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| <main class="article default-main full-800" data-ceiling="topbar"> |
| <div class="article-banner"> |
| <p>You are viewing the documentation for a prerelease version.</p> |
| </div> |
| <div class="article-header"> |
| <nav class="crumbs" aria-label="breadcrumbs"> |
| <ul> |
| <li class="crumb">Cassandra</li> |
| <li class="crumb"><a href="index.html">Data Modeling</a></li> |
| <li class="crumb"><a href="data-modeling_refining.html">Evaluating and refining data models</a></li> |
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| <li class="tool edit"><a href="https://github.com/apache/cassandra/edit/cassandra-5.0/doc/modules/cassandra/pages/developing/data-modeling/data-modeling_refining.adoc" title="Edit Page" target="_blank" rel="noopener">Edit</a></li> |
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| </div> |
| <article class="doc"> |
| <h1 class="page">Evaluating and refining data models</h1> |
| <div id="preamble"> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>Once you’ve created a physical model, there are some steps you’ll want |
| to take to evaluate and refine table designs to help ensure optimal |
| performance.</p> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="calculating-partition-size"><a class="anchor" href="#calculating-partition-size"></a>Calculating Partition Size</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>The first thing that you want to look for is whether your tables will |
| have partitions that will be overly large, or to put it another way, too |
| wide. Partition size is measured by the number of cells (values) that |
| are stored in the partition. Cassandra’s hard limit is 2 billion cells |
| per partition, but you’ll likely run into performance issues before |
| reaching that limit.</p> |
| </div> |
| <div class="paragraph"> |
| <p>In order to calculate the size of partitions, use the following formula:</p> |
| </div> |
| <div class="stemblock"> |
| <div class="content"> |
| \[N_v = N_r (N_c - N_{pk} - N_s) + N_s\] |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>The number of values (or cells) in the partition (N<sub>v</sub>) is equal to the |
| number of static columns (N<sub>s</sub>) plus the product of the number of rows |
| (N<sub>r</sub>) and the number of of values per row. The number of values per row |
| is defined as the number of columns (N<sub>c</sub>) minus the number of primary |
| key columns (N<sub>pk</sub>) and static columns (N<sub>s</sub>).</p> |
| </div> |
| <div class="paragraph"> |
| <p>The number of columns tends to be relatively static, although it is |
| possible to alter tables at runtime. For this reason, a primary driver |
| of partition size is the number of rows in the partition. This is a key |
| factor that you must consider in determining whether a partition has the |
| potential to get too large. Two billion values sounds like a lot, but in |
| a sensor system where tens or hundreds of values are measured every |
| millisecond, the number of values starts to add up pretty fast.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Let’s take a look at one of the tables to analyze the partition size. |
| Because it has a wide partition design with one partition per hotel, |
| look at the <code>available_rooms_by_hotel_date</code> table. The table has four |
| columns total (N<sub>c</sub> = 4), including three primary key columns (N<sub>pk</sub> = |
| 3) and no static columns (N<sub>s</sub> = 0). Plugging these values into the |
| formula, the result is:</p> |
| </div> |
| <div class="stemblock"> |
| <div class="content"> |
| \[N_v = N_r (4 - 3 - 0) + 0 = 1N_r\] |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>Therefore the number of values for this table is equal to the number of |
| rows. You still need to determine a number of rows. To do this, make |
| estimates based on the application design. The table is storing a record |
| for each room, in each of hotel, for every night. Let’s assume the |
| system will be used to store two years of inventory at a time, and there |
| are 5,000 hotels in the system, with an average of 100 rooms in each |
| hotel.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Since there is a partition for each hotel, the estimated number of rows |
| per partition is as follows:</p> |
| </div> |
| <div class="stemblock"> |
| <div class="content"> |
| \[N_r = 100 rooms/hotel \times 730 days = 73,000 rows\] |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>This relatively small number of rows per partition is not going to get |
| you in too much trouble, but if you start storing more dates of |
| inventory, or don’t manage the size of the inventory well using TTL, you |
| could start having issues. You still might want to look at breaking up |
| this large partition, which you’ll see how to do shortly.</p> |
| </div> |
| <div class="paragraph"> |
| <p>When performing sizing calculations, it is tempting to assume the |
| nominal or average case for variables such as the number of rows. |
| Consider calculating the worst case as well, as these sorts of |
| predictions have a way of coming true in successful systems.</p> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="calculating-size-on-disk"><a class="anchor" href="#calculating-size-on-disk"></a>Calculating Size on Disk</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>In addition to calculating the size of a partition, it is also an |
| excellent idea to estimate the amount of disk space that will be |
| required for each table you plan to store in the cluster. In order to |
| determine the size, use the following formula to determine the size S<sub>t</sub> |
| of a partition:</p> |
| </div> |
| <div class="stemblock"> |
| <div class="content"> |
| \[S_t = \displaystyle\sum_i sizeOf\big (c_{k_i}\big) + \displaystyle\sum_j sizeOf\big(c_{s_j}\big) + N_r\times \bigg(\displaystyle\sum_k sizeOf\big(c_{r_k}\big) + \displaystyle\sum_l sizeOf\big(c_{c_l}\big)\bigg) +\] |
| </div> |
| </div> |
| <div class="stemblock"> |
| <div class="content"> |
| \[N_v\times sizeOf\big(t_{avg}\big)\] |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>This is a bit more complex than the previous formula, but let’s break it |
| down a bit at a time. Let’s take a look at the notation first:</p> |
| </div> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p>In this formula, c<sub>k</sub> refers to partition key columns, c<sub>s</sub> to static |
| columns, c<sub>r</sub> to regular columns, and c<sub>c</sub> to clustering columns.</p> |
| </li> |
| <li> |
| <p>The term t<sub>avg</sub> refers to the average number of bytes of metadata |
| stored per cell, such as timestamps. It is typical to use an estimate of |
| 8 bytes for this value.</p> |
| </li> |
| <li> |
| <p>You’ll recognize the number of rows N<sub>r</sub> and number of values N<sub>v</sub> |
| from previous calculations.</p> |
| </li> |
| <li> |
| <p>The <strong>sizeOf()</strong> function refers to the size in bytes of the CQL data |
| type of each referenced column.</p> |
| </li> |
| </ul> |
| </div> |
| <div class="paragraph"> |
| <p>The first term asks you to sum the size of the partition key columns. |
| For this example, the <code>available_rooms_by_hotel_date</code> table has a single |
| partition key column, the <code>hotel_id</code>, which is of type <code>text</code>. Assuming |
| that hotel identifiers are simple 5-character codes, you have a 5-byte |
| value, so the sum of the partition key column sizes is 5 bytes.</p> |
| </div> |
| <div class="paragraph"> |
| <p>The second term asks you to sum the size of the static columns. This |
| table has no static columns, so the size is 0 bytes.</p> |
| </div> |
| <div class="paragraph"> |
| <p>The third term is the most involved, and for good reason—it is |
| calculating the size of the cells in the partition. Sum the size of the |
| clustering columns and regular columns. The two clustering columns are |
| the <code>date</code>, which is 4 bytes, and the <code>room_number</code>, which is a 2-byte |
| short integer, giving a sum of 6 bytes. There is only a single regular |
| column, the boolean <code>is_available</code>, which is 1 byte in size. Summing the |
| regular column size (1 byte) plus the clustering column size (6 bytes) |
| gives a total of 7 bytes. To finish up the term, multiply this value by |
| the number of rows (73,000), giving a result of 511,000 bytes (0.51 MB).</p> |
| </div> |
| <div class="paragraph"> |
| <p>The fourth term is simply counting the metadata that that Cassandra |
| stores for each cell. In the storage format used by Cassandra 3.0 and |
| later, the amount of metadata for a given cell varies based on the type |
| of data being stored, and whether or not custom timestamp or TTL values |
| are specified for individual cells. For this table, reuse the number of |
| values from the previous calculation (73,000) and multiply by 8, which |
| gives 0.58 MB.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Adding these terms together, you get a final estimate:</p> |
| </div> |
| <div class="stemblock"> |
| <div class="content"> |
| \[Partition size = 16 bytes + 0 bytes + 0.51 MB + 0.58 MB = 1.1 MB\] |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>This formula is an approximation of the actual size of a partition on |
| disk, but is accurate enough to be quite useful. Remembering that the |
| partition must be able to fit on a single node, it looks like the table |
| design will not put a lot of strain on disk storage.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Cassandra’s storage engine was re-implemented for the 3.0 release, |
| including a new format for SSTable files. The previous format stored a |
| separate copy of the clustering columns as part of the record for each |
| cell. The newer format eliminates this duplication, which reduces the |
| size of stored data and simplifies the formula for computing that size.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Keep in mind also that this estimate only counts a single replica of |
| data. You will need to multiply the value obtained here by the number of |
| partitions and the number of replicas specified by the keyspace’s |
| replication strategy in order to determine the total required total |
| capacity for each table. This will come in handy when you plan your |
| cluster.</p> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="breaking-up-large-partitions"><a class="anchor" href="#breaking-up-large-partitions"></a>Breaking Up Large Partitions</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>As discussed previously, the goal is to design tables that can provide |
| the data you need with queries that touch a single partition, or failing |
| that, the minimum possible number of partitions. However, as shown in |
| the examples, it is quite possible to design wide partition-style tables |
| that approach Cassandra’s built-in limits. Performing sizing analysis on |
| tables may reveal partitions that are potentially too large, either in |
| number of values, size on disk, or both.</p> |
| </div> |
| <div class="paragraph"> |
| <p>The technique for splitting a large partition is straightforward: add an |
| additional column to the partition key. In most cases, moving one of the |
| existing columns into the partition key will be sufficient. Another |
| option is to introduce an additional column to the table to act as a |
| sharding key, but this requires additional application logic.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Continuing to examine the available rooms example, if you add the <code>date</code> |
| column to the partition key for the <code>available_rooms_by_hotel_date</code> |
| table, each partition would then represent the availability of rooms at |
| a specific hotel on a specific date. This will certainly yield |
| partitions that are significantly smaller, perhaps too small, as the |
| data for consecutive days will likely be on separate nodes.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Another technique known as <strong>bucketing</strong> is often used to break the data |
| into moderate-size partitions. For example, you could bucketize the |
| <code>available_rooms_by_hotel_date</code> table by adding a <code>month</code> column to the |
| partition key, perhaps represented as an integer. The comparison with |
| the original design is shown in the figure below. While the <code>month</code> |
| column is partially duplicative of the <code>date</code>, it provides a nice way of |
| grouping related data in a partition that will not get too large.</p> |
| </div> |
| <div class="imageblock"> |
| <div class="content"> |
| <img src="../../_images/data-modeling_hotel_bucketing.png" alt="image"> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>If you really felt strongly about preserving a wide partition design, |
| you could instead add the <code>room_id</code> to the partition key, so that each |
| partition would represent the availability of the room across all dates. |
| Because there was no query identified that involves searching |
| availability of a specific room, the first or second design approach is |
| most suitable to the application needs.</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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