Creating a new backend

Annif backend code is in the annif/backend module. Each backend is implemented as a subclass of AnnifBackend, or its more specific subclass AnnifLearningBackend for backends that support online learning.

A backend must define these fields:

• name: a name for the backend (a single word, all lowercase)
• needs_subject_index: boolean value; defaults to False if not set; set to True if the backend makes use of the SubjectIndex passed as project.subjects to most methods
• needs_subject_vectorizer: boolean value; defaults to False if not set; set to True if the backend makes use of the TfIdfVectorizer passed as project.vectorizer to most methods

A backend needs to implement these key methods:

• initialize (optional): set up the necessary internal data structures
• train (optional): train the model on a given document corpus
• _suggest: this is the key method that is given a single document (text) and returns suggested subjects

Learning backends additionally implement:

• learn: continue training the model on the given corpus