configure/placement-rules.md)
   + Secure
     + Transport Layer Security (TLS)
       - [Enable TLS For MySQL Clients](/how-to/secure/enable-tls-clients.md)

how-to/configure/placement-rules.md

@@ -0,0 +1,345 @@
+---
+title: Placement Rules
+summary: Learn how to configure Placement Rules.
+category: how-to
+---
+
+# Placement Rules
+
+Placement Rules is an experimental feature of the Placement Driver (PD) introduced in v4.0. It is a replica rule system that guides PD to generate corresponding schedules for different types of data. By combining different scheduling rules, you can finely control the attributes of any continuous data range, such as the number of replicas, the storage location, the host type, whether to participate in Raft election, and whether to act as the Raft leader.
+
+## Rule system
+
+The configuration of the whole rule system consists of multiple rules. Each rule can specify attributes such as the number of replicas, the Raft role, the placement location, and the key range in which this rule takes effect. When PD is performing schedule, it first finds the rule corresponding to the Region in the rule system according to the key range of the Region, and then generates the corresponding schedule to make the distribution of the Region replica comply with the rule.
+
+The key ranges of multiple rules can have overlapping parts, which means that a Region can match multiple rules. In this case, PD decides whether the rules overwrite each other or take effect at the same time according to the attributes of rules. If multiple rules take effect at the same time, PD will generate schedules in sequence according to the stacking order of the rules for rule matching.
+
+In addition, to meet the requirement that rules from different sources are isolated from each other, the concept of "Group" is also introduced. If you do not want a rule to be affected by other rules in the system (such as being overridden), you can use a separate group for it.
+
+![Placement rules overview](/media/placement-rules-1.png)
+
+### Rule fields
+
+The following table shows the meaning of each field in a rule:
+
+| Field name           | Type and restriction                      | Description                                |
+| :---            | :---                           | :---                                |
+| `GroupID`         | `string`                         |  The group ID that marks the source of the rule.                |
+| `ID`              | `string`                         |  The unique ID of a rule in a group.                        |
+| `Index`           | `int`                            |   The stacking sequence of rules in a group.                     |
+| `Override`        | `true`/`false`                     | Whether to overwrite rules with smaller index (in a group).  |
+| `StartKey`        | `string`, in hexadecimal form                |  Applies to the starting key of a range.                |
+| `EndKey`          | `string`, in hexadecimal form                |  Applies to the ending key of a range.                |
+| `Role`            | `string` | Replica roles, including leader/follower/learner.                           |
+| `Count`           | `int`, positive integer                     |  The number of replicas.                            |
+| `LabelConstraint` | `[]Constraint`                    |  Filers nodes based on the label.               |
+| `LocationLabels`  | `[]string`                        |  Used for physical isolation.                       |
+
+`LabelConstraint` is similar to the function in Kubernetes that filters labels based on these four primitives: `in`, `notIn`, `exists`, and `notExists`. The meanings of these four primitives are as follows:
+
++ `in`: the label value of the given key is included in the given list.
++ `notIn`: the label value of the given key is not included in the given list.
++ `exists`: includes the given label key.
++ `notExists`: does not include the given label key.
+
+The meaning and function of `LocationLabels` are the same with those earlier than v4.0. For example, if you have deployed `[zone,rack,host]` that defines a three-layer topology: the cluster has multiple zones (Availability Zones), each zone has multiple racks, and each rack has multiple hosts. When performing schedule, PD first tries to place the Region's peers in different zones. If this try fails (such as there are three replicas but only two zones in total), PD guarantees to place these replicas in different racks. If the number of racks is not enough to guarantee isolation, then PD tries the host-level isolation.
+
+## Configure rules
+
+The operations in this section are based on [pd-ctl](/reference/tools/pd-control.md), and the commands involved in the operations also support calls via HTTP API.
+
+### Enable Placement Rules
+
+By default, the Placement Rules feature is disabled. To enable this feature, you can modify the PD configuration file as follows before initializing the cluster:
+
+{{< copyable "" >}}
+
+```toml
+[replication]
+enable-placement-rules = true
+```
+
+In this way, PD enables this feature after the cluster is successfully bootstrapped and generates corresponding rules according to the `max-replicas` and `location-labels` configurations:
+
+{{< copyable "" >}}
+
+```json
+{
+  "group_id": "pd",
+  "id": "default",
+  "start_key": "",
+  "end_key": "",
+  "role": "voter",
+  "count": 3,
+  "location_labels": ["zone", "rack", "host"]
+}
+```
+
+For a bootstrapped cluster, you can also enable Placement Rules online through pd-ctl:
+
+{{< copyable "shell-regular" >}}
+
+```bash
+pd-ctl config placement-rules enable
+```
+
+PD also generates default rules based on the `max-replicas` and `location-labels` configurations.
+
+### Disable Placement Rules
+
+You can use pd-ctl to disable the Placement Rules feature and switch to the previous scheduling strategy.
+
+{{< copyable "shell-regular" >}}
+
+```bash
+pd-ctl config placement-rules disable
+```
+
+> **Notes:**
+>
+> After disabling Placement Rules, PD uses the original `max-replicas` and `location-labels` configurations. The modification of rules (when Placement Rules is enabled) will not update these two configurations in real time. In addition, all the rules that have been configured remain in PD and will be used the next time you enable Placement Rules.
+
+### Set rules using pd-ctl
+
+> **Note：**
+>
+> The change of rules affects the PD scheduling in real time. Improper rule setting might result in fewer replicas and affect the high availability of the system.
+
+pd-ctl supports using the following methods to view rules in the system, and the output is a JSON-format rule or a rule list.
+
+- **To view the list of all rules:**
+
+    {{< copyable "shell-regular" >}}
+
+    ```bash
+    pd-ctl config placement-rules show
+    ```
+
+- **To view the list of all rules in a PD Group:**
+
+    {{< copyable "shell-regular" >}}
+
+    ```bash
+    pd-ctl config placement-rules show --group=pd
+    ```
+
+- **To view the rule of a specific ID in a Group:**
+
+    {{< copyable "shell-regular" >}}
+
+    ```bash
+    pd-ctl config placement-rules show --group=pd --id=default
+    ```
+
+- **To view the rule list that matches a Region:**
+
+    {{< copyable "shell-regular" >}}
+
+    ```bash
+    pd-ctl config placement-rules show --region=2
+    ```
+
+    In the above example, `2` is the Region ID.
+
+Adding rules and editing rules are similar. You need to write the corresponding rules into a file and then use the `save` command to save the rules to PD:
+
+{{< copyable "shell-regular" >}}
+
+```bash
+cat > rules.json <<EOF
+[
+    {
+        "group_id": "pd",
+        "id": "rule1",
+        "role": "voter",
+        "count": 3,
+        "location_labels": ["zone", "rack", "host"]
+    },
+    {
+        "group_id": "pd",
+        "id": "rule2",
+        "role": "voter",
+        "count": 2,
+        "location_labels": ["zone", "rack", "host"]
+    }
+]
+EOF
+pd-ctl config placement save --in=rules.json
+```
+
+The above operation writes `rule1` and `rule2` to PD. If a rule with the same `GroupID` + `ID` already exists in the system, this rule is overwritten.
+
+To delete a rule, you only need to set the `count` of the rule to `0`, and the rule with the same `GroupID` + `ID` will be deleted. The following command deletes the `pd / rule2` rule:
+
+{{< copyable "shell-regular" >}}
+
+```bash
+cat > rules.json <<EOF
+[
+    {
+        "group_id": "pd",
+        "id": "rule2"
+    }
+]
+EOF
+pd-ctl config placement save --in=rules.json
+```
+
+pd-ctl also supports directly saving rules to the file by using the `load` command for easier modification:
+
+{{< copyable "shell-regular" >}}
+
+```bash
+pd-ctl config placement-rules load
+```
+
+Executing the above command saves all rules to the `rules.json` file.
+
+{{< copyable "shell-regular" >}}
+
+```bash
+pd-ctl config placement-rules load --group=pd --out=rule.txt
+```
+
+The above command saves the rules of a PD Group to the `rules.json` file.
+
+### Use tidb-ctl to query the table-related key range
+
+If you need special configuration for metadata or a specific table, you can execute the [`keyrange` command](https://github.com/pingcap/tidb-ctl/blob/master/doc/tidb-ctl_keyrange.md) in [tidb-ctl](https://github.com/pingcap/tidb-ctl) to query related keys. Remember to add `--encode` at the end of the command.
+
+{{< copyable "shell-regular" >}}
+
+```bash
+tidb-ctl keyrange --database test --table ttt --encode
+```
+
+```text
+global ranges:
+  meta: (6d00000000000000f8, 6e00000000000000f8)
+  table: (7400000000000000f8, 7500000000000000f8)
+table ttt ranges: (NOTE: key range might be changed after DDL)
+  table: (7480000000000000ff2d00000000000000f8, 7480000000000000ff2e00000000000000f8)
+  table indexes: (7480000000000000ff2d5f690000000000fa, 7480000000000000ff2d5f720000000000fa)
+    index c2: (7480000000000000ff2d5f698000000000ff0000010000000000fa, 7480000000000000ff2d5f698000000000ff0000020000000000fa)
+    index c3: (7480000000000000ff2d5f698000000000ff0000020000000000fa, 7480000000000000ff2d5f698000000000ff0000030000000000fa)
+    index c4: (7480000000000000ff2d5f698000000000ff0000030000000000fa, 7480000000000000ff2d5f698000000000ff0000040000000000fa)
+  table rows: (7480000000000000ff2d5f720000000000fa, 7480000000000000ff2e00000000000000f8)
+```
+
+> **Note:**
+>
+> DDL and other operations can cause table ID changes, so you need to update the corresponding rules at the same time.
+
+## Typical usage scenarios
+
+This section introduces the typical usage scenarios of Placement Rules.
+
+### Scenario 1: Use three replicas for normal tables and five replicas for the metadata to improve cluster disaster tolerance
+
+You only need to add a rule that limits the key range to the range of metadata, and set the value of `count` to `5`. Here is an example of this rule:
+
+{{< copyable "" >}}
+
+```json
+{
+  "group_id": "pd",
+  "id": "meta",
+  "index": 1,
+  "override": true,
+  "start_key": "6d00000000000000f8",
+  "end_key": "6e00000000000000f8",
+  "role": "voter",
+  "count": "5",
+  "location_labels": ["zone", "rack", "host"]
+}
+```
+
+### Scenario 2: Place five replicas in three data centers in the proportion of 2:2:1, and the Leader should not be in the third data center
+
+Create three rules. Set the number of replicas to `2`, `2`, and `1` respectively. Limit the replicas to the corresponding data centers through `label_constraints` in each rule. In addition, change `role` to `follower` for the data center that does not need a Leader.
+
+{{< copyable "" >}}
+
+```json
+[
+    {
+        "group_id": "pd",
+        "id": "zone1",
+        "start_key": "",
+        "end_key": "",
+        "role": "voter",
+        "count": 2,
+        "label_constraints": [
+            {"key": "zone", "op": "in", "values": ["zone1"]}
+        ],
+        "location_labels": ["rack", "host"]
+    },
+    {
+        "group_id": "pd",
+        "id": "zone2",
+        "start_key": "",
+        "end_key": "",
+        "role": "voter",
+        "count": 2,
+        "label_constraints": [
+            {"key": "zone", "op": "in", "values": ["zone2"]}
+        ],
+        "location_labels": ["rack", "host"]
+    },
+    {
+        "group_id": "pd",
+        "id": "zone3",
+        "start_key": "",
+        "end_key": "",
+        "role": "follower",
+        "count": 1,
+        "label_constraints": [
+            {"key": "zone", "op": "in", "values": ["zone3"]}
+        ],
+        "location_labels": ["rack", "host"]
+    }
+]
+```
+
+### Scenario 3: Add two TiFlash replicas for a table
+
+Add a separate rule for the row key of the table and limit `count` to `2`. Use `label_constraints` to ensure that the replicas are generated on the node of `engine = tiflash`. Note that a separate `group_id` is used here to ensure that this rule does not overlap or conflict with rules from other sources in the system.
+
+{{< copyable "" >}}
+
+```json
+{
+  "group_id": "tiflash",
+  "id": "learner-replica-table-ttt",
+  "start_key": "7480000000000000ff2d5f720000000000fa",
+  "end_key": "7480000000000000ff2e00000000000000f8",
+  "role": "learner",
+  "count": 2,
+  "label_constraints": [
+    {"key": "engine", "op": "in", "values": ["tiflash"]}
+  ],
+  "location_labels": ["host"]
+}
+```
+
+### Scenario 4: Add two follower replicas for a table in the Beijing node with high-performance disks
+
+The following example shows a more complicated `label_constraints` configuration. In this rule, the replicas must be placed in the `bj1` or `bj2` machine room, and the disk type must not be `hdd`.
+
+{{< copyable "" >}}
+
+```json
+{
+  "group_id": "follower-read",
+  "id": "follower-read-table-ttt",
+  "start_key": "7480000000000000ff2d00000000000000f8",
+  "end_key": "7480000000000000ff2e00000000000000f8",
+  "role": "follower",
+  "count": 2,
+  "label_constraints": [
+    {"key": "zone", "op": "in", "values": ["bj1", "bj2"]},
+    {"key": "disk", "op": "notIn", "values": ["hdd"]}
+  ],
+  "location_labels": ["host"]
+}
+```