Service Granularity Principle
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By definition a coarse-grained service operation has broader scope than a fine-grained service, although the terms are relative. The former typically requires increased design complexity but can reduce the number of calls required to complete a task.
The four key factors to consider when designing for optimal granularity are performance, message size, transaction and business function.
Web services are accessed remotely and calls to web service operation create more network overhead. Reducing the number of service requests reduces that overhead.
Message size 
Coarse-grained services may pass more data than fine-grained services, including data that is not specifically required for the task. Reducing message size may require adding a more fine-grained operation.
For conceptual clarity each service operation should perform a single transaction. This also simplifies error recovery, and typically eases design.
Business Function 
Ideally, each service operation maps to a single business function, although if a single operation can provide multiple functions without adding design complexity or increasing message sizes, this generality can reduce implementation and usage costs.
- On the Definition of Service Granularity and its Architectural Impact
- Cooking EA - Service Granularity
- IEEE Case Study on The Role of Service Granularity in A Successful SOA Realization
- A Metrics Framework for Evaluating SOA Service Granularity
- Finding an optimum in service granularity
- Research on the SOA-based Service Granularity Control
- SOA Design Principles provides more information about SOA realization using Service design principles
- Getting Web Service Granularity Right provides information about Service Granularity Principle and useful advice about how to get it right.
- SOA Practitioner's Guide
- Managing Granularity in design and Implementation
- SOA Glossary
- Solving the Service Granularity Challenge
- Coarse-grained interfaces