Documentation > Administration > HTTP Bridge > HTTP Bridge Caller

HTTP Bridge Caller


The HTTP Caller feature is available starting with Crossbar 0.10.3.

The HTTP Caller is a service that allows clients to perform WAMP calls via HTTP/POST requests. Crossbar will forward the call to the performing server and return the result.

Try it

Clone the examples repository, and go to the rest/caller subdirectory.

Now start Crossbar:

crossbar start

This will register a simple procedure that takes two integers, adds them together, and returns the result.

To test this out, you can use curl:

curl -H "Content-Type: application/json" \
    -d '{"procedure": "com.example.add2", "args": [1, 2]}' \

...or any other HTTP/POST capable tool or library.


The HTTP Caller is configured on a path of a Web transport - here is part of a Crossbar configuration:

   "workers": [
         "type": "router",
         "transports": [
               "type": "web",
               "paths": {
                  "call": {
                     "type": "caller",
                     "realm": "realm1",
                     "role": "anonymous"

The service dictionary has the following parameters:

option description
type MUST be "caller" (required)
realm The realm to which the forwarding session is attached that will inject the submitted events, e.g. "realm1" (required)
role The fixed (authentication) role the forwarding session is authenticated as when attaching to the router-realm, e.g. "role1" (required)
options A dictionary of options (optional, see below).

The options dictionary has the following configuration parameters:

option description
key A string that when present provides the key from which request signatures are computed. If present, the secret must also be provided. E.g. "myapp1".
secret A string with the secret from which request signatures are computed. If present, the key must also be provided. E.g. "kkjH68GiuUZ").
post_body_limit An integer when present limits the length (in bytes) of a HTTP/POST body that will be accepted. If the request body exceed this limit, the request is rejected. If 0, accept unlimited length. (default: 0)
timestamp_delta_limit An integer when present limits the difference (in seconds) between a signature's timestamp and current time. If 0, allow any divergence. (default: 0).
require_ip A list of strings with single IP addresses or IP networks. When given, only clients with an IP from the designated list are accepted. Otherwise a request is denied. E.g. ["", ""] (default: -).
require_tls A flag that indicates if only requests running over TLS are accepted. (default: false).
debug A boolean that activates debug output for this service. (default: false).

Making Requests

To call WAMP procedures through Crossbar, issue a HTTP/POST request to the URL of the Crossbar HTTP Caller service with:

  1. Content type application/json
  2. Body containing a JSON object
  3. Two query parameters: timestamp and seq

For a call to a HTTP Caller service, the body MUST be a JSON object with the following attributes:

  • procedure: A string with the URI of the procedure to call.
  • args: An (optional) list of positional event payload arguments.
  • kwargs: An (optional) dictionary of keyword event payload arguments.

Signed Requests

Signed requests work like unsigned requests, but have the following additional query parameters. All query parameters (below and above) are mandatory for signed requests.

  • key: The key to be used for computing the signature.
  • nonce: A random integer from [0, 2^53]
  • signature: See below.

The signature computed as the Base64 encoding of the following value:

HMAC[SHA256]_{secret} (key | timestamp | seq | nonce | body)

Here, secret is the secret shared between the publishing application and Crossbar. This value will never travel over the wire.

The HMAC[SHA256] is computed w.r.t. the secret, and over the concatenation

key | timestamp | seq | nonce | body

The body is the JSON serialized event. You can look at working code here.