EAV vs Document DB vs XML Column for a system with arbitrary entities?

When designing a system with arbitrary entities, choosing the right data storage model is essential for the system's flexibility, performance, and ease of querying. In this context, three common data storage models are often considered: EAV (Entity-Attribute-Value), Document DB, and XML Column. Each has its strengths and weaknesses, so let's compare them:

1. EAV (Entity-Attribute-Value):

   • Model Description: EAV is a data storage model that represents entities as rows with key-value pairs (attributes) in a database table. It is commonly used when entities have different sets of attributes, and the schema is not known in advance.
   • Pros:
     • Flexibility: Can handle entities with varying attributes without schema changes.
     • Extensible: New attributes can be added dynamically without altering the table structure.
   • Cons:
     • Complex Queries: Retrieving data requires joining multiple tables, leading to complex queries and potentially poor performance.
     • Lack of Constraints: Since attributes are stored as values, enforcing constraints (e.g., data types) can be challenging.
     • Limited Performance: EAV models may suffer from performance issues, especially as the data grows and requires many joins.

2. Document DB:

   • Model Description: Document DBs store data in JSON-like documents, allowing each document to have different fields. Each document represents an entity, and collections group related documents.
   • Pros:
     • Schema Flexibility: Entities can have varying fields without a fixed schema.
     • Performance: Retrieving data for a specific entity is efficient as it's stored as a self-contained document.
     • Native JSON: Works well with modern applications that use JSON natively.
   • Cons:
     • Lack of Transactions: Some document databases may lack full ACID transactions.
     • No Joins: Most document databases do not support joins, requiring denormalization or multiple queries to gather related data.
     • Data Integrity: Ensuring data integrity can be challenging, as there are no strict constraints on fields.

3. XML Column:

   • Model Description: XML Column is a data storage approach where entities with different attributes are stored in a single table with an XML column. Each entity's attributes are serialized as XML in the column.
   • Pros:
     • Schema Flexibility: Allows storing entities with varying attributes.
     • XML Querying: Provides powerful XML querying capabilities.
     • Familiarity: Developers familiar with XML can work with the data easily.
   • Cons:
     • Complexity: Handling XML structures can be complex, especially for complex data.
     • Performance: XML queries may not be as efficient as other storage models, especially for large datasets.
     • Lack of Constraints: Constraints on data may be more challenging to enforce due to the flexible schema.

Ultimately, the choice between EAV, Document DB, or XML Column depends on various factors, such as the nature of the data, performance requirements, querying complexity, development team's familiarity with the technology, and the overall system's architecture.

If possible, consider other data storage models as well, such as NoSQL document databases like MongoDB or graph databases like Neo4j, as they may offer more efficient solutions for handling entities with varying attributes. Additionally, considering the specific use case and the expected growth of the system will help make a more informed decision.