Flow of control in the pipeline containing ESearch tool and EFetch/ESummary tool. An initial query is executed against a chosen database by using the ESearch tool. The query is entered on the term input of the ESearch tool, which also requires a database name to be provided on the db input. If there are records in the chosen database that satisfy the query, the ESearch tool generates a list of record identifiers (UID s), which is available on the IdList output (a) or posted onto the Entrez History Server (b), if the usehistory parameter is set to y. Based on the list of UIDs, the EFetch or ESummary programs retrieve appropriate records from the specified database (db input) and return them as an XML document. The list of UIDs can be entered on the id input of the EFetch or ESummary or can be taken directly from the Entrez History Server using a pair of WebEnv-QueryKey parameters.