1. Database Structure and Model: MongoDB is a document database that stores data in flexible, JSON-like documents with dynamic schemas. It allows for the storage of structured, semi-structured, and unstructured data. On the other hand, etcd is a distributed key-value store that provides a hierarchical data model for storing and retrieving data in a tree-like structure. It is designed to handle large amounts of small, often transient data.
2. Consistency and Availability: MongoDB provides strong consistency, which means that data changes are immediately visible to all users. It also supports high availability through replication, allowing for automatic failover and continuous operation in case of failures. Etcd, on the other hand, provides strong consistency guarantees by using a consensus algorithm. However, it sacrifices availability in the event of network partitions or failures to maintain consistency.
3. Querying and Indexing: MongoDB provides a rich query language that supports a wide range of operators and capabilities for querying documents. It also supports indexing to improve query performance. Etcd, on the other hand, does not provide a query language but allows for simple key-based retrieval and manipulation of data.
4. Data Replication and Clustering: MongoDB supports horizontal scalability through sharding, allowing for distributing data across multiple servers. It also provides automatic data replication and failover to ensure high availability. Etcd, on the other hand, uses a distributed consensus algorithm to replicate data across multiple nodes, providing fault tolerance and durability.
5. Concurrency Control: MongoDB uses optimistic concurrency control, allowing multiple clients to read and write data simultaneously. It uses a versioning system to handle conflicts and ensures consistency during concurrent operations. Etcd, on the other hand, uses a distributed locking mechanism to handle concurrency control and prevent conflicts between multiple clients accessing the same key.
6. Use Cases and Scalability: MongoDB is well-suited for applications that require flexible data models and high performance for read-heavy workloads. It is widely used in web applications, content management systems, and real-time analytics. Etcd, on the other hand, is often used for coordination and configuration purposes in distributed systems, such as service discovery, distributed locking, and dynamic configuration updates.

In Summary, MongoDB and etcd differ in terms of their database structure and model, consistency and availability guarantees, querying and indexing capabilities, data replication and clustering mechanisms, concurrency control approaches, and their use cases and scalability profiles.