LOOKUP matches records in its primary input stream with records in its secondary input stream and writes matched and unmatched records to different output streams. The records are matched on the basis of a key field (the contents of a specified range of columns in the records). LOOKUP reads records from its primary and secondary input streams and writes records to its primary, secondary, and tertiary output streams, if each is connected. The secondary input stream must be defined and connected. The records in the secondary input stream are the master records. LOOKUP first reads the master records into a buffer, where records with duplicate key fields are discarded; the first occurrence of a key is retained. The records in the buffer are referred to as the reference. The records in the primary input stream are the detail records. LOOKUP compares detail records to records in the reference. LOOKUP writes records to three output streams, if each is connected:

• The primary output stream contains matching records. You can specify the sequence of the master and detail records written to the primary output stream and what is written to the primary output stream: both detail and master records, only detail records, or only master records.
• The secondary output stream contains detail records that do not have a matching master record.
• The tertiary output stream contains master records in ascending order by their key fields. The primary and secondary output streams are severed at the end of file on the primary input stream before records are written to the tertiary output stream.

This arrangement allows one to use other filters to prepare the dictionary, or master records for input to LOOKUP from whatever source is required. The many Input/Output filters, or drivers, allow a Hartmann Pipe to interact directly with a variety data sources, from files, to the system itself, and such things as TCP/IP ports. The repertoire of filters and drivers is rich enough that one could, for example, write a server that consisted solely of a Hartmann pipeline.