Splitting a Docker deployment can make frontend and API services easier to release or scale independently, but it does not automatically make the runtime image smaller or the application faster. The extra work is mostly in the request path and operations: configure SPA route fallback, decide how browsers reach the API, manage port exposure and environment settings, and coordinate more services. A multi-stage Docker build can keep build tools out of a production image without splitting the app into separate services.
First, decide what “splitting a Docker image” means
The phrase can refer to three different changes, and they solve different problems:
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- Separate build and runtime stages: one Dockerfile builds the application, then copies production-ready output into a runtime stage. This keeps development and build tools out of the final image; it does not require multiple running services. Docker Docs describes multi-stage builds as a way to separate build-time dependencies from the runtime image.
- Separate frontend and backend images: the frontend and API are built into distinct images, but they might still be deployed together.
- Run separate services: frontend and API containers operate as independently configured services, with networking and deployment relationships to manage.
Docker’s guidance that “Each container should have only one concern” is a design principle, not a rule that every application component must have a separate image. A multi-stage build and a service split are independent choices.
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Think in terms of the actual request path. A browser loads the page from a public host; it then requests the API either through a same-origin proxy path such as /api or from a separately reachable API origin. Separately, the frontend-serving container may make container-to-container requests to an API service. Those are different network hops with different rules.
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| Concern | Combined deployment unit | Separate services |
|---|---|---|
| Release and scaling | Frontend and API changes are generally tied to the combined deployment unit. | They can be released or scaled independently if the platform and design support it. |
| Browser request path | A same-origin arrangement may be simpler. | Requires a public gateway or ingress, or a separately reachable API origin with explicit proxy or CORS configuration. |
| SPA routing | The server still needs to serve the entry document for client-side routes. | The frontend-serving service must own and configure that fallback. |
| Build and runtime images | Can use multi-stage builds. | Can also use multi-stage builds; separate services do not replace that build technique. |
| Operations | Fewer service relationships to configure. | More network, configuration, and runtime relationships, which Compose or a deployment platform can centralize. |
Docker’s documentation explains that decoupled services can support independent scaling and reuse, but does not establish a universal cost, speed, or image-size advantage for every application.
Surprise 1: SPA routes can fail on refresh
A React app can navigate to /settings/profile in the browser while the static server has no file at that path. If a user opens that URL directly or refreshes it, the server may return a 404 before the client-side router loads. The frontend-serving server needs a fallback that returns the SPA entry document for client routes. Docker’s React guide shows the NGINX pattern try_files $uri /index.html. Keep genuine missing asset requests distinguishable from client routes so asset problems do not silently return the HTML page. See Docker’s React.js guide.
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Surprise 2: containers talking does not make CORS disappear
CORS is a browser policy about origins: scheme, host, and port. If a page at one origin makes a script request to an API at another, the browser checks the API response’s CORS headers; some request types also cause an OPTIONS preflight. Putting the containers on a shared Docker network gives them a route to each other, but it does not grant a browser permission to read a cross-origin response. MDN explains CORS and preflight behavior.
To diagnose a browser error, compare the page origin with the API origin, then inspect the API or proxy response headers and whether it handles the preflight. A reverse proxy that serves the API under the page’s origin, such as /api, can avoid a cross-origin browser request. Otherwise, configure the API’s CORS responses for the browser origin and the requests the app actually makes.
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Surprise 3: a Compose service name is not a public API hostname
A service name on a shared Docker network is useful to another container on that network. A visitor’s browser is outside that network and generally cannot resolve an internal service name. Have browser code call an externally reachable API hostname or route requests through a public frontend or gateway; reserve internal service names for container-to-container communication. Docker’s networking overview distinguishes container networking from access outside the network.
Surprise 4: publishing a port changes who can reach it
A process can listen on a container port, and Docker’s EXPOSE instruction can document that port, without making it publicly reachable. Publishing a port with -p or --publish makes it available outside the host network boundary, subject to Docker’s rules and the host or deployment platform’s controls. Publish only services that need outside access, and verify how the platform handles exposure. Docker documents port publishing in its networking overview.
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Surprise 5: Compose centralizes service relationships; it does not remove them
With separate services, dependencies, networks, volumes, environment variables, configuration, and runtime status must line up. Docker Compose lets you define and manage these relationships declaratively rather than relying on a collection of manually maintained commands. That makes a multi-container app easier to coordinate, but it still has more components and decisions than a single deployment unit. Docker’s multi-container application guide and Compose application model documentation describe how those pieces fit together.
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1Clear out junk files and repair common Windows errors2Fix the driver behind crashes, sound loss and screen glitches3Repair Windows errors before they cause bigger problemsSurprise 6: a frontend image does not decide server behavior
A build stage can produce static assets and copy them into a runtime image that serves them. That image still needs server configuration for client-side route fallback and any desired caching behavior. Building the JavaScript bundle does not itself configure NGINX or another static server. Docker’s React guide demonstrates the distinction between building the app and serving it.
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Surprise 7: build-time API settings may not change at runtime
If the API URL is compiled into static JavaScript during the frontend build, changing an environment variable on the running container will not, by itself, rewrite the already-built bundle. Pick and document an explicit approach: use a same-origin relative API path, deliver a runtime configuration file, or build separately for each environment. These choices have different deployment implications; Docker’s documentation does not prescribe one universal frontend runtime-configuration pattern.
How to choose without guessing at savings
Choose based on operational fit rather than a blanket rule about splitting. A combined deployment unit is often the simpler arrangement when frontend and API releases move together and independent scaling is not important. Separate services are more attractive when independent ownership, release cadence, or scaling is valuable enough to justify the added request-path and configuration work.
- Map the browser’s public route to the frontend and API; decide whether API requests use a same-origin proxy or a distinct origin.
- Confirm who configures SPA fallback and how missing asset requests are handled.
- Identify which services need host or external access before publishing ports.
- Choose how frontend API configuration changes across environments.
- Use multi-stage builds where the goal is to keep build dependencies out of a runtime image, regardless of whether services are split.
There is no broadly applicable benchmark establishing a percentage image-size reduction, added latency, or cost saving from splitting a frontend and backend. Those outcomes depend on the application and deployment. Measure the images and operating costs in your own environment rather than treating tutorial image sizes or architectural advice as a forecast.
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