[设计 / Design] CDC、订阅与物化视图评估 / evaluate CDC, subscriptions, and materialized views

中文说明

版本、顺序与当前进度

版本待定 / P2 / deferred，不再将探索池命名为 v1.0 发布承诺。有限 NDJSON stream 不等于 CDC；须用真实增量订阅场景定义日志、重放、积压和预算后另排版本。

问题

数据库不负责 CDC、订阅或物化视图（APPLICATION_DATA.md 明确边界）；有限 NDJSON stream 是单次查询结果，不是实时更新流。中等业务常需要“数据变更后驱动缓存失效/事件总线/派生视图”。当前只能轮询或应用双写，容易产生不一致。

范围

• 评估最小形态：按 sequence/RowId 的有界变更日志读取、订阅游标、以及物化/派生视图的候选方案；明确与幂等 receipt、cursor、migration、compact 的交互。
• 明确投递语义（at-least-once/游标消费）、保留窗口与清理，不承诺 exactly-once 推送。
• 评估在单机单写者下的可行性，以及未来复制场景的衔接。
• 先 RFC 与证据；不把实时流作为当前版本范围。

验收条件

• RFC 冻结变更日志/订阅/物化视图的最小语义、游标与保留策略。
• 一个原型证明消费者可从某个 sequence 恢复并观察到一致变更。
• 明确与备份、migration、compact 及现有 stream 的关系。
• 若不采纳，说明替代方案（轮询/应用事件表）。

English Description

Version, sequence, and current progress

Version TBD / P2 / deferred, not a v1.0 release promise. Finite NDJSON streams are not CDC; require real incremental-subscription needs and log/replay/backlog/budget contracts before scheduling a release.

Problem

The database owns no CDC, subscriptions, or materialized views (explicitly out of scope in APPLICATION_DATA.md); the finite NDJSON stream is a single query result, not a live update stream. Medium businesses often need "on change, invalidate cache / drive an event bus / maintain a derived view". Today this requires polling or dual writes and drifts.

Scope

• Evaluate a minimal shape: a bounded change-log read by sequence/RowId, subscription cursors, and candidate materialized/derived views; define interaction with idempotency receipts, cursors, migration, and compaction.
• Define delivery semantics (at-least-once/cursor consumption), retention windows, and cleanup; do not promise exactly-once push.
• Evaluate feasibility under single-node single-writer and the path to future replication.
• RFC and evidence first; live streaming is not in the current scope.

Acceptance Criteria

• An RFC freezes minimal change-log/subscription/materialized-view semantics, cursors, and retention.
• A prototype shows a consumer can resume from a sequence and observe consistent changes.
• The relationship with backup, migration, compaction, and the existing stream is stated.
• If rejected, alternatives (polling/event table) are documented.

Start by reading APPLICATION_DATA.md to confirm the current database boundary, then inspect the existing finite NDJSON stream and related concepts named in the issue: receipts, cursors, migration, and compaction. Done means an RFC defines the minimal semantics and retention policy, a prototype demonstrates sequence-based recovery, and the relationships to backup, migration, compaction, and alternatives are documented.