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| <article class="doc"> |
| <h1 class="page">Physical Data Modeling</h1> |
| <div id="preamble"> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>Once you have a logical data model defined, creating the physical model |
| is a relatively simple process.</p> |
| </div> |
| <div class="paragraph"> |
| <p>You walk through each of the logical model tables, assigning types to |
| each item. You can use any valid <code>CQL data type <data-types></code>, including |
| the basic types, collections, and user-defined types. You may identify |
| additional user-defined types that can be created to simplify your |
| design.</p> |
| </div> |
| <div class="paragraph"> |
| <p>After you’ve assigned data types, you analyze the model by performing |
| size calculations and testing out how the model works. You may make some |
| adjustments based on your findings. Once again let’s cover the data |
| modeling process in more detail by working through an example.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Before getting started, let’s look at a few additions to the Chebotko |
| notation for physical data models. To draw physical models, you need to |
| be able to add the typing information for each column. This figure shows |
| the addition of a type for each column in a sample table.</p> |
| </div> |
| <div class="imageblock"> |
| <div class="content"> |
| <img src="../_images/data_modeling_chebotko_physical.png" alt="image"> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>The figure includes a designation of the keyspace containing each table |
| and visual cues for columns represented using collections and |
| user-defined types. Note the designation of static columns and secondary |
| index columns. There is no restriction on assigning these as part of a |
| logical model, but they are typically more of a physical data modeling |
| concern.</p> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="hotel-physical-data-model"><a class="anchor" href="#hotel-physical-data-model"></a>Hotel Physical Data Model</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>Now let’s get to work on the physical model. First, you need keyspaces |
| to contain the tables. To keep the design relatively simple, create a |
| <code>hotel</code> keyspace to contain tables for hotel and availability data, and |
| a <code>reservation</code> keyspace to contain tables for reservation and guest |
| data. In a real system, you might divide the tables across even more |
| keyspaces in order to separate concerns.</p> |
| </div> |
| <div class="paragraph"> |
| <p>For the <code>hotels</code> table, use Cassandra’s <code>text</code> type to represent the |
| hotel’s <code>id</code>. For the address, create an <code>address</code> user defined type. |
| Use the <code>text</code> type to represent the phone number, as there is |
| considerable variance in the formatting of numbers between countries.</p> |
| </div> |
| <div class="paragraph"> |
| <p>While it would make sense to use the <code>uuid</code> type for attributes such as |
| the <code>hotel_id</code>, this document uses mostly <code>text</code> attributes as |
| identifiers, to keep the samples simple and readable. For example, a |
| common convention in the hospitality industry is to reference properties |
| by short codes like "AZ123" or "NY229". This example uses these values |
| for <code>hotel_ids</code>, while acknowledging they are not necessarily globally |
| unique.</p> |
| </div> |
| <div class="paragraph"> |
| <p>You’ll find that it’s often helpful to use unique IDs to uniquely |
| reference elements, and to use these <code>uuids</code> as references in tables |
| representing other entities. This helps to minimize coupling between |
| different entity types. This may prove especially effective if you are |
| using a microservice architectural style for your application, in which |
| there are separate services responsible for each entity type.</p> |
| </div> |
| <div class="paragraph"> |
| <p>As you work to create physical representations of various tables in the |
| logical hotel data model, you use the same approach. The resulting |
| design is shown in this figure:</p> |
| </div> |
| <div class="imageblock"> |
| <div class="content"> |
| <img src="../_images/data_modeling_hotel_physical.png" alt="image"> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>Note that the <code>address</code> type is also included in the design. It is |
| designated with an asterisk to denote that it is a user-defined type, |
| and has no primary key columns identified. This type is used in the |
| <code>hotels</code> and <code>hotels_by_poi</code> tables.</p> |
| </div> |
| <div class="paragraph"> |
| <p>User-defined types are frequently used to help reduce duplication of |
| non-primary key columns, as was done with the <code>address</code> user-defined |
| type. This can reduce complexity in the design.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Remember that the scope of a UDT is the keyspace in which it is defined. |
| To use <code>address</code> in the <code>reservation</code> keyspace defined below design, |
| you’ll have to declare it again. This is just one of the many trade-offs |
| you have to make in data model design.</p> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="reservation-physical-data-model"><a class="anchor" href="#reservation-physical-data-model"></a>Reservation Physical Data Model</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>Now, let’s examine reservation tables in the design. Remember that the |
| logical model contained three denormalized tables to support queries for |
| reservations by confirmation number, guest, and hotel and date. For the |
| first iteration of your physical data model design, assume you’re going |
| to manage this denormalization manually. Note that this design could be |
| revised to use Cassandra’s (experimental) materialized view feature.</p> |
| </div> |
| <div class="imageblock"> |
| <div class="content"> |
| <img src="../_images/data_modeling_reservation_physical.png" alt="image"> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>Note that the <code>address</code> type is reproduced in this keyspace and |
| <code>guest_id</code> is modeled as a <code>uuid</code> type in all of the tables.</p> |
| </div> |
| <div class="paragraph"> |
| <p><em>Material adapted from Cassandra, The Definitive Guide. Published by |
| O’Reilly Media, Inc. Copyright © 2020 Jeff Carpenter, Eben Hewitt. All |
| rights reserved. Used with permission.</em></p> |
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