add notes about chapter 04

system-design/system-design-interview/chapter04/README.md

@@ -0,0 +1,130 @@
+# A Framework for System Design Interviews
+System design interviews are intimidating.
+
+How could you possible design a popular system in an hour when it has taken other people decades?
+ * It is about the problem solving aspect, not the final solution you came up with.
+ * Goal is to demonstrate your design skills, defend your choices, respond to feedback constructively.
+
+What's the goal of a system design interview?
+ * It is much more than evaluating a person's technical design skills.
+ * It is also about evaluating collaboration skills, working under pressure, resolving ambiguity, asking good questions.
+ * Catching red flags - over-engineering, narrow mindedness, stubborness, etc.
+
+# A 4-step process for effective system design interview
+Although every interview is different, there are some steps to cover in any interview.
+
+## Step 1 - understand the problem and establish design scope
+Don't rush to give answers before understanding the problem well enough first.
+In fact, this might be a red flag - answering without a thorough understanding of requirements.
+
+Don't jump to solutions. Ask questions to clarify requirements and assumptions.
+
+We have a tendency as engineers to jump ahead and solve a hard problem. But that often leads to designing the wrong system.
+
+When you get your answers (or you're asked to make assumptions yourself), write them down on the whiteboard to not forget.
+
+What kind of questions to ask? - to understand the exact requirements. Some examples:
+ * What specific features do we need to design?
+ * How many users does the product have?
+ * How fast is the company anticipated to scale up? - in 3, 6, 12 months?
+ * What is the company's tech stack? What existing services can you leverage to simplify the design?
+
+For example, say you have to design a news feed. Here's an example conversation:
+ * Candidate: Is it mobile, web, both?
+ * Interviewer: both.
+ * C: What's the most important features?
+ * I: Ability to make posts & see friends' news feed.
+ * C: How is the feed sorted? Just chronologically or based on eg some weight to posts from close friends.
+ * I: To keep things simple, assume posts are sorted chronologically.
+ * C: Max friends on a user?
+ * I: 5000
+ * C: What's the traffic volume?
+ * I: 10mil daily active users (DAU)
+ * C: Is there any media in the feed? - images, video?
+ * I: It can contain media files, including video & images.
+
+## Step 2 - Propose high-level design and get buy-in
+The goal of this step is to develop a high-level design, while collaborating with the interviewer.
+ * Come up with a blueprint, ask for feedback. Many good interviewers involve the interviewer.
+ * Draw boxes on the whiteboard with key components - clients, APIs, web servers, data stores, cache, cdn, message queue, etc.
+ * Do back-of-the-envelope calculations to evaluate if the blueprint fits the scale constraints. Communicate with interviewer if this type of estimation is required beforehand.
+
+You could go through some concrete use-cases, which can help you refine the design. It might help you uncover edge cases.
+
+Should we include API schema and database design? Depends on the problem. For larger scale systems, this might be too low level.
+Communicate with the interviewer to figure this out.
+
+Example - designing a news feed.
+
+High-level features:
+ * feed publishing - user creates a post and that post is written in cache/database, after which it gets populated in other news feeds.
+ * news feed building - news feed is built by aggregating friends' posts in chronological order.
+
+Example diagram - feed publishing:
+![feed-publishing](images/feed-publishing.png)
+
+Example diagram - news feed building:
+![news-feed-building](images/news-feed-building.png)
+
+## Step 3 - Design deep dive
+Objectives you should have achieved so far:
+ * Agreed on overall goals & features
+ * Sketched out a high-level blueprint of the design
+ * Obtained feedback about the high-level design
+ * Received initial ideas on areas you need to focus on in the deep dive
+
+At this stage, you should work with interviewer to prioritize which components you should focus on.
+
+Example things you might have to focus on:
+ * High-level design.
+ * System performance characteristics.
+ * In most cases, dig into the details of some system component.
+
+What details could you dig into? Some examples:
+ * For URL shortener - the hash function which converts long URLs into small ones
+ * For Chat system - reducing latency and supporting online/offline status
+
+Time management is essential - don't get carried away with details which don't show off your skills.
+For example, don't get carried away about how facebook's EdgeRank algorithm works as it doesn't demonstrate your design skills.
+
+Example design deep dive for feed publishing:
+![feed-publishing-deep-dive](images/feed-publishing-deep-dive.png)
+
+Example design deep dive for news feed building:
+![news-feed-building-deep-dive](images/news-feed-building-deep-dive.png)
+
+## Step 4 - Wrap up
+At this stage, the interviewer might ask you some follow-up questions or give you the freedom to discuss anything you want.
+
+A few directions to follow:
+ * Identify system bottlenecks and discuss improvements. No design is perfect, there are always things to improve.
+ * Give a recap of your design.
+ * Failure modes - server failure, network loss, etc.
+ * Operational issues - monitoring, alerting, rolling out the system.
+ * What needs to change to support the next scale curve? Eg 1mil -> 10mil users
+ * Propose other refinements.
+
+Do's:
+ * Ask for clarification, don't assume your assumption is correct.
+ * Understand problem requirements.
+ * There is no right or perfect answer. A solution for a small company is different from one for a larger one.
+ * Let interviewer know what you're thinking.
+ * Suggest multiple approaches.
+ * Agree on blueprint and then design the most critical components first.
+ * Share ideas with interviewer. They should work with you.
+
+Dont's:
+ * Come unprepared for typical interview questions.
+ * Jump into a solution without understanding the requirements first.
+ * Don't go into too much details on a single component at first. Design at a high-level initially.
+ * Feel free to ask for hints if you get stuck.
+ * Communicate, don't think silently.
+
+## Time allocation on each step
+45 minutes are not enough to cover any design into sufficient detail.
+
+Here's a rough guide on how much time you should spend on each step:
+ * Understand problem & establish design scope - 3-10m
+ * Propose high-level design & get buy-in - 10-15m
+ * Design deep dive - 10-25m
+ * Wrap-up - 3-5m