Change Data Capture Pipeline  ·  View 15 of 21  ·  Runtime

Schema Drift — Four Paths

Five change classes, two of which stop exactly one table and nothing else.

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Detect Classify Pipeline action Sink action Outcome Added nullable column Relation metadata from the stream Compatible Register new version effective LSN Additive DDL before first event No pause ≤ 60 s New table in allow-list Registry change Compatible Chunked snapshot view 14 Create sink table Live after backfill Dropped or renamed column DDL event Breaking Pause this table only ≤ 10 s Sink untouched no destructive DDL Operator decision resume from position Primary-key change DDL event Breaking Re-snapshot required keys no longer match Shadow then swap Rebuild, not migrate Unregistered column seen Decode mismatch Not a third option Propagate or exclude never silently drop Evolve or ignore Explicit either way Schema Drift — Four Changes, Four Paths Only the breaking rows stop anything, and they stop exactly one table. v 1.0 · owner Data Platform Architecture · date 2026-10

Decisions

  • Compatible changes propagate without a human; breaking ones pause one table and raise a decision (ADR-08).
  • A primary-key change is a re-snapshot, not a migration: the sink's keys no longer mean what they meant.
  • There is no third outcome for an unregistered column — propagate it or exclude it by allow-list. Silently dropping it is the failure this row exists to forbid.

Numbers

  • Compatible change propagates within 60 s; breaking change pauses within 10 s (assumptions).
  • A pause holds the table's position; the platform refuses a pause that would outlive source log retention.

Risks

  • Classification is the single point of judgement in the pipeline: a widened type misread as narrowed pauses needlessly, and the reverse corrupts a sink.
  • Detection depends on the source emitting decodable relation metadata; a source that does not is a polling fallback and loses this whole view.
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Solution Architecture v1.0 · Google Cloud · Data Platform Architecture · 2026-10 · 21 views · 15 architecture decision records15-schema-change-paths