WSGI v2.0 reference implementation
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WSGI v2.0


Initial work on a WSGI v2.0 specification. Will include a reference implementation and tests.

To Specify, or not to Specify

There is obviously a balance between providing a small complete specification and providing the core features that are common to a large majority of application code. As a test of whether a feature should be part of this specification I propose the following question, "Can this be implemented as a middleware?" I believe that if a feature can be implemented as middleware we should avoid specifying it, and instead rely on convention and community consensus.

An example of a good feature idea that would be best left to middleware are the logging facilities introduced by Rack. They provide "log.error", "log.critical", (etc..) keys in the environ to provide a standardized method for application logging. This feature could be implemented strictly as a middleware, and hence does not belong in this specification.

The WSGI Interface

In a nutshell, this specification revolves around a simple method for handling HTTP requests and generating HTTP responses. The basic application is a simple callable that accepts a single parameter that describes the request and returns a three-tuple that describes the response. All values referred to in this document are assumed to be the most direct equivalent for a given interpreter.

A Simple Example

CPython 2.5:

def application(environ):
    status = "200 OK"
    headers = [("Content-Type", "text/plain")]
    body = "Hello, world!\n"
    return status, headers, [body]

CPython 3.1:

def application(environ):
    status = b"200 OK"
    headers = [(b"Content-Type", b"text/plain")]
    body = b"Hello, world!\n"
    return status, headers, [body]

Obviously, variable names are not part of the specification. The current names are only used to be illustrative of their intent and to have a common name by which we can refer to them.


CPython 2.6 and 2.7 should be able to use either format because of the following condtion:

>>> type(b"") is type("")

The environ dict

The environ should provide a complete description of the HTTP request. A fundamental goal should be to provide WSGI applications the ability to produce a semantically equivalent request. For instance, in WSGI 1.0 there was no standardized way to access the original request URI before irreversible mutations (%HH escape decoding for instance). This is an example of following the CGI spec that should be avoided in the new specification.

Along with providing a description of the HTTP request, the environ should also provide the capability to pass along data that is out-of-band to the HTTP protocol. For instance, information regarding the server (port, hostname, ip), information regarding the client (ip, port, etc), and information regarding the execution environment (multithreading, multiprocessing).

Keys in the environ dict should be considered metadata about a given request. As such, they are not part of the request or the response. Since they are clearly not I/O related, they should be the default character type for the given interpreter. Ie, whatever type a bare string literal defaults to. This prevents the need for all code using an environ to have to worry about the key type.

Deciding whether a key should be part of the requirements for adherence to this specification should follow the same criteria for deciding what should be a feature. Each key should be required to correctly interpret an HTTP request or provide information that would not otherwise be detectable by application code.

HTTP Related

All HTTP values must be raw byte values. These values are pulled off a socket and may end up being sent to another socket. Unfortunately, HTTP predates much of the push for character encodings. Some may point that it specifies using latin-1 encoding in most places but I would argue that people do weird things with HTTP. As much as I would love to take a hardline on encodings, I am unable to justify specifying latin-1 when clients may ignore this and some application developers will be expected to handle such bad clients.

  • http.method - Request method (bytes)
  • http.uri.raw - The original unmodified request URI. (bytes)
  • http.uri.path - The path part of the request URI. (bytes)
  • http.uri.query_string - The query string part of the request URI (bytes)
  • http.version - The HTTP protocol version specified in the request. (two-tuple of integers)
  • http.headers - A dict of lists. keys are header names. Values are lists of header values. Most often these lists will have a length of one, but HTTP specifies that headers can be repeated. As such our representation should reflect this fact. (keys are bytes, values are lists of bytes)
  • http.trailers - A dict of lists. Keys are trailer names and values are lists of trailer values. (keys are bytes, values are lists of bytes)
  • http.body - A readable stream that provides any data that was sent with the request. This stream must respect the HTTP protocol in the data it provides. For instance, it must respect content-length, transfer- encoding, and request protocol version. (readable stream)

OOB Related

This set of keys deals with information that an application requires from the server that cannot be derived from the request.

  • wsgi.version - The tuple (2, 0) if the server complies with this specification.
  • wsgi.url_scheme - The original URL scheme used to make this request so applications can generate correct links to other resources it controls. Should generally be "http" or "https". (bytes)
  • wsgi.script_name - Servers invoking a WSGI application often reserve a section of its URI path hierarchy for request dispatching. Ie, if a server is hosting two WSGI applications, one application my get requests that have a paths matching "/foo.*" while the other application may get requests matching "/bar.*". "wsgi.script_nmae" can be used to inform the application of this path information so that it can generate URLs that will be compatible with differing path configurations. (bytes)
  • wsgi.multithread - If the application must handle running in an execution environment where application code may be called simultaneously from multiple threads. (True or False)
  • wsgi.multiprocess - An indication if the application must handle running in an execution environment where application code may be called simultaneously from multiple processes. (True or False)
  • wsgi.errors - A stream available for application logging. (ie, it must be writable). (Writable stream)
  • wsgi.upgrade - A callable that returns an object that represents the raw socket connection. This mechanism will replace the start_response callable and allow applications to remove the current request from the normal HTTP processing loop. (callable that takes no parameters and returns an upgrade stream)
  • wsgi.upgraded - A callable that returns a boolean specifying if the connection has been upgraded or not. This should be used by middleware that create responses (ie, error handling middleware) to determine if the exception should be intercepted. If this returns True, the exception should be re-raised so that the server can log the error and clean up the connection.
  • conn.server_name - They name of the server that the application may wish to provide to the application. If there is a "Host" header in the HTTP request, this should reflect that value. (bytes)
  • conn.server_port - The port of the socket that the server is listening on. If the Host header is present with a specified port, this value should reflect that. (int)
  • conn.remote_ip - The IP address of the remote HTTP client. (bytes)
  • conn.remote_port - The port of the remote HTTP client. (int)

Encoding Related

To allow applications to dictate how they want various byte types decoded there are a set of environ key/value pairs that should be respected when retrieving values from the environ.

  • enc.default - A tuple that specifies the default encoding and error type as native strings. The default is ("latin-1", "strict")

[ed: I plan on hashing this out more, but my current thought is to allow people to specify something like "enc.http.*" which would affect all environ bytes values with environ keys starting with "http.*". Though this is funny for things like headers and trailers that are complex values as well as when you may want to specify the encoding of a specific header or header value (Cookies being the motivating edge case for that). Maybe a callable that decodes the value? Something like that. Its time for beer.]

The "http.body" Readable Stream

The body readable stream must support the following methods:

  • read(size) - Returns up to size bytes from the request body. If size is negative or None, it returns all remaining bytes in the request body. This may return fewer bytes than requested if there are not enough bytes left in the request body to satisfy the request. When zero bytes remain in the request body, this function should return an empty string. All values returned from this function must be of the bytes type specified by the current interpreter. No character decoding should be applied.
  • readline(size) - Provides the same semantics as read(size) but will also limit the data returned to the next newline "n" character in the request body. Unlike WSGI v1.0, the size parameter is requierd to be supported. When no more data remains, the empty byte object should be returned.
  • readlines([size]) - Return all remaining lines in the request body as a list of byte objects. The optional size argument is only specified so that the body object matches the built-in file object API. When no more bytes are left in the request body, the empty bytes object should be returned.
  • __iter__() - Should yield all strings remaining in the request body. When no more data exists in the request body, an iterator that yields zero values should be returned.

Server implementations must not allow calling code to break the HTTP protocol by by reading beyond the end of the request body when using this stream object.

Server implementations do not need to implement the ability to rewind request bodies in any form.

[note: Should I specify a readchunk() method that returns a two-tuple of a size and iterable for reading requests that were sent with a chunked transfer- encoding so that apps can proxy chunked requests nearly exactly? Theoretically an HTTP endpoint that relies on the semantics of such things is mis-behaving, but its a possibility of specing something out of the realm of possibility. Granted, most HTTP parsers do not support chunk length parameters, which probably should not be in this spec.]

The "wsgi.errors" Writable Stream

This error stream is intended for use in a logging system. It must support enough of the file API to be used with the standard library logging module.

  • write(value) - Write the byte represented by value to the stream.
  • writelines(seq) - Iterate over seq writing each yieled bytes value to the underlying stream.
  • flush() - Ensure that any data has been sent through to the underlying stream. It is possible that this is a no-op depending on implementation. It should merely allow applications to ensure that some data has been logged.

The "upgrade" Stream

This stream is a readable and writable stream that can be used for direct communication with the client. It is returned from a call to "wsgi.upgrade" and should support the following methods:

  • recv(size) - Retrieve at most size bytes from the underlying connection. If size is not specified, negative, or None, it should return any available data or block until data becomes available.
  • send(value) - Send the bytes object value to the client immediately. The server should not attempt to buffer data. Servers should be careful to comply with this condition even if they are using TCP options like TCP_CORK or TCP_NOPUSH.
  • sendall(data) - Send all bytes in data? Should I smush this with send?
  • makefile() - Should I specify this?
  • setblocking() - Should I specify this?

Application Use of Streams

Only the methods specified in this document are allowed to be called by any application code regardless of what the actual object provides. For instance, if one of the streams has a close() call, it must not be called by any application.

The "wsgi.upgrade" Callable

The "wsgi.upgrade" callable must take zero parameters and return an object that represents the underlying client connection. This will provide applications the ability to continue using the "push" paradigm that the old write(data) callable returned from start_response() provided. Although, in this case the application becomes responsible for the entire response and must format its own status message and headers before sending its response.

This extra burden allows developers to use the same mechanics to support actual HTTP connection upgrades to other protocols if they so desire. For example, this is necessary to support WebSocket connections.

Once the "wsgi.upgrade" callable has been invoked, the server must not attempt to send any other data to the client. If they application invokes the upgrade callable and returns True, the server may optionally attempt to continue reading HTTP requests from the connection. If False is returned the server must close the underlying socket connection.

The HTTP Response

The basic HTTP response returned by a WSGI application must be a three-tuple containing a status code, a list of headers, and an iterable that represents the respones body.

Status Code

That staus code should be a byte object that represents an HTTP status code and the associated status message. This should match the regular expression: r"^[1-9]dd[ ]+[A-Za-z][A-Za-z ]+$".

Response Headers

The response headers must be a list of two-tuples. Each two-tuple must be a pair of byte objects that specify the name and value of the header. If header values span multiple lines, the application code is repsonisble for ensuring that the continuation indentation is properly specified. A server may enforce some constraints on header data, but this is not a requirement.

Response Body

The respones body returned from an application should be an iterable that yields byte objects that will be forwarded to the client.

Server Handling of the Response

A server should not attempt to send any data to a client until the first value is accessed from the iterator. This will allow for the maximum amount of time to be able to report any errors.

Once the server starts sending data it will not be possible to recover the connection. In this situation the server should close the client connection and report the error out of band in a server log or via some other implementation defined manner.

A WSGI server should attempt to ensure that the HTTP response complies with the HTTP protocol. For instance, if an application returns an response description that contains no Content-Length or Transfer-Encoding, the server should send the response and then close the underlying client connection to indicate the end of the response. A WSGI server should not attempt to modify the response in any way as this is the responsibility of middleware.

Error Handling

Now that there is no start_response() callable, the method for error handling has changed. If an application raises an exception while handling a request, the server should attempt to inform the HTTP client of this error. Generally this is accomplished using a simple "500 Internal Server Error" response status. If the server does send such a message it should be very careful to not send too much information. For instance, the Python traceback should not be included by default (although, it may be included in a "debug" mode). This is to prevent too much information leakage to possibly nefarious clients.

A server should also consider whether a request has been partially sent to the client when handling errors. If a request has already been started and no error message can be sent, the server should close terminate the current response and underlying connection and then notify the application developer with some other out of band communication (ie, a server error log). A connection that has been upgraded should be considered as started (regardless if any data has been sent to the client [ed: double check on this caveat]).

Guidelines for Middleware

When an application is preparing a response it is likely and common that it will call a second application as part of this processing. Applications that defer to sub-applications are known as "middleware". Common uses of middleware include routing, response compression, authentication, or session management.

All middlware must obey they requirements of the server (minus the prohibition on request/respones modifications) as well as requirements of applications (accepting an environ parameter, returning a three-tuple).

Common HTTP Behaviors

This is a list of common HTTP behavior that needs to be addressed by WSGI servers and applications.

Expect: 100-Continue

WSGI servers should automatically handle the "Expect: 100-Continue" header transparently as it is responsible for direct socket access and thus the best place to handle this logic. There are three valid methods for handling this behavior listed below in order from best to worst:

  • When an application attempts to read data from the client, send the "HTTP/1.1 100 Continuernrn" response before reading data.
  • Immediately send the "HTTP/1.1 100 Continuernrn" response before invoking the WSGI application
  • Do nothing and wait for the client to timeout and send the body on its own. This is very undesirable because the client may wait many seconds before timing out.


If a request has trailers these should be placed into the "http.trailers" environ value when they become available.

There may be better ways to deal with this. I have considered specifying a function on the "http.body" stream that is "read_headers" that will read the rest of the response to get to the headers if there are any. Before writing too much code for it I am waiting for feedback.


  • CGI compliance is not good enough. Most of the complaints I have seen revolve around the lack of HTTP compliance.
  • The major motivation for decision making should be to give applications the ability to have complete control of the HTTP protocol. The WSGI spec should merely serve to provide a standardized interface to the interaction to decouple server and app code.
  • The original outline of a function that creates an HTTP response given a description of the request has proven to be popular.
  • Other languages implementing WSGI inspired systems have universally done away with the start_response callable. This is a good idea as it reduces the complexity of the specification substantially.
  • Character encoding issues are a pain in the rear. The WSGI spec is not the place to make a decision on this issue because it is application specific. Although, specifying a standard way to access various parts of the request may provide recommendations on character decoding.
  • The keys in an environ dict should be considered metadata that point at subsections of the request. It was a mistake to have HTTP_$(HEADER_NAME) semantics as this blurs the line between data and metadata.
  • The original specification made a good decision in limiting the spec to only using builtin types. This simplifies the implementation greatly for other interpreters and implementations using the C-API.