PostHog India Software Engineer Interview: Rounds, Questions and Preparation

What the job involves

Software Engineers at fast-growing global tech companies build, test, and ship product features across the stack, often owning small services or components end to end. In a product-led company like PostHog India, you might work on event ingestion, analytics dashboards, or SDKs that thousands of developers use. The role blends coding, debugging, code review, and close collaboration with product and design in short release cycles.

Role and technical knowledge

1. 1

   How would you design a simple event tracking API that receives millions of events per day from client SDKs?

   Cover ingestion endpoint, batching, message queue, storage choice, and how you would handle duplicate events. Mention trade-offs between latency and durability.

2. 2

   Explain the difference between a session cookie and a persistent user ID when tracking analytics events across page loads.

   Discuss session scope, expiry, privacy considerations, and how a persistent anonymous ID helps stitch sessions. Show awareness of cookie limitations and local storage.

3. 3

   What happens when a user types a URL into a browser and presses Enter, and where could you add caching in that flow?

   Walk through DNS, TCP, TLS, HTTP request, server rendering, and browser cache. Suggest CDN or application cache points and explain cache invalidation briefly.

4. 4

   Given a table of events with user_id, event_name, and timestamp, write a SQL query to find the number of unique users who performed a specific event each day.

   Use GROUP BY on date and event_name with COUNT(DISTINCT user_id). Mention date truncation and filtering by event name. Be ready to discuss indexes.

Practical scenarios and problem solving

1. 5

   A dashboard shows a sudden drop in event volume for one customer. How would you debug this from the frontend SDK to the database?

   Start with recent deploys, SDK version, network errors, then check ingestion pipeline, queue backlog, and database writes. Suggest adding logging and a health check.

2. 6

   You need to add a new feature flag to a product used by thousands of customers. How would you roll it out safely?

   Describe feature flagging, gradual rollout percentage, monitoring key metrics, and a kill switch. Mention backward compatibility and customer communication.

3. 7

   Your API response time increased from 100ms to 2 seconds after a new release. What steps would you take to find and fix the bottleneck?

   Use profiling, slow query logs, and APM tools. Check N+1 queries, missing indexes, or external calls. Suggest a rollback and a fix with tests.

4. 8

   How would you write a unit test for a function that calculates the conversion rate from a list of events?

   Cover normal cases, empty lists, division by zero, and rounding. Mention test framework, mocking time or external data, and edge cases.

Behavioural and teamwork

1. 9

   Tell me about a time you had to learn a new technology quickly for a project. How did you approach it?

   Use a specific project example. Show a structured learning plan, hands-on practice, and a measurable outcome. Highlight curiosity and speed.

2. 10

   Describe a situation where you disagreed with a teammate on a technical approach. How did you resolve it?

   Focus on listening, data, and compromise. Show that you can disagree respectfully and commit to the team decision. Avoid blaming others.

3. 11

   Give an example of a bug you introduced and how you handled it.

   Own the mistake, explain the fix, and what you changed in your process to prevent it. Show accountability and learning.

4. 12

   How do you prioritize when you have multiple tasks with tight deadlines?

   Mention impact, urgency, and communication with your manager. Use a simple framework like important versus urgent. Give a real example.

HR and company fit

1. 13

   Why do you want to work at a fast-growing product company rather than a large established tech firm?

   Show interest in ownership, speed, and direct product impact. Connect to your own preference for learning broadly and shipping quickly.

2. 14

   What do you know about our product and how developers use it?

   Mention specific features like event capture, funnels, or session replay. Show you have tried the product or read the docs. Avoid vague praise.

3. 15

   Where do you see yourself in three years as a software engineer?

   Align your growth with the company: deeper technical skills, ownership of a service, or mentoring. Show ambition but stay realistic and flexible.

4. 16

   What are your salary expectations for this role?

   Give a researched range based on market data for freshers in India. Mention flexibility and interest in the overall opportunity. Avoid naming a single number too early.