The state delta subscriber (SDS), also referred to as state delta export, provides a mechanism for exporting on-chain state values from a Sawtooth validator to an external data store (e.g. a Postgres database). This allows applications to efficiently query their state values.
The SDS implements an event subscription client that subscribes to block commit events and Sawtooth state deltas at specific addresses (in this case we will subscribe to all state delta events at the certificate_registry transaction family namespace). Sawtooth sends these events whenever a validator’s state is updated. The events contain the raw state data at the updated addresses. The event subscription client processes the event data and uses it to update the reporting database, an off-chain copy of blockchain state. The REST API can query this database when a client needs to get information from the blockchain.