It is always a good idea to estimate the scale of the system as it help reflect if the designed system could fulfill the functional requirements. The requirements might include:
- Number of users
- Number of active users(NAU)
- Requrests per second(RPS)
- Logins per seconds
- Transactions per second(TPS) for E-commerce
- Likes/dislikes per second, shares per second, comments per second for social media sites
- Searches per second for sites with a search feature
- Storage needed
- Servers needed
- Network bandwidth needed
To estimate hardware resource needed, we need to understand that there are four major resources in a computer system.
Estimate servers needed
The modern computer system is a multi-processor system. It varies from single CPU core to multiple CPU cores. The following is a 32 CPU threads system.
$ lscpu Architecture: x86_64 CPU op-mode(s): 32-bit, 64-bit Byte Order: Little Endian CPU(s): 32 On-line CPU(s) list: 0-31 Thread(s) per core: 2 Core(s) per socket: 8 Socket(s): 2 NUMA node(s): 2 Vendor ID: GenuineIntel CPU family: 6 Model: 63 Model name: Intel(R) Xeon(R) CPU E5-2630 v3 @ 2.40GHz
In order to estimate how many servers are needed, we can approach in the following manner.
- How much work can single CPU do?
- How much work can single server do?
- How many servers are needed?
Let's have an example to go through this approach.
Let's say it takes 100ms for a sinlge-core CPU system to handle single client request. It means the system can handle 10 requests per second. So, we can extrapolate a 32-core system can handle 320 requests per second. Let's say we have to handle 320,000 requests per second(RPS). It means 1000 servers are needed.
Notice that this is a rough calculation only considering CPU needs. In real case, there might be other performance bottleneck to handle 320 requests per second in a system. For example, the system might be already I/O bound before running out of CPU bandwidth. But this method still gives us a estimation at the high level.
Estimate storage needed
To estimate storage needed, we can approach as below.
- Identify the different data types
- Estimate the space needed for each data type
- Get the total space needed
Let's take YouTube as an example to understand this approach.
- Data types: videos, thumbnail images and comments.
- Let's assume there are roughly 2B users and 5% users(100M users) upload videos consistently. On average, each user has a weekly upload(~50 videos per year). Roughly, 13M videos(100M*50/365) are uploaded daily. Let's assume the video is 10 minutes long on average and it takes 50MB storage space after compression. Let's say each video has a thumbnail image of 20KB. Each video has about 5 comments and the size of each comment is 200 bytes. In total, the space need for each video is 50MB + 20KB + 1KB, roughly 50MB. By multiplying 13M videos, it needs 619TB storage in a day.
Estimate network bandwidth needed
Determine the incoming and outgoing data for network bandwidth estimation
- We already know there would be ~619TB data uploaded to YouTube in a day. Dividing this by the number of seconds in a day(619TB/86400 seconds), the incoming data to YouTube would be 7.3GB/s.
- Let's say 10% of YouTube users are daily active users. With approximately 200M daily users, let's assume a user watches 10 videos a day. Then YouTube would have 2B views in a day. This would result in ~93PB outgoing data in a day. Dividing this by the number of seconds in a day(93PB/86400 seconds), the outgoing speed would be 1128GB/s.