Backend Developer cover letter example

Backend Developer cover letter example: opening on the team's own migration problem, proof from a zero-downtime ledger split, and a design-exercise close.

Resume for this role

Ciarán Doyle

Senior Backend Engineer | Go, PostgreSQL, Kafka | Payments and High-Volume APIs

Dublin, Ireland

30 October 2026

Aoife ByrneEngineering Manager, Core BankingTolka BankDublin

Re: Senior Backend Engineer, Ledger Platform

Dear Ms Byrne,

Your posting for a Senior Backend Engineer at Tolka Bank describes a ledger that has outgrown its monolith and a team that needs to split it without anyone noticing. I did exactly that at Lanyard Payments, and I would like to do it again with the lessons learned.

Over nine months I moved our ledger module out of a 400,000-line monolith into three services while it kept processing live card traffic. We ran old and new paths side by side, compared every balance nightly, and switched over one merchant group at a time. There was no customer-facing downtime, and the reconciliation reports we built for the cut-over are still used by the finance team.

I also own our card authorisation service in Go, which handles peaks of 2,000 requests per second at a p99 latency of 85 ms. Adding idempotency keys and an outbox pattern for retried writes took its uptime from 99.7% to 99.95% over the past year. Along the way I cut our team's monthly AWS bill by €11,000 by right-sizing RDS instances and moving cold data to S3.

I have 6 years of backend experience, mostly in Go, Python and PostgreSQL, and I care about the less visible work: migration safety, cost control and alerting. Your stack is Go on Postgres with event sourcing on Kafka, which matches what I use today. I also run our backend interview loop and have hired five engineers since 2024, so I can help you grow the team as well as the system.

I would be glad to take on a design exercise or walk your team through the ledger migration in more detail.

Kind regards,

Ciarán Doyle

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How this letter works

What each part of the letter above does, and how to do the same in yours.

The opening

The letter opens by restating the hard problem in the posting, splitting a ledger out of a monolith without downtime, and claims it has been done before. Engineering managers respond to someone who has read their actual problem, not a list of languages.

Whom to address

Address the engineering manager for the team, who is often named on the posting or findable on LinkedIn. In Ireland and the UK, "Dear Ms Byrne" with "Kind regards" is the usual, slightly less formal register for tech roles.

Requirements matched to proof

The posting asks forDecompose a monolith safely
The letter showsLedger moved into three services over nine months with parallel runs and nightly balance checks, no customer-facing downtime.
The posting asks forRun reliable high-volume services
The letter shows2,000 requests per second at 85 ms p99; uptime raised from 99.7% to 99.95%.
The posting asks forGo, PostgreSQL and Kafka experience
The letter showsSix years on the same stack, named against the posting's own description.

Length

Five paragraphs inside the usual 250 to 400 words. Technical letters tempt you to list everything; this one explains one migration properly and keeps the rest to a sentence each.

This letter: 273 words.

The closing

It closes by offering a design exercise or a walk-through of the migration, which is what the technical interview will ask for anyway. "Kind regards" fits the register of the greeting.

Mistakes to avoid in this letter

  1. A list of technologies

    The CV already has the stack. The letter should explain one system you built or changed, with the constraint and the result.

  2. Not reading the posting's problem

    Backend postings usually describe a real pain: scaling, a migration, reliability. A letter that ignores it reads as a mass application.

  3. Overclaiming team results

    Engineering managers will ask what you personally did in the interview. Say "I" for your part and "we" for the team's, and keep them straight.

Questions about this cover letter

Do backend developers need a cover letter?
Many tech companies make it optional, but a short, specific letter helps when the posting describes a problem you have solved. It gives the hiring manager a reason to open your CV first.
Should I link my GitHub in the cover letter?
Put it in the header with your contact details, and mention a project in the body only if it is relevant to the role.
How technical should the cover letter be?
Technical enough for an engineering manager to judge: name the pattern, the scale and the result. Leave architecture diagrams and code for the interview.

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