10.5.11. /db/_design/design-doc/_rewrite/path

ANY /{db}/_design/{ddoc}/_rewrite/{path}

Rewrites the specified path by rules defined in the specified design document. The rewrite rules are defined in array or string field of the design document called rewrites.

Rewrite section a is stringified function

‘Rewrite using JS’ feature was introduced in CouchDB 1.7. If the rewrites field is a stringified function, query server is used to pre-process and route a request.

The function receives truncated version of req object as a single argument and must return object, containing new information about request.

Returned object should include properties as:

  • path (string): Rewritten path, mandatory if no code provided
  • query (array): Rewritten query, if omitted original query keys are used
  • headers (object): Rewritten headers. If omitted, original request headers are used
  • method (string): Method of rewritten request. If omitted, original request method is used
  • code (number): Returned code. If provided, request is not rewritten and user immidiately receives response with the code
  • body (string): Body for POST/PUT requests, or for returning to user if code field provided. If POST/PUT request is being rewritten and no body returned by rewrite function, original request body is used

Example A. Restricting access.

function(req2) {
  var path = req2.path.slice(4),
    isWrite = /^(put|post|delete)$/i.test(req2.method),
    isFin = req2.userCtx.roles.indexOf("finance")>-1;
  if (path[0] == "finance" && isWrite && !isFin) {
    // Deny writes to  DB "finance" for users
    // having no "finance" role
    return {
      code: 403,
        reason:"You are not allowed to modify docs in this DB"
  // Pass through all other requests
  return {path:"../../../"+path.join("/")}

Example B. Different replies for JSON and HTML requests.

  function(req2) {
    var path = req2.path.slice(4),
      h = headers,
      wantsJson = (h.Accept||"").indexOf("application/json")>-1,
      reply = {};
    if (!wantsJson) {
      // Here we should prepare reply object
      // for plain HTML pages
    } else {
      // Pass through JSON requests
      reply.path = "../../../"+path.join("/");
    return reply;

The req2 object rewrites is called with is a slightly truncated version
of req object, provided for list and update functions. Fields *info*,
*uuid*, *id* and *form* are removed to speed up request processing.
All other fields of the req object are in place.

Rewrite section is an array

Each rule is an object with next structure:

  • from (string): The path rule used to bind current uri to the rule. It use pattern matching for that
  • to (string): Rule to rewrite an url. It can contain variables depending on binding variables discovered during pattern matching and query args (url args and from the query member)
  • method (string): HTTP request method to bind the request method to the rule. Default is "*"
  • query (object): Query args you want to define they can contain dynamic variable by binding the key

The to``and ``from paths may contains string patterns with leading : or * characters.

For example: /somepath/:var/*

  • This path is converted in Erlang list by splitting /
  • Each var are converted in atom
  • "" are converted to '' atom
  • The pattern matching is done by splitting / in request url in a list of token
  • A string pattern will match equal token
  • The star atom ('*' in single quotes) will match any number of tokens, but may only be present as the last pathterm in a pathspec
  • If all tokens are matched and all pathterms are used, then the pathspec matches

The pattern matching is done by first matching the HTTP request method to a rule. method is equal to "*" by default, and will match any HTTP method. It will then try to match the path to one rule. If no rule matches, then a 404 Not Found response returned.

Once a rule is found we rewrite the request url using the to and query fields. The identified token are matched to the rule and will replace var. If '*' is found in the rule it will contain the remaining part if it exists.


Rule Url Rewrite to Tokens
{“from”: “/a”,
“to”: “/some”}
/a /some  
{“from”: “/a/*”,
“to”: “/some/*}
/a/b/c /some/b/c  
{“from”: “/a/b”,
“to”: “/some”}
/a/b?k=v /some?k=v k=v
{“from”: “/a/b”,
“to”: “/some/:var”}
/a/b /some/b?var=b var=b
{“from”: “/a/:foo/”, “to”: “/some/:foo/”} /a/b/c /some/b/c?foo=b foo=b
{“from”: “/a/:foo”,
“to”: “/some”, “query”: { “k”: ”:foo” }}
/a/b /some/?k=b&foo=b foo=b
{“from”: “/a”,
“to”: “/some/:foo”}
/a?foo=b /some/?b&foo=b foo=b

Request method, header, query parameters, request payload and response body are depended on endpoint to which url will be rewritten.

param db:Database name
param ddoc:Design document name
param path:URL path to rewrite

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