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GTFS feeds contain schedule data that is published by an agency or operator in advance. The feed does not account for unexpected service changes or traffic disruptions that occur from day to day. Thus, this kind of data is also referred to as 'static' data or 'scheduled' arrival and departure times.
GTFS-Realtime complements GTFS with additional kinds of feeds. In contrast to the base GTFS schedule feed, they provide real-time updates ('dynamic' data) and are updated from minute to minute.
Alerts
Alerts are text messages attached to GTFS objects, informing riders of disruptions and changes. The information is downloaded in a single HTTP request and polled regularly.
TripUpdates via HTTP(S)
TripUpdates report on the status of scheduled trips as they happen, providing observed and predicted arrival and departure times for the remainder of the trip. The information is downloaded in a single HTTP request and polled regularly.
Streaming TripUpdates via MQTT
This updater connects to an MQTT broker and processes TripUpdates in a streaming fashion. This means that they will be applied individually in near-real-time rather than in batches at a certain interval.
This system powers the real-time updates in Helsinki and more information can be found on Github.
Vehicle Positions
VehiclePositions give the location of some or all vehicles currently in service, in terms of geographic coordinates or position relative to their scheduled stops. The information is downloaded in a single HTTP request and polled regularly.