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e2i Software Engineer Interview Guide

e2i Software Engineer interview guide: reported rounds, questions and how candidates describe the process.

The 60-second brief

At e2i, a Software Engineer—often titled as a Controls Engineer or Validation Engineer—serves as a vital link between complex industrial systems and the manufacturing processes that drive our clients' success. You will operate within the specialized field of systems integration, where your technical expertise in automation, PLC, HMI, and SCADA platforms directly influences the reliability and efficiency of high-stakes environments, such as pharmaceutical and automotive manufacturing. This role is not just about writing code; it is about solving real-world, physical-world problems. You will be responsible for designing and implementing control systems that ensure manufacturing lines run smoothly, safely, and accurately.

This guide is scoped to a Software Engineer candidate at e2i.

Focus areas

PLC (Programmable Logic Controllers)
Deviation Investigation / Root Cause Analysis
CAPA (Corrective and Preventive Actions)

The e2i interview process

e2i candidates report 3 rounds over 3-5 weeks. The stages below are what candidates describe, not a published process.

Initial Assessment

Assessment of your technical background to gauge foundational knowledge.

What to demonstrate

Assessment of your technical background to gauge foundational knowledge
Depth in PLC (Programmable Logic Controllers)

How to prepare

Answer aloud and timed: Describe your experience with PLC or SCADA platforms like Rockwell, DeltaV, or Siemens.
Answer aloud and timed: How do you approach the troubleshooting process when an automation system fails in a production environment?

Deeper Technical Discussions

In-depth technical interviews focusing on practical application and specific platforms.

What to demonstrate

In-depth technical interviews focusing on practical application and specific platforms
Depth in PLC (Programmable Logic Controllers)

How to prepare

Answer aloud and timed: Can you explain your process for reading and interpreting a complex Piping and Instrumentation Diagram (P&ID)?
Answer aloud and timed: How have you handled the implementation of Manufacturing Execution Systems (MES) in previous projects?

Cultural Fit Review

Evaluation of your alignment with the collaborative, project-based culture of e2i.

What to demonstrate

Evaluation of your alignment with the collaborative, project-based culture of e2i
Depth in PLC (Programmable Logic Controllers)

How to prepare

Answer aloud and timed: What are the key differences you consider when designing an HMI interface for an operator versus a maintenance engineer?
Answer aloud and timed: Tell me about a time you identified a critical bug in a production line; how did you isolate the issue?

Where candidates lose points

Show your work

When answering technical questions, explain your thought process. We are more interested in how you arrive at a solution than just the solution itself.

Be ready for the "Why

We don't just want to know what you did; we want to know why you chose a specific design or troubleshooting approach over alternatives.

Align with our Mantra

Keep "Quality People, Quality Results" in mind. Everything you share should reflect your dedication to high-quality work and professional integrity.

Do not underestimate the importance of soft skills

A brilliant engineer who cannot communicate effectively with a client will struggle in this role.

Questions & practice

Collapse a redelivered event batch into per-aggregate high-water marks

Easy · hashing · at-least-once · aggregation

You drain a batch of up to 5,000,000 events, each (aggregate_id BIGINT, aggregate_version INT, event_type, payload). The log guarantees order within one aggregate only; the batch merges 64 partitions, and a relay failover has redelivered a range, so an older version for an aggregate can appear after a newer one. Given a map of last_applied_version per aggregate, produce the events worth applying, at most one per (aggregate_id, version), plus the count discarded. Target O(n) time. State the memory for 2,000,000 distinct aggregates and what you do when it does not fit.

Approach

One pass, one hash map from aggregate_id to the highest version kept, and a discard counter. An event whose version is at or below last_applied_version for its aggregate is dropped without further work, which is the whole reason the event carries its version rather than a delta. O(n) expected time, O(d) space in distinct aggregates.
Keep the maximum, never the last occurrence. The redelivered range means the final appearance of an aggregate in the batch can be an older version than one seen earlier in the same batch, so last-wins applies stale state over newer state and the projection regresses with no error anywhere.
Cost the memory instead of calling it large: an 8-byte key plus a 4-byte version is 12 bytes of payload, and an open-addressed table held at a 0.7 load factor costs roughly 17 bytes per entry before per-slot metadata, so 2,000,000 aggregates is tens of megabytes in a native layout and several times that in a runtime that boxes both key and value.
If the distinct set exceeds memory, partition on hash(aggregate_id) mod P and reduce each partition independently. Every event for one aggregate hashes to the same partition, so the per-partition result is exact and the merge is concatenation rather than a second reduction.

Follow-up

The payload is a patch rather than a snapshot, so applying only the highest version loses the intermediate changes. What changes in your reduction?
How do you detect that version 7 arrived while version 6 was never delivered, and what should the consumer do about the gap?

Archive a resource graph without breaking live references or recursing

Medium · graph traversal · topological order · tenant isolation

Resources reference other resources within a tenant; for the largest tenant the reference table holds up to 2,000,000 nodes and 8,000,000 edges. Archiving a resource must archive everything reachable from it that nothing outside the set still references, refuse when a live external referrer exists, and terminate when references form cycles, which they legitimately do. Produce the archive order and the refusal list, targeting O(V+E). Say what stops the traversal crossing a tenant boundary, and why recursion is the wrong control structure at this size.

Approach

Load the subgraph with the tenant predicate on both endpoints of the edge, not only on the side you started from. Scoping the left table alone is the classic cross-tenant leak: one mis-entered edge then pulls another tenant's resources into the traversal and, worse, into the archive.
Traverse iteratively with an explicit stack. A 2,000,000-node graph can hold a chain deep enough to exhaust a native stack in the low tens of thousands of frames, and that failure is a process crash rather than an error you can return.
Treat cycles as data rather than corruption: compute strongly connected components with Tarjan in O(V+E) using its own explicit stack, then condense. The condensation is a DAG, so a topological order over it gives the archive order, and every member of a component archives in one transaction because no order within a cycle is valid.
Decide refusals with reverse edges. A candidate is archivable only if every in-edge originates inside the candidate set, so build the transpose or count in-degrees restricted to the visited set, and emit each blocked resource with the id of the external referrer, which is the only part of the answer an operator can act on.

Follow-up

The graph is read in one query and the archive writes a minute later. What can change in between, and how do you make the write safe?
The candidate set is 400,000 resources. Is that one transaction, and if not, what does a half-finished archive look like to a reader?

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?

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?

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?

Can you explain your process for reading and interpreting a complex Piping and Instrumentation Diagram (P&ID)?

Medium · Technical Proficiency and Domain Knowled

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 have you handled the implementation of Manufacturing Execution Systems (MES) in previous projects?

Medium · Technical Proficiency and Domain Knowled

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?

What are the key differences you consider when designing an HMI interface for an operator versus a maintenance

Medium · Technical Proficiency and Domain Knowled

What are the key differences you consider when designing an HMI interface for an operator versus a maintenance engineer?

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 balance the need for system performance with the necessity of maintaining rigorous safety and valid

Medium · Problem-Solving and Methodology

How do you balance the need for system performance with the necessity of maintaining rigorous safety and validation standards?

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 situation where your initial design for a control system needed to be changed due to unforeseen har

Medium · Problem-Solving and Methodology

Describe a situation where your initial design for a control system needed to be changed due to unforeseen hardware limitations.

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 your code is maintainable and well-documented for future support teams?

Medium · Problem-Solving and Methodology

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 communicate complex technical issues to a client who does not have an engineering background?

Medium · Client Relations and Soft Skills

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?

What is your approach to working within a team-based project environment where priorities might shift quickly?

Medium · Client Relations and Soft Skills

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 the troubleshooting process when an automation system fails in a production environment?

Medium · Technical Proficiency and Domain Knowled

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 e2i candidates report.

Day 1 — Map the e2i loop

Write out the reported sequence: Initial Assessment, Deeper Technical Discussions, Cultural Fit Review.
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 PLC (Programmable Logic Controllers)

Spend the session on PLC (Programmable Logic Controllers), which e2i candidates report being tested on.
Write one worked example in PLC (Programmable Logic Controllers) and time yourself on it.

Deliverable: One timed worked example in PLC (Programmable Logic Controllers).

Day 3 — Work Deviation Investigation / Root Cause Analysis

Spend the session on Deviation Investigation / Root Cause Analysis, which e2i candidates report being tested on.
Write one worked example in Deviation Investigation / Root Cause Analysis and time yourself on it.

Deliverable: One timed worked example in Deviation Investigation / Root Cause Analysis.

Day 4 — Work CAPA (Corrective and Preventive Actions)

Spend the session on CAPA (Corrective and Preventive Actions), which e2i candidates report being tested on.
Write one worked example in CAPA (Corrective and Preventive Actions) and time yourself on it.

Deliverable: One timed worked example in CAPA (Corrective and Preventive Actions).

Day 5 — Answer out loud: Technical Proficiency and Domain Knowledge

Answer aloud, timed: Describe your experience with PLC or SCADA platforms like Rockwell, DeltaV, or Siemens.
Answer aloud, timed: How do you approach the troubleshooting process when an automation system fails in a production environment?

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

Day 6 — Answer out loud: Problem-Solving and Methodology

Answer aloud, timed: Tell me about a time you identified a critical bug in a production line; how did you isolate the issue?
Answer aloud, timed: How do you balance the need for system performance with the necessity of maintaining rigorous safety and validation standards?

Deliverable: Spoken answers to 2 reported Problem-Solving and Methodology question(s), under time.

Day 7 — Answer out loud: Client Relations and Soft Skills

Answer aloud, timed: How do you communicate complex technical issues to a client who does not have an engineering background?
Answer aloud, timed: Tell me about a time you had to manage a difficult client request or a change in project scope.

Deliverable: Spoken answers to 2 reported Client Relations and Soft Skills question(s), under time.

Behavioural preparation

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

Describe your experience with PLC or SCADA platforms like Rockwell, DeltaV, or Siemens.

Medium · Technical Proficiency and Domain Knowled

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?

Tell me about a time you identified a critical bug in a production line; how did you isolate the issue?

Medium · Problem-Solving and Methodology

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?

Tell me about a time you had to manage a difficult client request or a change in project scope.

Medium · Client Relations and Soft Skills

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 on-call responsibilities while maintaining a high level of service for the client?

Medium · Client Relations and Soft Skills

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?