go get

Imposter is a lightweight and versatile tool for the mocking of web applications.

by Nicola Baldi

v0.0.0-20180809064045-76f5f5b66ec2 (see all)License:MIT
go get

naighes/imposter Build Status Go Report Card License: MIT

imPOSTer Logo

imPOSTer is a lightweight and versatile tool for the mocking of web applications.


You need go installed and GOBIN in your PATH. Once that is done, run the command:

$ go get -u

Precompiled binaries

Precompiled binaries for released versions are available in the releases section. Using the latest production release binary is the recommended way of installing imPOSTer.

Docker images

Docker images are available on Docker Hub.
You can launch an imPOSTer container for trying it out with:

$ docker run --name imposter -d -p 8080:8080 --rm naighes/imposter start

imPOSTer will now be reachable at http://localhost:8080.

Start command

Run a new instance of imPOSTer.


  • -config-file <string>: the configuration file path
  • -graceful-timeout <duration>: the duration for which the server gracefully wait for existing connections to finish - e.g. 15s or 1m (default 15s)
  • -port <int>: the listening TCP port (default 8080)
  • -tls-cert-file-list <string>: a comma separated list of x.509 certificates to secure communication
  • -tls-key-file-list <string>: a comma separated list of private key files corresponding to the x.509 certificates listed in -tls-cert-file-list <string>
  • -record <string>: Enable the recording of PUT requests (select multiple values from {scheme, host, path, query} separated by pipe (|))
  • -cors: Enable the support for CORS


$ ./imposter start --config-file ./config.yaml --port 3000 --record "scheme|host|path"

Validate command

Validate and type-check any rule_expression within a configuration file.


  • -config-file <string>: the configuration file path
  • -json <string>: enable JSON output instead of plain text


$ ./imposter validate --config-file ./config.yaml --json
found 2 errors:
could not find a built-in function with name 'eqrequest_http_method'
could not find a parser for the current token
gex_match(request_url_path(), ^/myredirect$")

Configuration file


We're going to write our first configuration now to launch an instance with a single catch-all rule:

  "pattern_list" : [{
    "rule_expression": "${true}",
    "response": {
      "body": "Hello, default body!",
      "status_code": "${200}"

A YAML parser is also available and you can write your configuration by the YAML syntax as well:

- rule_expression: ${true}
    body: Hello, default body!
    status_code: ${200}

pattern_list is a list of rules defining how imPOSTer will handle incoming requests. Every rule requires a boolean expression. That is, if an incoming request URL matches one of the rule_expression the corresponding response is served.

Let's suppose you need to catch all requests issued by POST HTTP method, every URL path containing the string hello and just a Content-Type header of type application/json:

- rule_expression: ${
        contains(request_url_path(), "hello"),
        eq(request_http_method(), "POST"),
        eq(request_http_header("Content-Type"), "application/json")
    body: Hello, complex rule!
    status_code: ${200}

Last but not least, you need to define the HTTP status_code to be returned (200 when not specified). Note that status_code is an expression itself and so it needs to be wrapped into the block marker (${...}). Furthermore, a positive integer value is expected from its evaluation (e.g. status_code: ${if(contains(request_http_header("Accept-Language"), "it")) 200 else 404}).
Rules are tested in the order they were added to the pattern_list collection. If two rules match, the first one wins:

The response object

There are two ways of defining a response object and it basically depends on the level of granularity you really need.
For example, you can define it in a computed manner:

- rule_expression: ${regex_match(request_url_path(), "^/myredirect$")}
  response: ${redirect("http://examp.lecom/foo", 301)}

The above snippet shows how you can benefit from built-in functions (redirect) to achieve interesting results (e.g. redirecting to different URLs).
Note: the computed version of the response object requires functions returning HTTPRsp (e.g. link, redirect, …).

Alternatively, you can rely on a full structured version of the response object:

  "pattern_list" : [{
    "rule_expression": "${regex_match(request_url_path(), \"^/posts$\")}",
    "response": {
      "body": "Hello, post!",
      "headers": {
        "Content-Type": "text/plain; charset=utf-8"
      "status_code": "${202}"

That will match the URL path /posts when an HTTP request will be issued by any HTTP method. The match will be handled by returning a body containing the Hello, post! string and just the Content-Type header.


Input variables serve as parameters for built-in functions.

- rule_expression: ${
        regex_match(request_url_path(), "^/imposter$"),
        eq(request_http_method(), "GET")
  response: ${redirect(var("imposter_link"), 301)}

Built-in functions

You can "combine" values with other values. These combinations are wrapped into the evaluation block marker (${…}), such as ${link("")}.
imPOSTer ships with built-in functions. Functions are called with the syntax function_name(arg1, arg2, ...). For example, to read a file: ${file("path.txt")}.

Supported built-in functions

The supported built-in functions are:

  • var(name: string) -> string - Reads the content of a variable with the specified name into a string.
  • and(arg1: bool, arg2: bool, …) -> bool - Evaluates all arguments by using the AND logical operator.
  • or(arg1: bool, arg2: bool, …) -> bool - Evaluates all arguments by using the OR logical operator.
  • not(arg: bool) -> bool - Negates its argument.
  • eq(arg1: any, arg2: any) -> bool - Determines whether the two specified arguments are equal.
  • ne(arg1: any, arg2: any) -> bool - Determines whether the two specified arguments are not equal.
  • contains(source: string, value: string) -> bool - Determines whether value substring occurs within this source string.
  • request_url() -> string - Returns the URL for the current request.
  • request_url_path() -> string - Returns the path component of the URL for the current request.
  • request_url_query() -> string - Returns any query information included in the URL for the current request.
  • request_url_query(name: string) -> string - Returns the first value associated with the given name.
  • request_http_method() -> string - Returns the HTTP method for the current request.
  • request_http_host() -> string - Returns the HTTP Host for the current request.
  • request_http_header(name: string) -> string - Returns the value of the HTTP header with the specified name for the current request.
  • regex_match(source: string, pattern: string) -> bool - Searches the specified source string for the first occurrence of the specified regular expression pattern and returns a value indicating whether the match is successful.
  • file(path: string) -> string - Reads the content of a file into a string.
  • link(url: string) -> HTTPRsp - Forwards a client to a new URL.
  • redirect(url: string, status_code: int) -> HTTPRsp - Redirects a client to a new URL with the specified status_code (it must be a 3XX value).
  • in(source: array, item: string|bool|int|flota64) -> bool - Determines whether the specified item exists as an element within the source array object.
  • to_string(obj: any) -> string - Returns a string that represents obj.

Conditional statements

A conditional statement identifies which statement to run based on the value of a boolean expression.


if (<boolean_expression>) <expression> else <expression>


- rule_expression: ${regex_match(request_url_path(), "^/myfile$")}
  body: testing if statement
    Content-Type: text/plain; charset=utf-8
    Content-Language: ${
      if(contains(request_http_header("Accept-Language"), "en"))
  status_code: ${200}

Induced latency

We're used to mock our application dependencies so we can test the application's behaviour by the proper isolation degree. Furthermore, sometimes it's useful to test how our application behaves due to the change of certain conditions (UX, load/stress testin, etc).
imPOSTer has the capability of inducing some latency before a response object is returned back to the client:

- rule_expression: ${regex_match(request_url_path(), "^/myredirect$")}
  response: ${redirect("http://examp.lecom/foo", 301)}
  latency: 2000

The above rule_expression introduces a 2 seconds delay.
Note: latency MUST be expressed in milliseconds.


imPOSTer can be configured to dynamically define rules at runtime. Once you issue an HTTP request by the HTTP PUT method, a copy of the original payload will be internally stored. You'll be able to subsequently retrieve the previously defined payload by requesting the same URL with the HTTP GET method. Recording needs to be explicitly enabled by the record flag (see the documentation above) and its value defines how incoming URLs will be matched.


Let's start imPOSTer by enabling recording:

$ ./imposter start --config-file ./config.yaml --record "scheme|host|path|query"

Send then an HTTP PUT request:

$ curl -il \
    -X PUT \
    -d "Hello, PUT!" \
    -H "Content-Type: text/plain" \

HTTP/1.1 202 Accepted
Date: Fri, 03 Aug 2018 18:37:47 GMT
Content-Length: 0

You can now retrieve the above resource as well:

$ curl -il \
    -X GET \

HTTP/1.1 200 OK
Content-Type: text/plain
Date: Fri, 03 Aug 2018 20:37:47 GMT
Last-Modified: Fri, 03 Aug 2018 20:37:47 GMT
Content-Length: 10

Hello, PUT!

Pretty good, isn't it? Now we're gonna do the same as above, but by adding a query parameter to the URL:

$ curl -il \
    -X GET \

HTTP/1.1 404 Not Found
Content-Type: text/plain; charset=utf-8
X-Content-Type-Options: nosniff
Date: Fri, 03 Aug 2018 18:42:43 GMT
Content-Length: 19

404 page not found

This time we get a 404 status code (the default one, unless any other rule_expression was matched). This is happening due to the strict matching requirement we imposed by running the server with --record "scheme|host|path|query". We can avoid this behaviour by ignoring the query parameter:

$ ./imposter start --config-file ./config.yaml --record "scheme|host|path"

Note: recording takes precedence over any rule_expression.


MIT licensed. See the LICENSE file for details.

GitHub Stars



4yrs ago








1yr ago
No alternatives found
No tutorials found
Add a tutorial