Choose Apache Cassandra when you need a database that keeps running across several data centres, scales by adding nodes and can survive a whole data centre going down. Choose MongoDB when your data fits naturally into flexible JSON-style documents and you want a managed cloud service with a free tier to start. Both are NoSQL, but they're built for different strengths.
I read the Apache Cassandra home, basics and architecture pages, and MongoDB's documents manual page, licensing page and Atlas plans page. Here's what each project says about how it works.
Cassandra and MongoDB at a glance
| Apache Cassandra | MongoDB | |
|---|---|---|
| Data model | Distributed NoSQL with partitions and CQL | BSON documents of field-and-value pairs |
| Architecture | Masterless: any node does the same job | Atlas managed service or self-managed Community Edition |
| Consistency | AP by default, tunable per query | Not covered on the pages I read |
| Licence | Apache open source | Server Side Public License for versions after 16 Oct 2018 |
| Managed cloud | Not part of the project | Atlas on AWS, Azure and Google Cloud |
| Free option (as of October 2026) | Yes, open source | Atlas Free (512MB) and Community Edition |
Why Cassandra suits multi-region systems
Cassandra's basics page explains that it has a masterless architecture: any node can provide exactly the same function as any other, and nodes talk to each other through a peer-to-peer protocol called gossip. Its home page says this design means it can withstand an entire data centre outage with no data loss, across public clouds, private clouds and on-premises.
Scaling is linear. Rather than buying bigger servers, you add nodes, and the project says this works essentially indefinitely. Data is spread automatically by hashing the partition key to decide which node owns each row, which is why designing your partition keys is the most important modelling job in Cassandra.
Tunable consistency, explained simply
Under the CAP theorem, Cassandra's docs describe it as AP by default, meaning it favours availability and partition tolerance. But you can set the consistency level per query. With three replicas and a QUORUM level, for example, two replicas must acknowledge a read or write before it counts as successful. You can also choose synchronous or asynchronous replication for each update.
Where MongoDB feels easier
MongoDB stores records as BSON documents, a binary form of JSON with more data types. Each document is a set of field-and-value pairs, and values can be other documents, arrays or arrays of documents. Its manual points out that documents map naturally to the objects in your code, which removes the object-relational mapping layer relational databases need.
Getting started is quick. MongoDB's Atlas Free tier is described as free forever for learning and exploring, with 512MB of storage on shared resources, and Atlas runs on AWS, Microsoft Azure and Google Cloud. If you'd rather run it yourself, Community Edition is available for local development for free. For a wider look at what else is out there, here are some free database hosting options and their limits.
What MongoDB gives you at no charge is covered in our MongoDB free plan guide.
The licence difference
This trips up some companies. Cassandra is an Apache project. MongoDB's licensing page says the database server uses MongoDB's own Server Side Public License for versions released after 16 Oct 2018, with GNU AGPL for earlier versions, and its drivers use Apache 2.0. If you plan to offer MongoDB as a service to others, read the SSPL carefully first.
A managed AWS alternative is compared in our DynamoDB vs MongoDB guide.
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Five quick questions
- Must the database keep running if a whole data centre fails? Cassandra was designed for that.
- Is your data shaped like nested objects that change often? MongoDB's documents fit.
- Do you know your read patterns in advance? Cassandra rewards careful partition key design.
- Do you want a managed service with a free tier today? MongoDB Atlas offers one.
- Does licensing matter for your product? Compare the Apache project with MongoDB's SSPL.
If you're hiring for this kind of work, my guide to remote database analyst roles is useful, and so is this piece on managing your business data online.
My advice on Cassandra or MongoDB
For most product teams building an app with changing data, MongoDB is the friendlier starting point, and Atlas Free lets you try it without running servers. Pick Cassandra when uptime across regions and massive amounts of data are the main requirement, and you have people who'll design the data model around your queries. Don't choose either on hype; choose on how your data is written and read.
Do you make database tools? Tool makers can ask to be considered for a featured place in comparisons like this one.
Frequently asked questions
Is Cassandra open source?
Yes. Apache Cassandra is an open source NoSQL distributed database run by the Apache Software Foundation.
What licence does MongoDB use?
MongoDB's database server uses the Server Side Public License for versions released after 16 Oct 2018, and GNU AGPL for earlier versions. Its drivers use Apache 2.0.
Is MongoDB Atlas free?
MongoDB's Atlas Free tier is described as free forever for learning and exploring, with 512MB of storage on shared RAM and vCPU.
Does Cassandra have a master node?
No. Cassandra's docs say it has a masterless architecture where any node can provide the same functionality as any other.
Can I change consistency in Cassandra?
Yes. Cassandra is available and partition-tolerant by default, but you can configure the consistency level per query, such as QUORUM. Fast aggregate queries over fresh data are a different job, and real-time analytics databases are built for it.
