A service level agreement (SLA) is a commonly used term for service providers. This is an agreement between you (the service provider) and your customer, and this agreement formally defines the level of uptime your service will deliver. Cloud providers Amazon , Google  and Microsoft  set their SLAs at 99.9% or above. Uptime is traditionally measured in nines. The more the nines, the better.
To obtain correct calculations, it is critical to know the data volume unit using the power of 2.
According to Jeff Dean, Google Senior Fellow, “back-of-the-envelope calculations are estimates you create using a combination of thought experiments and common performance numbers to get a good feel for which designs will meet your requirements”
Avoid disk seeks if possible.
By analyzing the numbers in Figure 1, we get the following conclusions: Memory is fast but the disk is slow. Avoid disk seeks if possible. Simple compression algorithms are fast. Compress data before sending it over the internet if possible. Data centers are usually in different regions, and it takes time to send data between them.
Back-of-the-envelope estimation is all about the process. Solving the problem is more important than obtaining results. Interviewers may test your problem-solving skills.
Latency numbers every programmer should know
is an agreement between you (the service provider) and your customer, and this agreement formally defines the level of uptime your service will deliver.
It is a good idea to write down your assumptions to be referenced later
QPS, peak QPS, storage, cache, number of servers, etc.
High availability is the ability of a system to be continuously operational for a desirably long period of time.
Use round numbers and approximation to your advantage. The division question can be simplified as follows: “100,000 / 10”.
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