Sommerville-Software-Engineering-10ed

516 Chapter 17 ■ Distributed software engineering
Tenant Key Name Address
234 C100 XYZ Corp 43, Anystreet, Sometown
234 C110 BigCorp 2, Main St, Motown
Figure 17.17 A
multi-tenant database
435 X234 J. Bowie 56, Mill St, Starville
592 PP37 R. Burns Alloway, Ayrshire
when considering scalability in the context of SaaS, you are considering “scaling
out” rather than “scaling up.” Recall that scaling out means adding additional servers
and so also increasing the number of transactions that can be processed in parallel.
Scalability is a complex topic that I cannot cover in detail here, but following are
some general guidelines for implementing scalable software:
1. Develop applications where each component is implemented as a simple stateless
service that may be run on any server. In the course of a single transaction,
a user may therefore interact with instances of the same service that are running
on several different servers.
2. Design the system using asynchronous interaction so that the application does
not have to wait for the result of an interaction (such as a read request). This
allows the application to carry on doing useful work while it is waiting for the
interaction to finish.
3. Manage resources, such as network and database connections, as a pool so that
no single server is likely to run out of resources.
4. Design your database to allow fine-grain locking. That is, do not lock out whole
records in the database when only part of a record is in use.
5. Use a cloud PaaS platform, such as Google App Engine (Sanderson 2012) or
other PaaS platform for system implementation. These include mechanisms that
will automatically scale out your system as the load increases.
The notion of software as a service is a major paradigm shift for distributed computing.
We have already seen consumer software and professional applications, such
as Photoshop, move to this model of delivery. Increasingly, businesses are replacing
their own systems, such as CRM and inventory systems, with cloud-based SaaS systems
from external providers such as Salesforce. Specialized software companies
that implement business applications prefer to provide SaaS because it simplifies
software update and management.
SaaS represents a new way to think about the engineering of enterprise systems.
It has always been helpful to think of systems delivering services to users, but, until
SaaS, this function has involved using different abstractions, such as objects, when
implementing the system. Where there is a closer match between user and system
abstractions, the resultant systems are easier to understand, maintain, and evolve.