Every time a connection is re-used in the pool - which happens is you're doing a dbquery - DB Connect will try to validate that the database connection is actually working. If validation fails, you will likely see an error message like the one above. There are two ways DB Connect tries to validate a connection: 1) If a testQuery is specified in database_types.conf, DB Connect will execute that query, and a response will validate that the database connection is working. 2) If testQuery is not specified, DB Connect will try to use the Java method connection.isValid(), and rely on the JDBC driver to answer. Some JDBC drivers do not implement this API call (seems like Derby is build against Java 1.5 source, where JDBC doesn't have the method isValid). The workaround is to specify a manual testQuery. The simplest one is SELECT 1 . ... View more