DDS Models simultaneously system's Things, their Metadata, and Relationships

From the design perspective, DDS models the words' entities or the words' things as they are, with their unique identity (key), their well-defined complex structures, data-types, their states, events, and metadata. Metadata can indicate if the entity represented by the data is new in the system, if it has been already seen, or if it is still alive or disposed and released by its writers.

This entity modeling capability and data-type system allow designers to build more effectively a type-safe system so that applications exchange data where the types are agreed upon at compilation time or runtime before exchanging any data content. When data is safely exchanged and stored, either in memory or on a persistent store, it can be queried and analyzed. From that perspective, DDS can be seen as a Real-time Distributed Relational database.

Relational databases typically require each cell of the table to contain a single scalar value. Furthermore, in First Normal Form databases (1NF), attributes must be atomic; that is, an attribute must not be an n-tuple and therefore cannot be a set, list, or complex data structure. In such cases, to rebuild the full data structure, designers need to split their data type into several tables and operate SQL JOIN operations. Running JOIN operations leads to random disk seeks on several tables and has tremendous performance degradation. Performance degradation can be worse if the DBMS needs to make distributed JOINS. Databases that only support storage of atomic values, such as MySQL or SQL Server [ROCKDB] give the designer no choice but to conform to 1NF.

DDS does not have such restriction, every data type attribute can be a complex structure, including the key attribute definition. In much the same way as DDS, the so-called Nested Relational Database, such as Rocket MultiValue databases systems [ROCKDB], mitigate the atomic attribute limitation, the usage of nested relation DBs remains marginal.

On the other hand, NoSQL databases such as MongoDB for instance store data in a dictionary of key-value pairs, where the value is a JSON document stored in Binary JSON for efficiency purposes. NoSQL DBs are also referred to as schema-less databases, as you can store any data in the same document regardless of its structure. NoSQL DBs do not know what attributes are present in each document, nor does it know their types, therefore this information must be stored on a per-document basis which comes at the cost of storage efficiency.

In DDS, data types can also be unstructured and modeled as key-value pairs. The value type can be bounded or an unbounded sequence of octets. The key-value modeling capability allows DDS to be seen and compared to the most popular NoSQL databases.

If you model your data as a keyless sequence of headers and payloads, you are modeling Messages and limiting the DDS usage to a Message Oriented Middleware (MOM). In MQTT for instance, messages are not bound to any type of structure. If a developer wants to mimic the concept of a key that uniquely identifies an entity in the system, they can either code the identifier in the message structure at the application level, or for each entity, create a dedicated topic ( e.g /Building1/Temperature_Sensors/SensorID), which can lead to topic proliferation. In all cases, the middleware is neither able to manage the entity life cycle nor to optimize its processing.

Whether you use DDS as an in-memory database, or as a MOM, the DDS data type definition can evolve over time [X-types] in new (version n) without breaking interworking and interoperability with the rest of the system, which can still see data with its previous Type-definition (version n-1). This capability is unique in the industry. Backward compatibility of data type is useful when you are building a dynamic system where the structure of the data evolves over time or evolves during the system runtime. Such data needs to be tagged as extensible, appendable, or even mutable. Extensible and appendable means you can add, remove, or change, any field of the data-type; while mutable means you can change the order of the data attributes in the data type definition. With DDS, you can even make your data type elastic, in the sense that some data fields can be considered optional and may or may not be instantiated by applications during runtime.

If you are building a mission-critical system, where safety and Real-time data exchange are important, data extensibility is not recommended. To help the DDS infrastructure ensure type safety at compilation time and to improve determinism, you can make an up-front commitment on the data type to use. In this case, knowing in advance the data type and structure helps the DDS kernel to optimize and pre-allocate memory to minimize runtime time jitters, assure better determinism and handle more efficiently application real-time constraints. In less critical systems where evolvability is desirable, the DDS data extensibility and elasticity features address the requirement.

The standard DDS data definition language is mainly a subset of the OMG Interface definition languages [IDL4], although it is possible to represent data in XML, XSD, Binary mode in Google Proto Buffer, or in JSON.

With DDS, you can model both the data objects and entities, as well as their relationship. In practice, you simply need to unify the data keys you want to relate into another dedicated relation represented by a dedicated data topic. DDS can therefore easily support Entity-Relationships or UML based designs. From this perspective, you can, to some extent, get Graph-database-like capabilities, where graph nodes represent the data entity (as a key-value pair) and the graph edges represent relationships. Relationships have a type and label, a direction, and properties. DDS can easily represent such relationships and answer simple queries that directly involve those relationships.

However, although it is feasible, to answer complex queries that involve multi-tier transitive relationships, DDS will not be as efficient and performant as graphical DBs (i.e when (x~1~ RelatedTo x~2~ ) and (x~2~ RelatedRTo ...x~n~ }) ⟹ (x~1~RelatedTo x~n~) ).

It is also worth mentioning, that many Model-Driven Architecture tools automatically translate UML designs into DDS data entities and application code. Such approaches minimize the DDS learning curve, avoid accidental coding errors, and make the application less error prone.

None of the known MOMs or Distributed BDs technologies can achieve such versatility in modeling world things and their interactions as DDS.

By Ramzi KAROUI, posted on 2022-11-08.