ExternalSource
SPI for streaming transactions into a database from an upstream feed (a CDC source, message log, etc.) instead of XTDB's own DML.
The source owns the read side of its upstream: it receives events (via polling or other methods), maps each to a transaction, and submits it through TxIndexer.executeTx.
XTDB drives the source via onPartitionAssigned and persists the per-tx ExternalSourceToken so the source can resume after the last indexed event.
A source that also confirms progress upstream — advancing a Postgres replication slot, committing a consumer-group offset — must gate that on TxIndexer.latestBlock rather than on a transaction's own durability handle. Confirmation tells the upstream it may discard everything behind the confirmed position, so confirming a transaction that is only on the replica log leaves XTDB the sole holder of data nobody can re-send. Resuming is different: resume from the furthest position indexed, which may legitimately be ahead of the last block.
Types
Opens an ExternalSource for a database, from the source config.
Makes a Factory persistable. A factory whose class is claimed by a ServiceLoader-discovered registration serialises (via protoTag / toProto) and so can travel as a stored secondary-database config; a programmatic factory with no registration still runs in-process, but Factory.toProto rejects it.
Functions
Called when partition is assigned to this node, to run the source's poll loop.