Glossary and high level overview

Source

for example 'Drupal 6 nodes in a SQL database'. Or 'products in a CSV file'. Implementation: a plugin in Plugin/migrate/source. Will have a base class and a SQL base class. Core likely will only have SQL-derived plugins.

Process

For example 'rename language to langcode' or 'rename field_alt_text to field_image:alt_text' which puts the alt text into the alt text subfield of image. This is a process plugin; by default we likely ship only the 'column map' plugin (default) and perhaps a 'skip' plugin so that we can indicate that certain columns are deliberately skipped.

Destination

For example, 'A Drupal 8 node as stored by entity API'.

Migration

This is a configuration entity containing the configuration for all of the above.For eg

source:
  id: drupal6_node
process:
  - 
    source: language
    destination: langcode
  - 
    source: field_alt_text
    destination: image:alt_text
  - 
    id: skip
    source: field_foobar
  -
    source: image
    file_plugin: fid
destination:
  id: drupal8_node

As you can see, every process item might contain arbitrary configuration data which will be passed to the destination plugin.

MigrateExecuteable

This actually runs the migration: reads a row from source, runs the process plugins on it then hands over to destination.

Note: more to come! This doesn't yet contain id mapping.