Substreams is a streaming engine, that can be compared to Fluvio, Kafka, Apache Spark, RabbitMQ and other such technologies, where a blockchain node (a deterministic data source) acts as the producer.
It has a logs-based architecture (the Firehose), and allows for user-defined custom code to be sent to a Substreams server(s), for streaming and/or ad-hoc querying of the available data.
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