Deployment environment

In software deployment, an environment or tier is a computer system in which a computer program or software component is deployed and executed. In simple cases, such as developing and immediately executing a program on the same machine, there may be a single environment, but in industrial use the development environment (where changes are originally made) and production environment (what end users use) are separated, often with several stages in between, in order to allow phased deployment (rollout), testing, and rollback in case of problems.

Environments may vary significantly in size: the development environment is typically an individual developer's workstation, while the production environment may be a network of many geographically distributed machines in data centers, or virtual machines in cloud computing. Code, data, and configuration may be deployed in parallel, and need not connect to the corresponding tier – for example, pre-production code might connect to a production database.

Deployment architectures vary significantly, but broadly, the tiers are bookended by starting at development (DEV) and ending at production (PROD). A common 4-tier architecture is development, testing, staging, production (DEV, TEST, STAGE, PROD), with software being deployed to each in order. Other common environments include Quality Assurance (QA), for acceptance testing; sandbox or experimental (EXP), for experiments that are not intended to proceed to production; and Disaster Recovery, to provide an immediate fallback in case of problems with production. Another common architecture is development, testing, acceptance and production (DTAP), consisting of dev/test/qa/prod.

This language is particularly suited for server programs, where servers run in a remote data center; for code that runs on an end user's device, such as applications (apps) or clients, one can refer to the user environment (USER) or local environment (LOCAL) instead.

Exact definitions and boundaries between environments vary – test may be considered part of dev, QA may be considered part of test, part of stage, or be separate, etc. The main tiers are progressed through in order, with new releases being deployed (rolled out or pushed) to each in turn. Experimental and recovery tiers, if present, are outside this flow – experimental releases are terminal, while recovery is typically an old or duplicate version of production, deployed after production. In case of problems, one can roll back to the old release, most simply by pushing the old release as if it were a new release. The last step, deploying to production ("pushing to prod") is the most sensitive, as any problems result in immediate user impact. For this reason this is often handled differently, at least being monitored more carefully, and in some cases having phased rollout or only requiring flipping a switch, allowing rapid rollback.

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