Documentation > Administration > Web Services > Long-poll Service

Long-poll Service

The default transport for WAMP is WebSocket. For clients not supporting WebSocket, or for blocking clients, the WAMP specification defines WAMP-over-Longpoll, a WAMP transport that runs over regular HTTP 1.0 requests.

The HTTP Long-pool transport can come in handy to support old browsers lacking WebSocket like IE9 and earlier or old Android WebKit. It is also useful to integrate with clients that cannot work asynchronously or have an inherent blocking, synchronous execution environment like PostgreSQL background processes for database sessions.

Configuration

To configure a Long-poll Service, attach a dictionary element to a path in your Web transport:

option description
type MUST be "longpoll" (required)
options A dictionary of options (optional, see below).

The options dictionary has the following configuration parameters:

option description
request_timeout An integer which determines the timeout in seconds for Long-poll requests. If 0, do not timeout. (default: 10). After this period, the request is returned even if there is no data to transmit. Note that clients may have their own timeouts, and that this should be set to a value greater than the request_timeout.
session_timeout An integer which determines the timeout on inactivity of sessions. If 0, do not timeout. (default: 30)
queue_limit_bytes Limit the number of total queued bytes. If 0, don't enforce a limit. (default: 131072)
queue_limit_messages Limit the number of queued messages. If 0, don't enforce a limit. (default: 100)
debug A boolean that activates debug output for this service. (default: false).
debug_transport_id If given (e.g. "kjmd3sBLOUnb3Fyr"), use this fixed transport ID. (default: null).

Example

The Long-poll Service is configured on a path of a Web transport - here is part of a Crossbar configuration:

{
   "workers": [
      {
         "type": "router",
         ...
         "transports": [
            {
               "type": "web",
               ...
               "paths": {
                  ...
                  "lp": {
                     "type": "longpoll",
                     "options": {
                        "session_timeout": 30
                     }
                  }
               }
            }
         ]
      }
   ]
}

Test using curl

For developers that want to add WAMP-over-Longpoll support to their WAMP client library, we have an example which demonstrates the transport using plain curl only.

This example can be useful during development and debugging. It is not intended for end-users.

Use with AutobahnJS

AutobahnJS fully supports WAMP-over-Longpoll and you can find a complete working example in the Crossbar.io examples here.

Use with AutobahnPostgres

upcoming

AutobahnPostgres uses WAMP-over-Longpoll to natively integrate PostgreSQL with Crossbar.io.