Deploy Preview
Definition
A deploy preview is a deployment that is deliberately not the live one. The host publishes the changed version exactly as it would publish the real thing, then serves it at a separate address — a name like 'preview-nav-fix' on a subdomain, or a generated one — while the public address carries on serving the current version. Reviewers open the preview link and use the site properly: click the navigation, submit the form, load it on a phone on a train. Nothing about it is a mock-up, a screenshot or a staging approximation; it is the same files that will go live, served the same way. Previews are usually short-lived, and many hosts remove them automatically once the change ships or after a set number of days.
Why It Matters
Reading a diff tells you what changed in the code. Opening a preview tells you what changed on the screen, and those are different questions. The second one catches what the first never will: a heading that wraps badly at 375 pixels wide, a link that now points at the wrong page, a hero image someone exported at 4 MB. Finding that on a preview costs a comment; finding it on the live site costs a rollback plus however many visitors saw it first. Previews also let people who do not read code approve work — a client can be sent one link and asked one question.
How It Works
Each candidate version is published as its own independent release with its own hostname, and the host keeps a mapping from hostname to release. Static files carry no shared state, so two releases sit side by side on the same edge network without interfering: a request for the preview hostname reads one directory, a request for the production hostname reads another. Preview hostnames are normally kept out of search results, either with a robots file that disallows everything or an 'X-Robots-Tag: noindex' response header, so the preview never competes with the real page. Access can be narrowed further by putting a password in front of the preview address. When the change is approved, the release the preview was already serving is promoted — the production hostname is simply repointed at it.
Real-World Example
An agency is rebuilding a client's one-page site. Rather than trade screenshots over email, they publish the new version to 99helpers as 'lawson-v2.99helpers.site' and put a password on it, while the current site stays exactly where it is. The client opens the link on a phone during a meeting and asks for a bigger telephone number in the header. A corrected preview is up twenty minutes later. Only when the client says yes does the new version go to the real address, and only three people ever saw the preview.
Common Mistakes
- ✕Leaving previews open to search engines — duplicate copies of the site get indexed and compete with the page you actually want ranked
- ✕Reviewing a preview built from different data or a different branch than the one that will ship — the approval then covers something that was never published
- ✕Keeping one long-lived staging address instead of a preview per change — two changes land on it at once and nobody can tell which one broke the page
Related Terms
Deployment
A deployment is the act of putting a new version of a site or file where the public can reach it. It is the moment the version you have been working on becomes the version visitors get.
Site Preview
A look at a site exactly as it will be served, at a real address, before you send that address to anyone. It catches the faults that only appear once files leave your own machine.
Continuous Deployment
Continuous deployment means every accepted change is published automatically, with nobody pressing a publish button. A change that passes its checks goes live on its own.
Atomic Deployment
An atomic deployment switches a site from the old version to the new one in a single step, so visitors never see a mixture of the two. Either every file is new, or every file is still the old one.
Password-Protected Link
A hosted address that asks for a shared secret before it will show the file or site behind it. One password covers everyone who has the link; there are no individual accounts.
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