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| <div class="section" id="physical-data-modeling"> |
| <h1>Physical Data Modeling<a class="headerlink" href="#physical-data-modeling" title="Permalink to this headline">¶</a></h1> |
| <p>Once you have a logical data model defined, creating the physical model |
| is a relatively simple process.</p> |
| <p>You walk through each of the logical model tables, assigning types to |
| each item. You can use any valid <a class="reference internal" href="../cql/types.html#data-types"><span class="std std-ref">CQL data type</span></a>, |
| 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> |
| <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> |
| <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> |
| <img alt="../_images/data_modeling_chebotko_physical.png" src="../_images/data_modeling_chebotko_physical.png" /> |
| <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 class="section" id="hotel-physical-data-model"> |
| <h2>Hotel Physical Data Model<a class="headerlink" href="#hotel-physical-data-model" title="Permalink to this headline">¶</a></h2> |
| <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 class="docutils literal notranslate"><span class="pre">hotel</span></code> keyspace to contain tables for hotel and availability |
| data, and a <code class="docutils literal notranslate"><span class="pre">reservation</span></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> |
| <p>For the <code class="docutils literal notranslate"><span class="pre">hotels</span></code> table, use Cassandra’s <code class="docutils literal notranslate"><span class="pre">text</span></code> type to |
| represent the hotel’s <code class="docutils literal notranslate"><span class="pre">id</span></code>. For the address, create an |
| <code class="docutils literal notranslate"><span class="pre">address</span></code> user defined type. Use the <code class="docutils literal notranslate"><span class="pre">text</span></code> type to represent the |
| phone number, as there is considerable variance in the formatting of |
| numbers between countries.</p> |
| <p>While it would make sense to use the <code class="docutils literal notranslate"><span class="pre">uuid</span></code> type for attributes such |
| as the <code class="docutils literal notranslate"><span class="pre">hotel_id</span></code>, this document uses mostly <code class="docutils literal notranslate"><span class="pre">text</span></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 class="docutils literal notranslate"><span class="pre">hotel_ids</span></code>, while acknowledging they are not necessarily globally |
| unique.</p> |
| <p>You’ll find that it’s often helpful to use unique IDs to uniquely |
| reference elements, and to use these <code class="docutils literal notranslate"><span class="pre">uuids</span></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> |
| <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> |
| <img alt="../_images/data_modeling_hotel_physical.png" src="../_images/data_modeling_hotel_physical.png" /> |
| <p>Note that the <code class="docutils literal notranslate"><span class="pre">address</span></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 class="docutils literal notranslate"><span class="pre">hotels</span></code> and <code class="docutils literal notranslate"><span class="pre">hotels_by_poi</span></code> tables.</p> |
| <p>User-defined types are frequently used to help reduce duplication of |
| non-primary key columns, as was done with the <code class="docutils literal notranslate"><span class="pre">address</span></code> |
| user-defined type. This can reduce complexity in the design.</p> |
| <p>Remember that the scope of a UDT is the keyspace in which it is defined. |
| To use <code class="docutils literal notranslate"><span class="pre">address</span></code> in the <code class="docutils literal notranslate"><span class="pre">reservation</span></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 class="section" id="reservation-physical-data-model"> |
| <h2>Reservation Physical Data Model<a class="headerlink" href="#reservation-physical-data-model" title="Permalink to this headline">¶</a></h2> |
| <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> |
| <img alt="../_images/data_modeling_reservation_physical.png" src="../_images/data_modeling_reservation_physical.png" /> |
| <p>Note that the <code class="docutils literal notranslate"><span class="pre">address</span></code> type is reproduced in this keyspace and |
| <code class="docutils literal notranslate"><span class="pre">guest_id</span></code> is modeled as a <code class="docutils literal notranslate"><span class="pre">uuid</span></code> type in all of the tables.</p> |
| <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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