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:

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

JMSReplyTo

rt

ReplyTo

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.