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Choosing dealers

Once Snorky receives all relevant deltas, the next step is to re-send them to the interested clients, if any. This is controlled with dealer classes.

What is a dealer

Dealers are classes which track client subscriptions to certain kinds of models.

Dealers also manage the delivery of deltas, by determining which clients are subscribed to the information that they carry.

What is a subscription

In Snorky, clients acquire subscriptions to dealers. Each subscription conforms one or more subscription items. Each subscription item specifies a dealer, and a query to that dealer.

For example, a dealer may be called CommentsByBlogEntry. A subscription may contain one subscription item having CommentsByBlogEntry as dealer and 15 as query, in order to get notified of new comments in the blog entry with id 15.

The Dealer API

.. automodule:: snorky.services.datasync.dealers

    The most basic dealer API is the :py:class:`Dealer` class. You can subclass it to make new dealers.

    .. autoclass:: Dealer

        .. autoattribute:: name
        .. autoattribute:: model
        .. automethod:: add_subscription_item
        .. automethod:: remove_subscription_item
        .. automethod:: get_subscription_items_for_model

Simple dealers

Often your dealer only has to filter models by a certain field which clients subscribe to.

For example, the CommentsByBlogEntry dealer would receive subscriptions that specify a blog entry id as query, and each time it receives a delta of model Comment, it would look which blog entry id it is for, and forward it to those clients which subscribed to it.

.. automodule:: snorky.services.datasync.dealers
    :noindex:

    Snorky comes with a :py:class:`SimpleDealer` class that leverages this pattern.

    .. autoclass:: SimpleDealer

        .. automethod:: get_key_for_model

Example

from snorky.services.datasync.dealers import SimpleDealer

class CommentsByBlogEntry(SimpleDealer):
    name = "CommentsByBlogEntry" # optional
    model = "Comment"

    def get_key_for_model(self, model):
        return model["entryId"]

Broadcast dealers

.. automodule:: snorky.services.datasync.dealers
    :noindex:

    Sometimes you want the clients to receive all deltas for a certain model class, unfiltered. For this purpose there is the :py:class:`BroadcastDealer` class.

    .. autoclass:: BroadcastDealer

Example

from snorky.services.datasync.dealers import BroadcastDealer

class AllTasks(BroadcastDealer):
    name = "AllTasks" # optional
    model = "Task"

Filter dealers

.. automodule:: snorky.services.datasync.dealers
    :noindex:

    For cases where clients need to ask for data filtered to complex criteria :py:class:`FilterDealer` provides an advanced dealer which supports complex filter expressions.

Filter syntax

The subscription query for this dealer must be a JSON list specifying a filter expression in prefix notation. These are some examples:

  • ['==', 'color', 'blue']

    color is 'blue'.

  • ['<', 'age', 21]

    age is less than 21.

  • ['>=', 'age', 21]

    age is greater than or equal to 21.

  • ['and', ['==', 'service', 'prosody'], ['>=', 'severity_level', 3]]

    service is 'prosody' and severity_level is greater than or equal to 3.

  • ['or', ['not', ['==', 'service', 'java']], ['>=', 'severity_level', 3]]

    service is not 'java' or severity_level is greater than or equal to 3.

  • ['==', 'player.color', 'blue']

    player is a dictionary which contains a property color, and the value of that property is 'blue'.

Example

from snorky.services.datasync.dealers import BroadcastDealer

class FilteredTasks(FilterDealer):
    name = "FilteredTasks" # optional
    model = "Task"