Ordering & Delivery Guarantees

Delivery semantics

Laredo provides at-least-once delivery. The engine defers ACK until all targets confirm durability. On restart:

• Ephemeral sources: full re-baseline (no resume capability)
• Stateful sources: resume from last ACKed position, which may redeliver some changes

Targets must be idempotent for the overlap window after a restart.

Ordering by source type

Source	Guarantee	Implication
PostgreSQL	Total order	All changes across all tables in commit order
S3 + Kinesis	Per-partition order	Ordered within a shard, no cross-shard guarantee

Baseline-to-stream consistency

There is no gap between the baseline snapshot and the change stream. PostgreSQL's exported snapshot mechanism ensures the baseline represents a consistent point-in-time, and the change stream picks up from exactly that point.

ACK coordination

When a table fans out to multiple targets, the engine ACKs the source position only after all targets have confirmed IsDurable(). The ACK position is the minimum confirmed position across all pipelines sharing that source.

Source position: 100
  Pipeline A (indexed-memory): durable at 100  ✓
  Pipeline B (http-sync):      durable at 97   ✓ (batch not yet flushed)
  Pipeline C (http-sync):      durable at 100  ✓

ACK position = min(100, 97, 100) = 97

The source retains data from position 97 onward. If the process crashes, changes 98-100 will be redelivered to pipeline B on restart.

Schema changes

When a source detects a column schema change, it notifies each target via OnSchemaChange. Targets respond with one of:

Response	Meaning
CONTINUE	Target adapted, keep streaming
RE_BASELINE	Target needs a full reload
ERROR	Target can't handle this change