Picking the right NoSQL database for a .NET project in 2026 means navigating two very mature, cloud-native offerings that look similar on the surface but diverge dramatically the moment your workload scales. Azure Cosmos DB and MongoDB Atlas are both document databases, both globally distributed, and both well-supported in the ASP.NET Core ecosystem - but they make different bets on your team's priorities: Cosmos DB optimises for SLA predictability and Azure integration, while MongoDB Atlas optimises for developer familiarity, query expressiveness, and multi-cloud portability. If you want the complete implementation - including how to wire up both databases inside a production ASP.NET Core service alongside resilience patterns and test fixtures - that full source is available on Patreon, where the code walks every trade-off covered here with a real running system.
Understanding where these two databases pull in opposite directions is exactly what the ASP.NET Core Web API: Zero to Production course unpacks in its data layer chapters - showing how clean architecture keeps your domain model free of vendor lock-in regardless of which database you choose underneath.
What Each Database Is Trying to Solve
Azure Cosmos DB is a multi-model, globally distributed database built entirely inside Microsoft Azure. It was designed for scenarios where predictable single-digit-millisecond latency at any scale matters more than raw query flexibility. It supports multiple APIs - NoSQL, MongoDB, Cassandra, Gremlin, and Table - but the native API (formerly called Core SQL) is the most capable and the one Microsoft actively develops.
MongoDB Atlas is the managed cloud version of MongoDB, deployable on Azure, AWS, or GCP. It targets teams who already know MongoDB's document model and query language, or who want to stay independent of any single cloud provider. Atlas adds automated backups, scaling, search, and multi-region replication on top of the open-source engine.
For .NET teams, both have well-maintained client SDKs. Cosmos DB ships the Microsoft.Azure.Cosmos SDK, which is deeply integrated with the Azure ecosystem. MongoDB Atlas is consumed via the official MongoDB.Driver NuGet package, which supports the same driver API whether you run self-hosted or cloud.
How Does Each One Integrate with ASP.NET Core?
Azure Cosmos DB in ASP.NET Core
The Microsoft.Azure.Cosmos SDK is idiomatic for dependency injection. You register a CosmosClient singleton in Program.cs and inject it into repositories. The SDK handles connection pooling, retry logic, and circuit-breaking internally.
EF Core 9+ ships with a Cosmos DB provider (Microsoft.EntityFrameworkCore.Cosmos) that lets you map domain entities to Cosmos containers using the familiar Fluent API. The provider works, but it abstracts away partition keys and RU budgets - which are the two most important performance levers in Cosmos DB. Most experienced teams that go all-in on Cosmos DB drop the EF Core provider and work directly with the Cosmos SDK for performance-sensitive reads, even if they keep EF Core for other persistence concerns.
MongoDB Atlas in ASP.NET Core
The MongoDB.Driver SDK integrates with DI via the IMongoClient interface. MongoClient is thread-safe and designed to be a singleton. Unlike Cosmos DB, MongoDB has no concept of throughput units billed separately from storage - the query model is simpler to reason about for teams new to document databases.
EF Core does not have an official MongoDB provider from Microsoft (the MongoDB.EntityFrameworkCore package exists as a community driver, but it is not production-grade for complex scenarios as of 2026). This means MongoDB teams typically use the native driver pattern with repository abstraction, which is actually more aligned with how the Mongo ecosystem works.
Side-by-Side Comparison
| Dimension |
Azure Cosmos DB |
MongoDB Atlas |
| Pricing model |
Request Units (RUs) per second, provisioned or serverless |
vCPU-based clusters + storage; no throughput units |
| Query language |
SQL-like (NoSQL API) or MongoDB wire protocol |
MongoDB Query Language (MQL) |
| .NET EF Core support |
Official EF Core provider (limited) |
Community provider (limited) |
| Native .NET SDK |
Microsoft.Azure.Cosmos (Microsoft-maintained) |
MongoDB.Driver (MongoDB Inc.) |
| Global distribution |
Built-in, configurable per region with one click |
Atlas Global Clusters; multi-region writes require zone sharding |
| SLA |
99.999% availability (multi-region active-active) |
99.995% (multi-region) |
| Partition strategy |
Mandatory partition key; affects every query |
Sharding optional; single-server model scales vertically first |
| Consistency levels |
5 configurable levels (Strong → Eventual) |
Single consistency model per operation type |
| Multi-cloud |
Azure-only |
Azure, AWS, GCP |
| Local development |
Cosmos DB Emulator (Docker-friendly) |
MongoDB community edition or Atlas free tier |
| Search |
Integrated Azure Cognitive Search |
Atlas Search (Lucene-based, built-in) |
| ACID transactions |
Supported (within a single logical partition) |
Supported (multi-document, multi-collection) |
| Change feed |
Native change feed API |
Change Streams |
| Vendor lock-in risk |
High (Azure-only, proprietary RU model) |
Low (open-source engine, multi-cloud) |
When Does Cosmos DB Win?
Cosmos DB earns its place when your workload has these characteristics:
You are already deep in Azure. If your team uses Azure App Service, Azure Functions, Azure Service Bus, and Azure Active Directory, Cosmos DB fits without any integration tax. The SDK integrates with Azure Managed Identity for keyless auth, Azure Monitor for diagnostics, and Azure Private Link for network isolation. None of those require any code - they are infrastructure concerns.
You need guaranteed single-digit-millisecond latency at scale. Cosmos DB's SLA covers P99 latency, not just availability. At high RPS on hot partitions, its in-memory execution tier stays predictable in a way that a provisioned MongoDB Atlas cluster may not.
Your access patterns are well-defined and stable. Cosmos DB rewards teams who model their data around partition keys and access patterns at design time. If your team knows that 90% of reads are by tenantId, Cosmos DB with a tenantId partition key delivers extremely efficient point lookups. Change those patterns later and you may need to remodel entire containers.
You are building serverless workloads. Cosmos DB Serverless mode bills per operation, making it cost-effective for APIs with spiky or unpredictable traffic - think a startup that has 100 requests per day during beta and could suddenly receive 100,000 after launch. Serverless scales to zero when idle.
When Does MongoDB Atlas Win?
MongoDB Atlas is the pragmatic choice when:
Your team comes from a MongoDB background. The MongoDB Query Language is richer and more expressive than Cosmos DB's NoSQL API for complex aggregations, text search, and ad-hoc queries. Teams who already know \(lookup, \)group, and the aggregation pipeline will be productive immediately.
Multi-cloud or cloud-agnostic strategy matters. MongoDB Atlas runs identically on Azure, AWS, and GCP. If your SaaS product serves enterprise customers who mandate data residency on a specific cloud, Atlas handles that without re-architecting your data layer. Cosmos DB cannot move off Azure.
Your data model evolves frequently. MongoDB's schema flexibility and lack of forced partition key decisions let teams iterate on their data model faster. Adding a new field or restructuring a sub-document does not require a migration plan the way that a partition key change in Cosmos DB does.
You need strong full-text search. Atlas Search is a mature Lucene-based search engine built into Atlas. It supports fuzzy matching, facets, autocomplete, and vector search with a familiar MongoDB integration. Cosmos DB has improved its search story, but Atlas Search remains more feature-complete for search-heavy workloads.
Cost predictability matters more than raw SLA. Cosmos DB's RU pricing can surprise teams at scale. A complex aggregation that works fine on a development cluster may consume 50× more RUs than expected in production. MongoDB Atlas's vCPU-based pricing model is easier to budget for.
What Are the Real Trade-Offs for .NET Teams?
The Partition Key Problem in Cosmos DB
Cosmos DB forces every container to have a partition key. Getting this wrong is expensive - Cosmos DB does not let you change the partition key after creation. For multi-tenant SaaS applications, using tenantId as the partition key is the textbook answer, but it creates a hot partition problem when your largest tenant has 100× the traffic of your smallest. The solution is hierarchical partition keys (supported since 2023), but this adds design complexity that MongoDB Atlas simply does not have.
The RU Budget Problem in Cosmos DB
Every operation in Cosmos DB consumes Request Units. A simple document read might cost 1 RU. A fan-out query across multiple partitions might cost 400 RUs. Teams that come from a relational or MongoDB background are often surprised to discover that their application is triggering expensive queries they did not design for. Cosmos DB's query explorer shows RU consumption, but teams need to tune queries at a level of detail most are not used to.
MongoDB Atlas's Operational Overhead
MongoDB Atlas handles most operational concerns automatically, but there are edge cases. Automated balancing of shards across a sharded cluster is eventually consistent - during a rebalancing event, some queries may route to slightly stale replicas. For most SaaS workloads this is acceptable, but it is a consideration for systems that require strict read-after-write consistency everywhere.
.NET SDK Ergonomics
The Microsoft.Azure.Cosmos SDK is well-designed for asynchronous .NET code. It returns FeedIterator<T> for paginated queries, which pairs naturally with IAsyncEnumerable<T> in C#. The MongoDB.Driver SDK supports LINQ-style queries against IMongoCollection<T>, which is familiar to .NET developers. Neither SDK significantly outperforms the other in developer ergonomics - the decision comes down to operational concerns, not SDK quality.
Which Database Should Your Team Choose?
Here is the decision framework for 2026:
Choose Cosmos DB if:
Your entire infrastructure is Azure and you want zero cross-cloud management overhead
Your access patterns are well-understood before you design the schema
You need the 99.999% SLA with active-active multi-region writes as a contractual requirement
You are building serverless or event-driven workloads where per-operation billing is cost-effective
Choose MongoDB Atlas if:
Your team already knows MongoDB or you want to stay cloud-agnostic
Your data model is likely to evolve significantly during the first year
You need rich full-text search without integrating a separate search service
Your cost model works better with predictable vCPU pricing than RU-based billing
Choose neither - consider PostgreSQL - if:
Your data is mostly relational with a few JSON blobs
Your team is small and the overhead of managing a document database model is not worth the flexibility
You are already using EF Core and want to stay in the familiar migration-based workflow
The honest answer for most .NET SaaS teams in 2026: if you are already on Azure and your access patterns are known, Cosmos DB is a defensible choice. If you are not tied to Azure or your product is still evolving, MongoDB Atlas gives you more room to manoeuvre.
FAQ
Is Azure Cosmos DB supported by EF Core?
Yes, EF Core has an official Cosmos DB provider via the Microsoft.EntityFrameworkCore.Cosmos package. It allows you to map entities to Cosmos DB containers using the familiar Fluent API. However, it abstracts over partition keys and RU consumption in ways that can hide important performance issues. For production workloads that are throughput-sensitive, most teams use the native Microsoft.Azure.Cosmos SDK directly and reserve EF Core for relational data stores.
Can I use MongoDB Atlas in an ASP.NET Core API without EF Core?
Yes, and this is actually the common pattern. You use the MongoDB.Driver NuGet package, register a singleton IMongoClient in your DI container, and inject IMongoCollection<T> into your repository classes. The driver supports async/await natively and integrates cleanly with ASP.NET Core's cancellation token model.
How does Cosmos DB pricing compare to MongoDB Atlas for a typical SaaS application?
This depends heavily on your access patterns. Cosmos DB Serverless is often cheaper for low-traffic APIs because it charges per operation and scales to zero. For consistently high-traffic APIs, a provisioned Cosmos DB cluster can become expensive relative to a MongoDB Atlas M30 cluster. Before committing, model your expected read/write RPS and use the Cosmos DB pricing calculator alongside Atlas cluster pricing to compare at your expected scale.
Does MongoDB Atlas support transactions like a relational database?
MongoDB Atlas supports multi-document ACID transactions across collections and even across shards. Transactions in MongoDB are designed for cases where you need atomic updates spanning multiple documents - for example, updating an order and its corresponding inventory record in a single operation. For the majority of document-level operations, you do not need explicit transactions because MongoDB guarantees atomicity at the document level.
What is the best local development experience for .NET teams using Cosmos DB?
Microsoft provides an official Azure Cosmos DB Emulator that runs as a Docker container. It supports the NoSQL API and the MongoDB compatibility API. You can add it to a docker-compose.yml alongside your ASP.NET Core application for a fully local dev loop. For integration tests, Testcontainers for .NET has a Cosmos DB module that spins up the emulator automatically in test runs, keeping your tests isolated and reproducible.
Can Azure Cosmos DB run on AWS or GCP?
No. Azure Cosmos DB is Azure-exclusive. If your organisation has a multi-cloud mandate or if your customers require data residency on AWS or GCP, Cosmos DB is not viable. MongoDB Atlas is the document database that runs natively and identically on all three major clouds.
What is the difference between Cosmos DB's NoSQL API and its MongoDB API?
Cosmos DB's MongoDB API emulates the MongoDB wire protocol, allowing you to connect with a standard MongoDB.Driver client. However, it does not support every MongoDB feature - complex aggregation pipeline operators, certain index types, and change stream behaviours differ from native MongoDB. Microsoft maintains a compatibility matrix on learn.microsoft.com. If your application relies on advanced MongoDB features, using native Atlas rather than Cosmos DB's MongoDB emulation API is safer.
About the Author
Celin Daniel is Co-founder of Coding Droplets with 13+ years of hands-on experience building, shipping, and operating .NET and ASP.NET Core systems in production. The guidance here comes from real projects and production incidents, not theory.
