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Xapian::PostingSource

Table of contents

Introduction

Xapian::PostingSource is an API class which you can subclass to feed data to Xapian's matcher. This feature can be made use of in a number of ways - for example:

As a filter - a subclass could return a stream of document ids to filter a query against.

As a weight boost - a subclass could return every document, but with a varying weight so that certain documents receive a weight boost. This could be used to prefer documents based on some external factor, such as age, price, proximity to a physical location, link analysis score, etc.

As an alternative way of ranking documents - if the weighting scheme is set to Xapian::BoolWeight, then the ranking will be entirely by the weight returned by Xapian::PostingSource.

Anatomy

When first constructed, a PostingSource is not tied to a particular database. Before Xapian can get any postings (or statistics) from the source, it needs to be supplied with a database. This is performed by the init() method, which is passed a single parameter holding the database to use. This method will always be called before asking for any information about the postings in the list. If a posting source is used for multiple searches, the init() method will be called before each search; implementations must cope with init() being called multiple times, and should always use the database provided in the most recent call:

virtual void init(const Xapian::Database & db) = 0;

Three methods return statistics independent of the iteration position. These are upper and lower bounds for the number of documents which can be returned, and an estimate of this number:

virtual Xapian::doccount get_termfreq_min() const = 0;
virtual Xapian::doccount get_termfreq_max() const = 0;
virtual Xapian::doccount get_termfreq_est() const = 0;

These methods are pure-virtual in the base class, so you have to define them when deriving your subclass.

It must always be true that:

get_termfreq_min() <= get_termfreq_est() <= get_termfreq_max()

PostingSources must always return documents in increasing document ID order.

After construction, a PostingSource points to a position before the first document id - so before a docid can be read, the position must be advanced by calling next(), skip_to() or check().

The get_weight() method returns the weight that you want to contribute to the current document. This weight must always be >= 0:

virtual double get_weight() const;

The default implementation of get_weight() returns 0, for convenience when deriving "weight-less" subclasses.

You also need to specify an upper bound on the value which get_weight() can return, which is used by the matcher to perform various optimisations. You should try hard to find a bound for efficiency, but if there really isn't one then you can set DBL_MAX:

void get_maxweight(double max_weight);

This method specifies an upper bound on what get_weight() will return from now on (until the next call to init()). So if you know that the upper bound has decreased, you should call set_maxweight() with the new reduced bound.

One thing to be aware of is that currently calling set_maxweight() during the match triggers an recursion through the postlist tree to recalculate the new overall maxweight, which takes a comparable amount of time to calculating the weight for a matching document. If your maxweight reduces for nearly every document, you may want to profile to see if it's beneficial to notify every single change. Experiments with a modified FixedWeightPostingSource which forces a pointless recalculation for every document suggest a worst case overhead in search times of about 37%, but reports of profiling results for real world examples are most welcome. In real cases, this overhead could easily be offset by the extra scope for matcher optimisations which a tighter maxweight bound allows.

A simple approach to reducing the number of calculations is only to do it every N documents. If it's cheap to calculate the maxweight in your posting source, a more sophisticated strategy might be to decide an absolute maximum number of times to update the maxweight (say 100) and then to call it whenever:

last_notified_maxweight - new_maxweight >= original_maxweight / 100.0

This ensures that only reasonably significant drops result in a recalculation of the maxweight.

Since get_weight() must always return >= 0, the upper bound must clearly also always be >= 0 too. If you don't call get_maxweight() then the bound defaults to 0, to match the default implementation of get_weight().

If you want to read the currently set upper bound, you can call:

double get_maxweight() const;

This is just a getter method for a member variable in the Xapian::PostingSource class, and is inlined from the API headers, so there's no point storing this yourself in your subclass - it should be just as efficient to call get_maxweight() whenever you want to use it.

The at_end() method checks if the current iteration position is past the last entry:

virtual bool at_end() const = 0;

The get_docid() method returns the document id at the current iteration position:

virtual Xapian::docid get_docid() const = 0;

There are three methods which advance the current position. All of these take a Xapian::Weight parameter min_wt, which indicates the minimum weight contribution which the matcher is interested in. The matcher still checks the weight of documents so it's OK to ignore this parameter completely, or to use it to discard only some documents. But it can be useful for optimising in some cases.

The simplest of these three methods is next(), which simply advances the iteration position to the next document (possibly skipping documents with weight contribution < min_wt):

virtual void next(double min_wt) = 0;

Then there's skip_to(). This advances the iteration position to the next document with document id >= that specified (possibly also skipping documents with weight contribution < min_wt):

virtual void skip_to(Xapian::docid did, double min_wt);

A default implementation of skip_to() is provided which just calls next() repeatedly. This works but skip_to() can often be implemented much more efficiently.

The final method of this group is check(). In some cases, it's fairly cheap to check if a given document matches, but the requirement that skip_to() must leave the iteration position on the next document is rather costly to implement (for example, it might require linear scanning of document ids). To avoid this where possible, the check() method allows the matcher to just check if a given document matches:

virtual bool check(Xapian::docid did, double min_wt);

The return value is true if the method leaves the iteration position valid, and false if it doesn't. In the latter case, next() will advance to the first matching position after document id did, and skip_to() will act as it would if the iteration position was the first matching position after did.

The default implementation of check() is just a thin wrapper around skip_to() which returns true - you should use this if skip_to() incurs only a small extra cost.

There's also a method to return a string describing this object:

virtual std::string get_description() const;

The default implementation returns a generic answer. This default is provided to avoid forcing you to provide an implementation if you don't really care what get_description() gives for your sub-class.

Examples

Here is an example of a Python PostingSource which contributes additional weight from some external source (note that in Python, you call next() on an iterator to get each item, including the first, which is exactly the semantics we need to implement here):

class ExternalWeightPostingSource(xapian.PostingSource):
"""
A Xapian posting source returning weights from an external source.
"""
def __init__(self, db, wtsource):
    xapian.PostingSource.__init__(self)
    self.db = db
    self.wtsource = wtsource

def init(self, db):
    self.alldocs = db.postlist('')

def get_termfreq_min(self): return 0
def get_termfreq_est(self): return self.db.get_doccount()
def get_termfreq_max(self): return self.db.get_doccount()

def next(self, minweight):
    try:
    self.current = self.alldocs.next()
    except StopIteration:
    self.current = None

def skip_to(self, docid, minweight):
    try:
    self.current = self.alldocs.skip_to(docid)
    except StopIteration:
    self.current = None

def at_end(self):
    return self.current is None

def get_docid(self):
    return self.current.docid

def get_maxweight(self):
    return self.wtsource.get_maxweight()

def get_weight(self):
    doc = self.db.get_document(self.current.docid)
    return self.wtsource.get_weight(doc)

ExternalWeightPostingSource doesn't restrict which documents match - it's intended to be combined with an existing query using OP_AND_MAYBE like so:

extwtps = xapian.ExternalWeightPostingSource(db, wtsource)
query = xapian.Query(query.OP_AND_MAYBE, query, xapian.Query(extwtps))

The wtsource would be a class like this one:

class WeightSource(object):
def get_maxweight(self):
    return 12.34;

def get_weight(self, doc):
    return some_func(doc.get_docid())

Multiple databases, and remote databases

In order to work with searches across multiple databases, or in remote databases, some additional methods need to be implemented in your Xapian::PostingSource subclass. The first of these is clone(), which is used for multi database searches. This method should just return a newly allocated instance of the same posting source class, initialised in the same way as the source that clone() was called on. The returned source will be deallocated by the caller (using "delete" - so you should allocate it with "new").

If you don't care about supporting searches across multiple databases, you can simply return NULL from this method. In fact, the default implementation does this, so you can just leave the default implementation in place. If clone() returns NULL, an attempt to perform a search with multiple databases will raise an exception:

virtual PostingSource * clone() const;

To work with searches across remote databases, you need to implement a few more methods. Firstly, you need to implement the name() method. This simply returns the name of your posting source (fully qualified with any namespace):

virtual std::string name() const;

Next, you need to implement the serialise and unserialise methods. The serialise() method converts all the settings of the PostingSource to a string, and the unserialise() method converts one of these strings back into a PostingSource. Note that the serialised string doesn't need to include any information about the current iteration position of the PostingSource:

virtual std::string serialise() const;
virtual PostingSource * unserialise(const std::string &s) const;

Finally, you need to make a remote server which knows about your PostingSource. Currently, the only way to do this is to modify the source slightly, and compile your own xapian-tcpsrv. To do this, you need to edit xapian-core/bin/xapian-tcpsrv.cc and find the register_user_weighting_schemes() function. If MyPostingSource is your posting source, at the end of this function, add these lines:

Xapian::Registry registry;
registry.register_postingsource(MyPostingSource());
server.set_registry(registry);