Sizing And Capacity Planning

Software sizing is an activity in software engineering that is used to estimate the size of a software application or component in order to be able to implement other software project management activities (such as estimating or tracking). Size is an inherent characteristic of a piece of software just like weight is an inherent characteristic of a tangible material.

Capacity planning is the process of determining the production capacity needed by an organization to meet changing demands for its products.^[1] In the context of capacity planning, design capacity is the maximum amount of work that an organization is capable of completing in a given period. Effective capacity is the maximum amount of work that an organization is capable of completing in a given period due to constraints such as quality problems, delays, material handling, etc. The phrase is also used in business computing as a synonym for capacity management.

 Sizing is to estimate the change in the software application by adding extra software, where as capacity  planning is find amount of extra hardware required to me the future business needs.

Difference between scaling horizontally and vertically for databases

Horizontal scaling means that you scale by adding more machines into your pool of resources where Vertical scaling means that you scale by adding more power (CPU, RAM) to your existing machine.

In a database world horizontal-scaling is often based on partitioning of the data i.e. each node contains only part of the data , in vertical-scaling the data resides on a single node and scaling is done through multi-core i.e. spreading the load between the CPU and RAM resources of that machine.

A good example for horizontal scaling is Cassandra , MongoDB .. and a good example for vertical scaling is MySQL - Amazon RDS (The cloud version of MySQL) provides an easy way to scale vertically by switching from small to bigger machines this process often involves downtime.

In-Memory Data Grids such as GigaSpaces XAP, Coherence etc.. are often optimized for both horizontal and vertical scaling simply because they're not bound to disk. Horizontal-scaling through partitioning and vertical-scaling through multi-core support.