Here’s what actually stuck with me…

🤔 The first thing I misunderstood: It’s not about getting the “right” answer

In the interview, they're not grading you against a reference architecture. They're watching how you think - how you break down a vague prompt like "design Youtube," what questions you ask before touching the whiteboard.

🛠️ Lesson 1 : Always choose the simplest thing that works

I assumed system design was about knowing the fanciest tools and sharing the same in interviews. It’s the opposite.

Sure, I could throw an amazon EC2 instance behind a load balancer with autoscaling at a problem. But if I’m building an internal tool for a few hundred users, why would I?? Every extra service you add is something you own: patching, monitoring, capacity you’re paying for and not using

The skill was never knowing the big tools. It’s knowing when you don’t need them.

🎯 Lesson 2 : Start with the customer, not the tech

This is basically Amazon's “working backwards” principle applied to architecture and it did reframe how I approach every design later in my projects. Before anything goes on the whiteboard, a few questions decide the entire stack:

• Where are the users? - whether you need a CDN for a multi-region replication or a single DB could be enough for one region.

• Hundreds of users or millions? - decision between vertical or horizontal scaling behind a LB.

• How fast does it need to feel? - “fast” isn’t a spec. p99 latency under 200ms and p99 under 2s are two completely different systems.

• What are the traffic peaks? - steady load or spikes lead to very different calls on autoscaling and queuing.

These answers decide everything. And before you build, get agreement on these requirements with your PM / interviewer before a single box goes on the diagram of building a new system or a new feature for an existing system.

🌐 Lesson 3: Distributed systems are everywhere

Once your data outgrows one machine, you’re in the distributed systems world. This is when things get interesting.

• Sharding = split one big DB into smaller ones, each on its own server. More read/write capacity, less load per node.

• Replication is the actual heartbeat of resilience or in other words prevention for single point of failure. Same idea whether it’s your DB, your cache, or HDFS blocks - copy data across racks so one dying rack doesn’t take your data with it.

• CAP theorem, practically: you can’t max out consistency, availability, and partition tolerance at once - pick what your system needs. Its a requirement conversation.

💸 Lesson 4: Resilience is a spending decision disguised as an engineering one

Plan for failure in layers: single host, rack, data center, region. The tech (hot/warm/cold standby) is the easy part. The hard part is asking “how much downtime can the business actually afford” because that answer decides how much redundancy you’re paying for.

✅ Lesson 5 : Every architecture pattern is a tool, not a checkbox

Somewhere in the course I stopped listing components and started asking what failure mode each one prevents.

• Load balancer - round robin if your servers are identical and stateless, least-connections if request cost varies, hash-based if you need session affinity.

• CDN - only for what actually needs to sit close to users. Static assets, video, images - yes. Your write path - no. It gets expensive fast if you’re not deliberate about what’s cached at the edge.

• Message queues - decouple producer from consumer so a slow downstream service doesn’t take the upstream one down with it. SQS-style if you need exactly one consumer per message, Kafka-style pub/sub if multiple services need to react to the same event independently.

• Idempotency, the quiet requirement - the moment you add retries or a queue, the same message can get processed twice. If your handler isn’t idempotent, “at least once” delivery becomes a bug generator, not a reliability feature.

None of these five lessons are separate from each other, really. They're the same question asked in five different ways.

The course gave me the vocabulary. The back-and-forth rereading, sketching, breaking my own implementations - iis what's slowly turning it into something that actually sticks.

Still figuring plenty of it out 😅. If you’ve been down this road, I’d genuinely like to know: which system design tradeoff took you the longest to actually trust in production, not just get right on a whiteboard???

So for everyone who does not know, I work at a Muslim org and a lot of our features and work are connected to major Islamic months and moments that hold deep importance for us as Muslims. So when Dhul Hijjah started, we had our work planned, and Alhamdulillah, things at work were going as expected. But while work was moving, something inside me was moving too. I started thinking about how, in our South Asian / Pakistani culture, we give so much importance to the nights of Ramadan, but we do not always give the same importance to the first ten days of Dhul Hijjah. Sometimes, these days are treated like ordinary days - and I say this including myself. Even I did not fully understand their importance before learning more about them. And honestly, that realization made me a little sad, because these are the best ten days of the year. Around the same time, something very beautiful happened at work. Coming from a South Asian background, making duaas out loud is not very common among young people or at least that has been my experience. It is usually something we associate more with older people. But this Dhul Hijjah, I saw it happen so naturally. During our standups, we had this culture of making duaa out loud -thanks to my team lead. Different people made duaa on different days, and every time it happened, it felt so sincere and heart touching. It was simple. No overthinking, no awkwardness, just a bunch of people asking Allah swt together. And I loved that! 💙

With this, during the best ten days of the year, Alhamdulillah, I felt greater khushoo in my prayers. These daily duaas during Dhul Hijjah made me feel more connected to Allah swt. Alhamdulillah Summa Alhamdulillah 😭 I also realized there is still so much room to improve the way I make duaa. But even that awareness felt like a blessing. Alhamdulillah! This year, I tried to structure my long and short duaas in a kind of sandwich method:

Praise Allah swt.
Send salawat upon Prophet Muhammad صَلَّى ٱللّٰهُ عَلَيْهِ وَسَلَّمَ
Thank Allah swt.
Ask for forgiveness and tawbah.
Then ask for whatever is in my heart. And the trick is to go all in - with all the faith, all the hope, and all the neediness.
Send salawat upon Prophet Muhammad صَلَّى ٱللّٰهُ عَلَيْهِ وَسَلَّمَ
Praise Allah swt.

Then came the most blessed day: the Day of Arafah. There are so many beautiful references that show the importance of this day. I felt terrible for not giving it this kind of importance earlier. I remember my Day of Arafah in previous years used to be something like: I would fast, do some ibadah, zikr, and duaa - and that was it. Maybe I just did not realize the magnitude of importance this day holds... I mean, fasting on the Day of Arafah expiates the sins of the previous year and the coming year [Sahih Muslim 1162]. And then there are so many moments on this day where you can ask Allah swt for just anything. Here, credit also goes to the kind of work I do and the colleagues I have around me. Alhamdulillah, I cannot thank Allah swt enough for that. And when the day passes, the duaas we forgot to make start coming back to us like post-exam regrets. 😅 In those duaas, while saying takbeer and tahmeed, there was this deep urge in my heart for Hajj 2027 InshaAllah. So dear reader, maybe sneak in a duaa for me and my family too. 😊

Alhamdulillah, this time I also learned a few new comprehensive Arabic duaas and integrated them into my duaa structure. And all of this made me realize how dependent and needy we are of Allah swt. Even if we spent an entire day asking Him, it still would not be enough for everything our hearts carry. And yet, in our normal routines, we sometimes struggle to sit and make duaa for even five minutes.

Who do we think we are???
So full of ourselves.
So convinced by our own efforts.
So caught up in thinking we are in control.

When in reality, it has always been Allah swt. And still, we forget to ask Him properly. We forget to make detailed duaas. We forget that Allah SWT loves when we turn to Him, ask Him, beg Him, and trust Him like a child trusts the one who takes care of them. 💙

And then came the beautiful Eid - Eid ul Adha. Alhamdulillah, my company gave us holidays till the weekend. That itself felt like such a blessing, especially getting to enjoy Eid properly with family. ☺️

And before Eid, on my last working day, we had a conversation around qurbani. Since I work with a multicultural team, everyone had different experiences and perspectives. Some colleagues from North America were genuinely surprised when I told them that, in Pakistan, many families bring their qurbani goats or cows home, take care of them, love them, and feel the essence of qurbani closely - sometimes themselves and sometimes with butchers doing qurbani in the backyards.

That conversation stayed with me. It made me realize how different our experiences of Eid can be across cultures.

And maybe that is why Eid ul Adha touched me so differently this year. All of it came together in a way I had not experienced before. Alhamdulillah! May Allah swt accept our fasts, duaas, sacrifices, prayers, hajj and all our tiny imperfect efforts. And may He invite us all to Hajj. Ameen. 🤍The Dhul Hijjah Challenge