Pub/Sub Topics
A Topic contains the message itself, along with additional subscriber and publisher information. The ITopic/Topic
interface in Pub/Sub facilitates to create a topic, set topic priority while creation, get a topic, delete a topic simply, and asynchronously.
Note
This feature is also available in NCache Professional.
Prerequisites
- To learn about the standard prerequisites required to work with all NCache client-side features including Pub/Sub, please refer to the given page on Client-Side API Prerequisites.
- For API details, refer to: ICache, CacheItem, ITopic, CreateTopic, GetTopic, DeleteTopic, TopicPriority, DeleteTopicAsync.
Create Topic
The CreateTopic method creates a Topic in the cache with a specified name. If the Topic already exists, an instance of the Topic is returned as ITopic
. Whenever a message is published on a Topic, it is delivered based on message preference to subscribers who are registered on that Topic. The following example creates a Topic orderTopic.
// Precondition: Cache is already connected
// Mention the name of the topic
string topicName = "orderTopic";
// Create the topic
ITopic topic = cache.MessagingService.CreateTopic(topicName);
Note
To ensure the operation is fail-safe, it is recommended to handle any potential exceptions within your application, as explained in Handling Failures.
Create Topic with Priority
Note
This feature is only available in NCache 5.2 and onwards.
NCache Pub/Sub allows you to set topic priority to prioritize the delivery of critical messages. Priority is specified using the TopicPriority
property as Low
, Normal
, and High
. In case eviction is enabled, the topics with the least priority are evicted first, and the ones with the high priority are evicted last. Topics created with no priority specified will be created with the default priority, i.e., Normal.
Important
Priority of a topic can be specified at the time of topic creation only and cannot be modified afterwards.
The following example creates a topic, orderTopic, with priority as high to avoid early eviction (if eviction is enabled).
// Mention the name of the topic
string topicName = "orderTopic";
// Create the topic with priority
ITopic topic = cache.MessagingService.CreateTopic(topicName, Alachisoft.NCache.Runtime.Messaging.TopicPriority.High);
Get Topic
The GetTopic method fetches an instance of the specified topic from the cache. If the topic exists, it is returned, otherwise an exception is thrown. The following example gets an existing topic orderTopic from the cache.
// Mention the name of the topic
string topicName = "orderTopic";
// Get the topic from the cache
ITopic orderTopic = cache.MessagingService.GetTopic(topicName);
// Verify successful topic retrieval
if (orderTopic != null)
{
// orderTopic will be used for receiving and/or publishing messages
}
else
{
// No topic exists
}
Delete Topic
The DeleteTopic method unregisters the topic from the cache and removes all messages associated with that Topic. If registered, a Topic deletion callback OnTopicDeleted
will be triggered upon this method call. The following example deletes the existing Topic orderTopic from the cache and removes all messages associated with that Topic. If the OnTopicDeleted
callback is registered, it will be triggered upon this method call.
// Define the topic to be deleted
string topicName = "orderTopic";
// Delete the topic "orderTopic"
cache.MessagingService.DeleteTopic(topicName);
// Callback will be triggered if registered
Delete Topic Asynchronously
The DeleteTopicAsync method deletes the Topic asynchronously. Whenever a Topic is deleted asynchronously, a Task
is returned to the user for performing further tasks without waiting for the Topic to be deleted. The following example shows the asynchronous deletion of the Topic orderTopic.
// Define the topic to be deleted
string topicName = "orderTopic";
// Delete the topic "orderTopic"
Task task = cache.MessagingService.DeleteTopicAsync(topicName);
// Use task to perform further operations according to business logic
// Callback will be triggered if registered
Properties of ITopic Interface
Members | Type | Description |
---|---|---|
MessageDeliveryFailure |
MessageDeliveryFailureCallback |
Event on the Topic so that publisher receives all failed messages that are not delivered to any subscriber or maybe messages are expired or evicted before delivery. |
Name |
string |
Name of the Topic specified during Topic creation. |
OnTopicDeleted |
TopicDeletedCallback |
Event to handle Topic deletion by publisher and subscriber. |
ExpirationTime |
TimeSpan |
If message level expiration is not provided, then this Topic level expiration expires messages in the Topic by default. The value is TimeSpan.MaxValue by default. |
IsClosed |
bool |
Check whether Topic is disposed, before performing any operation. |
Dispose |
IDisposable |
Removes registered Topic's subscription from cache server. |
Additional Resources
NCache provides a sample application for Pub/Sub on GitHub.
See Also
.NET: Alachisoft.NCache.Runtime.Caching namespace.
Java: com.alachisoft.ncache.runtime.caching namespace.
Node.js: Topic class.
Python: ncache.client.services class.