# tibemsSession_CreateDurableSubscriber

Function

## Purpose

Create a durable topic subscriber.

## C Declaration

    tibems_status tibemsSession_CreateDurableSubscriber(
        tibemsSession session,
        tibemsMsgConsumer* msgConsumer,
        tibemsTopic topic,
        const char* name,
        const char* messageSelector,
        tibems_bool noLocal );

## COBOL Call

    CALL "tibemsSession_CreateDurableSubscriber"
         USING BY VALUE session,
               BY REFERENCE msgConsumer,
               BY VALUE topic,
               BY REFERENCE name,
               BY REFERENCE messageSelector,
               BY VALUE noLocal,
               RETURNING tibems-status
    END-CALL.

Note: `session`, `msgConsumer`, and `topic` have usage pointer.

## Parameters

Parameter | Description
--- | ---
`session` | Create the topic subscriber in this session.
`msgConsumer` | Store the new message consumer object in this location. Note that the message consumer must be a topic subscriber.
`topic` | Create a durable subscriber for this topic (which cannot be a `tibemsTemporaryTopic`).
`name` | Unique name that the server uses to associate the subscriber with a subscription.
`messageSelector` | When non-null, the server filters messages using this selector, so the subscriber receives only matching messages; see `Message Selectors`. When null, or the empty string, the subscriber receives messages without filtering.
`noLocal` | When true, the server filters messages so the subscriber does not receive messages that originate locally—that is, messages sent through the same connection. When false, the consumer receives messages with local origin.

## Remarks

The server associates a durable subscription with at most one subscriber object at a time. When a subscriber object exists, the subscription is active, and the server delivers messages to it; when no subscriber object exists, the subscription is inactive.

Durable subscriptions guarantee message delivery across periods during which the subscriber is inactive. The server retains unacknowledged messages until the subscriber acknowledges them, or until the messages expire.

## Subscription Continuity

Continuity across inactive periods uses two data items from the client:

- Subscription Name- A parameter of this call.
- Client ID - an optional property of the `tibemsConnection` (used only when supplied)

The server uses one or both of these two items to match a subscriber object with its subscription. If a matching subscription exists, and it is inactive, then the server associates it with the subscriber (and the subscription becomes active). The server delivers unacknowledged messages to the subscriber.

If a matching subscription exists, but it is already active, this function fails with `TIBEMS_INVALID_CONSUMER`.

If a matching subscription to the topic does not yet exist, the server creates one.

## Matching Client ID

- If the `tibemsConnection`’s client ID is non-null when a session creates a durable subscription, then only sessions of a connection with the same client ID can attach to that subscription.
- If the `tibemsConnection`’s client ID is null when a session creates a durable subscription, then any session can attach to that subscription (to receive its messages).

## Changing Topic or Selector

Notice that the server does not use the topic and message selector arguments to match a subscriber to an existing subscription. As a result, client programs can change a subscription by altering either or both of these arguments. The effect is equivalent to deleting the existing subscription (from the server) and creating a new one (albeit with the same client ID and subscription name).

## See Also

`tibemsTopic`

`tibemsMsgConsumer`

`tibemsConnection`

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