Glossary.
Terms that actually come up in projects, briefly explained.
Every entry ends with the catch the textbook leaves out.
Legacy and modernization
Strangler fig
A pattern for replacing a legacy system step by step: new functionality grows around it, the old system shrinks, until nothing of it is left.
Technical debt
The deferred effort from earlier shortcuts in code or architecture. It does not disappear, it simply falls due later and at a higher price.
Anti-corruption layer
A translation layer between new and old, so that the quirks of the legacy system do not seep into new code.
Feature flag
A switch in the code that turns functionality on and off at runtime, without shipping a new release.
Modular monolith
An application that ships as one unit but has clearly separated modules with real boundaries inside.
Expand and contract
A procedure for database changes without downtime: expand first, switch second, remove the old thing last.
Bus factor
The number of people who would have to disappear before a system can no longer be developed or operated. For many legacy systems it is one.
End of life
The point from which a version no longer receives security updates. From then on every known hole stays open permanently.
PHP upgrade
Moving an application to a PHP version that still receives security updates. The effort rarely sits in the language, it sits in the dependencies.
Zend Framework
A PHP framework that continues as Laminas since early 2020 and is no longer developed under its old name. The route across depends on the major version you run.
Brownfield and greenfield
Greenfield means building on an empty site. Brownfield means building while an existing system keeps running around you.
Big bang migration
Switching a system on a fixed date, usually over a weekend, with no intermediate state and only one attempt.
Legacy code
Code without tests that earns money. The definition by Michael Feathers is useful because it names the real problem: missing safety, not age.
Characterization tests
Tests that record what a system does today, rather than checking what it should do. The safety net before the first change.
Test coverage
The share of code executed during a test run. A number that says a lot about gaps and little about quality.
Refactoring
Improving the internal structure of code without changing its externally visible behaviour.
Monolith
An application built, deployed and operated as a single unit.
Microservices
An application is split into several independently deployable services, each with its own storage and lifecycle.
Data migration
Moving existing data into a new system, usually the underestimated and most schedule-critical part of a replacement.
API first
The interface is designed and agreed first, implementation and user interface are built against it afterwards.
AWS and cost
Landing zone
The prepared foundation of an AWS environment: accounts, networking, permissions, logging and cost allocation, before the first application moves in.
Cost and Usage Report
The most detailed cost report AWS offers: every resource, every hour, every pricing component as raw data instead of a chart.
FinOps
The practice of treating cloud cost as an ongoing technical decision rather than a monthly invoice that lands in accounting.
Savings Plans and Reserved Instances
Two ways to pay considerably less than on-demand pricing in exchange for a usage commitment over one or three years.
Rightsizing
Bringing resources to the size actually needed rather than the one chosen when they were created.
NAT gateway
The service through which resources in private subnets reach the internet. Billed per hour and per gigabyte processed.
Egress cost
Charges for data leaving an AWS region, an availability zone or the cloud. Inbound data is usually free.
Tagging strategy
A binding scheme that assigns every resource to its team, its environment and its cost centre.
Well-Architected Framework
A question catalogue from AWS for assessing an architecture along six pillars, from operational excellence to sustainability.
Spot instances
Spare capacity that AWS sells at up to 90 percent off and can reclaim at short notice.
Auto scaling
Capacity follows load automatically, so that neither users wait nor idle resources get paid for.
S3 storage classes
Tiers of storage graded by access frequency, from instantly available to archive, at very different prices.
Lift and shift
A system moves to the cloud almost unchanged, without adapting to cloud characteristics.
Replatforming
A system is deliberately adapted to the platform during the move, without being rewritten.
Serverless
An operating model where the provider handles provisioning and scaling and only actual usage is billed.
AWS Lambda
The service that runs individual functions without your own servers, triggered by events or calls.
VPC
Your own virtual network in AWS, where subnets, routes and access rules are defined.
IAM
Access management in AWS: who or what may perform which action on which resource.
Amazon RDS
Managed relational databases on AWS: backup, patching and failover are handled by the provider.
CloudFront
The AWS delivery network: content is cached at locations close to the user.
Backend and operations
RTO and RPO
Two numbers for the bad day: how long recovery may take (RTO), and how much data may be lost in the process (RPO).
Blue-green deployment
Two complete environments side by side: the new version runs quietly, traffic is switched over, and on failure you switch back.
Multi-tenancy
One system serves several customers on the same infrastructure, without the data of one appearing for another.
Multi-region
A system runs in several geographic regions, so the failure of one region does not end the service, and users have shorter distances.
Idempotency
An operation is idempotent when running the same request several times has the same result as running it once.
Circuit breaker
A protective mechanism that stops calling a failed service for a while, instead of running into every timeout.
Rate limiting
An upper bound on requests per period and caller, so that individual users cannot take a system down for everyone else.
Observability
The property of a system to reveal from the outside what is happening inside, including for questions nobody asked in advance.
SLO, SLI and SLA
A measurement (SLI), a self-imposed target for it (SLO), and a contractual promise with consequences (SLA).
Event-driven architecture
Services exchange events instead of calling each other directly. The sender does not know who is listening.
Infrastructure as code
Infrastructure is described as versioned code instead of being clicked together in a web console.
Graceful degradation
A system loses individual features under partial failure instead of going down entirely.
gRPC
A method for calls between services with a declared interface and binary transport, considerably leaner than JSON over HTTP.
Saga pattern
A way to run operations across several services when there is no shared transaction: every step has a compensating step.
Canary release
A new version goes to a small share of traffic first. If the numbers hold, the share increases.
Sharding
Data is split across several databases by a key, when a single one no longer carries the volume or the load.
Container
An application together with its dependencies in a self-contained package that runs the same everywhere.
Kubernetes
A system that distributes containers across machines, restarts them, scales them and manages their reachability.
CI/CD
An automated path from change to deployment: build, test, ship, with no manual steps in between.
Load balancer
A distributor that spreads incoming requests across instances and takes failed ones out of rotation.
Caching
Results are stored temporarily so that expensive computations or queries do not have to repeat.
Message queue
A buffer between sender and receiver: messages wait until someone processes them.
API gateway
A service in front that accepts requests, validates them and forwards them to the services behind.
REST
A style for HTTP interfaces: resources with clear addresses, standard methods and stateless calls.
Webhook
A reversed call: instead of polling, you get notified by the other system as soon as something happens.
Database index
An additional structure that lets the database find rows without reading the whole table.
Chaos engineering
Controlled disruption in a running system, to find out whether the safeguards actually hold.
Runbook
A short instruction for a recurring operational case: what to check, what to do, whom to call.
AI in production
RAG
A language model receives matching excerpts from your own data before answering, instead of relying on its training.
Model Context Protocol
An open standard through which language models reach tools and data sources: one shared interface instead of one integration per vendor.
Prompt injection
An attack that hides instructions inside data a language model processes, so it does something other than intended.
Embeddings
Number sequences that represent the meaning of a text. Similar content sits close together in vector space, even without shared words.
Guardrails
Technical boundaries around a language model that define what may go in, what may come out, and what it can trigger.
LLM evaluation
A repeatable procedure for measuring whether answers from a language model are getting better or worse.
Context window
The amount of text a language model can consider at once for a request, measured in tokens.
Hallucination
An answer that sounds plausible and is factually wrong. The model does not invent deliberately, it strings likely words together.
Fine-tuning
An existing model is further trained on your own examples, to hit style, format or domain language reliably.
AI agent
A language model that does not only answer but calls tools over several steps, judges intermediate results and continues.
Inference cost
The running cost per request to a language model, billed by tokens in and tokens out.
Human in the loop
A flow in which a person confirms, corrects or rejects at defined points before an action takes effect.
Vector database
A database that stores vectors and finds similar entries quickly, the basis for search by meaning.
Chunking
Splitting documents into sections that can be searched individually and handed to a model.
Reranking
A second step in retrieval: the first hits are reordered by a more accurate model before they reach the language model.
Prompt engineering
Deliberately phrasing instructions to a language model so that results are reliable rather than accidentally good.
System prompt
The base instruction that precedes every user input and sets role, boundaries and output format.
Function calling
Instead of text, a model returns a structured request for which tool to call with which parameters.
Open-weight models
Models whose weights are freely available and which can therefore be run yourself, in your data centre or your own cloud.
EU AI Act
The European regulation for artificial intelligence, classifying applications by risk and attaching obligations to that class.
Process and automation
Process automation
A recurring workflow is taken over by software instead of being carried between systems by people.
Media break
The point where data changes format or system and a human has to step in.
Straight-through processing
A transaction runs end to end without human intervention, from intake to booking.
Workflow orchestration
A service that drives multi-step workflows, holds state, handles retries and continues in a defined way after failures.
ETL
Data is extracted from source systems, transformed and loaded into a target, usually on a schedule.
ERP integration
Connecting a system to inventory management, so that stock, prices, orders and documents are not maintained twice.
RPA
Software operates existing applications the way a human would, through their user interface rather than an API.
Security and operational risk
Least privilege
Every identity gets exactly the permissions its task requires, and none beyond that.
Secrets management
Managing passwords, keys and credentials outside of code and configuration files.
Zero trust
No access is trusted merely because it comes from the internal network. Every request is verified.
Audit log
An immutable record of who performed which action and when.
Encryption at rest and in transit
Data is encrypted both while being transferred and while being stored.
Patch management
The organized handling of security updates: detect, assess, apply, evidence.
Penetration test
A commissioned attack on your own system, to find weaknesses before somebody else does.