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How Does Data Modeling Change in Apache Cassandra® 5.0 With Storage-Attached Indexes?

Data modeling in Apache Cassandra® is an important topic—how you model your data can affect your cluster’s performance, costs, etc. Today I’ll be looking at a new feature in Cassandra 5.0 called Storage -Attached Indexes (SAI), and how they affect the way you model data in Cassandra databases. 

First, I’ll briefly cover what SAIs are (for more information about SAIs, check out this post). Then I’ll look at 3 use cases where your modeling strategy could change with SAI. Finally, I’ll talk about benefits and constraints of SAIs. and constraints of SAIs. 

What Are StorageAttached Indexes? 

From the Cassandra 5.0 Documentation, Storage Attached Indexes (SAIs) “[provide] an indexing mechanism that is closely integrated with the Cassandra storage engine to make data modeling easier for users. Secondary Indexing, which is indexing values on properties that are not part of the Primary Key for that table, has been available for Cassandra in the past (called SASI and 2i). However, SAIs will replace the existing functionality, as it will be deprecated in 5.0, and then tentatively removed in Cassandra 6.0. 

This is because SAIs improve upon the older methods in a lot of key ways. For one, according to the devs, SAIs are the fastest indexing method for Cassandra clusters. This performance boost was a plus for using indexing in production environments. It also lowered the data storage overhead over prior implementations, which lowers costs by reducing the need for database storage, which induces operational costs, and by reducing latency when dealing with indexes, lowering a loss of user interaction due to high latency. 

How Do SAIs work? 

SAIs are implemented as part of the SSTables, or Sorted String Tables, of a Cassandra database. This is because SAIs index Memtables and SSTables as they are written. It filters from both in-memory and on-disk sources, filtering them out into a series of indexed columns at read time. I’m not going to go into too much detail here because there are a lot of existing resources on this exciting topic: see the Cassandra 5.0 Documentation and the Instaclustr site for examples. 

The main thing to keep in mind is that SAIs are attached to Cassandra’s storage engine, and it’s much more performant from speed, scalability, and data storage angles as a result. This means that you can use indexing reliably in production beginning with Cassandra 5.0, which allows data modeling to be improved very quickly. 

To learn more about how SAIs work, check out this piece from the Apache Cassandra blog. 

What Is SAI For? 

SAI is a filtering engine, and while it does have some functionality overlap with search engines, it directly says it is not an enterprise search engine” (source). 

SAI is meant for creating filters on non-primary-key or composite partition keys (source), essentially meaning that you can enable a ‘WHERE’ clause on any column in your Cassandra 5.0 database. This makes queries a lot more flexible without sacrificing latency or storage space as with prior methods.  

How Can We Use SAI When Data Modeling in Cassandra 5.0? 

Because of the increased scalability and performance of SAIs, data modeling in Cassandra 5.0 will most definitely change 

You will be able to search collections more thoroughly and easily, for instance, indexing is more of an option when designing your Cassandra queries. This will also allow new query types, which can improve your existing querieswhich by Cassandra’s design paradigm changes your table design. 

But what if you’re not on a greenfield project and want to use SAIs? No problem! SAI is backwards-compatible, and you can migrate your application one index at a time if you need. 

How Do StorageAttached Indexes Affect Data Modeling in Apache Cassandra 5.0? 

Cassandra’s SAI was designed with data modeling in mind (source). It unlocks new query patterns that make data modeling easier in quite a few cases. In the Cassandra team’s words: “You can create a table that is most natural for you, write to just that table, and query it any way you want.” (source) 

I think another great way to look at how SAIs affect data modeling is by looking at some queries that could be asked of SAI data. This is because Cassandra data modeling relies heavily on the queries that will be used to retrieve the data. I’ll take a look at 2 use cases: indexing as a means of searching a collection in a row and indexing to manage a one-to-many relationship. 

Use Case: Querying on Values of Non-Primary-Key Columns 

You may find you’re searching for records with a particular value in a particular column often in a table. An example may be a search form for a large email inbox with lots of filters. You could find yourself looking at a record like: 

  • Subject 
  • Sender 
  • Receiver 
  • Body 
  • Time sent 

Your table creation may look like: 

If you allow users to search for a particular subject or sender, and the data set is large, not having SAIs could make query times painful: 

To fix this problem, we can create secondary indexes on our sender, receiver, and body fields: 

Once you’ve established the indexes, you can run the same query and it will automatically use the SAI index to find all emails with a sender of “sam.example@examplemail.com OR by subject match/body match.  Note that although the data model changed with the inclusion of the indexes, the SELECT query does not change, and the fields of the table stayed the same as well! 

Use Case: Managing One-To-Many Relationships 

Going back to the previous example, one email could have many recipients. Prior to secondary indexes, you would need to scan every row in the collection of every row in the table in order to query on recipients. This could be solved in a few ways. One is to create a join table for recipients that contains an id, email id, and recipient. This becomes complicated when the constraint that each email should only appear once per email is added. With SAI, we now have an index-based solutioncreate an index on a collection of recipients for each row. 

The script to create the table and indices changes a bit: 

The text type of receivers changes to a set<text>. A set is used because each email should only occur once. This takes the logic you would have had to implement for the join table solution and moves it to Cassandra.  

The indexing code remains mostly the same, except for the creation of the index for receivers: 

That’s it! One line of CQL and there’s now an index on receivers. We can query for emails with a particular receiver: 

There are many one-to-many relationships that can be simplified in Cassandra with the use of secondary indexes and SAI. 

What Are the Benefits of Data Modeling With Storage Attached Indexes? 

There are many benefits to using SAI when data modeling in Cassandra 5.0: 

  • Query performance: because of SAI’s implementation, it has much faster query speeds than previous implementations, and indexed data is generally faster to search than unindexed data. This means you have more flexibility to search within your data and write queries that search non-primary-key columns and collections. 
  • Move over piecemeal: SAI’s backwards compatibility, coupled with how little your table structure has to change to add SAIs, means you can move over your data models piece by piece, meaning moving is easier.  
  • Data storage overhead: SAI has much lower data overhead than previous secondary index implementations, meaning more flexibility in what you can store in your data models without impacting overall storage needs. 
  • More complex queries/features: SAI allows you to write much more thorough queries when looking through SAIs, and offers up a lot of new functionality, like: 
    • Vector Search 
    • Numeric Range queries 
    • AND queries within indexes 
    • Support for map/set/ 

What Are the Constraints of StorageAttached Indexes? 

While there are benefits to SAI, there are also a few constraints, including: 

  • Because SAI is attached to the SSTable mechanism, the performance of queries on indexed columns will be “highly dependent on the compaction strategy in use” (per the Cassandra 5.0 CEP-7) 
  • SAI is not designed for unlimited-size data sets, such as logs; indexing on a dataset like that would cause performance issues. The reason for this is read latency at higher row counts spread across a cluster. It is also related to consistency level (CL), as the higher the CL is, the more nodes you’ll have to ping on larger datasets. (Source). 
  • Query complexity: while you can query as many indexes as you like, when you do so, you incur a cost related to the number of index values processed. Be aware when designing queries to select from as few indexes as necessary. 
  • You cannot index multiple columns in one index, as there is a 1-to-1 mapping of an SAI index to a column. You can however create separate indexes and query them simultaneously. 

This is a v1, and some features, like the LIKE comparison for strings, the OR operator, and global sorting are all slated for v2. 

Disk usage: SAI uses an extra 20-35% disk space over unindexed data; note that over previous versions of indexing, it consumes much less (source). You shouldn’t just make every column an index if you don’t need to, saving disk space and maintaining query performance. 


SAI is a very robust solution for secondary indexes, and their addition to Cassandra 5.0 opens the door for several new data modelling strategiesfrom searching non-primary-key columns, to managing one-to-many relationships, to vector search. To learn more about SAI, read this post from the Instaclustr by NetApp blog, or check out the documentation for Cassandra 5.0. 

If you’d like to test SAI without setting up and configuring Cassandra yourself, Instaclustr has a free trial and you can spin up Cassandra 5.0 clusters today through a public preview! Instaclustr also offers a bunch of educational content about Cassandra 5.0. 


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