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Because it operates on the i2b2 database directly rather then than using the i2b2 Web Service API, you need a physical connection to that database. This includes network access to the database server (check your firewall when in doubt), a JDBC database driver (for PostgreSQL, it is supplied within the tool) and a database account that was granted the right to write to the required i2b2 project tables. If you used the i2b2 wizard to create a project, you already provided the credentials.

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