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docs: update placement policy limits #30202

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Nov 29, 2021
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docs: update placement policy limits #30202

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docs: update placement policy limits
morgo Nov 29, 2021
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Merge branch 'master' into update-placement-proposal2
ti-chi-bot Nov 29, 2021
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55 changes: 25 additions & 30 deletions docs/design/2020-06-24-placement-rules-in-sql.md
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Original file line number Diff line number Diff line change
@@ -1,7 +1,7 @@
# Defining placement rules in SQL

- Author(s): [djshow832](https://github.com/djshow832) (Ming Zhang), [morgo](https://github.com/morgo) (Morgan Tocker)
- Last updated: 2021-09-13
- Last updated: 2021-11-28
- Discussion PR: https://github.com/pingcap/tidb/pull/26221
- Tracking Issue: https://github.com/pingcap/tidb/issues/18030
- Original Document (Chinese): https://docs.google.com/document/d/18Kdhi90dv33muF9k_VAIccNLeGf-DdQyUc8JlWF9Gok
Expand Down Expand Up @@ -175,25 +175,25 @@ A new system table `information_schema.placement_rules` is added to view all exp
The table definition is as follows:

```sql
+----------------------+--------------+------+------+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------------------+--------------+------+------+---------+-------+
| POLICY_ID | bigint(64) | NO | | NULL | |
| CATALOG_NAME | varchar(512) | NO | | NULL | |
| POLICY_NAME | varchar(5) | YES | | NULL | |
| SCHEMA_NAME | varchar(5) | YES | | NULL | |
| TABLE_NAME | varchar(5) | YES | | NULL | |
| PARTITION_NAME | varchar(5) | YES | | NULL | |
| PRIMARY_REGION | varchar(5) | NO | | NULL | |
| REGIONS | varchar(5) | NO | | NULL | |
| CONSTRAINTS | varchar(5) | NO | | NULL | |
| LEADER_CONSTRAINTS | varchar(5) | NO | | NULL | |
| FOLLOWER_CONSTRAINTS | varchar(5) | NO | | NULL | |
| LEARNER_CONSTRAINTS | varchar(5) | NO | | NULL | |
| SCHEDULE | varchar(20) | NO | | NULL | |
| FOLLOWERS | bigint(64) | NO | | NULL | |
| LEARNERS | bigint(64) | NO | | NULL | |
+----------------------+--------------+------+------+---------+-------+
+----------------------+---------------+------+------+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------------------+---------------+------+------+---------+-------+
| POLICY_ID | bigint(64) | NO | | NULL | |
| CATALOG_NAME | varchar(512) | NO | | NULL | |
| POLICY_NAME | varchar(64) | YES | | NULL | |
| SCHEMA_NAME | varchar(64) | YES | | NULL | |
| TABLE_NAME | varchar(64) | YES | | NULL | |
| PARTITION_NAME | varchar(64) | YES | | NULL | |
| PRIMARY_REGION | varchar(1024) | NO | | NULL | |
| REGIONS | varchar(1024) | NO | | NULL | |
| CONSTRAINTS | varchar(1024) | NO | | NULL | |
| LEADER_CONSTRAINTS | varchar(1024) | NO | | NULL | |
| FOLLOWER_CONSTRAINTS | varchar(1024) | NO | | NULL | |
| LEARNER_CONSTRAINTS | varchar(1024) | NO | | NULL | |
| SCHEDULE | varchar(20) | NO | | NULL | |
| FOLLOWERS | bigint(64) | NO | | NULL | |
| LEARNERS | bigint(64) | NO | | NULL | |
+----------------------+---------------+------+------+---------+-------+
15 rows in set (0.00 sec)
```

Expand All @@ -203,7 +203,7 @@ The information_schema tables for `tables` and `partitions` should be modified t

```golang
{name: "TIDB_PLACEMENT_POLICY_NAME", tp: mysql.TypeVarchar, size: 64},
{name: "TIDB_DIRECT_PLACEMENT", tp: mysql.TypeVarchar, size: types.UnspecifiedLength}
{name: "TIDB_DIRECT_PLACEMENT", tp: mysql.TypeVarchar, size: 1024}
```

This helps make the information match what is available in `SHOW CREATE TABLE`, but in a structured format.
Expand Down Expand Up @@ -418,15 +418,6 @@ Explanation:

`+any` changes an earlier proposal where the `FOLLOWERS` count could also be specified. This has been removed to reduce the risk of discrepancies and misconfiguration. See also "Policy Validation" below.

#### Built-in Placement Policies

By default every system will have two placement policies, which can be modified via `ALTER PLACEMENT POLICY` but never dropped:

* `default`: This policy is used only in the event that a policy has not been specified.
* `system`: This policy is used for internal TiDB system tables.

Some common applications might be to increase the replica count on system or default tables. It is not typically recommended to add constraints to these policies as it will lead to cluster inbalance, but it is possible.

#### Schedule Property

When using either the syntactic sugar or list format for placement rules, PD is free to schedule followers/leaders wherever it decides. For example:
Expand Down Expand Up @@ -1040,6 +1031,10 @@ This specific semantic will be the hardest to implement because of the other dep

## Changelog

* 2021-11-29:
- Updated limits on object length.
- Removed built-in placement policies (not supported for now, need additional discussion due to `DEFAULT` conflicts.)

* 2021-10-29:
- Add more description to 'scheduling_state'.

Expand Down