CDX for non-GET requests

Originally CDX files were only used to index web archives containing GET requests. As browser based capture methods can record non-GET requests such as those generated by JavaScript a way for CDX records to differentiate based on request method and request body is needed. This document describes the mechanism used for encoding the request method and body in the CDX key by appending additional query parameters as originally implemented by pywb.

Compatibility Note

This document aims to describe the behaviour of pywb 2.6.7 running on Python 3.7 or later. Older versions of pywb or Python can produce different output.


[TODO: To be written]

Encoding the request method

If the request method is not GET it must be appended as the value of query parameter __wb_method.

If the URL does not have a query string a ? must be added:

http://example.org/         => http://example.org/?__wb_method=POST

If the URL already has a query string the __wb_method parameter must be added at the end after a & separator:

http://example.org/?page=1  => http://example.org/?page=1&__wb_method=POST

Even if the query string already ends in & another separator must still be added:

http://example.org/?foo&    => http://example.org/?foo&&__wb_method=POST

Encoding the request body

Encoding the request body depends on the content-type.

Content-Type Primary Encoding Fallback Encoding
application/json JSON  
application/x-amf AMF  
application/x-www-form-urlencoded urlencoded form binary
multipart/* multipart form binary
text/plain JSON binary
* binary  

AMF request body encoding

[TODO: To be written]

Binary request body encoding

The request body is encoded as Base64 (RFC 4648) and appended to the query string as the __wb_post_data parameter.


Original request:

POST /chat HTTP/1.0
Host: example.org
Content-Length: 5


Encoded URL:


Encoding a urlencoded form request body

Decode the body to a string using UTF-8, percent decoded the string, percent plus encode it and then append the result to the output. If a UTF-8 decoding error occurs then the binary encoding method must be used instead.

[TODO: example]

Encoding a multipart form request body

The body must be decoded as form data per RFC 2388 and then percent plus encoded. If the body is not a valid multipart/form-data message then the binary encoding method must be used instead.

[TODO: example]

Encoding a JSON request body

The request must be parsed as JSON (RFC 8259) and then apply the following algorithm with an empty string as the initial value of name.

To encode a JSON value, given a name and an initially-empty map nameCounts of strings to integers:

  1. If value is a JSON object:
    1. Recursively encode each member of the object passing member’s name as name and the member’s value as value.
  2. If value is a JSON array:
    1. Recursively encode each element of the array passing the current value of name as name and the value of the element as value.
  3. Otherwise:
    1. Define the string encodedValue as:
      1. If value is JSON true then the string “True”.
      2. If value is JSON false then the string “False”.
      3. If value is JSON null then the string “None”.
      4. If value is a JSON string then the result of percent plus encoding the string.
      5. If value is a JSON number then the number encoded as a decimal string.
    2. If nameCounts contains the integer count for name:
      1. Increment count by 1.
      2. Store count as the new count for name in nameCounts.
      3. Append the string “&name.count_=encodedValue” to the output.
    3. Otherwise, if nameCounts does not contain name:
      1. Store the integer 1 in nameCounts for name.
      2. Append the string “&name=encodedValue” to the output.


Original request:

POST /events HTTP/1.0
Host: example.org
Content-Type: application/json

   "type": "event",
   "id": 44.0,
   "values": [true, false, null],
   "source": {
      "type": "component",
      "id": "a+b&c= d",
      "values": [3, 4]

Encoded URL:



Percent plus encoding

To percent plus encode a string, first encode it as UTF-8 and then percent plus encode the resulting byte sequence.

To percent plus encode a byte sequence, for each byte in the input sequence:

  1. If the byte falls within the following ASCII character ranges append it to the output as is.

    '0'-'9', 'a'-'z', 'A'-'Z', '-', '.', '_', '~'
  2. If the byte is the ASCII space character (‘ ‘) append the ASCII plus character (‘+’) to the output.

  3. Otherwise, append ASCII percent character (‘%’) to the output and followed by the value of the byte formatted as two uppercase hexadecimal digits.

Compatibility Note

Prior to Python 3.7 the character “~” was percent encoded.