Logical design and physical design are two important concepts in network architecture and computer systems. Logical design refers to the abstract layout and functional features of a system, focusing on what it should do. Physical design refers to the actual components, connections and physical layout of the system, focusing on how it’s implemented. Both stages are crucial in developing functional and efficient computer networks and systems. Though related, logical and physical design have distinct differences in their purpose, scope, audience, flexibility and tools used. This article will compare and contrast these two phases of design and showcase their unique roles.
Logical Design
Logical design is a high-level, abstract design focused on the overall functionality and flow of the system, rather than the details of how data is stored or managed. As defined in the Oracle documentation, logical design pertains to an abstract representation of the data flow, inputs, and outputs of the system. It describes the inputs (sources), outputs (destinations), and various processes that transform the inputs into outputs.
The logical design represents data structures in a simple, understandable format that you can show to users at any stage of development, as noted in the Broadcom TechDocs overview. It focuses on what the system needs to fulfill the requirements rather than how it will be physically implemented. The logical design includes high-level data models that describe the key entities, relationships, and attributes needed.
Overall, logical design deals with the functionality of the system and does not consider physical constraints such as performance requirements or hardware limitations. It is technology-independent and focused on defining the essential components and logical flow of information in the system.
Physical Design
Physical design relates to the actual implementation of the system in terms of input and output processes (Tutorials Point, 2023). It focuses on specifying how data will be input into the system, verified, processed, and output in a concrete way. Physical design transforms the logical design into a real, functioning system.
The physical design phase involves selecting the technology, hardware and software needed to implement the logical design (Khan, 2021). This includes things like:
- Choosing the database management system
- Selecting the programming language(s)
- Determining the physical network architecture
- Deciding on the user interface technology
Whereas logical design focuses on functional requirements, physical design focuses on non-functional requirements like performance, reliability, availability, recoverability, etc. There are many technical decisions to be made in order to optimize these factors.
Physical design produces detailed specifications for the hardware, software, people, network infrastructure and data resources required to implement the system. The deliverables often include schema diagrams, screen layouts, report layouts, hardware specifications, and network topology diagrams.
Key Differences
The key differences between logical and physical design revolve around the level of abstraction and flexibility:
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Logical design is abstract, physical design is concrete – Logical design focuses on the overall system architecture and flow of information. It deals with logical data structures and does not define physical details. Physical design actually implements the logical design, specifying physical hardware, software, database models, etc.
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Logical design is flexible, physical design is fixed – Logical design can more easily adapt to changing business needs or new technologies, since it is conceptual and high-level. Physical design is concrete and specific, making changes more difficult and expensive once implemented.
In summary, logical design emphasizes what the system must accomplish while physical design determines how the system will implement the logical design.
Order of Development
Logical design comes before physical design in the database development process. The logical design focuses on the logical structure of the database and defines entities, attributes, relationships, and keys without regard for how the data will be physically implemented. The physical design takes the logical design and maps it to physical storage structures like tables, indexes, and files. It focuses on performance optimization by figuring out the physical configuration and data storage details.
Logical design models the data and defines database objects like tables, views, and stored procedures. Physical design then implements those objects using the physical storage mechanisms supported by the database platform, like tablespaces, partitions, and clusters. The logical design is more conceptual and abstract, while the physical design is concrete and specific.
Doing the logical design first allows the development team to focus solely on the logical relationships and structure of the data, without being constrained by physical limitations. This leads to a clean, optimized logical design. That logical design can then be physicalized in a flexible way to meet performance and storage requirements.
In summary, logical design models the logical view of the data, while physical design implements the physical storage and configuration of the data. Logical design comes first in the typical development process.
Focus
A key difference between logical and physical design is their focus. Logical design focuses on what the system should do and what problem it should solve. It aims to meet the functional requirements of the system. Logical design is concerned with the processes the system needs to carry out.
In contrast, physical design focuses on how the system will meet the functional requirements specified in the logical design. It determines the most suitable architecture, hardware, software, network infrastructure, etc. to implement the logical design. Physical design deals with the physical constraints and specifics of the system, such as system performance, security, reliability, and maintainability.
In summary, logical design is focused on the functional aspects and specifications of the system, while physical design deals with the physical implementation details. Logical design focuses on what the system should do, physical design on how the system will do it.
Audience
The key difference in the audience for logical vs physical design is that logical design is focused on customers and management, while physical design is focused on developers.
The logical design describes the system from the users’ perspective, using terminology that business stakeholders can understand according to Conceptual, Logical and Physical Design – Grant Orchard. It is meant for customers and management who need to validate the requirements and functionality before developers build out the system.
In contrast, the physical design uses technical terminology tailored for developers who will be responsible for implementing the system according to Understanding Logical and Physical Architecture. The physical design provides the technical blueprint that developers need for coding, database design, infrastructure, etc.
Flexibility
One of the key differences between logical and physical design is flexibility. Logical design tends to be more adaptable to changes, whereas physical design is more rigid. As the ThoughtSpot article explains, logical design focuses on the overall structure of the database and relationships between entities. It does not specify physical storage details. This higher level of abstraction means that logical models can more easily evolve as requirements change.
In contrast, physical design maps the logical model directly to database structures. As the LinkedIn article describes, physical design provides a detailed specification of database schema, indexes, partitions, etc. Once implemented, it is much harder to change physical structures. Altering a physical design often requires modifying table structures, migrating data, and updating queries and applications.
Therefore, logical design provides more flexibility upfront to adapt to new business needs. Physical design trades flexibility for performance optimization of the implemented system.
Tools Used
Logical design focuses more on conceptual modeling using diagrams and documentation to represent the data elements, entities, relationships, and processes. Common tools used in logical design include entity-relationship diagrams (ERDs), flowcharts, data flow diagrams (DFDs), data dictionaries, and business process models. These tools help capture the logical view of the system before physical implementation.
In contrast, physical design relies heavily on actual coding and database schema. Tools used in physical design include database schema diagrams, Data Definition Language (DDL) to create database objects like tables and indexes, and scripts to populate the database. Programming languages and IDEs are needed to code any required modules, interfaces, APIs, etc. The designer focuses on the internal storage, access structures, and physical implementation of the system.
Overall, logical design emphasizes conceptual modeling while physical design relies on concrete schemas, code, and scripts to construct the physical database and software components.
Conclusion
In summary, logical design and physical design are two important stages in the database design process. Logical design focuses on the abstract structure and relationships between data, while physical design deals with the actual implementation of the database on storage devices.
The main differences between the two are:
- Logical design happens first and establishes the entities, attributes, and relationships. Physical design happens after and translates the logical model into a schema optimized for a DBMS technology.
- Logical design aims for high-level organization and optimization, while physical design focuses on performance considerations like indexes, partitions, storage details.
- Logical design requires a deep understanding of business needs and rules. Physical design requires expertise in DBMS-specific implementation.
- Logical design emphasizes flexibility and adaptability. Physical design prioritizes performance and scalability.
While different in focus, logical and physical designs are closely connected. The physical implementation strives to remain faithful to the intent of the logical model. Changes at the logical level often necessitate changes in the physical schema. A skilled database designer will iterate between logical and physical to produce a database optimized for the software and underlying infrastructure.
