1. Data Scalability: One key difference between Azure Monitor and Thanos is that Azure Monitor is a cloud-based monitoring service by Microsoft that offers scalability through its cloud infrastructure, whereas Thanos is an open-source project that provides a highly scalable and efficient global-scale monitoring system for Prometheus metrics. Azure Monitor is limited to the scalability provided by the Azure cloud, while Thanos can handle massive amounts of data beyond what a single Prometheus server can manage.

2. Storage Backend: Another difference is that Azure Monitor stores its data in Azure storage services like Azure Log Analytics and Azure Application Insights, providing a fully managed solution for storing and analyzing monitoring data. In contrast, Thanos uses object storage services like Amazon S3 or Google Cloud Storage as its backend storage, giving users more flexibility in choosing where their data is stored and enabling better cost optimization based on storage options available from different providers.

3. Multi-Tenancy Support: Azure Monitor supports multi-tenancy, allowing organizations to segregate their monitoring data and access controls based on different user groups or departments within the same Azure environment. Thanos, on the other hand, focuses on horizontal scalability and federation to provide a global view of Prometheus metrics across multiple clusters and regions, making it suitable for organizations with distributed infrastructure and the need for a unified monitoring solution.

4. Querying Capabilities: Azure Monitor offers a powerful query language, Kusto Query Language (KQL), for analyzing monitoring data and generating insights through its rich set of functions, operators, and visualization options. Thanos, leveraging the query language of Prometheus, provides similar capabilities for querying, aggregating, and visualizing metrics, making it a preferred choice for users familiar with Prometheus metrics and queries.

5. Cost Model: Azure Monitor follows a pay-per-usage pricing model, where customers are charged based on the volume of data ingested, stored, and analyzed in the Azure environment, providing predictable cost management for monitoring resources. Thanos, being an open-source project, offers a cost-effective solution for storing and querying massive amounts of Prometheus data without any additional licensing fees, making it ideal for organizations looking to optimize their monitoring costs while maintaining scalability and performance.

6. Integration Ecosystem: Azure Monitor integrates seamlessly with other Azure services and third-party tools through its extensible APIs and connectors, enabling users to incorporate monitoring data into their existing workflows and analytics platforms. Thanos, being compatible with Prometheus ecosystem tools and libraries, provides a rich set of integrations for data visualization, alerting, and automation, making it a preferred choice for users already using Prometheus for monitoring their infrastructure.

In Summary, Azure Monitor and Thanos differ in terms of data scalability, storage backend, multi-tenancy support, querying capabilities, cost model, and integration ecosystem, catering to different use cases and preferences in the monitoring and observability space.