Interview guides · Cowen

Cowen Software Engineer Interview Guide

Cowen Software Engineer interview guide: reported rounds, questions and how candidates describe the process.

The 60-second brief

A Software Engineer at Cowen plays a pivotal role in shaping the technology that powers the firm’s financial services. This position is integral to developing and maintaining software solutions that enhance the efficiency, accuracy, and scalability of Cowen's operations. You will be working alongside talented professionals to tackle complex problems in finance, making your contributions essential to the firm's success in a highly competitive industry. In this role, you will engage with various teams to design and implement robust software that supports trading systems, risk management, and client services. The complexity of financial products and the need for rapid innovation create an exciting environment where your skills will directly influence the company’s ability to serve its clients effectively.

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

Focus areas

Quantitative background (quant)
Finance domain knowledge
Face-to-face technical interviews

The Cowen interview process

Cowen candidates report 4 rounds over 3-5 weeks. The stages below are what candidates describe, not a published process.

Recruiter Call

Initial screening call with a recruiter to discuss the candidate's background and fit for the role.

What to demonstrate

Initial screening call with a recruiter to discuss the candidate's background and fit for the role
Depth in Quantitative background (quant)

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 Assessments

Candidates undergo technical assessments to evaluate their coding skills and problem-solving abilities.

What to demonstrate

Candidates undergo technical assessments to evaluate their coding skills and problem-solving abilities
Depth in Quantitative background (quant)

How to prepare

Answer aloud and timed: Describe a complex software project you worked on. What challenges did you face?
Answer aloud and timed: How do you ensure the quality and performance of your code?

Behavioral Questions

Interviews include behavioral questions to assess cultural fit and teamwork capabilities.

What to demonstrate

Interviews include behavioral questions to assess cultural fit and teamwork capabilities
Depth in Quantitative background (quant)

How to prepare

Prepare three examples from your own work, each with a decision you made and an outcome you can quantify.
Re-read the description of the behavioral questions above and write down what you would ask to confirm before it.

Team Discussions

Candidates engage in discussions with team members to further evaluate mutual fit and collaboration.

What to demonstrate

Candidates engage in discussions with team members to further evaluate mutual fit and collaboration
Depth in Quantitative background (quant)

How to prepare

Answer aloud and timed: Describe a situation where you had to learn a new technology quickly. What was your approach?
Answer aloud and timed: How do you prioritize tasks when you have multiple deadlines?

Where candidates lose points

Understand the financial domain

Familiarize yourself with key financial concepts relevant to your role. This knowledge can greatly enhance your discussions during the interview.

Practice coding problems

Use platforms like LeetCode or HackerRank to sharpen your coding skills in preparation for technical interviews.

Review your past projects

Be ready to discuss specific examples from your past work that showcase your problem-solving abilities and technical expertise.

Prepare for behavioral questions

Reflect on your experiences and how they align with Cowen’s values. Use the STAR method (Situation, Task, Action, Result) to structure your responses.

Questions & practice

How would you approach optimizing an existing algorithm for better performance?

Medium · Problem-Solving / Case Studies

Approach

Restate the input: its shape, its size, and what is guaranteed about it.
Name the brute-force solution and its complexity before improving on it.
Choose the data structure from the access pattern, not from familiarity.
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

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.
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.
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.
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?

Merge partitioned event streams into one ordered feed with bounded lateness

Hard · k-way merge · watermarks · out-of-order streams

The read-model service consumes 64 log partitions carrying about 4,000 events per second in total. Each partition is ordered within itself, but partitions drift by up to 30 seconds, and the activity feed must present a tenant's events in occurred_at order. Produce the merge. State its complexity, the buffer it requires in events and in bytes, what happens when one partition is idle, and what you do with an event that arrives after you have already emitted its position. Payloads average 1 KB.

Approach

Merge with a min-heap over the 64 partition heads keyed on (occurred_at, event_id): O(log P) per event and O(n log P) overall. The tie-break on event_id is what makes the output deterministic when two partitions carry the same millisecond, which matters because the feed is paginated and a non-deterministic order reorders pages under the reader.
Emitting the heap head is only correct once every partition has produced everything up to that timestamp, so the emit condition is a watermark: the minimum across partitions of the highest occurred_at seen, less the allowed lateness. Events are held until the watermark passes them, which is what turns individually ordered streams into a jointly ordered one.
Size the buffer from the lateness rather than guessing: 4,000 events per second times 30 seconds is 120,000 buffered events, and at 1 KB each about 120 MB of heap. That number is the real price of the ordering guarantee and belongs in front of whoever asked for it.
Handle the idle partition explicitly, because it fails the feed rather than corrupting it: a partition with no traffic never advances its own maximum, so the watermark freezes and output stops entirely. Either every partition emits a periodic idle marker carrying the broker's current time, or the watermark falls back to wall clock for a partition silent beyond a threshold.

Follow-up

The lateness budget is raised to five minutes. What is the new buffer, and what besides memory changes?
The consumer restarts. Where does it resume from, and what does the feed look like for the first 30 seconds?

Can you explain the differences between SQL and NoSQL databases?

Medium · Technical / Domain Questions

Approach

Name the grain you start from and join outward from it.
Check whether any join is one-to-many before aggregating, or the sums inflate.
Say which index the query would use, and what makes it unusable.
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?

Find version gaps and relay lag with window functions

Medium · window functions · event tables · frame semantics

outbox_event holds event_id, aggregate_type, aggregate_id, aggregate_version, event_type, payload, status ('pending','published','dead'), attempts, created_at, published_at. A projection is missing rows and you must decide whether the relay skipped events or the consumer dropped them. Write three queries over the last seven days: one listing every aggregate_id whose published aggregate_version sequence has a hole, one giving per-day counts with a running total, and one returning the newest published event per aggregate. For each, say where the window function is evaluated relative to WHERE and LIMIT. PostgreSQL 16.

Approach

Gaps: compute lead(aggregate_version) OVER (PARTITION BY aggregate_id ORDER BY aggregate_version) in a subquery, then filter next_version <> aggregate_version + 1 in the outer query. Window functions are evaluated after WHERE, GROUP BY and HAVING and before the outer ORDER BY and LIMIT, so the predicate cannot sit in the same WHERE clause and PostgreSQL 16 has no QUALIFY.
Say what the seven-day filter does to the answer: it truncates every partition, so the first row per aggregate has no predecessor inside the window and a hole spanning the boundary is invisible. Widen the window, or join to resource.version as the authority for the true maximum.
Running total: SELECT date_trunc('day', created_at) AS d, count() AS n, sum(count()) OVER (ORDER BY date_trunc('day', created_at) ROWS UNBOUNDED PRECEDING). An aggregate inside a window call is legal because grouping runs before windowing. The grouping key is unique per row here so ROWS and RANGE agree, but write the frame anyway — over ungrouped rows with tied timestamps the default RANGE frame pulls in every peer row and the total jumps.
Newest per aggregate: DISTINCT ON (aggregate_id) ... ORDER BY aggregate_id, aggregate_version DESC is the cheap PostgreSQL-only form when an index matches that order; row_number() OVER (PARTITION BY aggregate_id ORDER BY aggregate_version DESC) = 1 is the portable form and needs a subquery for the same evaluation-order reason as the gap query.

Follow-up

Relay failover redelivers events. Does a duplicate break the gap query, and how would you detect one from this table alone?
Turn the gap check into a continuous monitor rather than a query someone runs after an incident. What does it watch?

What programming languages are you most comfortable with, and why?

Medium · Technical / Domain Questions

Approach

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

Follow-up

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

Describe a complex software project you worked on. What challenges did you face?

Medium · Technical / Domain Questions

Approach

Clarify what is being asked and what a complete answer contains.
State your assumptions explicitly before working the problem.
Say what you would check first and why it is the highest-information step.
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 ensure the quality and performance of your code?

Medium · Technical / Domain Questions

Approach

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

Follow-up

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

Discuss a financial concept you're familiar with and its significance in software development.

Medium · Technical / Domain Questions

Approach

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

Follow-up

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

Here’s a specific financial problem; how would you approach solving it?

Medium · Problem-Solving / Case Studies

Approach

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

Follow-up

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

Explain how you would design a system to handle high-frequency trading.

Medium · Problem-Solving / Case Studies

Approach

Fix the scope first: who calls this, how often, and what they do when it fails.
Name the read and write paths separately; they rarely have the same bottleneck.
Choose a partition key and say what query it makes expensive.
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?

Discuss a hypothetical scenario where you need to balance speed and accuracy in software deployment.

Medium · Problem-Solving / Case Studies

Approach

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

Follow-up

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

Describe your thought process when debugging a complex software issue.

Medium · Problem-Solving / Case Studies

Approach

Establish what changed and when, before forming any theory.
Pick a bisection that eliminates candidates whichever way it turns out.
Check the instrumentation before believing the symptom.
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 Cowen candidates report.

Day 1 — Map the Cowen loop

Write out the reported sequence: Recruiter Call, Technical Assessments, Behavioral Questions, Team Discussions.
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 4 reported rounds, with the weakest marked.

Day 2 — Work Quantitative background (quant)

Spend the session on Quantitative background (quant), which Cowen candidates report being tested on.
Write one worked example in Quantitative background (quant) and time yourself on it.

Deliverable: One timed worked example in Quantitative background (quant).

Day 3 — Work Finance domain knowledge

Spend the session on Finance domain knowledge, which Cowen candidates report being tested on.
Write one worked example in Finance domain knowledge and time yourself on it.

Deliverable: One timed worked example in Finance domain knowledge.

Day 4 — Work Face-to-face technical interviews

Spend the session on Face-to-face technical interviews, which Cowen candidates report being tested on.
Write one worked example in Face-to-face technical interviews and time yourself on it.

Deliverable: One timed worked example in Face-to-face technical interviews.

Day 5 — Answer out loud: Technical / Domain Questions

Answer aloud, timed: What programming languages are you most comfortable with, and why?
Answer aloud, timed: Can you explain the differences between SQL and NoSQL databases?

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

Day 6 — Answer out loud: Behavioral / Leadership

Answer aloud, timed: Tell me about a time you faced a conflict in a team. How did you resolve it?
Answer aloud, timed: Describe a situation where you had to learn a new technology quickly. What was your approach?

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

Day 7 — Answer out loud: Problem-Solving / Case Studies

Answer aloud, timed: Here’s a specific financial problem; how would you approach solving it?
Answer aloud, timed: Describe your thought process when debugging a complex software issue.

Deliverable: Spoken answers to 2 reported Problem-Solving / Case Studies question(s), under time.

Behavioural preparation

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

Tell me about a time you faced a conflict in a team. How did you resolve it?

Medium · Behavioral / Leadership

Approach

Pick a story where you made the decision, not one where you watched it.
State the situation in two sentences and spend the rest on the reasoning.
Give the blast radius: what could have broken, and what you measured.
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 situation where you had to learn a new technology quickly. What was your approach?

Medium · Behavioral / Leadership

Approach

Pick a story where you made the decision, not one where you watched it.
State the situation in two sentences and spend the rest on the reasoning.
Give the blast radius: what could have broken, and what you measured.
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 when you have multiple deadlines?

Medium · Behavioral / Leadership

Approach

Pick a story where you made the decision, not one where you watched it.
State the situation in two sentences and spend the rest on the reasoning.
Give the blast radius: what could have broken, and what you measured.
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?

Share an example of how you’ve contributed to a team’s success.

Medium · Behavioral / Leadership

Approach

Pick a story where you made the decision, not one where you watched it.
State the situation in two sentences and spend the rest on the reasoning.
Give the blast radius: what could have broken, and what you measured.
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 handle feedback and criticism?

Medium · Behavioral / Leadership

Approach

Pick a story where you made the decision, not one where you watched it.
State the situation in two sentences and spend the rest on the reasoning.
Give the blast radius: what could have broken, and what you measured.
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?