Historical data format is the same as provided by real-time bitFlyer lightningJSON-RPC 2.0 over WebSocket API with addition of local timestamps. If you'd like to work with normalized data format instead (same format for each exchange) see downloadable CSV files or official client libs that can perform data normalization client-side.
Tardis-machine is a locally runnable server that exposes API allowing efficiently requesting historical market data for whole time periods in contrast to HTTP API that provides data only in minute by minute slices.
lightning_board_snapshotlightning_board_snapshot channel is subscribed only in order to get initial order book snapshot, after that it's automatically unsubscribed as all order book update are being provided via lightning_board channel.
Market data collection details
Market data collection infrastructure for bitFlyer since 2020-05-28 is located in GCP asia-northeast1 (Tokyo, Japan), before that it was located in GCP europe-west2 region (London, UK).
Real-time market data is captured via single WebSocket connection.
Bitflyer servers are located in AWS ap-northeast-1 region (Tokyo, Japan).