So, some time ago, I was working for a complete puckered anus of a cosmetics company on their ecommerce product. Won't name names, but they're shitty and known for MLM. If you're clever, go you ;)

Anyways, over the course of years they brought in a competent firm to implement their service layer. I'd even worked with them in the past and it was designed to handle a frankly ridiculous-scale load. After they got the 1.0 released, the manager was replaced with some absolutely talentless, chauvinist cuntrag from a phone company that is well known for having 99% indian devs and not being able to heard now. He of course brought in his number two, worked on making life miserable and running everyone on the team off; inside of a year the entire team was ex-said-phone-company.

Watching the decay of this product was a sheer joy. They cratered the database numerous times during peak-load periods, caused $20M in redis-cluster cost overrun, ended up submitting hundreds of erroneous and duplicate orders, and mailed almost $40K worth of product to a random guy in outer mongolia who is , we can only hope, now enjoying his new life as an instagram influencer. They even terminally broke the automatic metadata, and hired THIRTY PEOPLE to sit there and do nothing but edit swagger. And it was still both wrong and unusable.

Over the course of two years, I ended up rewriting large portions of their infra surrounding the centralized service cancer to do things like, "implement security," as well as cut memory usage and runtimes down by quite literally 100x in the worst cases.

It was during this time I discovered a rather critical flaw. This is the story of what, how and how can you fucking even be that stupid. The issue relates to users and their reports and their ability to order.

I first found this issue looking at some erroneous data for a low value order and went, "There's no fucking way, they're fucking stupid, but this is borderline criminal." It was easy to miss, but someone in a top down reporting chain had submitted an order for someone else in a different org. Shouldn't be possible, but here was that order staring me in the face.

So I set to work seeing if we'd pwned ourselves as an org. I spend a few hours poring over logs from the log service and dynatrace trying to recreate what happened. I first tested to see if I could get a user, not something that was usually done because auth identity was pervasive. I discover the users are INCREMENTAL int values they used for ids in the database when requesting from the API, so naturally I have a full list of users and their title and relative position, as well as reports and descendants in about 10 minutes.

I try the happy path of setting values for random, known payment methods and org structures similar to the impossible order, and submitting as a normal user, no dice. Several more tries and I'm confident this isn't the vector.

Exhausting that option, I look at the protocol for a type of order in the system that allowed higher level people to impersonate people below them and use their own payment info for descendant report orders. I see that all of the data for this transaction is stored in a cookie. Few tests later, I discover the UI has no forgery checks, hashing, etc, and just fucking trusts whatever is present in that cookie.

An hour of tweaking later, I'm impersonating a director as a bottom rung employee. Score. So I fill a cart with a bunch of test items and proceed to checkout. There, in all its glory are the director's payment options. I select one and am presented with:

"please reenter card number to validate."

Bupkiss. Dead end.

OR SO YOU WOULD THINK.

One unimportant detail I noticed during my log investigations that the shit slinging GUI monkeys who butchered the system didn't was, on a failed attempt to submit payment in the DB, the logs were filled with messages like:

"Failed to submit order for [userid] with credit card id [id], number [FULL CREDIT CARD NUMBER]"

One submit click later and the user's credit card number drops into lnav like a gatcha prize. I dutifully rerun the checkout and got an email send notification in the logs for successful transfer to fulfillment. Order placed. Some continued experimentation later and the truth is evident:

With an authenticated user or any privilege, you could place any order, as anyone, using anyon's payment methods and have it sent anywhere.

So naturally, I pack the crucifixion-worthy body of evidence up and walk it into the IT director's office. I show him the defect, and he turns sheet fucking white. He knows there's no recovering from it, and there's no way his shitstick service team can handle fixing it. Somewhere in his tiny little grinchly manager's heart he knew they'd caused it, and he was to blame for being a shit captain to the SS Failboat. He replies quietly, "You will never speak of this to anyone, fix this discretely." Straight up hitler's bunker meme rage.

There's very little good use cases for mongo, change my mind.

Prototyping maybe? Rails can prototype, create/update/destroy db schemas really quickly anyways.

If you're doing a web app, there's tons of libs that let you have a store in your app, even a fake mongo on the browser.

Are the reads fast? When I need that, use with redis.

Can it be an actual replacement for an app's db? No. Safety mechanisms that relational dbs have are pretty much must haves for a production level app.
Data type checks, null checks, foreign key checks, query checks.
All this robustness, this safety is something critical to maintain the data of an app sane.

Screw ups in the app layer affecting the data are a lot less visible and don't get noticed immediately (things like this can happen with relational dbs but are a lot less likely)

Let's not even get into mutating structures. Once you pick a structure with mongo, you're pretty much set.
Redoing a structure is manual, and you better have checks afterwards.

But at the same time, this is kind of a pro for mongo, since if there's variable data, as in some fields that are not always present, you don't need to create column for them, they just go into the data.

But you can have json columns in postgres too!

Is it easier to migrate than relational dbs? yes, but docker makes everything easy also.

One problem for CS education is that the salaries for academia are so low compared to industry that if someone is even vaguely competent, they can at least double their income by working a 'real' job. Now this may be different at higher levels of colleges but generally those folks are such bullshitters they wouldn't last outside of academia.

As what to improve?
Depends if it's a research or vocational course.

For vocational; heavy on group projects, common tools, methodologies and architectures. All demonstrated in something like c#/java/python. And one project must have a web app (db, app layer and JavaScript from end)
Basic knowledge of algorithms, runtime analysis (O notation) and some data structures and you're an instahire.

For research, go wild. Deep dive into the math, algorithmic side. Read up lots of research papers. Try out different programming paradigms. You would aim for a career in academia, AI, quant finance etc...

Hey, Do you think it's common to get an interview take-home test like this...

Write a Python script that:- Downloads the Equity bhavcopy zip from the above page- Extracts and parses the CSV file in it- Writes the records into Redis into appropriate data structures(Fields: code, name, open, high, low, close)
Write a simple CherryPy python web application that:- Renders an HTML5 + CSS3 page that lists the top 10 stock entries from the Redis DB in a table- Has a searchbox that lets you search the entries by the 'name' field in Redis and renders it in a table- Make the page look nice!

Commit the code to Github. Host the application on AWS or Heroku or a similar provider and share both the links.