View source
<p>To get the most out of the migrate module, one should understand the following concepts:</p>
<dl>
<dt>Content set</dt>
<dd>This represents a set of source content (embodied in a view) to be migrated.
It includes mappings from columns in the view
to fields within Drupal objects.</dd>
<dt>Destination</dt>
<dd>A Drupal object designated to receive migrated data from a source view.
Most destinations are simple - roles, terms, users - but nodes are more
complex - each distinct node type defined in your Drupal site is a
distinct destination in the migrate module.</dd>
<dt>Clear</dt>
<dd>An operation which deletes previously-migrated content, generally in
preparation for remigration.</dd>
<dt>Import</dt>
<dd>An operation which imports content represented by a content set into
Drupal objects. By default this operation only processes source data
which has not yet been migrated, but optionally it can also update
previously-imported destination objects with the current source data.</dd>
<dt>Map tables</dt>
<dd>A table associated with each content set, holding mappings from the unique
ID of a source view row to the unique ID of the Drupal object created from
that row. This is used to keep track of which rows have been processed, and
can also be used to develop audit views directly comparing source data to
resulting Drupal data.</dd>
<dt>Primary key</dt>
<dd>In relational databases, a table's <em>primary key</em> consists of one or more
columns which are unique for each row and never null, explicitly designated
to uniquely identify each row. The migrate module uses this term to
describe a unique non-null column for a source view - in most cases, this
will actually be the primary key of the base table of the view, but you
may designate another unique column to function as a primary key for
the purpose of migration. This column is used as the source ID in map
tables.</dd>
</dl>
