Performance

Question 1

What is PAAS? What does it offer?

Answer 1

Platform as a service.

Easy horizontal scaling
Component level performance tuning
Infrastructure security.

Question 2

Is Response Time Important?

Answer 2

Yes, Constitutes to a drop in sales, traffic, searches. If it takes more than 100ms it is instantaneous and then greater than 7 seconds is abandonment time. Speed is a feature.

Question 3

Where does time go during Web Request?

Answer 3

Request Sent -> Web Server -> App Server (controller & model) -> Database access -> View rendering -> OS/Network Stack -> Back to user, render on user. All assets loaded, run scripts, rendering done.

Question 4

What Client side of request takes the most time?

Answer 4

CSS Selectors & JavaScript = 41% of client rendering time.

Question 5

What hinders client side of request speed

Answer 5

JavaScript interpreter, javascript is single threaded,
Html Parse - 21%
Selector Engine 29%

Question 6

Why is it bad to draw a bell curve distribution of users response time?

Answer 6

The response time distribution is not a bell curve and is actually extremely Right skewed ^___ So you should not draw a bell curve, but a skewed curve. Most users experience good connection and availability.

Question 7

Scalability

Answer 7

Means different things in different context.

For Saas: as number of users increases how to does app availability change

Question 8

Service Level Objective

Answer 8

Time to Satisfy user request.
SLO: Instead of worse case or average, what % of users get acceptable performance?
Specify percentile, target response time, time window.

Question 9

ApDex Score

Answer 9

Given a latency T for user satisfaction:
Satisfaction <= T
Tolerable T <= user <= 4T
Apdex = #Satisfactory + (.5)(tolerable) / requests
0.85 to .93 generally good
But it can hide systematic outliers. Eg critical action occurs once every 15 clicks but takes 10x as long. SO your Apdex is being scored well, but you have a systematic problem with your app taking too long

Question 10

What is a good Threshold?

Answer 10

1000 - 2000ms is best

Question 11

What do you do if site is slow?

Answer 11

Overprovision: Just add more servers since they are cheaper, but only good for smaller sites

Large Site: Dangerous since you have so many idol servers now

Question 12

App wants 99.9% uptime and it was down for an hour. Can it make its goal?

Answer 12

We don’t have enough information because we don’t know what the time frame is for the uptime goal, like over a year or over a day. Then you also don’t know if they had outages in the past.

Question 13

What does a 99.999% up time per year entitle?

Answer 13

<= 5 minutes down per year

Question 14

What are the kinds of monitoring?

Answer 14

At development time (profiling)
Probing while the site is down using an external monitor.
Both are normally used.

Question 15

What is internal monitoring?

Answer 15

Info collected in your app but stored locally.
Server on Heroku injected measurement points in the stack and then in background gets this data shipped to another site and then gives you historical data. You can just use Relic as a gem and that’s all you need to do to test it. It has a free tier.

Question 16

What are options for monitoring?

Answer 16

Google Analytics for clicks and time in between user action, Slow Controller and DB queries with NewRelic. Availability with pingdom. Unhandled exceptions airbrake

Question 17

What should we measure for our app health?

Answer 17

Stress Testing or load testing: how far can I push my system before performance becomes unacceptable and before it grasps and dies.
Usually one component will be bottleneck.

Question 18

What are Longevity Bugs?

Answer 18

They are bugs that happen a while after production and it is usually stuff like memory leaks where over time they get worse until they get over manageable. The way around this is to just reset the servers every set time period so it never waits long enough to crash from memory leaks.

Question 19

Running out of sessions is an example of?

Answer 19

Self-denial of service attacks like hitting homepage and such -> Where your homepage takes some effort to load and then a massive amount of users would hinder it. You can avoid this with not expensive home pages.

Question 20

what is not a high interest metric to the app operator?

Answer 20

Maximum CPU Utilization which does not affect the user.

Question 21

What can you do to your application that helps with responsiveness?

Answer 21

You can define the queries and how users get presented. Since the database is hard to scale this is a point to worry about. Find ways to optimize queries and such.

Question 22

What is n + 1 Query problem?

Answer 22

Traversing an association through an immediate table. You call fans and then you call fans.movies For each of these records you retrieve 1 query and then an additional n queries for each one query. You can avoid this with active record. Tell it @fans = Moviegoer.where(‘’’’’’’).includes(:movies) this will stop from needing to do additional queries and it helps with speed. You also don’t want to preload associations that you don’t use. It will only waste time.

Question 23

How to fix n + 1?

Answer 23

Bullet gem can help recognize them. Solution is to rewrite them.

Question 24

What are indices?

Answer 24

Speed up access when searching DB table by column other than primary key. It is just like hash table. Always index foreign keys.

Question 25

Where should you put a feature flag?

Answer 25

Not in a file, Put it in a database or a place you can change at run time.