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Multiple Indexes

Much like Django's multiple database support, Haystack has "multiple index" support. This allows you to talk to several different engines at the same time. It enables things like master-slave setups, multiple language indexing, separate indexes for general search & autocomplete as well as other options.

Specifying Available Connections

You can supply as many backends as you like, each with a descriptive name. A complete setup that accesses all backends might look like:

HAYSTACK_CONNECTIONS = {
    'default': {
        'ENGINE': 'haystack.backends.solr_backend.SolrEngine',
        'URL': 'http://localhost:9001/solr/default',
        'TIMEOUT': 60 * 5,
        'INCLUDE_SPELLING': True,
        'BATCH_SIZE': 100,
        'SILENTLY_FAIL': True,
    },
    'autocomplete': {
        'ENGINE': 'haystack.backends.whoosh_backend.WhooshEngine',
        'PATH': '/home/search/whoosh_index',
        'STORAGE': 'file',
        'POST_LIMIT': 128 * 1024 * 1024,
        'INCLUDE_SPELLING': True,
        'BATCH_SIZE': 100,
        'SILENTLY_FAIL': True,
    },
    'slave': {
        'ENGINE': 'xapian_backend.XapianEngine',
        'PATH': '/home/search/xapian_index',
        'INCLUDE_SPELLING': True,
        'BATCH_SIZE': 100,
        'SILENTLY_FAIL': True,
    },
    'db': {
        'ENGINE': 'haystack.backends.simple_backend.SimpleEngine',
        'SILENTLY_FAIL': True,
    }
}

You are required to have at least one connection listed within HAYSTACK_CONNECTIONS, it must be named default & it must have a valid ENGINE within it.

Management Commands

All management commands that manipulate data use ONLY one connection at a time. By default, they use the default index but accept a --using flag to specify a different connection. For example:

./manage.py rebuild_index --noinput --using=whoosh

Automatic Routing

To make the selection of the correct index easier, Haystack (like Django) has the concept of "routers". All provided routers are checked whenever a read or write happens, stopping at the first router that knows how to handle it.

Haystack ships with a DefaultRouter enabled. It looks like:

class DefaultRouter(BaseRouter):
    def for_read(self, **hints):
        return DEFAULT_ALIAS

    def for_write(self, **hints):
        return DEFAULT_ALIAS

On a read (when a search query is executed), the DefaultRouter.for_read method is checked & returns the DEFAULT_ALIAS (which is default), telling whatever requested it that it should perform the query against the default connection. The same process is followed for writes.

If the for_read or for_write method returns None, that indicates that the current router can't handle the data. The next router is then checked.

The hints passed can be anything that helps the router make a decision. This data should always be considered optional & be guarded against. At current, for_write receives an index option (pointing to the SearchIndex calling it) while for_read may receive models (being a list of Model classes the SearchQuerySet may be looking at).

You may provide as many routers as you like by overriding the HAYSTACK_ROUTERS setting. For example:

HAYSTACK_ROUTERS = ['myapp.routers.MasterRouter', 'myapp.routers.SlaveRouter', 'haystack.routers.DefaultRouter']

Master-Slave Example

The MasterRouter & SlaveRouter might look like:

from haystack import routers


class MasterRouter(routers.BaseRouter):
    def for_write(self, **hints):
        return 'master'

    def for_read(self, **hints):
        return None


class SlaveRouter(routers.BaseRouter):
    def for_write(self, **hints):
        return None

    def for_read(self, **hints):
        return 'slave'

The observant might notice that since the methods don't overlap, this could be combined into one Router like so:

from haystack import routers


class MasterSlaveRouter(routers.BaseRouter):
    def for_write(self, **hints):
        return 'master'

    def for_read(self, **hints):
        return 'slave'

Manually Selecting

There may be times when automatic selection of the correct index is undesirable, such as when fixing erroneous data in an index or when you know exactly where data should be located.

For this, the SearchQuerySet class allows for manually selecting the index via the SearchQuerySet.using method:

from haystack.query import SearchQuerySet

# Uses the routers' opinion.
sqs = SearchQuerySet().auto_query('banana')

# Forces the default.
sqs = SearchQuerySet().using('default').auto_query('banana')

# Forces the slave connection (presuming it was setup).
sqs = SearchQuerySet().using('slave').auto_query('banana')

Warning

Note that the models a SearchQuerySet is trying to pull from must all come from the same index. Haystack is not able to combine search queries against different indexes.

Custom Index Selection

If a specific backend has been selected, the SearchIndex.index_queryset and SearchIndex.read_queryset will receive the backend name, giving indexes the opportunity to customize the returned queryset.

For example, a site which uses separate indexes for recent items and older content might define index_queryset to filter the items based on date:

def index_queryset(self, using=None):
    qs = Note.objects.all()
    archive_limit = datetime.datetime.now() - datetime.timedelta(days=90)

    if using == "archive":
        return qs.filter(pub_date__lte=archive_limit)
    else:
        return qs.filter(pub_date__gte=archive_limit)

Multi-lingual Content

Most search engines require you to set the language at the index level. For example, a multi-lingual site using Solr can use multiple cores and corresponding Haystack backends using the language name. Under this scenario, queries are simple:

sqs = SearchQuerySet.using(lang).auto_query(…)

During index updates, the Index's index_queryset method will need to filter the items to avoid sending the wrong content to the search engine:

def index_queryset(self, using=None):
    return Post.objects.filter(language=using)
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