Using Selectors

Selectors enable clients to specify which messages that they are interested in receiving, as determined by the messages’ header field and property values. A selector is a string up to a maximum of 2,000 bytes that uses a conditional expression syntax that is a subset of SQL92. For detailed information on message selector syntax, refer to the Java Message Service Specification – Version 1.1.

When a selector is used, a client only receives a message if the selector evaluates to true when the message’s header field and property values are substituted for their corresponding identifiers in the selector. The event broker filters out messages that do not match.

Selectors can be set for:

Flows bound to a queue or topic endpoint. Refer to Setting Selectors on Endpoints.
Browsers or browser flows. Refer to Browsing Guaranteed Messages.

Performance Considerations When Using Selectors

While selectors are fully supported, the best performance is typically achieved using topic matching without selectors. In many cases, it’s possible to include the message properties that would be selected upon within the topic hierarchy. Topic subscriptions and topic subscription exceptions can then be used to match against the relevant levels within the topic to implement selector expression-like matching.

Topic subscriptions and topic subscription exceptions support wildcards and prefix matching within levels. With wildcards, exceptions, and the ability to specify multiple subscriptions or exceptions for a queue or topic endpoint, it’s possible to implement matching operations like AND, OR, and NOT.

Consideration must also be given to performance when using selectors to retrieve messages from deep queues. The Solace event broker needs to retrieve each of the queue’s messages, potentially from disk, to evaluate its contents against the consumer’s selector expression. If the queue is very deep, it may take several seconds before the first matching message is found. Large messages also take longer to retrieve and evaluate against selector expressions. These challenges are eliminated when using topic subscriptions for filtering, because the filtering occurs before the event broker adds the messages into the queues.

In general, you should avoid using selectors whenever possible

Selector Identifier Mapping between JMS and Solace APIs

The following table lists standard JMS identifiers and their equivalent Solace identifiers. Solace identifiers are encoded in structured data maps of Solace SMF messages as reserved keys. Both types of identifiers can be used to define selectors.

For example, JMSCorrelatonID is encoded in the map with the key ci. If you are using the JMS API and you want to match only messages where the correlation ID, which is a string, is "myCorrelationId", you could write a selector expression in JMS like this:

JMSCorrelationID = 'myCorrelationId'

If you are using a Solace messaging API, you could write the same expression like this:

ci = 'myCorrelatonId'

JMS may have no set methods for some Solace identifiers, such as message sequence number (sn), but it is still possible to select on the sn identifier.

Selector Identifiers
	JMS Identifier	Solace Identifier	Relevant Solace Message Property
Message Header Fields	JMSCorrelationID	ci	CorrelationId
	JMSDeliveryMode	NA	DeliveryMode
	JMSMessageID	mi	ApplicationMessageId
	JMSPriority	NA	Priority
	JMSTimestamp	ts	SenderTimestamp
	JMSType	mt	ApplicationMessageType
	NA	si	SenderID
	NA	sn	SequenceNo
	NA	ex	Expiration
	NA	ct	HttpContentType
	NA	ce	HttpContentEncoding
JMS-Defined Properties	JMSXGroupSeq	Treated as user‑specified	Solace messaging APIs can access JMSX properties as user properties with the same property name as used in JMS.
	JMSXGroupID	Treated as user‑specified
	JMSXUserID	`<client-username>` The JMSXUserID property is only available when using Basic client authentication and if the `jmsx-user-id-enabled` Connection Factory property is enabled.
User-Defined Message Properties	User‑specified	User‑specified	NA

Setting Selectors on Endpoints

A selector can be set on a Flow that binds to either a queue or a topic endpoint. A selector functions differently on each endpoint type:

Queue
Selectors act as egress filters on queues.
When a flow binds to an exclusive queue, the selector defined for the flow filters the messages to be consumed by the client. Non-selected messages remain in the queue and can be consumed by clients on subsequent flows with different selectors that might bind.
When a flow binds to a non-exclusive queue, messages are consumed by bound clients in a round-robin fashion according to their selectors. Once a message is consumed, it is no longer available for subsequent clients.
Topic endpoint
Selectors act as ingress filters on topic endpoints.
When a flow binds to a topic endpoint, messages that match the topic subscription are received by the topic endpoint. When a selector is defined for the flow, messages whose message header fields and properties do not match the corresponding identifiers defined in the selector are not queued to the topic endpoint.
If a flow disconnects from a durable topic endpoint, and the selector is changed, all existing messages are removed when the flow binds again. To prevent removing messages for a topic endpoint when rebinding, the selector used must be an identical string to the one used in the previous bind.

To Set Selectors on a Flow
PubSub+ Messaging API	Use
JCSMP	ConsumerFlowProperties.setSelector(...)
Java RTO	FlowHandle.PROPERTIES.SELECTOR
C	SOLCLIENT_FLOW_PROP_SELECTOR
.NET	FlowProperties.Selector
JavaScript and Node.js	Not applicable

Related Samples

For an example of how to set selectors for a flow, refer to the messageSelectorsOnQueue sample for the appropriate messaging API.

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