6. Indexing with General Record IDs
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## 6. Indexing with General Record IDs

When using this method you construct an (almost) arbitrary, internal record key based on the contents of the record itself and other system information. If you have a group of records that explicitly associates an ID with each record, this method is convenient. For example, the record format may contain a title or a ID-number - unique within the group. In either case you specify the Z39.50 attribute set and use-attribute location in which this information is stored, and the system looks at that field to determine the identity of the record.

As before, the record ID is defined by the `recordId` setting in the configuration file. The value of the record ID specification consists of one or more tokens separated by whitespace. The resulting ID is represented in the index by concatenating the tokens and separating them by ASCII value (1).

There are three kinds of tokens:

Internal record info

  The token refers to a key that is extracted from the record. The syntax of this token is `(` set `,` use `)`, where set is the attribute set name use is the name or value of the attribute.

System variable

  The system variables are preceded by

              $

  and immediately followed by the system variable name, which may one of

  group

    Group name.

  database

    Current database specified.

  type

    Record type.

Constant string

  A string used as part of the ID — surrounded by single- or double quotes.

For instance, the sample GILS records that come with the Zebra distribution contain a unique ID in the data tagged Control-Identifier. The data is mapped to the BIB-1 use attribute Identifier-standard (code 1007). To use this field as a record id, specify `(bib1,Identifier-standard)` as the value of the `recordId` in the configuration file. If you have other record types that uses the same field for a different purpose, you might add the record type (or group or database name) to the record id of the gils records as well, to prevent matches with other types of records. In this case the recordId might be set like this:

        gils.recordId: $type (bib1,Identifier-standard)

(see Chapter 9, GRS-1 Record Model and Filter Modules for details of how the mapping between elements of your records and searchable attributes is established).

As for the file record ID case described in the previous section, updating your system is simply a matter of running `zebraidx` with the `update` command. However, the update with general keys is considerably slower than with file record IDs, since all files visited must be (re)read to discover their IDs.

As you might expect, when using the general record IDs method, you can only add or modify existing records with the `update` command. If you wish to delete records, you must use the, `delete` command, with a directory as a parameter. This will remove all records that match the files below that root directory.

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