A plug for parsing the request body.

This module also specifies a behaviour that all the parsers to be used with Plug should adopt.

This plug also fetches query params in the connection through Plug.Conn.fetch_query_params/2.

Once a connection goes through this plug, it will have :body_params set to the map of params parsed by one of the parsers listed in :parsers and :params set to the result of merging the :body_params and :query_params.

This plug will raise Plug.Parsers.UnsupportedMediaTypeError by default if the request cannot be parsed by any of the given types and the MIME type has not been explicitly accepted with the :pass option.

Plug.Parsers.RequestTooLargeError will be raised if the request goes over the given limit.

Parsers may raise a Plug.Parsers.ParseError if the request has a malformed body.

This plug only parses the body if the request method is one of the following:

• POST
• PUT
• PATCH
• DELETE

For requests with a different request method, this plug will only fetch the query params.

Options

• :parsers - a list of modules or atoms of built-in parsers to be invoked for parsing. These modules need to implement the behaviour outlined in this module.

• :pass - an optional list of MIME type strings that are allowed to pass through. Any mime not handled by a parser and not explicitly listed in :pass will raise UnsupportedMediaTypeError. For example:

  • ["*/*"] - never raises
  • ["text/html", "application/*"] - doesn't raise for those values
  • [] - always raises (default)

• :query_string_length - the maximum allowed size for query strings

• :body_reader - an optional replacement (or wrapper) for Plug.Conn.read_body/2 to provide a function that gives access to the raw body before it is parsed and discarded. It is in the standard format of {Module, :function, [args]} (MFA) and defaults to {Plug.Conn, :read_body, []}. Note that this option is not used by Plug.Parsers.MULTIPART which relies instead on other functions defined in Plug.Conn.

Examples

plug Plug.Parsers, parsers: [:urlencoded, :multipart]

plug Plug.Parsers, parsers: [:urlencoded, :json],
                   pass: ["text/*"],
                   json_decoder: Jason

Each parser also accepts options to be given directly to it by using tuples. For example, to support file uploads it is common to pass the :length, :read_length and :read_timeout option to the multipart parser:

plug Plug.Parsers,
     parsers: [
       :url_encoded,
       {:multipart, length: 20_000_000} # Increase to 20MB max upload
     ]

Built-in parsers

Plug ships with the following parsers:

• Plug.Parsers.URLENCODED - parses application/x-www-form-urlencoded requests (can be used as :urlencoded as well in the :parsers option)
• Plug.Parsers.MULTIPART - parses multipart/form-data and multipart/mixed requests (can be used as :multipart as well in the :parsers option)
• Plug.Parsers.JSON - parses application/json requests with the given :json_decoder (can be used as :json as well in the :parsers option)

File handling

If a file is uploaded via any of the parsers, Plug will stream the uploaded contents to a file in a temporary directory in order to avoid loading the whole file into memory. For such, the :plug application needs to be started in order for file uploads to work. More details on how the uploaded file is handled can be found in the documentation for Plug.Upload.

When a file is uploaded, the request parameter that identifies that file will be a Plug.Upload struct with information about the uploaded file (e.g. filename and content type) and about where the file is stored.

The temporary directory where files are streamed to can be customized by setting the PLUG_TMPDIR environment variable on the host system. If PLUG_TMPDIR isn't set, Plug will look at some environment variables which usually hold the value of the system's temporary directory (like TMPDIR or TMP). If no value is found in any of those variables, /tmp is used as a default.

Custom body reader

Sometimes you may want to customize how a parser reads the body from the connection. For example, you may want to cache the body to perform verification later, such as HTTP Signature Verification. This can be achieved with a custom body reader that would read the body and store it in the connection, such as:

defmodule CacheBodyReader do
  def read_body(conn, opts) do
    {:ok, body, conn} = Plug.Conn.read_body(conn, opts)
    conn = update_in(conn.assigns[:raw_body], &[body | (&1 || [])])
    {:ok, body, conn}
  end
end

which could then be set as:

plug Plug.Parsers,
  parsers: [:urlencoded, :json],
  pass: ["text/*"],
  body_reader: {CacheBodyReader, :read_body, []},
  json_decoder: Jason