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Subscribe for Filtered Messages

Backendless message filtering is a powerful mechanism enabling conditional message delivery, interest-based subscriptions and private (point-to-point) messaging. To enable filtering, the subscription API documented below requires a special condition called selector which must be included in the API's request body. Backendless applies the selector to every message published into a channel and if the condition is true, the message is delivered to the subscriber.

A selector is a query expressed in the SQL-92 syntax and formatted as the condition part of the SQL's WHERE clause. The condition references headers in the published messages. When a message is published and a subscriber has a selector, Backendless checks the condition of the selector against the headers of the published message. If the result of the condition is true, the message is delivered to the subscriber.

Message retrieval requires a separate API call documented in the Retrieve Messages section of this guide.

Method

POST

URL

https://api.backendless.com/<application-id>/<REST-api-key>/messaging/<channel-name>/subscribe

where:

Argument                Description
<application-id> the ID of your application generated upon its creation. You can get the value in the Manage > App Settings section of the Backendless Console.
<REST-api-key> REST API key of your application. You can get the value from the Manage > App Settings section of the Backendless Console.
<channel-name> name of the channel to subscribe to.

Request Headers

Content-Type: application/json  
user-token: optional value obtained as a result of the login operation.

where:

Argument                Description
Content-Type Must be set to application/json. This header is mandatory.
user-token Optional header. Contains a value returned by Backendless in a preceding user Login API call. If user-token is set in the request, the operation will be executed with the security policy associated with the currently logged in user. This means all permissions associated with the user and the roles assigned to the user will be enforced by Backendless.

Request Body

Optional parameters are shown in [square brackets]:

{  
  ["subscriberId" : value,]  
  ["selector":selector-query]  
}

Response Body

{  
  "subscriptionId" : value  
}

Returned subscriptionId must be used in the API call to retrieve messages for the subscription.

Errors

When the server-side reports an error, it returns a JSON object in the following format:

{  
  "message":error-message,  
  "code":error-code  
}

Example

For an example of channel subscription with a selector see the Conditional Pub/Sub section of this guide.