Search the practice set
275 questions, 991 terms and 600 topics in 30 areas.
60 results for “Database Migration Under CD”
Expand-Contract Migration
Changing a schema in additive steps that keep old and new code both working, so deployment and migration never have to be simultaneous.
Database Migration Strategy
The approach for moving data to a new store, which is usually the longest pole and the highest risk in any modernisation.
Airbnb's Service-Oriented Migration
Airbnb decomposed a large Rails monolith by first extracting a unified data-access layer, so that services were built on owned data rather than on shared database tables.
Zero-Downtime Migration
Changing a schema or system while it stays in service, by adding the new alongside the old, migrating, then removing the old.
Join Strategies
The three ways a database combines two row sets — nested loop, hash join and merge join — and the conditions under which each is correct.
Blue-Green Database Schema
The constraint that makes fast rollback actually work — both application versions must be able to run against one schema at the same time.
CI/CD
Merging work continuously into a shared trunk with automated verification, and keeping every commit in a state that could be released.
Database Index
A secondary structure that lets the engine find rows without scanning, trading write cost and storage for read speed.
Database per Service
Each service owning its own datastore, with no other service reading or writing it directly.
Decision-Making Under Uncertainty
Choosing well when the information is incomplete — by bounding the downside and buying information, rather than by waiting for certainty.
Embedding Model Migration
The process of moving a corpus to a new embedding model, which requires re-embedding everything because vectors from different models are not comparable.
Graph Database
A store whose first-class citizens are nodes and the relationships between them, making multi-hop traversal cheap.
Migration Risk Register
A maintained record of what could go wrong in a migration, with likelihood, impact, owner and mitigation, reviewed as the programme progresses.
Migration Wave
A batch of applications migrated together, sequenced so that dependencies move in a workable order and each wave delivers learning for the next.
Refactoring Under Test
Changing internal structure without changing behaviour, with tests as the mechanism that makes the claim verifiable.
Report Migration Inventory
The enumerated list of every report, extract and downstream consumer of the legacy warehouse, with usage evidence, which is what makes the migration finite.
Shared Database Integration
Two or more applications reading and writing the same database directly — the most damaging integration pattern and the hardest to unwind.
Six Rs of Migration
The standard menu of options for each application in a migration — rehost, replatform, refactor, repurchase, retire, retain.
Slack's Cellular Migration
After repeated availability-zone-level incidents, Slack rebuilt its infrastructure into per-zone cells with the ability to drain traffic away from a failing zone in minutes.
Vector Database
A store optimised for approximate nearest-neighbour search over high-dimensional embeddings.
A release must rename a heavily used database column and ships tonight. The team proposes doing the rename in the deployment. What is wrong, and what do you propose?
What is wrong A rename is not additive. During a rolling deployment both versions of the application run simultaneously: one expects the old name, one the new.
A serverless API works in testing and fails under load with connection errors. The database is at 5% CPU. Explain and fix.
The mechanism Serverless functions scale by creating independent execution environments , each with its own process and its own connection pool. Two hundred con
An estate has database passwords in environment variables across 200 services. Design the migration to a secrets manager.
Sequence it by risk, not by convenience Phase 0 — stop the bleeding. Secret scanning in CI and on the existing repositories, blocking new commits containing cre
A 15-year-old monolith must be modernised without a rewrite and without a feature freeze. Plan the migration.
Establish why, before deciding what "Modernise" is not an objective. The reasons that justify the cost are specific: delivery is too slow; the technology is uns
A brief database slowdown caused a two-hour full outage. Explain the likely amplification chain and the fixes at each stage.
The chain Slow database → queries that took 10 ms take 2 s. Thread pool exhaustion. Requests hold threads for 200× longer. A pool of 50 that handled the load ea
A downstream team needs to react to order changes. The order service can publish events, or they can consume CDC from its database. Which, and why?
The recommendation: published events, with CDC as the mechanism if needed The distinction that matters is what the consumer becomes coupled to . CDC consumed di
A multi-tenant SaaS product has outgrown one database. You must shard. How do you choose the partition key, and what makes this decision so expensive to get wrong?
What the interviewer is testing Whether you exhaust cheaper options first, and whether you understand that a shard key is close to irreversible. First: do not s
A product catalogue page does 40,000 reads per second against a database that can serve 5,000. Walk me through the caching design, including what happens at 3 AM when the cache is empty.
What the interviewer is testing Whether you can design a cache including its failure modes, rather than saying "put Redis in front of it". The base design Cache
A service has widely varying request costs — most complete in 10 ms, some take 5 s. Under round robin some instances are overwhelmed while others idle. What do you change?
Why round robin fails here It distributes by count , not by cost or by whether a backend is coping. With uniform requests that is fine. With a 500× cost spread,
A service intermittently times out connecting to a managed database. The database is healthy, nothing changed, and failures correlate with traffic volume. Where do you look?
The likely cause NAT gateway port exhaustion. Source NAT maps each outbound connection to a source port on the gateway's address. The number of simultaneous con
A service writes to its database and then publishes an event to Kafka. Sometimes consumers see an event for a record that does not exist, and sometimes a record exists with no event. Why, and how do you fix it?
What the interviewer is testing Recognition of the dual write problem — one of the most common defects in event driven systems and one that testing rarely catch
A team proposes exposing their service's database change stream via CDC so other teams can consume it, avoiding the work of building an event API. What is your assessment?
Name what is actually being proposed The proposal is to publish the service's internal schema as its integration contract. CDC does not emit domain events; it e
An order spans payment, inventory and shipping, each in its own service with its own database. How do you keep them consistent?
Rule out the distributed transaction Two phase commit gives atomicity and isolation across services, and its costs make it unusable at scale: locks held across
Anomalous access to a customer database is detected. Walk me through the first day, and say what determines whether you can answer the regulator.
The first hours Declare an incident and assign command. Named commander who does not debug, operations lead, communications lead, scribe. Security incidents add
Classify DynamoDB, Spanner and Cassandra under PACELC, and say which half of the classification you would actually design around.
The classifications Store Partition Else Why DynamoDB (default reads) PA EL Serves from any replica; eventually consistent reads avoid a quorum round trip Cassa
Production is degraded after a release. The team tries to roll back and discovers a migration has already run. What do you do now, and what do you change afterwards?
Now: stop trying to roll back Reverting the deployment would run the previous code against a schema it does not know, which is a second and less understood fail
TSB's 2018 core banking migration moved 1.9 million customers in a single weekend and failed publicly. What would you have required before approving that cutover?
The case, as publicly reported In April 2018 TSB migrated from a platform rented from Lloyds Banking Group to Proteo4UK, built by its parent Banco Sabadell. The
Two services share a database because it was faster to build that way. Both teams now block each other on every schema change. What do you do?
Name the actual coupling They are not two services. They are one deployable unit split across two repositories, with the coupling moved from code — where a comp
Users report seeing stale data intermittently. Replication lag is normally under a second but spikes to minutes twice a day. How do you handle it?
First: find the cause of the spikes Twice a day is a schedule, so look for one. The usual candidates, each with a different fix: A batch job or bulk write on th
You are asked to give an internal AI agent access to the customer database, the ticketing system and outbound email so it can resolve support tickets. What is your response?
What the interviewer is testing Whether you recognise a specific and well documented security pattern, and whether you can propose a workable design instead of
You are asked to lead a migration of 300 applications to cloud. What happens in the first ninety days?
Days 1–30: discovery, and the foundation in parallel Discovery from evidence, not interviews. Network flow logs for dependencies (the only trustworthy source),
You must make a breaking change to an API used by 200 internal services and 40 external partners. Design the change and the migration.
First, verify it must break Many "breaking" changes are avoidable. Adding a field is safe if clients ignore unknown fields — which should be a documented expect
Your cluster fails over spuriously under load, but a real leader failure takes 45 seconds to detect. How do you resolve the tension?
The tension Detection time is set by the heartbeat interval times the failure threshold. Shorten it and you detect real failures faster and mistake slow but ali
Your search index and your database disagree — some products appear in search that were deleted, and some new ones never appear. How do you make this reliable?
Why it drifts Almost always a dual write : the application writes to the database and then to the search index as two independent operations. One can succeed an
A sponsor wants a date for a programme with several unknowns. Refusing to estimate is not an option. How do you handle it?
Give a range with the reasons, not a point A single date implies precision you do not have and will be treated as a commitment. A range — with the specific unkn
A team wants to move a workload from PostgreSQL to a document store because "the schema keeps changing". What do you ask?
The questions 1. What is actually changing — the shape, or the schema management process? "The schema keeps changing" usually means migrations are painful, not
Database Migration Under CD
Expand-contract, backwards-compatible schema change, and migrations that cannot roll back.
Database Migration
Moving engines, versions and schemas without losing data or uptime.
CI/CD
Continuous integration and delivery, and the architecture that caps them.
Change Management vs CD
Reconciling CAB-era controls with continuous delivery without pretending either away.
Cloud Migration
Per-application disposition, sequencing and the capability change underneath.
Data Migration Strategies
Backfill, dual-write, reconciliation and verification.
Database Performance
Plans, indexes, contention and the pool in front of the database.
Deciding Under Uncertainty
Bounding the downside and buying information cheaply.
Judgement Under Uncertainty
Choosing the best worst case when information is missing.
Migration Risk
Bounding blast radius, staging by cohort, and honest readiness reporting.
Warehouse Migration
Moving off a legacy warehouse with thousands of reports pointed at it.
Zero-Downtime Migration
Expand, migrate, contract — and the contract phase that never happens.