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+# Black as a server (blackd)
+
+`blackd` is a small HTTP server that exposes _Black_'s functionality over a simple
+protocol. The main benefit of using it is to avoid the cost of starting up a new _Black_
+process every time you want to blacken a file.
+
+## Usage
+
+`blackd` is not packaged alongside _Black_ by default because it has additional
+dependencies. You will need to execute `pip install black[d]` to install it.
+
+You can start the server on the default port, binding only to the local interface by
+running `blackd`. You will see a single line mentioning the server's version, and the
+host and port it's listening on. `blackd` will then print an access log similar to most
+web servers on standard output, merged with any exception traces caused by invalid
+formatting requests.
+
+`blackd` provides even less options than _Black_. You can see them by running
+`blackd --help`:
+
+```{program-output} blackd --help
+
+```
+
+There is no official `blackd` client tool (yet!). You can test that blackd is working
+using `curl`:
+
+```sh
+blackd --bind-port 9090 &  # or let blackd choose a port
+curl -s -XPOST "localhost:9090" -d "print('valid')"
+```
+
+## Protocol
+
+`blackd` only accepts `POST` requests at the `/` path. The body of the request should
+contain the python source code to be formatted, encoded according to the `charset` field
+in the `Content-Type` request header. If no `charset` is specified, `blackd` assumes
+`UTF-8`.
+
+There are a few HTTP headers that control how the source code is formatted. These
+correspond to command line flags for _Black_. There is one exception to this:
+`X-Protocol-Version` which if present, should have the value `1`, otherwise the request
+is rejected with `HTTP 501` (Not Implemented).
+
+The headers controlling how source code is formatted are:
+
+- `X-Line-Length`: corresponds to the `--line-length` command line flag.
+- `X-Skip-String-Normalization`: corresponds to the `--skip-string-normalization`
+  command line flag. If present and its value is not the empty string, no string
+  normalization will be performed.
+- `X-Skip-Magic-Trailing-Comma`: corresponds to the `--skip-magic-trailing-comma`
+  command line flag. If present and its value is not the empty string, trailing commas
+  will not be used as a reason to split lines.
+- `X-Fast-Or-Safe`: if set to `fast`, `blackd` will act as _Black_ does when passed the
+  `--fast` command line flag.
+- `X-Python-Variant`: if set to `pyi`, `blackd` will act as _Black_ does when passed the
+  `--pyi` command line flag. Otherwise, its value must correspond to a Python version or
+  a set of comma-separated Python versions, optionally prefixed with `py`. For example,
+  to request code that is compatible with Python 3.5 and 3.6, set the header to
+  `py3.5,py3.6`.
+- `X-Diff`: corresponds to the `--diff` command line flag. If present, a diff of the
+  formats will be output.
+
+If any of these headers are set to invalid values, `blackd` returns a `HTTP 400` error
+response, mentioning the name of the problematic header in the message body.
+
+Apart from the above, `blackd` can produce the following response codes:
+
+- `HTTP 204`: If the input is already well-formatted. The response body is empty.
+- `HTTP 200`: If formatting was needed on the input. The response body contains the
+  blackened Python code, and the `Content-Type` header is set accordingly.
+- `HTTP 400`: If the input contains a syntax error. Details of the error are returned in
+  the response body.
+- `HTTP 500`: If there was any other kind of error while trying to format the input. The
+  response body contains a textual representation of the error.
+
+The response headers include a `X-Black-Version` header containing the version of
+_Black_.