Interview guides · Dexcom

Dexcom Software Engineer Interview Guide

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

The 60-second brief

As a Software Engineer at Dexcom, you are at the intersection of life-saving medical technology and high-scale software engineering. Your work directly impacts the lives of millions of users who rely on Dexcom’s Continuous Glucose Monitoring (CGM) systems to manage their health. You are not just building software; you are ensuring the reliability, accuracy, and security of data that informs critical medical decisions every single day. This role requires a unique blend of technical precision and user-centric empathy. You will contribute to the development of complex systems, ranging from mobile applications and cloud-based data platforms to the embedded software powering our hardware devices.

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

Focus areas

JavaScript
Problem Solving
Case Study / Engineering Scenario

The Dexcom interview process

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

Recruiter Screen

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

What to demonstrate

Initial screening call with a recruiter to assess candidate's background and fit for the role
Depth in JavaScript

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

In-depth technical interviews to evaluate coding skills and technical knowledge.

What to demonstrate

In-depth technical interviews to evaluate coding skills and technical knowledge
Depth in JavaScript

How to prepare

Answer aloud and timed: Explain your experience with [specific language/framework] and how you debug complex issues.
Answer aloud and timed: How would you design a system to handle high-frequency data from a wearable device?

Behavioral Sessions

Interviews focused on assessing cultural fit and behavioral competencies.

What to demonstrate

Interviews focused on assessing cultural fit and behavioral competencies
Depth in JavaScript

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 sessions above and write down what you would ask to confirm before it.

Final Panel

Final onsite or virtual panel interviews, which may involve multiple interviewers.

What to demonstrate

Final onsite or virtual panel interviews, which may involve multiple interviewers
Depth in JavaScript

How to prepare

Answer aloud and timed: Tell us about a time you had to deal with a strong or difficult personality in the workplace.
Answer aloud and timed: How do you handle tight deadlines or shifting requirements in a fast-paced environment?

Where candidates lose points

Own your story

Be ready to talk about every line on your resume. If you list a technology, be prepared to answer deep-dive questions about it.

Ask meaningful questions

Use your time at the end of the interview to ask about the team’s biggest challenges or how they measure success. It shows you are already thinking like an engineer on the team.

Practice communication

We value engineers who can explain complex solutions clearly. If you can't explain it simply, you may not understand it well enough yet.

Stay persistent

The process can be demanding. Keep your communication professional and proactive, even if you are waiting for updates.

Questions & practice

Tell us about a time you had to optimize code for performance or memory constraints.

Medium · Technical & Domain Proficiency

Approach

Say what the runtime actually does before reasoning about the code.
Name what is shared across threads and what owns each piece of state.
Identify the window where an invariant is briefly untrue.
Distinguish a value from a reference to it, and say which one you handed out.

Follow-up

What happens if two callers reach this at the same time?
Where could this allocate more than you expect?

If you were tasked with improving the latency of our data sync process, where would you start?

Medium · Case Studies & Problem Solving

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?

How would you prioritize features if you had limited engineering resources and a hard deadline?

Medium · Case Studies & Problem Solving

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?

Explain why the owner filter ignores the listing index

Medium · composite index · query plans · sort elimination

The only index on resource is (tenant_id, status, updated_at DESC, resource_id DESC). A new endpoint returns one user's resources across all statuses, newest created first: WHERE tenant_id = $1 AND owner_user_id = $2 ORDER BY created_at DESC LIMIT 20. On a tenant with 2M rows it takes 900 ms and EXPLAIN shows a sort above a large scan. Explain precisely why the existing index cannot serve it, give the index that can, and state which of these the new index still will not help: owner_user_id alone across tenants; the same query ordered by updated_at. PostgreSQL 16.

Approach

Separate the two jobs an index does. For filtering, a composite btree is seekable only on a left prefix, so with no predicate on status the scan can at best range over tenant_id and test owner_user_id per row; PostgreSQL 16 has no btree skip scan to jump the unconstrained column.
For ordering, the index is sorted by (status, updated_at) within a tenant and not by created_at, so the LIMIT cannot stop early: every matching row is read and then sorted. That is the 'Sort Method: top-N heapsort' line, and it is why the plan reads 2M rows to answer with 20.
Derive the replacement from the access path — equality, equality, then the ordering column: CREATE INDEX CONCURRENTLY ON resource (tenant_id, owner_user_id, created_at DESC). The scan seeks to the (tenant, owner) range and walks 20 entries in order, so the Sort node disappears along with the row-read.
Treat INCLUDE (title, status) as conditional, not free. An index-only scan still visits the heap for any row whose page is not marked all-visible, so on a table taking 1.2k writes/second the win depends on autovacuum keeping the visibility map current, and the wider index costs more on every insert.

Follow-up

90% of rows are status='active'. Would a partial index WHERE status = 'active' change your answer, and for which of the three queries?
A dashboard runs this for 40 owners in one page load. What changes about the design?

Replace offset paging on the resource feed with keyset

Easy · keyset pagination · composite index · sort order

resource holds resource_id, tenant_id, owner_user_id, title, body_ref, version, status ('draft','active','archived','deleted'), created_at, updated_at, deleted_at, with an index on (tenant_id, status, updated_at DESC, resource_id DESC). The listing endpoint returns active resources for one tenant, newest update first, 50 per page, today with LIMIT 50 OFFSET n. Tenants reach page 400 and rows are created while they read. Write the keyset query, define what the cursor carries and how it is encoded, and say which part of the index each predicate uses. Assume PostgreSQL 16.

Approach

Name the two failures separately. OFFSET 20000 makes the server produce and discard 20,000 rows, so page cost grows with depth rather than with page size. Independently, any write that changes how many rows sort above the offset moves the window between two fetches, and the direction decides which anomaly you get: an insert lands at the head of updated_at DESC and pushes already-returned rows down past the boundary, so they are returned a second time; a delete above the offset, or a row whose updated_at is bumped above the cursor, pulls rows up and one is never returned at all. Nothing in the response reveals either.
Write the seek: WHERE tenant_id = $1 AND status = 'active' AND (updated_at, resource_id) < ($2, $3) ORDER BY updated_at DESC, resource_id DESC LIMIT 50. The row-value comparison is one index range rather than a disjunction, and both columns are NOT NULL, which is what makes that comparison well defined.
Map each predicate onto the index: tenant_id and status are equality on the leading columns, (updated_at, resource_id) is the range, and the ORDER BY matches the index order so no Sort node appears and the scan stops after 50 rows. The DESC in the definition only matters for mixed directions — a plain ascending btree on the same columns is read backwards for this query.
Put both sort columns in the cursor and nothing the client can tamper with into another tenant: base64 of (updated_at, resource_id), validated server-side, with tenant_id taken from the principal.

Follow-up

The client asks for 'jump to page 400'. What do you offer instead, and what does the honest version cost?
Sort order becomes user-selectable across four columns. How many indexes is that, and which would you refuse to add?

Describe a challenging technical problem you solved in a previous project.

Medium · Technical & Domain Proficiency

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 approach testing and validation for software that interacts with hardware?

Medium · Technical & Domain Proficiency

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 would you design a system to handle high-frequency data from a wearable device?

Medium · Technical & Domain Proficiency

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?

Walk us through your thought process for designing a scalable architecture for [hypothetical feature].

Medium · Case Studies & Problem Solving

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?

Listing latency scales with page size, not with filters

Easy · n+1 · latency · query batching

The tenant listing endpoint reads resource filtered by tenant_id and status, ordered by updated_at DESC, and returns each row plus the owner's display name from app_user and the actor of that resource's latest resource_revision. p99 is 55 ms at 10 rows per page and 1.4 s at 200. Database telemetry shows 401 statements per request, each under 1 ms, and nothing in the slow-query log. Diagnose the cause and give the fix, stating the statement count per request and the p99 you expect afterwards.

Approach

Read the counters before forming a theory. 401 statements for 200 rows is one driver query plus two per row, and sub-millisecond execution with an empty slow-query log rules out a bad plan. The time is round trips, which is why it is invisible in every per-query metric and scales with rows returned rather than with filter selectivity.
Name the two per-row statements from their normalised text: a single-row app_user lookup by user_id, and a resource_revision lookup by resource_id ordered by version DESC LIMIT 1. Confirm by dropping those two response fields and watching the statement count fall to one. That locates the calls in the serialisation layer, not the repository.
Check that the arithmetic accounts for the whole gap. Measure one round trip to the replica in isolation; 400 trips at roughly 3 ms of network plus 0.2 ms of execution is about 1.3 s on top of a 55 ms baseline, which matches. If the multiplication had fallen short, the N+1 would only be part of the story and you would keep looking.
Batch both lookups. Collect owner_user_ids and resource_ids from the driver query, then issue WHERE tenant_id = $1 AND user_id = ANY($2) for the users, and PostgreSQL's SELECT DISTINCT ON (resource_id) ... WHERE resource_id = ANY($2) ORDER BY resource_id, version DESC for the latest revision, which the UNIQUE (resource_id, version) index serves directly. On an engine without DISTINCT ON, use a lateral join or a row_number window. Three statements per request at any page size.

Follow-up

The page size is capped at 200 today. What breaks first if it is raised to 2,000, and is it still this bug?
How do you stop the next N+1 from reaching production, given that no individual query is slow and the endpoint's tests pass?

Suggested practice checklist

Built from the rounds and topics Dexcom candidates report.

Day 1 — Map the Dexcom loop

Write out the reported sequence: Recruiter Screen, Technical Interviews, Behavioral Sessions, Final Panel.
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 JavaScript

Spend the session on JavaScript, which Dexcom candidates report being tested on.
Write one worked example in JavaScript and time yourself on it.

Deliverable: One timed worked example in JavaScript.

Day 3 — Work Problem Solving

Spend the session on Problem Solving, which Dexcom candidates report being tested on.
Write one worked example in Problem Solving and time yourself on it.

Deliverable: One timed worked example in Problem Solving.

Day 4 — Work Case Study / Engineering Scenario

Spend the session on Case Study / Engineering Scenario, which Dexcom candidates report being tested on.
Write one worked example in Case Study / Engineering Scenario and time yourself on it.

Deliverable: One timed worked example in Case Study / Engineering Scenario.

Day 5 — Answer out loud: Technical & Domain Proficiency

Answer aloud, timed: Describe a challenging technical problem you solved in a previous project.
Answer aloud, timed: How do you approach testing and validation for software that interacts with hardware?

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

Day 6 — Answer out loud: Behavioral & Leadership

Answer aloud, timed: Describe a time you worked with a cross-functional team. How did you handle conflicting priorities?
Answer aloud, timed: Tell us about a time you had to deal with a strong or difficult personality in the workplace.

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

Day 7 — Answer out loud: Case Studies & Problem Solving

Answer aloud, timed: Walk us through your thought process for designing a scalable architecture for [hypothetical feature].
Answer aloud, timed: If you were tasked with improving the latency of our data sync process, where would you start?

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

Behavioural preparation

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

Explain your experience with [specific language/framework] and how you debug complex issues.

Medium · Technical & Domain Proficiency

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?