UrbanSim gets its input data from either a baseyear database or a scenario database. The UrbanSim simulator treats baseyear and scenario databases as read-only databases, however other data preparation applications such as the estimators or the household synthesizer may write to them. In fact, when the run manager starts a new simulation, the first step is to copy the baseyear data into the baseyear cache. The simulation then reads all of its baseyear information from the baseyear cache and writes all results to the simulation cache. Data is only written to the output database when specified (currently done manually).
At the moment, UrbanSim only supports the MySQL database server (we will be adding support for other database servers).
A baseyear database contains a snapshot of the base information defining the initial state before the UrbanSim simulation. Most of the data typically is about a particular year, e.g., geographic information, initial household and job information, etc., for a given year.
A scenario database contains additional and augmenting information to alter the base year data when simulating a particular scenario e.g., new transportation links, an expanded urban growth boundary, etc. Any of the table may be placed in either of the database, although typically most are placed in the baseyear database. The scenario databases typically only contains tables specifying different possible futures, e.g. tables of exogenous events scheduled for future years.
The way that the scenario can modify the information in the baseyear database or another scenario database is determined by the scenario linking.