Basic OTP Architecture
At the core of OpenTripPlanner is a library of Java code that finds efficient paths through multi-modal transportation networks built from OpenStreetMap and GTFS data. Several different services are built upon this library:
The OTP Transit Index API is another RESTful web service that provides information derived from the input GTFS feed(s). Examples include routes serving a particular stop, upcoming vehicles at a particular stop, upcoming stops on a given trip, etc. More complex transit data requests can be formulated using a GraphQL API.
The term "OTP Analyst" refers to parts of OTP that apply the routing engine to transportation network analysis rather than end-to-end trip planning. OTP Analyst includes:
The OTP Analyst Web Services provide network analysis results such as travel time maps and isochrones as standard web Mercator tiles or GIS rasters via a WMS-derived API. These web services are conceptually separate from the routing API, but are provided by the same servlet: once you have a working OTP trip planner you can also use it to produce travel time maps and other visualizations of transit service. See this blog post for discussion and examples.
The OTP Analyst Batch Processor is a command-line tool that handles more complex one-off network analysis tasks. It uses the same core routing library and data sources as other OTP services, but allows for very open-ended configuration and the inclusion of population or opportunity data. While configuration and use are currently somewhat opaque for non-developers, the "Batch Analyst" is becoming a powerful tool for visualizing how transportation networks affect access to urban opportunities. See this article for an example case study on the effects of hurricane Sandy in New York.
The OTP Scripting API allow the execution of routing requests from within scripts (such as Python). It is composed of a stable internal API, and an embedded Jython interpreter. It can be used in different contexts, such as batch analysis or automated regression testing. More information here.