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Lucid Software Software Engineer Interview Guide

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

The 60-second brief

A Software Engineer at Lucid Software is responsible for building and scaling the highly interactive visual collaboration products that millions of users rely on daily, including Lucidchart, Lucidspark, and Lucidscale. This role sits at the intersection of complex web graphics, real-time collaborative state synchronization, and highly performant backend architectures. Unlike traditional web applications that deal primarily with static forms and documents, the engineering team here solves unique challenges related to rendering millions of canvas elements, managing complex user interactions, and maintaining real-time multiplayer consistency. As a developer in this space, your contributions directly impact how teams brainstorm, design systems, and visualize infrastructure.

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

Focus areas

  • Algorithms
  • Problem solving
  • Walkthroughs and explaining code

The Lucid Software interview process

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

Online Technical Assessment

Evaluate fundamental coding skills under a flexible timeline.

What to demonstrate

  • Evaluate fundamental coding skills under a flexible timeline
  • Depth in Algorithms

How to prepare

  • Answer aloud and timed: Implement an efficient algorithm to find the longest non-decreasing subsequence in an array.
  • Answer aloud and timed: Given an N x N matrix representing a grid, write a function to find the shortest path between two coordinates while avoiding obstacles.

Live Technical Screening

Practical video conference session focusing on implementing complex logic.

What to demonstrate

  • Practical video conference session focusing on implementing complex logic
  • Depth in Algorithms

How to prepare

  • Answer aloud and timed: Implement a depth-first search (BFS) or breadth-first search (DFS) to traverse a graph and identify connected components.
  • Answer aloud and timed: Write a function to manipulate and parse complex nested string patterns, ensuring optimal time and space complexity.

Virtual Onsite Loop

Comprehensive sessions evaluating algorithmic knowledge, system design, and behavioral alignment.

What to demonstrate

  • Comprehensive sessions evaluating algorithmic knowledge, system design, and behavioral alignment
  • Depth in Algorithms

How to prepare

  • Answer aloud and timed: Implement the core board-state logic and move-validation rules for the game of Reversi (Othello).
  • Answer aloud and timed: Write a program to simulate a Tic-Tac-Toe game on an arbitrary grid size, including a function to detect win conditions.

Where candidates lose points

Focus on code functionality first

During the game design and coding rounds, prioritize writing working, bug-free code over a highly optimized but incomplete solution. Ensure your core logic runs and passes basic test cases before trying to optimize.

Ask clarifying questions early

The design and modeling prompts are intentionally broad. Before writing any code or drawing diagrams, spend the first few minutes clarifying the scope, assumptions, and constraints with your interviewer.

Practice coding without external assistance

The live coding environments discourage or restrict the use of external Q&A forums. Practice solving algorithmic problems on a clean IDE without relying on autocomplete or web searches.

Questions & practice

Implement an efficient algorithm to find the longest non-decreasing subsequence in an array.

Medium · Algorithmic & Data Structures

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?

Given an N x N matrix representing a grid, write a function to find the shortest path between two coordinates

Medium · Algorithmic & Data Structures

Given an N x N matrix representing a grid, write a function to find the shortest path between two coordinates while avoiding obstacles.

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?

Implement a depth-first search (BFS) or breadth-first search (DFS) to traverse a graph and identify connected

Medium · Algorithmic & Data Structures

Implement a depth-first search (BFS) or breadth-first search (DFS) to traverse a graph and identify connected components.

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?

Write a function to manipulate and parse complex nested string patterns, ensuring optimal time and space compl

Medium · Algorithmic & Data Structures

Write a function to manipulate and parse complex nested string patterns, ensuring optimal time and space complexity.

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?

Create an in-memory class model to track and render front-end visual entities on a collaborative canvas.

Medium · Class Modeling & Object-Oriented Design

Approach

  1. Say what the runtime actually does before reasoning about the code.
  2. Name what is shared across threads and what owns each piece of state.
  3. Identify the window where an invariant is briefly untrue.
  4. 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?

Model a system to handle and process multiple asynchronous events in a collaborative document editor.

Medium · Class Modeling & Object-Oriented Design

Approach

  1. Say what the runtime actually does before reasoning about the code.
  2. Name what is shared across threads and what owns each piece of state.
  3. Identify the window where an invariant is briefly untrue.
  4. 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?

Denormalise tenant onto revisions and backfill it live

Hard · denormalisation · online migration · backfill

resource_revision (revision_id, resource_id, version, actor_user_id, change_kind, patch, request_id, created_at) has 400M rows and no tenant column; tenant_id lives only on resource. Two reads need it: a tenant-scoped audit feed ordered by created_at DESC, and an offboarding purge. Both join back to resource today. Justify adding tenant_id to resource_revision against those two reads, name the anomaly the copy introduces and the constraint that prevents it, then give the ordered migration for a live table taking 1.2k writes/second — the lock each step takes, how the backfill is batched, and where each step stops being reversible. PostgreSQL 16.

Approach

  1. Justify from the access path rather than from taste. Without the column, the audit feed either scans resource_revision by created_at and discards other tenants' rows, or resolves the tenant's resource_ids first and probes with them — both proportional to the tenant's whole history rather than to one page. With (tenant_id, created_at DESC, revision_id DESC) it is a seek that stops at 50 rows, and the purge becomes a ranged delete instead of a join.
  2. Name the cost exactly: a second copy of a fact can disagree with the first. Make the disagreement unwritable rather than documented — add UNIQUE (resource_id, tenant_id) on resource so it can serve as a foreign-key target, then FOREIGN KEY (resource_id, tenant_id) REFERENCES resource (resource_id, tenant_id) on the revision table. A revision can then only ever carry its parent's tenant.
  3. Step one, expand: ALTER TABLE resource_revision ADD COLUMN tenant_id BIGINT NULL, with no default, so it is a catalogue change and no rewrite. It still needs ACCESS EXCLUSIVE for an instant, and that instant queues behind the longest open transaction on the table while every later query queues behind it — set lock_timeout to 2s and retry rather than wait.
  4. Step two, dual-write: deploy the writer that populates tenant_id on every new revision while reads still use the join. Reversible by redeploying the previous build, because nothing reads the column yet.

Follow-up

  • The backfill is half finished and a rollback is required. What state is the table in, and what does the previous build do with a half-populated column?
  • How do you verify the backfill actually finished, given rows are still being inserted while it runs?

Write the update path that detects a concurrent edit

Medium · optimistic concurrency · transactional outbox · audit history

resource carries version INT NOT NULL DEFAULT 1. resource_revision holds revision_id, resource_id, version, actor_user_id, change_kind, patch JSONB, request_id, created_at with UNIQUE (resource_id, version). outbox_event holds aggregate_type, aggregate_id, aggregate_version, event_type, payload, status. A PUT carries the version the client read. Write the exact statements for the single transaction that applies the edit, records the revision and enqueues 'resource.updated', and give the handler's branch on zero affected rows. Then say what PostgreSQL 16 does under READ COMMITTED when two of these updates hit one row at once.

Approach

  1. One transaction, three writes, no network call inside it: UPDATE resource SET title = $3, version = version + 1, updated_at = now() WHERE resource_id = $1 AND tenant_id = $4 AND version = $2; then INSERT the resource_revision row at version $2 + 1; then INSERT the outbox_event row at the same aggregate_version. The event goes to a table rather than a broker because no transaction spans both.
  2. Branch on the affected-row count before doing anything else. Zero has three causes — stale version, wrong tenant, row gone — so re-read once and map to 409 carrying the current version, or 404 for an id outside the caller's tenant, which also stops the endpoint confirming that another tenant's id exists.
  3. State the engine behaviour instead of assuming it. Under READ COMMITTED the second UPDATE blocks on the row lock, and when the first commits PostgreSQL re-evaluates the WHERE clause against the newly committed row, so the version predicate now fails and the statement reports zero rows. Under REPEATABLE READ the identical collision raises SQLSTATE 40001 instead, so the handler must fold both shapes into one conflict response.
  4. Keep UNIQUE (resource_id, version) even though the predicate already serialises writers. It is what makes a lost update unwritable if any other path ever reaches the revision table, and it converts a logic bug into 23505 rather than into a silently missing history row.

Follow-up

  • A client sends the version it read ten minutes ago and the resource has moved three versions. What is in your 409 so it can resolve the conflict without a full re-fetch?
  • Two editors, two disjoint fields, no overlap. Does your answer still refuse the second write, and should it?

Implement the core board-state logic and move-validation rules for the game of Reversi (Othello).

Medium · Game Logic & Simulation

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?

Write a program to simulate a Tic-Tac-Toe game on an arbitrary grid size, including a function to detect win c

Medium · Game Logic & Simulation

Write a program to simulate a Tic-Tac-Toe game on an arbitrary grid size, including a function to detect win conditions.

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?

Implement the grid-shifting and tile-merging mechanics for a simulation of the game 2048.

Medium · Game Logic & Simulation

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 the board-generation and cell-reveal logic for a command-line version of Minesweeper.

Medium · Game Logic & Simulation

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 the class structure and interface relationships for a multiplayer chess game, ensuring scalability for

Medium · Class Modeling & Object-Oriented Design

Design the class structure and interface relationships for a multiplayer chess game, ensuring scalability for different rule variations.

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?

Design a parking lot system, defining classes for vehicles, parking spots, and payment terminals while justify

Medium · Class Modeling & Object-Oriented Design

Design a parking lot system, defining classes for vehicles, parking spots, and payment terminals while justifying your inheritance choices.

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?

Describe a significant engineering project you recently worked on, focusing on the architectural decisions and

Medium · Behavioral & Experience

Describe a significant engineering project you recently worked on, focusing on the architectural decisions and trade-offs you made.

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?

p99 jumped on one listing filter while p50 stayed flat

Medium · query planning · composite index · percentiles

After a release that added an owner_user_id filter to the resource listing, p99 rose from 90 ms to 1.9 s while p50 stayed at 40 ms. Traffic and row counts are unchanged. resource carries the index (tenant_id, status, updated_at DESC, resource_id DESC). The new query filters tenant_id and owner_user_id, orders by updated_at DESC, resource_id DESC, and takes 20 rows. On PostgreSQL, explain the shape of the regression, prove it from a query plan, and give the index you would add.

Approach

  1. Start from the shape. A flat p50 with a moved p99 means a subset of requests changed cost, not all of them, so the first job is naming the subset. Bucket the endpoint's latency by the tenant's row count; the natural hypothesis is that large tenants are a small share of requests and all of the tail.
  2. Get the plan for the new query on a large tenant with EXPLAIN (ANALYZE, BUFFERS). Expect an index scan over the tenant's range, a filter discarding most of it, then a Sort feeding the Limit, possibly reporting Sort Method: external merge Disk. Read actual rows on the scan node, not estimated.
  3. Explain why the existing index cannot serve it. A composite B-tree is seekable only as a left prefix, and with no equality predicate on status the scan cannot treat updated_at as an ordering, because rows in the tenant's range are ordered by status first. Everything matching must be read and sorted before LIMIT 20 can apply, so a tenant with 400,000 rows pays 400,000 rows to return 20.
  4. Add (tenant_id, owner_user_id, updated_at DESC, resource_id DESC). Equality on the first two columns leaves the index ordered by updated_at within that pair, so the plan becomes an index scan that stops after 20 rows with no Sort node. PostgreSQL can scan a B-tree backwards, so the DESC markers matter only if the two sort columns ever disagree in direction; keeping them explicit documents the order the keyset cursor depends on.

Follow-up

  • The endpoint paginates with OFFSET. What does page 500 cost with your index, and what does the keyset version cost?
  • How would you have caught this before release, given that a 10,000-row seed database produces the same plan shape at an unnoticeable cost?

Suggested practice checklist

Built from the rounds and topics Lucid Software candidates report.

Day 1 — Map the Lucid Software loop

  • Write out the reported sequence: Online Technical Assessment, Live Technical Screening, Virtual Onsite Loop.
  • 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 Algorithms

  • Spend the session on Algorithms, which Lucid Software candidates report being tested on.
  • Write one worked example in Algorithms and time yourself on it.

Deliverable: One timed worked example in Algorithms.

Day 3 — Work Problem solving

  • Spend the session on Problem solving, which Lucid Software 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 Walkthroughs and explaining code

  • Spend the session on Walkthroughs and explaining code, which Lucid Software candidates report being tested on.
  • Write one worked example in Walkthroughs and explaining code and time yourself on it.

Deliverable: One timed worked example in Walkthroughs and explaining code.

Day 5 — Answer out loud: Algorithmic & Data Structures

  • Answer aloud, timed: Implement an efficient algorithm to find the longest non-decreasing subsequence in an array.
  • Answer aloud, timed: Given an N x N matrix representing a grid, write a function to find the shortest path between two coordinates while avoiding obstacles.

Deliverable: Spoken answers to 2 reported Algorithmic & Data Structures question(s), under time.

Day 6 — Answer out loud: Game Logic & Simulation

  • Answer aloud, timed: Implement the core board-state logic and move-validation rules for the game of Reversi (Othello).
  • Answer aloud, timed: Write a program to simulate a Tic-Tac-Toe game on an arbitrary grid size, including a function to detect win conditions.

Deliverable: Spoken answers to 2 reported Game Logic & Simulation question(s), under time.

Day 7 — Answer out loud: Class Modeling & Object-Oriented Design

  • Answer aloud, timed: Design the class structure and interface relationships for a multiplayer chess game, ensuring scalability for different rule variations.
  • Answer aloud, timed: Create an in-memory class model to track and render front-end visual entities on a collaborative canvas.

Deliverable: Spoken answers to 2 reported Class Modeling & Object-Oriented Design question(s), under time.

Behavioural preparation

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

Walk me through a highly challenging technical situation you faced in a previous role and explain the exact st

Medium · Behavioral & Experience

Walk me through a highly challenging technical situation you faced in a previous role and explain the exact steps you took to resolve it.

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 handle a situation where you receive vague or conflicting requirements from stakeholders?

Medium · Behavioral & Experience

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?

Tell me about a time you had to learn a new technology or domain quickly to deliver a critical feature.

Medium · Behavioral & Experience

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?