# thermograph-frontend (Go)

The SSR frontend service, ported from the Python implementation one directory
up (`app.py` / `content.py` / `api_client.py` / `format.py`): server-rendered
content pages, the interactive tool's SPA shells, and every static asset. It
is I/O-bound glue over the backend's `/content/*` JSON API — no climate maths,
no database, no auth.

## Layout

    go.mod                        module thermograph/frontend (Go 1.26)
    main.go                       config + mux + static + graceful shutdown
    internal/config/              every env var the service reads (same names,
                                  defaults and required/optional split as the
                                  Python — see config.go's field docs)
    internal/contentapi/          backend /content/* client: TTL cache, bounded
                                  LRU, per-key single-flight, origin forwarding;
                                  typed payloads in types.go
    internal/render/              html/template engine over an embed.FS +
                                  response ETag helpers (W/"sha1[:20]",
                                  If-None-Match handling)
    internal/render/templates/    the page templates (embedded; *.tmpl)
    internal/contentdata/         glossary.yaml / pages.yaml loader (fail-loud
                                  validation, file order preserved)

Dependencies: stdlib plus `gopkg.in/yaml.v3` — the committed SSR copy in
`frontend/content/*.yaml` is shared with the rest of the repo and uses block/
folded scalars, so a YAML parser is genuinely required.

## Run locally

    cd frontend/server
    go build -o thermograph-frontend .
    cd ..            # static/ and content/ resolve relative to the working dir
    THERMOGRAPH_API_BASE_INTERNAL=http://127.0.0.1:8137 \
    THERMOGRAPH_BASE=/thermograph \
      ./server/thermograph-frontend

`THERMOGRAPH_API_BASE_INTERNAL` is required — the boot fails loudly without it,
same as the Python raised at import. Other env vars (all optional):
`THERMOGRAPH_BASE` (default `/thermograph`; the image sets `/`),
`THERMOGRAPH_API_VERSION` (default `v2` — bump only per the API-version pinning
contract in `frontend/CLAUDE.md`), `THERMOGRAPH_API_BASE_PUBLIC`,
`THERMOGRAPH_SSR_CACHE_TTL` (seconds, default 600),
`THERMOGRAPH_GOOGLE_VERIFY` / `THERMOGRAPH_BING_VERIFY`, and `PORT`
(default 8080).

The process expects `static/` and `content/` in its working directory
(`frontend/` locally, `/app` in the image). Templates are embedded in the
binary; static assets and the YAML copy are read from disk.

## Test / verify

    cd frontend/server
    go build ./... && go vet ./... && go test ./...

The deployed binary is `/usr/local/bin/thermograph-frontend` inside the
`emi/thermograph/frontend` image; the image name, `frontend-*` CI workflows and
deploy path are unchanged from the Python service.