netwatch/README.md

NetWatch is an MIT-licensed single-page web application by
[@sneak](https://sneak.berlin) that shows the latency from your browser to 26
well-known internet hosts. The page is JavaScript and does all its measuring in
the browser. A small Go backend, `netwatch-server`, stores the measurements the
page reports. The page and the backend ship together in one Docker image.

## Rationale

When a network misbehaves, it helps to see at a glance which hosts answer and
how fast. NetWatch is one page with a latency figure and a small graph for each
host, refreshed every few seconds. The page measures from the browser, so it
shows the path your own traffic takes. It can be served from any static file
host. The backend only stores what the page reports. Without it, the page works
the same and nothing is stored.

## Getting Started

```bash
# Install the dependencies and the git pre-commit hook
make setup

# Serve the page from the Vite dev server
make dev

# Build the page and netwatch-server, then run the server on port 8080
make run

# Run the tests, both linters and the format check
make check

# Build the page into dist/ and netwatch-server into the repo root
make build

# Build the Docker image and run it
make docker
docker run -p 8080:8080 netwatch
```

`make check` and `make docker` need Docker. `make dev` serves the page at the
address Vite prints and passes `/api` requests to `http://127.0.0.1:8080`.
`make run` starts `netwatch-server` there with its defaults, so the page's
reports are stored in `data/reports`.

## The page

The page shows one row per host. A row has the host's name, its URL, its latest
latency in milliseconds and a graph of its last 100 checks. Below the figure,
the row shows the minimum, median, average and maximum of the host's answers in
that history. After a failed check, it shows `timeout` or `unreachable` there
instead. A round of checks runs every 3 seconds by default. The header shows the
interval, how long the history spans, and whether the page is running or paused.

### Targets

The page checks 26 hosts on the internet, called the WAN hosts:

- `datavi.be`, pinned to the top at start
- the Anthropic API and the OpenAI API
- the AWS Console, the Google Cloud Console and Microsoft Azure
- Cloudflare, Fastly, Akamai, Google, GitHub and Backblaze B2
- 8 S3 regional endpoints: Cape Town, London, Bahrain, Tokyo, Singapore, Sydney,
  Oregon and São Paulo
- 6 Hetzner speed-test servers: Nuremberg, Falkenstein, Helsinki, Ashburn,
  Hillsboro and Singapore

Two local targets follow under the heading "Local Network". They are left out of
the summary and the health box.

- `Local CPE` is the cable modem at `http://192.168.100.1`. Its row is always
  shown.
- `Local Gateway` is found when the page loads. The page sends a request to
  `http://192.168.1.1`, `http://192.168.0.1`, `http://192.168.8.1` and
  `http://10.0.0.1`. The first to answer within 1.5 seconds becomes this target.
  If none answers, there is no row for it.

Browsers block a page served from a public address from reaching private
addresses (Private Network Access). So the local targets answer only when the
page is served from `localhost` or from a private address, and only on a network
that has them.

The host list is written in `web/src/main.js`. There is no setting for it.

### What a figure measures

A check is a request to the target's URL: GET, or HEAD for the S3 regions. The
figure is the time from sending the request until the response headers arrive,
in milliseconds. Unlike ping, it includes the time the server takes to answer.
When the browser opens a new connection, it also includes the DNS lookup, the
connection setup and the TLS handshake. Chrome opens a new connection for every
check of the Hetzner servers, and for more targets at longer intervals.

Any answer counts as reachable, an error page included. The request is made in
the browser's `no-cors` mode, so the page cannot read the answer, only time it.

The browser follows redirects, and the figure then includes the request to the
new URL too. So a target whose home page redirects is checked at `/robots.txt`
on the same host, which answers without a redirect. The S3 regions are checked
with a HEAD request. Chrome keeps the connection to an S3 region open after a
HEAD request, but closes it after most GET requests.

The request is sent with `cache: 'no-store'`, so the browser's cache stays out
of the measurement. It carries no `Referer` header and no query string: the
Hetzner servers close the connection without an answer when either is present.

A check times out after 80% of the interval: 2.4 seconds at the default of 3
seconds, 24 seconds at 30 seconds. So a round's checks have all finished before
the next round is due. A check that times out is recorded as a timeout, and so
is one that answers after that limit. A check that fails with a network error is
recorded as unreachable.

A round can start early, after an interval change or when the recovery probe
finds a target answering. It then gives up the last round's checks that are
still waiting, so rounds never overlap.

The first round after the page loads is discarded, as is the first round after
an interval change. Its connections are all new, so its figures run high.

The browser chooses between IPv4 and IPv6 for each target, as for any request.
The local targets are IPv4 addresses.

### Colors

A latency figure and its line in the graph take one color:

- green: under 50 ms
- lime: under 100 ms
- yellow: under 200 ms
- orange: under 500 ms
- red: 500 ms and over
- gray: no answer

### The graph

The graph in each row plots the host's last 100 checks. At the default interval
that is 5 minutes; the labels on its time axis say how long. The vertical axis
runs from 0 to 1000 ms. A latency above 1000 ms is drawn at the top, and the
figure still shows its real value. The rounds in which a check failed are shaded
red.

### Health and summary

Above the rows, a health box sums up the WAN hosts' latest results. A host
counts as failed when its last check timed out or found it unreachable.

- `OFFLINE` (red): more than 10 hosts failed and at most 4 answered
- `DEGRADED` (orange): otherwise, more than 4 hosts failed
- `SLOW` (yellow): otherwise, more than 3 hosts took over 1000 ms
- `HEALTHY` (green): otherwise

Below the health box, a summary line covers the WAN hosts only:

- `Reachable`: how many answered in the last round, out of 26
- `Now`: the minimum, median, average and maximum of their latest figures
- the history's span, such as `5m`, with the minimum and maximum over the whole
  history
- `Checks`: the number of rounds run since the page loaded or the interval
  changed

When no WAN host answers, the page starts a recovery probe. Every half second it
checks 4 WAN hosts and gives up the checks it started half a second before. The
4 are picked at random when the probe starts. As soon as one answers, a new
round starts at once.

### Row order

The WAN rows are sorted by their latest latency, fastest first and unreachable
last. The rows are sorted when the second round ends, and again in rounds 11,
21, 31 and so on. The pin button at the left of a row keeps the row at the top.
Pinned rows are sorted by name, and a line separates them from the rest.
`datavi.be` is pinned at start.

### Controls

- `Pause` stops the checks. The graphs keep scrolling and leave a gap for each
  paused round; no outage is drawn. The figures turn gray until `Resume`.
- The `Interval` menu sets the time between rounds: 1, 2, 3, 5, 10, 15, 30 or 60
  seconds. Changing it clears the history on the page and starts a new round at
  once.
- The pin button in each WAN row pins and unpins the row.
- Each host's URL is a link that opens in a new tab.
- The `Debug log` checkbox in the footer opens a panel with the page's last 1000
  log lines. Each line has a level: error, warning, notice, info or debug.
- The header shows the local time with its time zone, and the UTC time. The
  footer shows the color legend and links to the commit the page was built from.

When the page records a failed check, it writes one line to the browser console
with `console.error`, and the same line to the debug log. Nothing is written for
the discarded first round, for a check a round gives up, or by the recovery
probe. The line gives the target's name and URL, the time, what failed and how
long the request took. The three kinds of failure are a timeout, an answer over
the time limit, and a network error. For a network error the line quotes the
error the browser gives the page, such as `TypeError: Failed to fetch`, which
does not say why. When a target answers again after failed checks, one
`console.info` line gives its latency and how many checks in a row had failed.

### Browsers

The page needs a browser with ES modules, the Fetch API, the Canvas API and CSS
custom properties.

## Reporting

Every 60 seconds the page gathers its undelivered samples and posts them as a
report to `/api/v1/reports` on the origin it was served from. When there are
none, it posts nothing. The report is JSON with four fields:

- `clientId`: a random id, made once per browser and kept in `localStorage`.
  When the browser has no usable storage, a new id is made for the session.
- `geo`: always `null`
- `hosts`: one entry per host with new samples. The entry has the host's `name`,
  `url` and `status`, and `history`, a list of samples. A sample has the check's
  time `t` in Unix milliseconds, its `latency` (`null` for a failed check) and
  its `error` (`"timeout"`, `"unreachable"` or `null`).
- `timestamp`: the time of the report

Each report carries only the samples no earlier report has delivered. For each
host, the page remembers the time of the newest sample it delivered, and moves
that mark only when the server answers 2xx. Paused rounds are never sent. While
the page is paused, no report is sent at all. One report is in flight at a time.
A report with no answer after 30 seconds is abandoned, and its samples go in the
next report. So a server that stored them but answered late gets them twice.

A failed delivery is quiet. The page writes one debug log line when deliveries
start failing and one when they recover. The checks go on.

A sample waits for delivery only as long as the history holds it. The history
keeps each host's last 100 rounds: 5 minutes at the default interval of 3
seconds. At 1 second it spans 100 seconds, at 2 seconds 200 seconds, and at 60
seconds 100 minutes. Changing the interval clears the history. The cleared
samples are kept apart, the last 100 per host, and those not yet delivered go in
the next report.

So at the 1-second interval, one refused or failed report loses the oldest 20
seconds of each host's samples: the next delivered report comes 2 minutes after
the last. At 2 seconds and longer, one failed report loses nothing. Samples are
lost from the third failed report in a row at 2 seconds, from the fifth at 3
seconds, and later still at longer intervals.

Each paused round still adds an entry to the history, so each report due during
a pause counts as a failed one. A pause longer than the history spans loses the
samples recorded between the last delivered report and the pause: up to a
minute's worth for each host.

## Deployment

### Static files

`make build` writes the page to `dist/`: an `index.html` and an `assets/`
directory with one CSS file and one JavaScript file. Any static file host can
serve it. Without a backend at `/api/v1/reports` on that host, the reports fail
quietly and nothing is stored.

### The Docker image

`make docker` builds the image `netwatch` from `Dockerfile`. It is the whole
service in one container. nginx serves the page and passes `/api/`,
`/.well-known/healthcheck` and `/metrics` to `netwatch-server`, which runs in
the same container. The backend listens on `127.0.0.1:8081` only, as user
`netwatch` (uid 1000). `bin/entrypoint.sh` starts both and watches them.

The image:

- listens on port `8080`; the `PORT` variable changes it (see
  [Settings](#settings))
- stores the report files in `/data/reports`, on the `/data` volume. Before the
  backend starts, `netwatch-server prepare-data-dir` creates that directory and
  gives `/data` and the directories, regular files and symbolic links in it to
  user `netwatch`. A file with a second name (hard link) keeps its owner, and
  its path is logged. A host directory mounted at `/data` therefore ends up
  owned by uid 1000. The command changes nothing outside `/data`, whatever links
  it meets there.
- takes the client address from `X-Forwarded-For` only on a request from a
  reverse proxy named in `TRUSTED_PROXIES`. nginx resolves the address and
  passes that one address to the backend. The backend is started with
  `TRUSTED_PROXIES=127.0.0.1/32`, so it trusts nginx alone.
- writes nginx's access log to stdout, and the backend's log as JSON to stdout
- sends the page with `Cache-Control: no-cache`, so a browser that keeps a copy
  asks the server whether it is current before using it again. The files under
  `/assets/` are cached as immutable; their names change whenever their contents
  do.
- sends the security headers in `configs/security-headers.conf` on every
  response, in place of the backend's own
- refuses a request body over 1 MiB with nginx's own 413 error page; the backend
  never sees such a request
- requests `/.well-known/healthcheck` through nginx every 30 seconds, as its
  `HEALTHCHECK`. The check fails unless both nginx and the backend answer. It
  also fails while report files cannot be written. Three failures in a row make
  the container `unhealthy`; one pass makes it `healthy` again.
- on `docker stop`, writes the waiting reports to disk and exits 0. If that
  final write fails, the container exits non-zero. If nginx or the backend exits
  on its own, the other is stopped and the container exits non-zero, so the
  platform restarts it.

### Running under upaas

The [upaas](https://git.eeqj.de/sneak/upaas) app for netwatch needs:

- Port: container port `8080`.
- Volume: container path `/data`. The reports are kept in `/data/reports`.
- Environment variables: none is required. Every setting in
  [Settings](#settings) applies, with four differences in the image:
    - `BIND_ADDRESS` has no effect. The backend listens on `127.0.0.1` inside
      the container.
    - `PORT` is nginx's public port, from 1 to 65535 and not `8081`, where the
      backend listens inside the container.
    - `DATA_DIR` must be `/data` or a path below it, written in full, with no
      `.` or `..` part and no extra `/`. A path through a symbolic link is
      refused unless the link is relative and stays inside `/data`.
    - `TRUSTED_PROXIES` names the proxies nginx trusts, as IP addresses or CIDRs
      separated by commas. The backend's own setting is fixed to `127.0.0.1/32`.
      With `TRUSTED_PROXIES` set and empty, nginx trusts no proxy. Every client
      behind the proxy then shares the proxy's one allowance of
      `REPORTS_PER_MINUTE`. Any client that connects from an address in the set
      can write its own `X-Forwarded-For`. A connection through a port Docker
      publishes can arrive from the Docker network's gateway, such as
      `172.17.0.1`. The default includes that address. Where that matters, name
      only the addresses that nothing but the proxy connects from.
- Health: the image's `HEALTHCHECK`, described above.

The container stops at start when a variable holds a value it cannot use. The
reason is in its log.

## Settings

`netwatch-server` reads its settings from environment variables. It also reads a
`.env` file in the directory it starts in, and a YAML file named
`netwatch-server.yaml` in `/etc/netwatch-server/` or
`~/.config/netwatch-server/`, when one exists. A variable in the environment
wins over a file.

The server does not start when a variable holds a value it cannot use. Its error
names the variable. An empty variable counts as unset, except `TRUSTED_PROXIES`.

- `BIND_ADDRESS`, default empty: the IP address to listen on. Empty listens on
  every interface. The image sets it to `127.0.0.1`.
- `PORT`, default `8080`: the port to listen on, from 1 to 65535. In the image
  this is nginx's port; the backend listens on `8081`.
- `DATA_DIR`, default `./data/reports` (`/data/reports` in the image): the
  directory for the report files. The server creates it if it is missing, and
  stops at start if it cannot create a file in it.
- `DATA_DIR_MAX_BYTES`, default `1073741824` (1 GiB): the most bytes of report
  files kept in `DATA_DIR`, oldest deleted first. A positive whole number. See
  [Report limits](#report-limits).
- `REPORTS_PER_MINUTE`, default `60`: how many reports each client address may
  send a minute. A positive whole number. See [Report limits](#report-limits).
- `TRUSTED_PROXIES`, default `10.0.0.0/8,172.16.0.0/12,192.168.0.0/16`: the
  reverse proxies whose `X-Forwarded-For` and `X-Real-IP` headers are trusted,
  as CIDRs separated by commas. See [Trusted proxies](#trusted-proxies).
- `CORS_ALLOWED_ORIGINS`, default empty: other origins whose pages may call the
  API, separated by commas. See [CORS](#cors).
- `METRICS_USERNAME` and `METRICS_PASSWORD`, default empty: the basic auth
  credentials for `/metrics`. See [Metrics](#metrics).
- `SENTRY_DSN`, default empty: the DSN of a Sentry project to report crashes to.
  See [Sentry](#sentry).
- `DEBUG`, default empty: any non-empty value turns debug logging on, `false`
  and `0` included. No value of it stops the start.

### Trusted proxies

Each `TRUSTED_PROXIES` entry must be a CIDR, such as `127.0.0.1/32`; a lone
address is refused. A list replaces the default. Set and empty, the server
trusts no proxy. Loopback is not in the default, so a reverse proxy on the same
host has to be named.

A request from a peer outside this set has its forwarded headers ignored. The
peer's own address is logged and rate limited.

A request from a peer inside the set takes its client address from
`X-Forwarded-For`. Each proxy appends the address it took the request from, so
the entries on the right were written by trusted proxies and the ones further
left by the client. The client address is the right-most entry that is not in
the set. An entry may carry a port, such as `198.51.100.7:5000` or
`[2001:db8::1]:443`. It is read as the address alone. The peer's address is used
when every entry is in the set. It is also used when the search from the right
meets an entry that is not an address at all, such as `unknown`. `X-Real-IP` is
read only when `X-Forwarded-For` is missing or empty.

### Routes

- `POST /api/v1/reports` takes a report from the page. It needs no credentials;
  [Report limits](#report-limits) bound it instead.
- `GET /.well-known/healthcheck` answers 200 with a JSON object: `appname`,
  `version`, `now`, `uptime_seconds`, `uptime_human` and `status`, which is
  `"ok"`. While report files cannot be written, it answers 503 with the same
  fields and `"status":"error"`.
- `GET /metrics` serves Prometheus metrics, only when `METRICS_USERNAME` and
  `METRICS_PASSWORD` are set. See [Metrics](#metrics).

Every request body is capped at 1 MiB, and every request at 60 seconds.

### Report storage

The server keeps the reports it takes in memory and writes them to `DATA_DIR` as
files of JSON lines, compressed with zstd. It writes the waiting reports to a
new file every minute, whenever 10 MiB of them are waiting, and when it stops.
With none waiting, it writes no file.

The files are named `reports-<timestamp>-<number>.jsonl.zst`. The timestamp is
UTC to the millisecond, so the names sort by time. The number starts at 1 when
the server starts, and goes up by one for each file the server starts to write.
So two files started in the same millisecond still get different names. A file
is created with `O_EXCL`, so it never overwrites another.

A write can fail, as when the disk is full. The reports of a failed write stay
in memory. The next write tries them again, within a minute. One file is written
at a time, so a file holds its reports in the order they arrived, those of a
failed write included. A failed write uses up its number. A file it created is
removed. If removing it fails too, the file stays and counts toward
`DATA_DIR_MAX_BYTES` from the next start.

From a failed write until a write succeeds, `POST /api/v1/reports` refuses every
report with 500 and keeps nothing of it, and the health check answers 503. The
page sends the refused samples again in its next report, as long as it still
holds them (see [Reporting](#reporting)). Reports already answered 200 are lost
only if the server stops before a write succeeds. The final write at stop tries
them too, and if it fails, the process exits non-zero.

### Report limits

`POST /api/v1/reports` takes reports from anyone who can reach it, so it is
bounded in four ways. Each refusal by the backend answers with the body
`{"status":"error"}`.

- JSON only. A report must be sent with `Content-Type: application/json`, with
  or without parameters such as `charset`, as the page sends it. Any other
  content type is refused with 415, and nothing of it is stored. A page on
  another origin can send a `text/plain` POST, or a form, without a CORS
  preflight. Refusing those keeps any site from making its visitors' browsers
  store reports (see [CORS](#cors)).
- Body limit. A report body may be at most 1 MiB, the backend's limit for any
  request. A request that declares a longer body is answered 413 before it
  reaches the route. A body sent without a declared length is read up to 1 MiB.
  The route answers 413 only when the report runs past that; nothing after the
  end of the report is read. A body that stops being valid JSON sooner is
  answered 400. In the image, nginx refuses a body over 1 MiB itself, with its
  own HTML error page, and the backend never sees the request.
- Rate limit. Each client address may send `REPORTS_PER_MINUTE` reports a
  minute. [Trusted proxies](#trusted-proxies) says how that address is found.
  Past that it gets 429 with `Retry-After: 60`. The minute slides: reports from
  the minute before still count, fading out over the current one. An address
  that sends at most half the limit in any 60 seconds is never refused. Above
  that, it may be. The page sends one report a minute from each open tab. So the
  default of 60 refuses nothing from up to 30 tabs behind one address, however
  their reports bunch up. Every answer the route gives carries the headers
  `X-RateLimit-Limit`, `X-RateLimit-Remaining` and `X-RateLimit-Reset`.
- Size cap. The report files in `DATA_DIR` may total at most
  `DATA_DIR_MAX_BYTES`, counting the files already there at start. Reports
  waiting in memory count at their uncompressed size until they are written.
  When a report would take the total past the cap, the oldest report files are
  deleted to make room, and each deletion is logged with the file's name and
  size. A file still being written is never deleted. A report is refused with
  507 only when the reports waiting to be written fill the cap on their own;
  then no file is deleted. At start, report files past the cap are deleted the
  same way, as after lowering it.

So the cap decides how much of the newest reports is kept. The default of 1 GiB
fits on any host; set it to the space you can give `DATA_DIR`. At the defaults,
one address may send 60 reports of up to 1 MiB a minute. A report made up to
compress badly takes about as much room in the files as in its request body. So
in about 17 minutes one address can fill the 1 GiB cap with its own reports, and
every report stored before them is deleted to make room. A lower
`REPORTS_PER_MINUTE` or a higher `DATA_DIR_MAX_BYTES` makes that take longer.

### CORS

The page calls the API from the origin it is served from. By default the server
sends no CORS headers, so browsers let no other origin's pages call it. To serve
the page from elsewhere, list that origin in `CORS_ALLOWED_ORIGINS`, for example
`https://netwatch.example.com`. Pages from a listed origin may `GET` and `POST`
with a `Content-Type` header.

Each entry must be a plain origin as browsers send it: `scheme://host`, with an
optional `:port`, no path, no trailing `/` and no `*`. Any other entry stops the
server from starting.

### Metrics

With both `METRICS_USERNAME` and `METRICS_PASSWORD` set, the server serves
Prometheus metrics at `GET /metrics`. A request with those as its basic auth
credentials gets them; any other gets 401. With neither set, there are no
metrics and `/metrics` answers 404. One set without the other stops the start,
and so does a user name containing `:`.

The metrics record each request that reaches the health check or
`POST /api/v1/reports`. For each, they record its duration and response size,
labelled with its path, method and status, and they count such requests in
progress. Requests the rate limit refuses are included. Go's runtime and process
metrics are included too. No other request is recorded:

- not a request to `/metrics` itself
- not a CORS preflight
- not a 404 for an unknown path, or a 405 for the wrong method
- not a 413 answered before the route

Only those two routes are recorded because the labels are the request's path and
method. Recording every request would let clients add labels without bound.

`/metrics` has its own rate limit, so that its password cannot be guessed
quickly. Each client address may make 60 requests to it a minute, whatever its
credentials. The address is found as for reports. Past that it gets 429 with
`Retry-After: 60`, and its credentials are not checked. The minute slides as it
does for reports, so a scraper polling every 2 seconds or less often is never
refused.

### Sentry

With `SENTRY_DSN` set, each request whose handling crashes is reported to that
Sentry project, under the release `netwatch-server-` followed by the server's
version. The request still gets a 500 answer. The server does not start if
Sentry rejects the DSN. On shutdown the server waits up to 2 seconds for Sentry
to finish sending. Empty, the server sends nothing to Sentry.

### Logging

The backend logs JSON to stdout, or plain text when stdout is a terminal. Each
line names the source location it was written from. Each request is logged with
its method, URL, status, client address and duration, among other fields. Every
value taken from a request is cut to 128 bytes before it is logged.

## Design

The repository holds the page, the backend and the image:

- `web/`: the page's source. `index.html` loads `src/main.js` and
  `src/styles.css`.
- `cmd/netwatch-server/` and `internal/`: the Go backend. `cmd/` holds only
  `main`, which calls into `internal/cli`.
- `configs/`: nginx's configuration for the image, `nginx.conf` and
  `security-headers.conf`.
- `bin/entrypoint.sh`: the image's entrypoint.
- `Dockerfile`: the lint and test phases, the two build stages and the image.
- `script/`: the development commands; `Makefile` targets call them.
- `test/unit/`: the page's unit tests. `test/viewport/`: the layout test.

### The page

The page is built with Vite and Tailwind CSS v4. All its code is in
`web/src/main.js`:

- `CONFIG` holds the timing and display constants. The interval menu sets its
  `updateInterval`; the check timeout, the history span and the time axis are
  computed from it.
- `WAN_HOSTS` is the list of WAN targets, each with its URL. The S3 entries also
  name HEAD as their method.
- `HostState` is one host's history, latest latency and status.
- `AppState` holds the WAN hosts, the local hosts, the pause state and the round
  count, and computes the summary and the health.
- `measureLatency` runs one check.
- `tick` runs one round. It checks every host in parallel and draws each row as
  soon as its check ends. When the last check ends, it redraws every row, the
  summary and the health box, and sorts the rows when a sort is due. A discarded
  round draws nothing.
- `SparklineRenderer` draws a graph on a canvas, scaled to the device's pixel
  ratio.
- `buildUI` builds the page; `updateHostRow`, `updateSummary` and
  `updateHealthBox` redraw its parts.
- `Reporter` builds and posts the reports. `buildReport` is a pure function of
  the hosts' state.

### The backend

`netwatch-server` is a Go HTTP server. The `fx` library starts and stops its
parts in dependency order. The backend's packages are in `internal/`:

- `cli` runs the command line. Given no arguments, it runs the server. It also
  runs the `check-cidr` and `prepare-data-dir` subcommands, which
  `bin/entrypoint.sh` runs before the server starts. Any other arguments get a
  usage message and exit code 2.
- `config` reads the settings and refuses values the server cannot use.
- `server` builds the router and the HTTP server and registers the routes.
- `middleware` holds the request log, the security headers, CORS, the body
  limit, the rate limit, the client address resolution and the metrics.
- `handlers` answers the health check and takes reports.
- `reportbuf` is the in-memory buffer of reports and the files it writes them
  to. It also holds `PrepareDataDir`, which `prepare-data-dir` runs.
- `healthcheck`, `logger` and `globals` hold the health status, the logger and
  the name and version the build stamps in.

In the image, the `test` phase of `Dockerfile` tests the backend, the `builder`
stage builds it, and `bin/entrypoint.sh` runs it as user `netwatch` on
`127.0.0.1:8081`, behind nginx. Outside the image, `make run` builds it and runs
it on port 8080.

## Development

### Entrypoints

This repository adheres to the
[Scripts to Rule Them All](https://github.com/github/scripts-to-rule-them-all)
standard. The scripts in `script/` are the entrypoints for the development
workflow, and the `Makefile` targets are thin shims that call them. The scripts
cover both the page and the Go backend, so `make check` fails if either one is
broken. Every script that runs yarn on the host does so through `script/yarn`,
except `script/bootstrap`.

- `script/bootstrap`: install the dependencies. It uses the `node` on `PATH`
  when that node has the pinned major version and is new enough for the page's
  dependencies. Otherwise it installs nvm from a hash-verified archive, and the
  pinned node through it. It uses the `yarn` on `PATH` only when it is exactly
  the pinned version. Otherwise it installs that version through corepack, or
  through npm where there is no corepack. Then it runs
  `yarn install --frozen-lockfile`. It uses the `go` on `PATH` only when it is
  exactly the pinned version. Otherwise it installs that release from a
  hash-verified archive. Then it runs `go mod download`. What it installs itself
  is linked into `~/.local/bin`, which has to be on `PATH`. It installs no Go
  linter and no Docker; `make test` and `make lint` run in Docker.
- `script/setup`: make a fresh clone ready for development: `script/bootstrap`,
  then the git pre-commit hook.
- `script/dev`: run the Vite dev server, which passes `/api` to a
  `netwatch-server` running locally.
- `script/build`: build the page into `dist/`, then the static `netwatch-server`
  binary into the repo root. The binary's version is `VERSION` from the
  environment. When that is unset or empty, it is
  `git describe --tags --always --dirty`, as in `script/docker`, or `dev`
  outside a git checkout.
- `script/run`: `script/build`, then run `netwatch-server` with its defaults.
- `script/clean`: remove what `script/build` writes.
- `script/projectname`: print the project name, `netwatch`, which
  `script/docker` uses as the image tag.
- `script/yarn`: run yarn with the given arguments under the node
  `script/bootstrap` would use.
- `script/test`: build the `test` phase of `Dockerfile`, writing no image. The
  phase runs the backend's Go tests with the race detector and coverage, and the
  page's unit tests in its `frontend-test` stage.
- `script/lint`: build the `lint` phase of `Dockerfile`, writing no image. The
  phase runs golangci-lint over the Go packages, and eslint in its
  `frontend-lint` stage.
- `script/fmt`: run the formatters, which write: prettier over the JavaScript,
  CSS, HTML and Markdown, then gofmt over `cmd/` and `internal/`. It runs on the
  host, with `~/.local/bin` put on `PATH`.
- `script/fmt-check`: check the formatting (read-only), with the same scope as
  `script/fmt`.
- `script/check`: `script/test`, `script/lint` and `script/fmt-check`.
- `script/add-dependency`: add a page dependency, or move one to another
  version. `make add-dependency PACKAGE=<name>@<version>` runs `yarn add --dev`,
  which changes `package.json` and `yarn.lock` together, then
  `yarn install --frozen-lockfile`.
- `script/tidy`: run `go mod tidy`. To add a Go module, import it and run
  `make tidy`. To move one to another version, edit its `require` line in
  `go.mod`, then run `make tidy`.
- `script/frontend-test`: run the unit tests in `test/unit/` on the host, with
  Node's built-in test runner, then the production build. If a test fails, the
  tests run again listing every test, and the script fails. Each test run has a
  90-second timeout.
- `script/frontend-fmt` and `script/frontend-fmt-check`: the prettier half of
  `script/fmt` and `script/fmt-check`.
- `script/frontend-viewport-test`: check the built page's layout in a
  containerised headless Chrome; see
  [test/viewport/README.md](test/viewport/README.md). It is not part of
  `script/check`, as it takes minutes.
- `script/docker`: build the image from `Dockerfile`, tagged `netwatch`.
- `script/cibuild`: the CI entrypoint. It runs `script/bootstrap` and
  `script/check`, then builds the image as `script/docker` does. The Gitea
  workflow `.gitea/workflows/check.yml` runs it on every push.
- `script/precommit`: run by the git pre-commit hook; it runs `script/check`.
- `script/install-precommit`: install the git pre-commit hook. `make hooks`
  calls it.

### Build caching

`script/test`, `script/lint`, `script/docker` and `script/cibuild` each pass a
new `CACHEBUST` build argument. In `Dockerfile`, every step that lints, tests or
compiles comes after `ARG CACHEBUST`, so it runs again on every build, even when
nothing has changed. Docker keeps the layers above that line, the system
packages and the Go modules, until the base image, their own lines, or `go.mod`
and `go.sum` change. Go's build cache and golangci-lint's cache are kept between
builds too; each reuses a result only when everything it was made from is
unchanged. Both `go test` runs carry `-count=1`, so no test result is reused.
The yarn packages are installed again on every build.

### Tests

`make test` runs the backend's Go tests and the page's unit tests in
`test/unit/`, in Docker. The unit tests import `web/src/main.js` in Node and
call the functions it exports; no browser is involved.

`make frontend-viewport-test` builds the page and checks its layout at several
window widths in a headless Chrome. The page is served with the nginx image and
the `configs/nginx.conf` that the Docker image ships. It tests each breakpoint
declared in `web/src/styles.css`, today the one at `max-width: 768px`, and the
widths one pixel either side of it. It also tests four fixed widths: 320px,
1280px and two phone landscape sizes.
[test/viewport/README.md](test/viewport/README.md) says what it checks and what
it cannot.

## TODO

The to-do list is [TODO.md](TODO.md): where the work stands, the next step, the
open work, and what has been done.

## License

MIT. See [LICENSE](LICENSE).

## Author

[@sneak](https://sneak.berlin)