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"""a basic example of using the SQLAlchemy Sharding API.
Sharding refers to horizontally scaling data across multiple
databases.
The basic components of a "sharded" mapping are:
* multiple databases, each assigned a 'shard id'
* a function which can return a single shard id, given an instance
to be saved; this is called "shard_chooser"
* a function which can return a list of shard ids which apply to a particular
instance identifier; this is called "id_chooser". If it returns all shard ids,
all shards will be searched.
* a function which can return a list of shard ids to try, given a particular
Query ("query_chooser"). If it returns all shard ids, all shards will be
queried and the results joined together.
In this example, four sqlite databases will store information about
weather data on a database-per-continent basis. We provide example shard_chooser, id_chooser and query_chooser functions. The query_chooser illustrates inspection of the SQL expression element in order to attempt to determine a single shard being requested.
"""
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