Perchwell Software Engineer Interview Guide

The 60-second brief

As a Software Engineer at Perchwell, you play a pivotal role in developing innovative technology solutions that enhance the real estate experience for users. This position is critical to our mission of transforming how real estate is managed and understood, making it essential for delivering high-quality products that meet user needs effectively. Your contributions will directly impact the functionality and performance of our platforms, influencing not just our business outcomes but also the experiences of our clients and end-users. In this role, you will engage with complex problems across multiple domains, including data management, application performance, and user experience. You will collaborate closely with cross-functional teams, leveraging your technical skills to build and optimize systems that drive our real estate applications.

This guide is scoped to a Software Engineer candidate at Perchwell.

Focus areas

• Ruby on Rails
• Technical Interviewing (Live Coding/Technical Rounds)
• System Design

The Perchwell interview process

Perchwell candidates report 3 rounds over 3-5 weeks. The stages below are what candidates describe, not a published process.

Phone Screen

Initial call with HR to discuss background and assess role fit.

What to demonstrate

• Initial call with HR to discuss background and assess role fit
• Depth in Ruby on Rails

How to prepare

• Be able to walk your CV end to end in two minutes, and say why this company specifically.
• Have your salary expectations, notice period and location constraints ready, and ask for the rest of the loop in writing.

Technical Interviews

Interviews focusing on coding, system design, and behavioral assessments.

What to demonstrate

• Interviews focusing on coding, system design, and behavioral assessments
• Depth in Ruby on Rails

How to prepare

• Answer aloud and timed: Can you explain how you would optimize a database query for performance?
• Answer aloud and timed: How do you approach debugging a complex issue in production?

Stakeholder Meetings

Meet various team members to understand the collaborative nature of the engineering team.

What to demonstrate

• Meet various team members to understand the collaborative nature of the engineering team
• Depth in Ruby on Rails

How to prepare

• Answer aloud and timed: Design a simplified version of a URL shortening service (like TinyURL).
• Answer aloud and timed: Discuss the trade-offs between SQL and NoSQL databases in a real estate application.

Where candidates lose points

Prepare for a conversational interview style

Expect discussions rather than strict Q&A sessions. This approach encourages you to share your thought process and experiences freely.

Familiarize yourself with the company's products

Understanding Perchwell's offerings and their impact on the real estate market will help you frame your responses in context.

Showcase your learning mindset

Be prepared to discuss how you keep your skills current and how you embrace new technologies or methods.

Practice coding on a whiteboard

If you have coding challenges, practicing on a whiteboard can help simulate the interview environment and improve your problem-solving skills.

Questions & practice

Given an array of integers, write a function to find the two numbers that add up to a specific target.

Medium · Coding / Algorithms

Approach

1. Restate the input: its shape, its size, and what is guaranteed about it.
2. Name the brute-force solution and its complexity before improving on it.
3. Choose the data structure from the access pattern, not from familiarity.
4. State the target complexity and say which constraint rules the naive version out.

Follow-up

• How does this change if the input no longer fits in memory?
• What is the worst case, and how likely is it on real data?

How would you implement a basic search algorithm to find a property in a list?

Medium · Coding / Algorithms

Approach

1. Restate the input: its shape, its size, and what is guaranteed about it.
2. Name the brute-force solution and its complexity before improving on it.
3. Choose the data structure from the access pattern, not from familiarity.
4. State the target complexity and say which constraint rules the naive version out.

Follow-up

• How does this change if the input no longer fits in memory?
• What is the worst case, and how likely is it on real data?

Diff a projection against the primary without per-row point reads

Hard · reconciliation · range hashing · throttling

The listing projection has drifted and some rows show a stale version. The primary holds 40,000,000 resource rows across 12,000 tenants while serving 1,200 writes and 14,000 reads per second. The obvious repair, reading each resource row and comparing its version against the projection, is correct and would eventually finish. Explain precisely why it is unacceptable here, then give a diff that finds the differing rows, state its complexity, and make it safe to run against a live primary. Replication lag is usually under 100 ms and is not bounded.

Approach

1. Quantify the naive cost rather than calling it slow: 40,000,000 point reads at even 0.5 ms each is over five hours serialised, and the only lever is concurrency, which is exactly what you cannot spend. The primary's pool is sized for the write path, and 40,000,000 random reads evict the buffer cache that sustains the 85 percent cache hit rate, so the audit degrades the system it is auditing.
2. Replace random access with one ordered pass per side. Both sides can be read in (tenant_id, resource_id) order, which is a sequential scan on each and a merge join in O(n) time and O(1) memory. For a dense diff that is the whole answer, and it reads the primary once instead of 40,000,000 times.
3. For the expected sparse case, compare range hashes instead of rows: partition the key space, compute per range an order-independent aggregate over hash(resource_id, version), compare aggregates, and descend only into ranges that differ. With d differing rows and branching factor B, at most d ranges mismatch per level, so the drill-down examines O(d log_B(n/d)) ranges and reads full rows only in mismatching leaves.
4. Aggregate with a sum modulo 2^64 or a multiset hash, never XOR. XOR is order-independent but self-cancelling, so two rows wrong in the same way, or a row duplicated on one side, leave the range aggregate matching and the range is declared clean.

Follow-up

• The diff reports 900 stale rows. How do you decide between patching those rows and rebuilding the projection from resource_revision?
• Same job, but the projection lives in a search index that cannot be scanned in key order. What changes?

Can you explain how you would optimize a database query for performance?

Medium · Technical / Domain Questions

Approach

1. Name the grain you start from and join outward from it.
2. Check whether any join is one-to-many before aggregating, or the sums inflate.
3. Say which index the query would use, and what makes it unusable.
4. Handle the rows that do not match: that is usually the actual question.

Follow-up

• How does the query change if that join becomes one-to-many?
• What happens to this when the table is ten times larger?

Discuss the trade-offs between SQL and NoSQL databases in a real estate application.

Medium · System Design / Architecture

Approach

1. Name the grain you start from and join outward from it.
2. Check whether any join is one-to-many before aggregating, or the sums inflate.
3. Say which index the query would use, and what makes it unusable.
4. Handle the rows that do not match: that is usually the actual question.

Follow-up

• How does the query change if that join becomes one-to-many?
• What happens to this when the table is ten times larger?

What are some best practices you follow when writing unit tests?

Medium · Technical / Domain Questions

Approach

1. Clarify what is being asked and what a complete answer contains.
2. State your assumptions explicitly before working the problem.
3. Say what you would check first and why it is the highest-information step.
4. Work from the requirement backwards to the design.

Follow-up

• What assumption would you test first?
• How would you know your answer was wrong?

Design a simplified version of a URL shortening service (like TinyURL).

Medium · System Design / Architecture

Approach

1. Fix the scope first: who calls this, how often, and what they do when it fails.
2. Name the read and write paths separately; they rarely have the same bottleneck.
3. Choose a partition key and say what query it makes expensive.
4. State the consistency you need, and where you are willing to be stale.

Follow-up

• What breaks first when traffic grows ten times?
• How does this behave when that dependency is down for an hour?

How would you scale an application to handle increased user traffic?

Medium · System Design / Architecture

Approach

1. Fix the scope first: who calls this, how often, and what they do when it fails.
2. Name the read and write paths separately; they rarely have the same bottleneck.
3. Choose a partition key and say what query it makes expensive.
4. State the consistency you need, and where you are willing to be stale.

Follow-up

• What breaks first when traffic grows ten times?
• How does this behave when that dependency is down for an hour?

If you had to implement a feature to allow users to filter properties based on specific criteria, how would yo

Medium · Problem-Solving / Case Studies

If you had to implement a feature to allow users to filter properties based on specific criteria, how would you approach it?

Approach

1. Clarify what is being asked and what a complete answer contains.
2. State your assumptions explicitly before working the problem.
3. Say what you would check first and why it is the highest-information step.
4. Work from the requirement backwards to the design.

Follow-up

• What assumption would you test first?
• How would you know your answer was wrong?

Walk me through your process of creating a custom field for real estate properties.

Medium · Problem-Solving / Case Studies

Approach

1. Clarify what is being asked and what a complete answer contains.
2. State your assumptions explicitly before working the problem.
3. Say what you would check first and why it is the highest-information step.
4. Work from the requirement backwards to the design.

Follow-up

• What assumption would you test first?
• How would you know your answer was wrong?

How do you approach debugging a complex issue in production?

Medium · Technical / Domain Questions

Approach

1. Establish what changed and when, before forming any theory.
2. Pick a bisection that eliminates candidates whichever way it turns out.
3. Check the instrumentation before believing the symptom.
4. Separate the trigger from the cause; the deploy is rarely the bug.

Follow-up

• What would you look at first, and what would it rule out?
• How would you tell a cause from a coincidence here?

Suggested practice checklist

Built from the rounds and topics Perchwell candidates report.

Day 1 — Map the Perchwell loop

• Write out the reported sequence: Phone Screen, Technical Interviews, Stakeholder Meetings.
• For each round, write one sentence on what it is judging, from the description above, and mark the one you are least ready for.

Deliverable: A one-page map of the 3 reported rounds, with the weakest marked.

Day 2 — Work Ruby on Rails

• Spend the session on Ruby on Rails, which Perchwell candidates report being tested on.
• Write one worked example in Ruby on Rails and time yourself on it.

Deliverable: One timed worked example in Ruby on Rails.

Day 3 — Work Technical Interviewing (Live Coding/Technical Rounds)

• Spend the session on Technical Interviewing (Live Coding/Technical Rounds), which Perchwell candidates report being tested on.
• Write one worked example in Technical Interviewing (Live Coding/Technical Rounds) and time yourself on it.

Deliverable: One timed worked example in Technical Interviewing (Live Coding/Technical Rounds).

Day 4 — Work System Design

• Spend the session on System Design, which Perchwell candidates report being tested on.
• Write one worked example in System Design and time yourself on it.

Deliverable: One timed worked example in System Design.

Day 5 — Answer out loud: Technical / Domain Questions

• Answer aloud, timed: Describe your experience with Ruby on Rails and how you have used it in past projects.
• Answer aloud, timed: What are some best practices you follow when writing unit tests?

Deliverable: Spoken answers to 2 reported Technical / Domain Questions question(s), under time.

Day 6 — Answer out loud: System Design / Architecture

• Answer aloud, timed: Design a simplified version of a URL shortening service (like TinyURL).
• Answer aloud, timed: Discuss the trade-offs between SQL and NoSQL databases in a real estate application.

Deliverable: Spoken answers to 2 reported System Design / Architecture question(s), under time.

Day 7 — Answer out loud: Behavioral / Leadership

• Answer aloud, timed: Describe a time you faced a significant challenge at work. How did you overcome it?
• Answer aloud, timed: How do you prioritize tasks and manage deadlines in a fast-paced environment?

Deliverable: Spoken answers to 2 reported Behavioral / Leadership question(s), under time.

Behavioural preparation

Behavioural rounds judge the decision you made and what it cost.

Describe your experience with Ruby on Rails and how you have used it in past projects.

Medium · Technical / Domain Questions

Approach

1. Pick a story where you made the decision, not one where you watched it.
2. State the situation in two sentences and spend the rest on the reasoning.
3. Give the blast radius: what could have broken, and what you measured.
4. Name the disagreement and how you resolved it with evidence.

Follow-up

• What would you do differently if you ran that again?
• How did you know your change caused the improvement?

Describe a time you faced a significant challenge at work. How did you overcome it?

Medium · Behavioral / Leadership

Approach

1. Pick a story where you made the decision, not one where you watched it.
2. State the situation in two sentences and spend the rest on the reasoning.
3. Give the blast radius: what could have broken, and what you measured.
4. Name the disagreement and how you resolved it with evidence.

Follow-up

• What would you do differently if you ran that again?
• How did you know your change caused the improvement?

How do you prioritize tasks and manage deadlines in a fast-paced environment?

Medium · Behavioral / Leadership

Approach

1. Pick a story where you made the decision, not one where you watched it.
2. State the situation in two sentences and spend the rest on the reasoning.
3. Give the blast radius: what could have broken, and what you measured.
4. Name the disagreement and how you resolved it with evidence.

Follow-up

• What would you do differently if you ran that again?
• How did you know your change caused the improvement?

Give an example of how you’ve worked effectively within a team.

Medium · Behavioral / Leadership

Approach

1. Pick a story where you made the decision, not one where you watched it.
2. State the situation in two sentences and spend the rest on the reasoning.
3. Give the blast radius: what could have broken, and what you measured.
4. Name the disagreement and how you resolved it with evidence.

Follow-up

• What would you do differently if you ran that again?
• How did you know your change caused the improvement?