Recoll indexing can be performed along two different modes:

The choice between the two methods is mostly a matter of preference, and they can be combined by setting up multiple indexes (ie: use periodic indexing on a big documentation directory, and real time indexing on a small home directory). Monitoring a big file system tree can consume significant system resources.

The choice of method and the parameters used can be configured from the recoll GUI: Preferences → Indexing schedule

The parameters describing what is to be indexed and local preferences are defined in text files contained in a configuration directory.

All parameters have defaults, defined in system-wide files.

Without further configuration, Recoll will index all appropriate files from your home directory, with a reasonable set of defaults.

A default personal configuration directory ($HOME/.recoll/) is created when a Recoll program is first executed. It is possible to create other configuration directories, and use them by setting the RECOLL_CONFDIR environment variable, or giving the -c option to any of the Recoll commands.

In some cases, it may be interesting to index different areas of the file system to separate databases. You can do this by using multiple configuration directories, each indexing a file system area to a specific database. Typically, this would be done to separate personal and shared indexes, or to take advantage of the organization of your data to improve search precision.

The generated indexes can be queried concurrently in a transparent manner.

For index generation, multiple configurations are totally independant from each other. When multiple indexes need to be used for a single search, some parameters should be consistent among the configurations.

Recoll knows about quite a few different document types. The parameters for document types recognition and processing are set in configuration files.

Most file types, like HTML or word processing files, only hold one document. Some file types, like email folders or zip archives, can hold many individually indexed documents, which may themselves be compound ones. Such hierarchies can go quite deep, and Recoll can process, for example, a LibreOffice document stored as an attachment to an email message inside an email folder archived in a zip file...

Recoll indexing processes plain text, HTML, OpenDocument (Open/LibreOffice), email formats, and a few others internally.

Other file types (ie: postscript, pdf, ms-word, rtf ...) need external applications for preprocessing. The list is in the installation section. After every indexing operation, Recoll updates a list of commands that would be needed for indexing existing files types. This list can be displayed by selecting the menu option File → Show Missing Helpers in the recoll GUI. It is stored in the missing text file inside the configuration directory.

By default, Recoll will try to index any file type that it has a way to read. This is sometimes not desirable, and there are ways to either exclude some types, or on the contrary to define a positive list of types to be indexed. In the latter case, any type not in the list will be ignored.

Excluding types can be done by adding wildcard name patterns to the skippedNames list, which can be done from the GUI Index configuration menu. For versions 1.20 and later, you can alternatively set the excludedmimetypes list in the configuration file. This can be redefined for subdirectories.

You can also define an exclusive list of MIME types to be indexed (no others will be indexed), by settting the indexedmimetypes configuration variable. Example:

indexedmimetypes = text/html application/pdf
          

It is possible to redefine this parameter for subdirectories. Example:

[/path/to/my/dir]
indexedmimetypes = application/pdf
          

(When using sections like this, don't forget that they remain in effect until the end of the file or another section indicator).

excludedmimetypes or indexedmimetypes, can be set either by editing the main configuration file (recoll.conf), or from the GUI index configuration tool.

Indexing may fail for some documents, for a number of reasons: a helper program may be missing, the document may be corrupt, we may fail to uncompress a file because no file system space is available, etc.

Recoll versions prior to 1.21 always retried to index files which had previously caused an error. This guaranteed that anything that may have become indexable (for example because a helper had been installed) would be indexed. However this was bad for performance because some indexing failures may be quite costly (for example failing to uncompress a big file because of insufficient disk space).

The indexer in Recoll versions 1.21 and later do not retry failed file by default. Retrying will only occur if an explicit option (-k) is set on the recollindex command line, or if a script executed when recollindex starts up says so. The script is defined by a configuration variable (checkneedretryindexscript), and makes a rather lame attempt at deciding if a helper command may have been installed, by checking if any of the common bin directories have changed.

In the rare case where the index becomes corrupted (which can signal itself by weird search results or crashes), the index files need to be erased before restarting a clean indexing pass. Just delete the xapiandb directory (see next section), or, alternatively, start the next recollindex with the -z option, which will reset the database before indexing.