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System Design Flashcards

1
Q

What are CRUD operations?

A

Create, Read, Update, Delete

Often serves as the bedrock of a functioning system, thus at the core of many APIs

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2
Q

What is pagination and why is it necessary?

A

When a network request warrants a large response, the API might be designed to return a limited portion of that response accompanied by an identifier for the client to request the next page if desired

Often used when designing API list endpoints

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3
Q

What is an ACL?

A

Access Control List

Refers to a permissioning model about which users in a system can perform which operations

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4
Q

What is a DDoS attack?

A

Distributed Denial of Service attack

A DoS attack where the traffic flooding the target system comes from many different sources making it much harder to defend against

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5
Q

What is a DoS attack?

A

Denial of Service attack

Attack where a malicious user tries to bring down or damage a system in order to render it unavailable to users, often by flooding the target system

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6
Q

What is rate limiting?

A

The act of limiting the number of requests sent to/from a system

Often used to prevent DoS attacks

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7
Q

What is streaming in terms of networking?

A

The act of continuously getting a feed of information from a server by keeping an open connection between the two machines

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8
Q

What is polling?

A

The act of fetching data regularly at an interval to make sure the data is not too stale

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9
Q

What is a socket?

A

A kind of file that acts like a stream

Processes can read/write to sockets and communicate in this manner

Often fronts for TCP connection

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10
Q

What is the gossip protocol?

A

When a set of machines talk to each other in an uncoordinated manner in a cluster to spread information through a system without requiring a central source of data

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11
Q

What is a peer-to-peer network?

A

A collection of machines referred to as peers that divide workload between themselves to complete the workload faster

Often used in file distribution systems

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12
Q

What is blob storage?

A

Widely used storage that only allows the user to store and retrieve data based on the name of the blob

ie. GCS and AWS S3

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13
Q

What is a key value store?

A

A flexible NoSQL db often used for caching and dynamic configuration

Examples:

  • Etcd
  • Zookeeper
  • Redis
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14
Q

What is the difference between strong consistency vs eventual consistency?

A

Strong consistency
- Refers to consistency of ACID transactions

Eventual consistency

  • Database reads may return stale data
  • An eventually consistent database gives guarantees that the state of the db will eventually reflect writes within a certain time period
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15
Q

What is ACID transaction?

A

A type of db transaction that has the following properties

Atomicity
- The operations that constitute the transaction will either all succeed/fail

Consistency

  • Transaction cannot bring db to an invalid state
  • After the transaction is committed/rolled back, the rules for each record will still apply

Isolation
- Execution of multiple transactions concurrently will have the same effect as if they had been executed sequentially

Durability
- Any committed transaction is written to non-volatile storage; not be undone by a crash, power loss or network partition

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16
Q

What is the difference between relational vs non-relational databases?

A

Relational db

  • Structured db where data is stored following a tabular format
  • Often referred to as SQL dbs
  • Often supports powerful querying using SQL

Non-relational db

  • Database that is free from imposed, tabular-like structure
  • Often referred to as NoSQL dbs
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17
Q

What are the different ways that data can be stored? Explain the difference

A

Disk

  • Data will persist through power failures and machine crashes
  • Can refer to either HDD or SSD
  • AKA persistent storage

Memory

  • Refers to RAM (Random Access Memory)
  • Data stored in memory will be lost through power failures and machine crashes
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18
Q

What are databases?

A

Programs that either use disk/memory to store/query data

In general, they are themselves servers that are long-lived and interact with the rest of your application through network calls, with protocols on top of TCP or HTTP

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19
Q

What is the single responsibility principle?

A

A single component having one responsibility and executing it perfectly

This approach provides flexibility and makes management easier

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20
Q

What is separation of concerns?

A

Keeping components separate/loosely-coupled makes them reusable

This approach makes scaling the service easier

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21
Q

What is client-server architecture?

A

Uses the request-response model

The client sends a request to the server for information and the server responds with it

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22
Q

What is a client?

A

A machine/process that requests data from a server

A single machine can be both client and server at the same time
ie. act as a server for users and a client for a db

ie. web app, mobile app, web-based console running commands to interact with the backend server

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23
Q

What is a server?

A

A machine/process that provides data for a client, usually by listening to incoming network calls

A single machine can be both client and server at the same time
ie. act as a server for users and a client for a db

ie. app server, proxy server, mail server, file server

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24
Q

What is an IP address?

A

An address given to each machine connected to public internet

Special values

  • Localhost: 127.0.0.1
  • Your private network: 192.168.x.x
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25
Q

Why are there different ports?

A

In order for multiple programs to listen for new network connections on the same machine without colliding, they pick a port to listen on

Common ports and their uses

  • 22: SSH
  • 53: DNS lookup
  • 80: HTTP
  • 443: HTTPS
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26
Q

What is TCP?

A

Network protocol built on top of IP

Allows for ordered, reliable data delivery between machines over the internet by creating a connection

TCP is usually implemented in the kernel, which exposes sockets to applications that they can use to stream data through an open connection

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27
Q

What is HTTP?

A

HyperText Transfer Protocol

Common network protocol implemented on top of TCP

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28
Q

What is an IP network packet?

A

Data being sent over IP

Consists of:

  • IP header: contains source and destination IP addresses, and other network-related information
  • Payload: data being sent
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29
Q

What are application servers?

A

Servers that run web apps

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30
Q

What is a forward proxy?

A

A server that sits between a client and server and acts on behalf of the client

Typically used to mask the client’s identity (IP address)

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31
Q

What is a reverse proxy?

A

A server that sits between clients and servers and acts on behalf of the servers

Typically used for logging, load balancing and caching

ie. Nginx is a popular web server often used as a reverse proxy and load balancer

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32
Q

What is an API gateway?

A

Single entry point into the system

Handles all client requests, taking care of authorisation/authentication, sanitising input data and other necessary tasks before providing access to application resources

33
Q

What is the HTTP pull mechanism?

A

Default mode of HTTP communication

The clients pull data from the server whenever required - every request/response consumes bandwidth and every hit on the server costs the business money and adds more load on the server

34
Q

What is the HTTP push mechanism?

A

Clients sends a request for a particular information to the server once and after the initial request, the server keeps pushing new updates to the client whenever they are available

ie. web sockets, message queues, streaming over HTTP

35
Q

What is scalability?

A

Ability of an application to handle and withstand increased workload without sacrificing latency

Scalability in terms or Big O notation should be (O)1 constant time

ie. if an app takes x seconds to respond to a user request, then it should take the same x seconds to respond to each concurrent user requests on the app

36
Q

What is latency?

A

The time it takes for a certain operation to complete

Latency is generally divided into:

  • Network latency
  • Application latency
37
Q

What is throughput?

A

The number of operations that a system can handle properly per time unit

eg. the throughput of a server can be measured in requests per second

38
Q

What are some orders of magnitude in terms of latency?

A 
Reading 1MB from RAM = 0.25 ms
Reading 1MB from SSD = 1 ms
Reading 1MB from HDD = 20 ms
Transfer 1MB over network = 10 ms
Inter-continental round trip = 150 ms
39
Q

What is network latency?

A

Amount of time that the network takes to send a data packet from point A to point B - the network should be sufficient enough to handle the increased traffic load

40
Q

What strategies can be employed to reduce network latency?

A
  • Use CDN

- Deploy servers across the globe as close to the end user as possible

41
Q

What is application latency?

A

Amount of time the application takes to process a user request

42
Q

What strategies can be employed to reduce application latency?

A
  • Run load/stress tests to scan for bottlenecks
43
Q

What are the different ways of scaling an application?

A
  • Vertical scaling

- Horizontal scaling

44
Q

What is a persistent connection?

A

A network connection that exists between client and server that remains open for further requests and responses as opposed to being closed after a single communication

The connection stays open with the help of heartbeat interceptors

45
Q

What is a heartbeat interceptor?

A

Blank request/responses between client and server to prevent the browser from closing the connection after the TTL time

46
Q

Persistent connection: pros

A
  • Prevent browser from closing the connection after the TTL time
  • When frequency of request/response is high, a persistent connection averts the need to open and close a new connection
47
Q

Persistent connection: cons

A
  • Open connection consume resources
  • There is a limit to the number of open connections a server can handle at once
  • If connections don’t close and new ones are introduced, then the server will run out of memory over time
48
Q

What are web sockets?

A

Network protocol that allows two-way communication session between the client and server

A type of HTTP push based mechanism

Preferred when a persistent bi-directional, low latency data flow is needed

ie. chat apps, real-time social streams, bitcoin trading sites, browser-based multiplayer games

49
Q

What is long polling?

A

Technique to emulate a realtime server push feature

In long polling, the client requests are long-lived and the connection hangs until the server responds with data or a timeout threshold is reached

Reduces consumption bandwidth compared to polling as there are a smaller number of client requests sent

50
Q

What are server-sent events? What are some use cases?

A

The server automatically pushes data to the client whenever updates are available

Reduces a huge amount of blank request-response cycles

Uni-directional data flow from server to client as opposed to web sockets having bi-directional data flow

ie. Twitter feed, realtime notifications

51
Q

Why do browser requests have a TTL?

A

Because open connections between client and server are resource-intensive and there is a limit to the number of connections a server can manage

52
Q

What is the difference between client-side and server-side rendering?

A

Client-side rendering

  • The rendering engine (a component on the browser) constructs the DOM tree, renders and paints the construction
  • Used for dynamic and AJAX-based websites

Server-side rendering

  • Rendering the UI on the backend server and then sending the rendered HTML page to the client
  • Ensures faster rendering of the UI
  • Great for SEO because web crawlers can easily read the generated content
  • Used for static content (ie. Wordpress blogs)
53
Q

What is vertical scaling? What are some use cases?

A

Increasing the power of the hardware running the app (adding more power to the server)

ie. a server with 16GBs of RAM and to handle the increased load, you scale up to 32GBs of RAM

Use cases:

  • Minimal, consistent traffic
  • Internal app for the organisation
54
Q

Vertical scaling: pros

A
  • Simpler than horizontal scaling because we don’t need to touch the code or make any complex system configurations
55
Q

Vertical scaling: cons

A
  • Availability risk - the servers are powerful but few in number
  • There is a limit to the capacity we can increase for a single server (ie. for a multi-storey building, you can’t infinitely add more floors but you can build more buildings)
  • Unable to dynamically scale in realtime as it requires pre-planning
56
Q

What is horizontal scaling? What are some use cases?

A

Increasing more hardware to the existing resource pool to add more computational power to the system as a whole

Use cases:
- Social network app where traffic is expected to spike significantly

57
Q

Horizontal scaling: pros

A
  • No limit to augmenting the hardware capacity
  • Servers can be setup across the globe so data can be replicated across different geoghical regions
  • Able to dynamically scale in realtime as traffic increase/decrease
58
Q

Horizontal scaling: cons

A
  • Takes more administrative, monitoring and management effort to managing a distributed environment
  • Code needs to change - can’t have any static instances in the class as static instances hold application data so if a particular server goes down, all static data is lost and app is left in an inconsistent state (this is why functional programming became popular with distributed systems as functions don’t retain any state)
59
Q

What does cloud elasticitiy mean?

A

Ability to add/remove server nodes dynamically

Indicates the stretching and returning to the original infrastructural computational capacity

60
Q

What are the primary bottlenecks that hurt scalability?

A
  • Database is a single monolith
    • Db partitioning, sharding can make the app efficient
  • Database containing business logic
    • Unnecessary load on the db
    • Whole app becomes tightly coupled
  • Not picking the right database
  • Application architecture
  • Not using caching wisely
  • Inefficient setup of load balancers
    • Too many/too few impacts latency
  • Inefficient code
    • Tightly coupled code
    • Unnecessary/nested loops
61
Q

What strategies can be employed to fine tune the performance of an app?

A
  • Profiling
  • Caching
  • CDN
  • Data compression
  • Avoid unnecessary client-server requests
62
Q

How is the app’s performance and scalability related?

A

Performance of an app is directly related to scalability - if an app does not scale well, the increase in traffic will reduce the app’s performance significantly

63
Q

What is availability?

A

The odds of a server/service being up and running at any point in time

Usually measured in percentages

ie. a server that has 99% availability will be operational 99% of the time

64
Q

What are nines in terms of availability?

A

Refers to percentage of uptime

Expected downtimes per year for uptime percentages:

  • 99% (two nines): 87 hrs
  • 99.9% (three nines): 8 hrs
  • 99.99% (four nines): 52 mins
  • 99.999% (five nines): 5 mins
65
Q

What is meant by high availability?

A

Systems that have typically > five 9s

66
Q

What are some common reasons for failing systems?

A
  • Software crashes
    • OS crashing
    • Memory-hogging unresponsive processes
    • Overloaded CPU and RAM
  • Hardware failures
    • Hard disk failures
  • Human errors
    • Flawed config
  • Planned downtime
    • Routine maitenance
    • Patching software/hardware upgrades
67
Q

What strategies can be employed to achieve high availability?

A
  • Fault tolerance
  • Microservices
  • Redundancy
  • Replication
68
Q

What is fault tolerance?

A

A system’s ability to stay up despite taking hits

69
Q

What is redundancy?

A

The process of replicating parts of a system to make it more reliable

Ensures the system as a whole remains unimpacted

70
Q

What is replication?

A

The act of duplicating data from one database to others

Uses:

  • Increase system redundancy
  • Tolerate regional failures
  • Move data closer to clients to decrease latency
71
Q

What is sharding or data partitioning?

A

The act of splitting a database into >= 2 more pieces (shards) based on different strategies

Used to increase throughput of the db

72
Q

What are some sharding strategies?

A
  • Based on client’s region
  • Based on type of data being stored
    (ie. payments data in one shard, user data in another shard)
  • Based on the hash of a column (for structured data)
73
Q

What is a CDN?

A

Content Delivery Network

Third-party service that acts like a cache for your servers

A CDN has servers across the globe so the latency to a CDN will almost always be better than the latency to your servers

74
Q

What is a message queue?

A

Queue that routes messages from the source to the destination

Follows FIFO policy

Use cases:
- Notification systems

75
Q

What are the different patterns for message queues?

A
  • Pub/sub model

- Point-to-point model

76
Q

Message queue: pros

A
  • Facilitates async behaviour
  • Facilitates cross-module communication with each other in the background
    • This is key in service-oriented and microservices architecture
  • Provide temporary storage of messages until they are consumed
77
Q

What is the pub/sub model in a message queue?

A

Model where multiple subsribers receive the same message sent from a publisher

One-to-many relationship - one producer, multiple consumers

78
Q

What is the point-to-point model in a message queue?

A

Model where the message from one producer is consumed by only one consumer

One-to-one relationship - one producer, one consumer

79
Q

Why is a pull-based approach not ideal in implementing the user notifications feature?

A

Because a pull-based approach is resource intensive - application servers have to deal with unnecessary requests and it is also not in realtime

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