class WebEventService

File Information

Library: OSP/WebEvent
Package: WebEvent
Header: Poco/OSP/WebEvent/WebEventService.h


The WebEventService is a service that allows a web application server to notify clients running in a browser, using a RFC 6455 WebSocket.

The way this service works is as follows:

JavaScript code running in the browser will open a WebSocket connection to the OSP web server (using path "/webevent") and send a single-frame text subscription message specifying the subjects to subscribe to. The format of the subscription message is:

SUBSCRIBE <subjectList> WebEvent/1.0

where <subjectList> is a comma-separated list of subject names not containing white space characters.

Subject names use a hierarchical, reverse DNS-style syntax, with the parts of the subject delimited by periods. Subject names must not contain spaces and their parts should be valid C++ identifiers. However, the latter restriction is not enforced. Subject names are hierarchical. A subscriber to a "parent" subject (e.g., "com.appinf.events") will also receive all "child" subjects (e.g., "com.appinf.events.someEvent" and "com.appinf.events.someOtherEvent").

A client can unsubscribe from specific events by either sending an unsubscribe request or by simply closing the WebSocket. The format of the UNSUBSCRIBE message is similar to the SUBSCRIBE message:

UNSUBSCRIBE <subjectList>|* WebEvent/1.0

where <subjectList> is a comma-separated list of subject names, as with the SUBSCRIBE message. Instead of a subject list, a single asterisk can be specified, which means unsubscribe from all subjects.

After the initial SUBSCRIBE message the client can send additional SUBSCRIBE or UNSUBSCRIBE messages, and thus dynamically change its subscriptions without having to re-establish the WebSocket.

Once a client has opened a WebSocket and sent the SUBSCRIBE message, the WebSocket connection will be "parked" on the server and only used when an event message needs to be sent to the client. This allows a very high number of simultaneous subscribers, since a single subscriber does not consume much server resources, except for the socket and some data overhead. Specifically, a specific thread for the WebSocket connection will only be used during the initial connection setup.

Notification messages sent to clients are single-frame text messages with the following format:

NOTIFY <subjectName> WebEvent/1.0\r\n

where <subjectName> is the subjectName specified in the call to notify(), and <data> is the given data string, which typically should be a valid serialized JSON or XML document in UTF-8 encoding, although any valid UTF-8 string can be sent.

A client can also sent notifications to other clients by sending a NOTIFY message to the server.

A client can send a NOTIFY message using the special subject "system.ping" to the server, and the server will respond with a NOTIFY using the subject "system.pong". This can be used as a heartbeat mechanism to verify that the connection to the server is alive. Any data sent with the ping message will be sent back with the pong message.


Direct Base Classes: Poco::OSP::Service

All Base Classes: Poco::OSP::Service, Poco::RefCountedObject

Known Derived Classes: WebEventServiceImpl

Member Summary

Member Functions: isA, notify, subjectNotified, subscriberCount, type

Inherited Functions: duplicate, isA, referenceCount, release, type

Types Aliases


using NotificationEvent = std::pair < std::string, std::string >;


using Ptr = Poco::AutoPtr < WebEventService >;


WebEventService protected


Creates the WebEventService.


~WebEventService protected virtual


Destroys the WebEventService.

Member Functions

isA virtual

bool isA(
    const std::type_info & otherType
) const;

notify virtual

virtual void notify(
    const std::string & subjectName,
    const std::string & data
) = 0;

Notify all registered subscribers to the given subject, using the given data, which is typically a serialized JSON or XML document in UTF-8 encoding.

Sending the notification is done asynchronously. If a notification cannot be delivered to a subscriber due to a network issue, the subscriber will be removed and its WebSocket closed.

subjectNotified virtual

virtual Poco::BasicEvent < const NotificationEvent > & subjectNotified(
    const std::string & subject
) = 0;

Fired when a notification has been sent for the given subject, either by calling notify(), or by a client sending a NOTIFY message to the server.

subscriberCount virtual

virtual int subscriberCount(
    const std::string & subject
) = 0;

Returns the number of subscribers to the subject.

type virtual

const std::type_info & type() const;



static const std::string SERVICE_NAME;


Poco::BasicEvent < const NotificationEvent > notificationSent;

Fired when a notification has been sent, either by calling notify(), or by a client sending a NOTIFY message to the server.

This event is fired for all subjects. It's the responsibility of the delegate function to filter out ignore events not of interest.


Poco::BasicEvent < const Poco::Net::HTTPServerRequest * > requestReceived;

Fired when a HTTP WebSocket upgrade request has been received. This can be used for authentication and authorization, or to reject the client's web socket connection for other reasons. If the delegate function sets the status code of the HTTP response to anything other than 200 (OK), the WebSocket connection will not be established.


Poco::BasicEvent < const std::string > subscriptionRequestReceived;

Fired when a subscription request for a new subject arrives. This event is fired once per subject subscription on a socket. To find out the number of clients subscribed to a subject, use the subscriberCount() call.


Poco::BasicEvent < const std::string > unsubscriptionRequestReceived;

Fired when an unsubscription request for a subject arrives. This event is fired once per unsubscription on a socket. To find out the number of clients subscribed to a subject, use the subscriberCount() call.