While attending the Gartner BI Summit in Grapevine, TX yesterday I had a meeting with a client that made me facepalm a little.
The client in question was telling me about his need to educate his development staff in a number of critical areas, ranging from application lifecycle management, to data access patterns, to SOA governance, and on. The organization he works for is in the middle of a major hiring binge, and within the 200+ IT group there are few technologists with more than a year’s experience inside the company. This is a challenging situation, even when you can assume (as you should be able to) that most of your programmers know the difference between a library and a service! Many of the programmers in this organization prefer to create code libraries rather than services, and most of them don’t understand why they should build a service instead of a library. So, let’s get into that here.
In a nutshell: a library is shared code that you compile into your application. A service is a shared capability that you access from your application:
I am a (mostly) strong proponent of relying on services whenever possible, instead of building custom code libraries for inclusion in many programs. The agility, reuse, and transparency benefits of a service are just too hard to ignore. Having said that, I recognize that there are specific cases where creating libraries could be more appropriate:
- When you’re building an application that will be deployed to a device with inconsistent network access (making remote service consumption unreliable)
- When you’re building an application that will be hardened with wrapper technology for security reasons (and network access to remote services is seen as an unacceptable compromise)
A few more details that I quickly came up with which you might find worthwhile (let me know if you have any to add here):
|Characteristics||Executes locally inside of your program||Executes remotely outside of your program|
|Distributed/deployed with your program||Exists independently of your program|
|Internal compilation dependency for your program||External consumption dependency for your program|
|Cannot change unless you recompile/redeploy your program||Versioned independently of your program|
|Instance used in your program is used only by your program||Can be shared with many remote programs|
|Access to library routines cannot be mediated||Access can be mediated for security, QoS, or other reasons|
|Access to library routines cannot be audited||Access can be logged and audited by a mediator|
|Strengths||Program cannot be inadvertently broken by a dependency change||Many programs can benefit from service reuse and improvement|
|Programmers can easily understand all project dependencies||Service mediation can provide many benefits|
|Compiled program is entirely self-contained||Service-based design is more flexible to unanticipated changes|
|Weaknesses||Library improvements can only be used at program recompilation||Service oversubscription creates an availability bottleneck|
|Multiple versions of the same library can be used in many programs||Service mediation is a black art to most programmers|
|Patterns of library consumption are not clearly visible at runtime||Services may not be modeled with the right level of granularity for reuse|
A final word.
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