Projections

Built-in projections materialize views from streams or $all without changing the source event log. State lives under data/projections/<name>/.

Runtimes

Runtime	Program	Notes
builtin	count, count_by_type, sum_field, latest_by_key, group_count	Fast fixed aggregations
cel	CEL expression over event, state, position, version	JSON numbers are doubles; cast with double(...) / int(double(...))
starlark	def reduce(event, state): ...	Sandboxed; no FS/network

JavaScript is not supported. WASM is not available in the current documented runtime set.

Lifecycle

./bin/chronacta projection create -name orders-count -runtime builtin -program count \
  -source-stream orders-1 \
  -filter 'event.event_type == "OrderCreated" || event.event_type == "OrderPaid"'
./bin/chronacta projection rebuild -name orders-count
./bin/chronacta projection result -name orders-count
./bin/chronacta projection status -name orders-count
./bin/chronacta projection verify-checkpoint -name orders-count

Optional create flags: -event-types, -filter (CEL bool), -initial-state, -version.

Operational commands: enable, disable, resume, restore-checkpoint, migrate, detect-stalled, errors, dead-letters, delete.

Semantics

• Checkpoint advances only after successful processing (or intentional skip of non-matching source events)
• Stream-scoped projections ignore other streams but still scan $all by global position
• Prefer rebuild after create when historical events already exist
• Failures retry then poison into dead letters; source log is unchanged
• Projection files are included in versioned backups

Postgres read models

For SQL/UI/BI, project events into Postgres with a YAML schema and chronacta sqlgen (DDL/DML). JSON projections above stay for ops. See postgres-projections.md.

See also CLI and the examples/projection-test binary.