Alvan's digital garden

Non-Functional Requirements

Non-functional requirements are, but not limited to:


System must be fast. But fast is relative term. If there is an end user fast means requests taking less than 1 sec. On the other side if we are working B2B, fast can mean less than 100ms.


How much time does it take to perform a single task?


How many tasks can be performed in a given time unit?

Latency VS Throughput

Type Duration
Latency 1second
Throughput - Well designed app: > 1000
- Badly designed app: < 60


Quantity of work we can perform without crushing. (It depends on application type) E.g: WebAPI -> Number of concurrent requests we can handle without crashing.

Load VS Throughput

Type Value
Throughput 100 requests/sec
Load 500 requests without crashing
For e-commerce app it can be ~200 concurrent request, but system must be available on Black Fridays. So, we must be ready for ~2000 concurrent requests.

Data Volume

Concurrent users

Concurrent users VS Load

Key Value
Concurrent Users Including “Dead times”
Load Actual requests

For example, user requests for all products in category. Service handles request. After this for 5 minute user will be looking at this data. This duration is counted as “Dead Time” and appropriate user is included in concurrent users.
For average system, Concurrent = Load x 10

SLA (Service Level Agreement)

Required uptime for the system

Cloud providers are mostly competing in this field. For example, SLA for Azure Cosmos DB is notes as 99.99% This means less than an hour per year:

24 * 365 = 8760 hrs/year
8760 * 99.99% = 8759.12
8760 - 8759.12 = 0.88 hrs/year