vitess

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Vitess

Vitess is a MySQL-compatible, cloud-native database system originally built at YouTube to scale MySQL. PlanetScale runs Vitess as a managed service. Core capabilities:

Horizontal sharding: Built-in sharding transparent to the application — no sharding logic in app code.
Connection pooling: VTGate multiplexes client connections, scaling concurrent connections far beyond native MySQL limits.
High availability: Automatic primary failure detection and repair. Resharding and data migrations with near-zero downtime.
Query rewriting and caching: VTGate rewrites and optimizes queries before routing to shards.
Schema management: Apply schema changes across all shards consistently, in the background, without disrupting workloads.
Materialized views and messaging: Cross-shard materialized views and publish/subscribe messaging via VStream.

Vitess是一款兼容MySQL的云原生数据库系统，最初由YouTube开发用于扩展MySQL。PlanetScale将Vitess作为托管服务运行。核心功能：

Horizontal sharding：内置分片机制，对应用程序透明，无需在应用代码中实现分片逻辑。
Connection pooling：VTGate多路复用客户端连接，可支持的并发连接数远超原生MySQL的限制。
High availability：自动检测主节点故障并完成修复。分片重配置和数据迁移几乎不会导致停机。
Query rewriting and caching：VTGate在将查询路由到分片前会进行重写和优化。
Schema management：在后台跨所有分片一致地应用schema变更，不会中断业务负载。
Materialized views and messaging：跨分片物化视图，以及通过VStream实现的发布/订阅消息机制。

Key concepts

Concept	What it is
Keyspace	Logical database mapping to one or more shards. Analogous to a MySQL schema.
Shard	A horizontal partition of a keyspace, each backed by a separate MySQL instance.
VSchema	Configuration defining how tables map to shards, vindex (sharding) keys, and routing rules.
Vindex	Sharding function mapping column values to shards ( `hash` , `unicode_loose_xxhash` , `lookup` ).
VTGate	Stateless proxy that plans and routes queries to the correct shard(s).
Online DDL	Non-blocking schema migrations. On PlanetScale, use deploy requests for production changes.

Concept	定义
Keyspace	映射到一个或多个分片的逻辑数据库，类似于MySQL的schema。
Shard	Keyspace的水平分区，每个分片由独立的MySQL实例提供支持。
VSchema	定义表如何映射到分片、vindex（分片键）以及路由规则的配置文件。
Vindex	将列值映射到分片的分片函数（如 `hash` 、 `unicode_loose_xxhash` 、 `lookup` ）。
VTGate	无状态代理，负责规划并将查询路由到正确的分片。
Online DDL	无阻塞的schema迁移。在PlanetScale中，生产环境变更需使用部署请求（deploy requests）。

PlanetScale specifics

PlanetScale专属特性

Branching: Git-like database branches for development; deploy requests for production schema changes.
Connections: MySQL protocol, port
```
3306
```
(direct) or
```
443
```
(serverless). SSL always required.

Branching：类Git的数据库分支，用于开发环境；生产环境schema变更需通过部署请求完成。
Connections：支持MySQL协议，端口为
```
3306
```
（直连）或
```
443
```
（无服务器模式）。始终要求SSL加密。

SQL compatibility

SQL兼容性

Vitess supports nearly all MySQL syntax — most applications work without query changes. Standard DML, DDL, joins, subqueries, CTEs (including recursive CTEs as of v21+), window functions, and common built-in functions all work as expected.

Known limitations:

Stored procedures / triggers / events: Not supported through VTGate.
LOCK TABLES
/
GET_LOCK
: Not supported through VTGate.
SELECT ... FOR UPDATE
: Works within a single shard; cross-shard locking is not atomic.
Cross-shard joins: Supported but expensive (scatter-gather). Filter by vindex column for single-shard routing.
Correlated subqueries: May fail or perform poorly cross-shard. Rewrite as joins when possible.
AUTO_INCREMENT: Sequences are per-shard. Use vindexes or app-generated IDs (UUIDs, snowflake) to avoid collisions on sharded tables.
Aggregations on sharded tables:
```
GROUP BY
```
/
```
ORDER BY
```
/
```
LIMIT
```
merge in VTGate memory. Large result sets can be slow.
Foreign keys: Limited support. Prefer application-level referential integrity on sharded keyspaces.

Vitess几乎支持所有MySQL语法——大多数应用无需修改查询即可正常运行。标准DML、DDL、连接查询、子查询、CTE（包括v21+版本支持的递归CTE）、窗口函数以及常用内置函数均能按预期工作。

已知限制：

存储过程/触发器/事件：无法通过VTGate使用。
LOCK TABLES
/
GET_LOCK
：无法通过VTGate使用。
SELECT ... FOR UPDATE
：在单个分片内可正常工作；跨分片锁不具备原子性。
跨分片连接查询：支持但性能开销大（分散-聚合模式）。建议通过vindex列过滤以实现单分片路由。
关联子查询：跨分片场景下可能执行失败或性能低下。尽可能重写为连接查询。
AUTO_INCREMENT：序列按分片独立生成。在分片表上，建议使用vindex或应用生成的ID（如UUID、雪花算法ID）避免冲突。
分片表上的聚合操作：
```
GROUP BY
```
/
```
ORDER BY
```
/
```
LIMIT
```
操作在VTGate内存中合并。结果集过大会导致性能缓慢。
外键：支持有限。在分片Keyspace中，建议优先在应用层实现引用完整性。

References

参考资料

Topic	Reference	Use for
VSchema	references/vschema.md	VSchema design, vindexes, sequences, sharding strategies
Schema Changes	references/schema-changes.md	Online DDL, managed migrations, ddl strategies, migration lifecycle
VReplication	references/vreplication.md	MoveTables, Reshard, Materialize, VDiff, VStream
Architecture	references/architecture.md	VTGate, VTTablet, Topology Service, VTOrc, component interactions
Query Serving	references/query-serving.md	Query routing, MySQL compatibility, cross-shard performance, EXPLAIN

主题	参考链接	适用场景
VSchema	references/vschema.md	VSchema设计、vindex、序列、分片策略
Schema变更	references/schema-changes.md	Online DDL、托管迁移、DDL策略、迁移生命周期
VReplication	references/vreplication.md	MoveTables、Reshard、Materialize、VDiff、VStream
架构	references/architecture.md	VTGate、VTTablet、拓扑服务、VTOrc、组件交互
查询服务	references/query-serving.md	查询路由、MySQL兼容性、跨分片性能、EXPLAIN