Replacing Subscriptions
AMPS provides the ability to perform atomic subscription replacement. This allows you to replace the filter, change the topic, or update the options for a subscription.
The most common use for this capability is for an application to change the filter for a subscription. For example, a GUI that is providing a view of a set of orders may need to add or remove an order from the set of orders being displayed. By replacing the content filter with a filter that tracks the updated set of orders, the application can do this without missing messages, getting duplicate messages, or having to manage more than one subscription.
Replacing a filter is an atomic operation. That is, the application is guaranteed not to miss messages that are in both the original and replacement subscription, and is guaranteed to receive all messages for the new subscription as of the point at which the replacement happens.
To replace a subscription, applications re-submit the subscription using the subscription ID of the previous subscription. See the Developer Guide of the client library you are using and the AMPS Command Reference for details.
When replacing a sow_and_subscribe command (described later in the guide), AMPS runs the SOW command again and provides any messages that were not previously in the result set to the application. See the section called Replacing Subscriptions with SOW and Subscribe for details.
Notice that some options on an initial subscription limit the support for replace on a subscription. In those cases, the limitation is described when the option is described.
Replacing the Content Filter on a Subscription
AMPS allows you to replace the content filter on an existing subscription. When this happens, AMPS begins sending messages on the subscription that match the new filter. When an application needs to bring more messages into scope, this can be more efficient than creating another subscription.
For example, an application might start off with a filter such as the following:
The application might then need to bring other regions into scope, for example:
Replacing the Topic on a Subscription
AMPS allows a subscription to replace the topic on a subscription. When the topic is replaced, AMPS re-evaluates the subscription as it does when a filter is replaced. If the subscription is updated to include a topic that the user does not have permission to subscribe to, the replace operation succeeds, but no messages will be delivered on that topic.
Replacing the Options on a Subscription
AMPS allows a subscription to replace some of the options on the subscription. In this case, the subscription is evaluated as though the topic or filter has been replaced. Any new messages generated after the subscription is replaced use the new options. However, AMPS does not replay or re-query previous messages to apply the options.
For example, if a sow_and_subscribe command did not previously specify Out-of-Focus tracking and adds this option, AMPS generates the appropriate Out-of-Focus messages from the replace point forward. AMPS does not recreate Out-of-Focus messages that would have previously been generated by the subscription.
If the subscription uses pagination (see Managing Result Sets), the replacement must contain the full set of pagination options provided on the original subscription. For a paginated subscription, the replacement may not change the topic of the subscription. Instead, close the existing subscription and create a new subscription with a different topic.
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