Serialisation of Queries and Documents


In order to pass xapian.Query and xapian.Document objects to or from remote databases, Xapian includes support for serialising these objects to binary strings, and then converting these strings back into objects. This support may be accessed directly, and used for storing persistent representations of such objects. The representations used are not architecture dependent, so you can successfully unserialise an object on a machine with a different word size or endianness to the machine it was serialised on.

Be aware that the serialised representation may change between release series, so if you’re using serialised objects for long term storage you will need a strategy for dealing with this. Changes to the representation will be clearly noted in the release notes.

Serialising Documents

If you have a xapian.Document object which you want to serialise, you can call the xapian.Document.serialise() method on it, which returns a string.

Documents are often lazily fetched from databases: this method will first force the full document contents to be fetched from the database, in order to serialise them. The serialised document will have identical contents (data, terms, positions, values) to the original document.

To get a document from a serialised form, call the static xapian.Document.unserialise() method, passing it the string returned from serialise(), which will give you a new xapian.Document object.

Serialising Queries

Serialisation of queries is very similar to serialisation of documents: there is a xapian.Query.serialise() method to produce a serialised Query, and a corresponding xapian.Query.unserialise() method to produce a xapian.Query object from a serialised representation.

However, there is a wrinkle. A query can contain arbitrary user-defined xapian.PostingSource subqueries. In order to serialise and unserialise such queries, all the xapian.PostingSource subclasses used in the query must implement the name(), serialise() and unserialise() methods (see Posting sources for details).

In addition, a special form of unserialise must be used which takes a xapian.Registry object as an additional parameter, which must know all the custom posting sources used in the query. You need to call xapian.Registry.register_posting_source() to register each such class.

Note that xapian.Registry objects always know about built-in posting sources (such as xapian.ValueWeightPostingSource), so you don’t need to call register_posting_source() for them.