Why use services (REST/SOAP) instead of a library?

Question

Let's say you're looking at breaking up your applications into services. Are there any good reasons to adopt a SOA approach vs. just creating a library API that can be loaded by the applications that need it.

Answer 1

The difference may be subtle between both. For example in .NET world, you may have an application which would feel like monolithic to an end user and which will work on a same machine, but inside, would be separated into a bunch of WCF services. You may also have architectures where libraries are not strongly linked (addins/plugins) and are just following a protocol when talking to each other.

If we avoid talking about those intermediary cases, and deal only with strongly linked API library vs. a separate REST service, then you may want to consider following points:

1. A library API is called on the same machine. Services can be hosted anywhere and be called from anywhere. If you're counting hosting the application on multiple machines for performance/scalability/security reasons, chances are you'll need to use services.

2. The situation is similar when it goes to using one service by an application deployed on several machines. For example if you're doing an application for a bank doing some financial computations, one way is to deploy the whole large application to every desktop, and be forced to do large-size updates to every client every time; another approach would be to host computation part on servers, and deploy to the desktops only a lightweight app with just a UI and a bunch of calls to those servers.

3. If you're hosting a REST service, anyone can use it: a Mac user, a person who uses Linux, etc. If you've created a C# library with Visual Studio and you distribute it as a DLL, forget about users (customers?) who don't have Windows.

Answer 2

Another advantage of services is that when you update the service, it is immediately deployed to all consumers of the service. So if you fixed a bug, or performance issue, everyone gets the benefit as soon as the updated service goes live, instead of having to distribute an update that people may choose to ignore.

Answer 3

A SOA approach allows the various services to be hosted and maintained separately. In addition to code, deploying a particular service may require a lot of special configuration (passwords, ports, certificates, etc). Consuming a REST service has a finite amount of complexity that can be clearly documented and easily understood. It's also more secure because it means you don't need to grant access to DBs or other resources to clients.

Answer 4

Library Advantages:

• Native code = higher performance
• Simplest thing that could possibly work
• No risk of centralized service going down and impacting all consumers

Service Advantages:

• Everyone gets upgrades immediately and transparently (unless versioned API offerred)
• Consumers cannot decompile the code
• Can scale service hardware separately
• Technology agnostic. With a shared library, consumers must utilize a compatible technology.
• More secure. The UI tier can call the service which sits behind a firewall instead of directly accessing the DB.

Answer 5

If there is a change to an SOA service, then that SOA service will have to be redeveloped, retested and redeployed. All applications that consume that service can continue to do so. A change to a library in a DLL will mean all consumers of that library will have to be redeveloped to reference that DLL, they will all have to be retested and they will all have to be redeployed. There is also the danger this may not happen properly, and different applications will have different versions of the DLL. Sometimes this might not be a problem - perhaps every system should have the version of the library that was present at deployment time (you may have updated your logging system to have helpful new features - do you really need to update every system with it?) in this case a library is fine. But say you have a service for calculating tax rate, and the tax laws change. You don't want to have to update every system to incorporate this change, it would be better to do it in one place. In this case a service is a better option.