Using Rule Actions in Windows Azure Service Bus Subscriptions

Setting rule actions in Service Bus Subscriptions is a useful mechanism for automatically modify properties of BrokeredMessages when they are sent to a Topic.

The easiest way for assigning a rule action to a subscription is to do it when the subscription is being created:

namespaceManager.CreateSubscription(
    topicName, subscriptionName,
    new RuleDescription { Action = new SqlRuleAction("set dcount=[sys].DeliveryCount") });

Here we are setting the value of the DeliveryCount property of the BrokeredMessage object to a property in the header of the message (these properties can be used to route messages to specific subscriptions).

If we need to assign multiple rule actions to a single subscription, we have to create the subscription and afterwards create the rules and assign them to the subscription:

namespaceManager.CreateSubscription(topicName, subscriptionName);

var deliveryCountRule = new RuleDescription()
{
    Action = new SqlRuleAction("set dcount=[sys].DeliveryCount"),
Name = "DeliveryCountRule"
};

var sizeRule = new RuleDescription()
{
    Action = new SqlRuleAction("set sz=[sys].Size"),
Name = "SizeRule"
};

var subscriptionClient = messagingFactory.CreateSubscriptionClient(
    topicName, subscriptionName);

subscriptionClient.AddRule(deliveryCountRule);
subscriptionClient.AddRule(sizeRule);

We can combine rule actions and filters in order to set property values based on conditions:

namespaceManager.CreateSubscription(topicName, subscriptionName);

var ruleLowPrice = new RuleDescription()
{
    Action = new SqlRuleAction("set PriceRange='Low'"),
    Filter = new SqlFilter("TotalCost < 100"),
    Name = "LowPrice"
};

var ruleMediumPrice = new RuleDescription()
{
    Action = new SqlRuleAction("set PriceRange='Medium'"),
    Filter = new SqlFilter("TotalCost >= 100 AND TotalCost < 500"),
    Name = "MediumPrice"
};

var ruleHighPrice = new RuleDescription()
{
    Action = new SqlRuleAction("set PriceRange='High'"),
    Filter = new SqlFilter("TotalCost >= 500"),
    Name = "HighPrice"
};

var subscriptionClient = messagingFactory.CreateSubscriptionClient(
    topicName, subscriptionName);

subscriptionClient.AddRule(ruleLowPrice);
subscriptionClient.AddRule(ruleMediumPrice);
subscriptionClient.AddRule(ruleHighPrice);

We’re almost there… The last piece of code should work, except for the fact that when the subscription is created using the CreateSubscription method, a default rule with a TrueFilter is added, which means that every message will show up in that subscription.

To avoid that, we should remove existing rules before adding ours.

var rules = namespaceManager.GetRules(topicName, subscriptionName);

...

var subscriptionClient = messagingFactory.CreateSubscriptionClient(
    topicName, subscriptionName);

foreach (var rule in rules)
    subscriptionClient.RemoveRule(rule.Name);

subscriptionClient.AddRule(ruleLowPrice);
subscriptionClient.AddRule(ruleMediumPrice);
subscriptionClient.AddRule(ruleHighPrice);

...

For more detailed information about the expressions that can be used when defining rule actions, see http://msdn.microsoft.com/en-us/library/microsoft.servicebus.messaging.sqlruleaction.sqlexpression.aspx

Tags: Service Bus, Windows Azure