Managing Message Delivery

7 Managing Message Delivery

A Message Queue message is routed to its consumer clients by way of a physical destination on a message broker. The broker manages the memory and persistent storage associated with the physical destination and configures its behavior. The broker also manages memory at a system-wide level, to assure that sufficient resources are available to support all destinations.

Message delivery also involves the maintenance of state information needed by the broker to route messages to consumers and to track acknowledgements and transactions.

This chapter provides information needed to manage message delivery, and includes the following topics:

Configuring and Managing Physical Destinations
Managing Broker System-Wide Memory
Managing Durable Subscriptions
Managing Transactions

Configuring and Managing Physical Destinations

This section describes how to use the Message Queue Command utility (imqcmd) to manage physical destinations. It includes discussion of a specialized physical destination managed by the broker, the dead message queue, whose properties differ somewhat from those of other destinations.

Note

In a broker cluster, you create a physical destination on one broker and the cluster propagates it to all the others. Because the brokers cooperate to route messages across the cluster, client applications can consume messages from destinations on any broker in the cluster. However the persistence and acknowledgment of a message is managed only by the broker to which a message was originally produced.

This section covers the following topics regarding the management of physical destinations:

Command Utility Subcommands for Physical Destination Management
Creating and Destroying Physical Destinations
Pausing and Resuming a Physical Destination
Purging a Physical Destination
Updating Physical Destination Properties
Viewing Physical Destination Information
Managing Physical Destination Disk Utilization
Using the Dead Message Queue

Note

For provider independence and portability, client applications typically use destination administered objects to interact with physical destinations. Managing Administered Objects describes how to configure such administered objects for use by client applications. For a general conceptual introduction to physical destinations, see the Open Message Queue Technical Overview.

Command Utility Subcommands for Physical Destination Management

The Message Queue Command utility (imqcmd) enables you to manage physical destinations interactively from the command line. See Command Line Reference for general reference information about the syntax, subcommands, and options of the imqcmd command, and Physical Destination Property Reference for specific information on the configuration properties used to specify physical destination behavior.

Table 7-1 lists the imqcmd subcommands for physical destination management. For full reference information about these subcommands, see Table 16-7.

Table 7-1 Physical Destination Subcommands for the Command Utility

Subcommand Description

Subcommand	Description
`create dst`	Create physical destination
`destroy dst`	Destroy physical destination
`pause dst`	Pause message delivery for physical destination
`resume dst`	Resume message delivery for physical destination
`purge dst`	Purge all messages from physical destination
`compact dst`	Compact physical destination
`update dst`	Set physical destination properties
`list dst`	List physical destinations
`query dst`	List physical destination property values
`metrics dst`	Display physical destination metrics

create dst

Create physical destination

destroy dst

Destroy physical destination

pause dst

Pause message delivery for physical destination

resume dst

Resume message delivery for physical destination

purge dst

Purge all messages from physical destination

compact dst

Compact physical destination

update dst

Set physical destination properties

list dst

List physical destinations

query dst

List physical destination property values

metrics dst

Display physical destination metrics

Creating and Destroying Physical Destinations

The subcommand imqcmd create dst creates a new physical destination:

imqcmd create dst  -t destType  -n destName
                      [ [-o property=value] … ]

You supply the destination type (q for a queue or t for a topic) and the name of the destination.

Naming Destinations

Destination names must conform to the rules described below for queue and topic destinations.

Supported Queue Destination Names

Queue destination names must conform to the following rules:

It must contain only alphabetic characters (A-Z, a-z), digit characters (0-9), underscores (_), and dollar signs ($).
It must not contain spaces.
It must begin with an alphabetic character (A-Z, a-z), an underscore (_), or a dollar sign ($).
It must not begin with the characters mq.

For example, the following command creates a queue destination named XQueue:

imqcmd create dst  -t q  -n XQueue

Supported Topic Destination Names

Topic destination names must conform to the same rules as queue destinations, as specified in Supported Queue Destination Names, except that Message Queue also supports, in addition, topic destination names that include wildcard characters, representing multiple destinations. These symbolic names allow publishers to publish messages to multiple topics and subscribers to consume messages from multiple topics. Using symbolic names, you can create destinations, as needed, consistent with the wildcard naming scheme. Publishers and subscribers automatically publish to and consume from any added destinations that match the symbolic names. (Wildcard topic subscribers are more common than publishers.)

The format of a symbolic topic destination name consists of multiple segments, in which wildcard characters (*, **, >) can represent one or more segments of the name. For example, suppose you have a topic destination naming scheme as follows:

size.color.shape

where the topic name segments can have the following values:

size: large, medium, small, …
color: red, green, blue, …
shape: circle, triangle, square, …

Message Queue supports the following wildcard characters:

* matches a single segment
** matches one or more segments
> matches any number of segments and must not be proceeded by .

You can therefore indicate multiple topic destinations as follows:

large.*.circle would represent:

large.red.circle
large.green.circle
...

**.square would represent all names ending in .square, for example:

small.green.square
medium.blue.square
...

small> would represent the destination small and all destination names starting with small., for example:

small
small.blue.circle
small.red.square
...

It would not, for example, match smallyellow.

To use this multiple destination feature, you create topic destinations using a naming scheme similar to that described above. For example, the following command creates a topic destination named large.green.circle:

imqcmd create dst  -t t  -n large.green.circle

Client applications can then create wildcard publishers or wildcard consumers using symbolic destination names.

Note	Oracle recommends storing and looking up Topic objects in JNDI. It is the topic name which contains the wildcard, not the JNDI name

For example:

Example 7-1 Wildcard Publisher

...
String topicName = "large.*.circle";
Topic t = session.createTopic(topicName)
TopicPublisher myPublisher = mySession.createPublisher(t)
myPublisher.send(myMessage);
...

In this example, the broker will place a copy of the message in any destination that matches the symbolic name large.*.circle

Example 7-2 Wildcard Subscriber

...
String topicName = "**.square";
Topic t = session.createTopic(topicName);
TopicSubscriber mySubscriber = mySession.createSubscriber(t);
Message m = mySubscriber.receive();
...

In this example, a subscriber will be created if there is at least one destination that matches the symbolic name **.square and will receive messages from all destinations that match that symbolic name. If there are no destinations matching the symbolic name, the subscriber will not be registered with the broker until such a destination exists.

If you create additional destinations that match a symbolic name, then wildcard publishers created using that symbolic name will subsequently publish to that destination and wildcard subscribers created using that symbolic name will subsequently receive messages from that destination.

In addition, Message Queue administration tools, in addition to reporting the total number of publishers (producers) and subscribers (consumers) for a topic destination, will also report the number of publishers that are wildcard publishers (including their corresponding symbolic destination names) and the number of subscribers that are wildcard subscribers (including their symbolic destination names), if any. See Viewing Physical Destination Information.

Setting Property Values

The imqcmd create dst command may also optionally include any property values you wish to set for the destination, specified with the -o option. For example, the following command creates a topic destination named hotTopic with a maximum message length of 5000 bytes:

imqcmd create dst  -t t  -n hotTopic  -o maxBytesPerMsg=5000

See Physical Destination Property Reference for reference information about the physical destination properties that can be set with this option. (For auto-created destinations, you set default property values in the broker’s instance configuration file; see Table 17-3 for information on these properties.)

Destroying Destinations

To destroy a physical destination, use the imqcmd destroy dst subcommand:

imqcmd destroy dest  -t destType  -n destName

This purges all messages at the specified destination and removes it from the broker; the operation is not reversible.

For example, the following command destroys the queue destination named curlyQueue:

imqcmd destroy dest  -t q  -n curlyQueue  -u admin

Note	You cannot destroy the dead message queue.

Pausing and Resuming a Physical Destination

Pausing a physical destination temporarily suspends the delivery of messages from producers to the destination, from the destination to consumers, or both. This can be useful, for instance, to prevent destinations from being overwhelmed when messages are being produced much faster than they are consumed. You must also pause a physical destination before compacting it (see Managing Physical Destination Disk Utilization).

To pause the delivery of messages to or from a physical destination, use the imqcmd pause dst subcommand:

imqcmd pause dest  [-t destType  -n destName]
[-pst pauseType]

If you omit the destination type and name (-t and -n options), all physical destinations will be paused. The pause type (-pst) specifies what type of message delivery to pause:

PRODUCERS: Pause delivery from message producers to the destination
CONSUMERS: Pause delivery from the destination to message consumers
ALL: Pause all message delivery (both producers and consumers)

If no pause type is specified, all message delivery will be paused.

For example, the following command pauses delivery from message producers to the queue destination curlyQueue:

imqcmd pause dst  -t q  -n curlyQueue  -pst PRODUCERS  -u admin

The following command pauses delivery to message consumers from the topic destination hotTopic:

imqcmd pause dst  -t t  -n hotTopic  -pst CONSUMERS  -u admin

This command pauses all message delivery to and from all physical destinations:

imqcmd pause dst  -u admin

Note	In a broker cluster, since each broker in the cluster has its own instance of each physical destination, you must pause each such instance individually.

The imqcmd resume dst subcommand resumes delivery to a paused destination:

imqcmd resume dest  [-t destType  -n destName]

For example, the following command resumes message delivery to the queue destination curlyQueue:

imqcmd resume dst  -t q  -n curlyQueue  -u admin

If no destination type and name are specified, all destinations are resumed. This command resumes delivery to all physical destinations:

imqcmd resume dst  -u admin

Purging a Physical Destination

Purging a physical destination deletes all messages it is currently holding. You might want to do this when a destination’s accumulated messages are taking up too much of the system’s resources, such as when a queue is receiving messages but has no registered consumers to which to deliver them, or when a topic’s durable subscribers remain inactive for long periods of time.

To purge a physical destination, use the imqcmd purge dst subcommand:

imqcmd purge dst  -t destType  -n destName

For example, the following command purges all accumulated messages from the topic destination hotTopic:

imqcmd purge dst  -t t  -n hotTopic  -u admin

Note	In a broker cluster, since each broker in the cluster has its own instance of each physical destination, you must purge each such instance individually.

Tip	When restarting a broker that has been shut down, you can use the Broker utility’s `-reset messages` option to clear out its stale messages: for example, `imqbrokerd -reset messages -u admin` This saves you the trouble of purging physical destinations after restarting the broker.

Updating Physical Destination Properties

The subcommand imqcmd update dst changes the values of specified properties of a physical destination:

imqcmd update dst  -t destType  -n destName
-o property1=value1  [ [-o property2=value2] … ]

The properties to be updated can include any of those listed in Table 18-1 (with the exception of the isLocalOnly property, which cannot be changed once the destination has been created). For example, the following command changes the maxBytesPerMsg property of the queue destination curlyQueue to 1000 and the maxNumMsgs property to 2000:

imqcmd update dst  -t q  -n curlyQueue  -u admin
-o maxBytesPerMsg=1000
-o maxNumMsgs=2000

Note	The type of a physical destination is not an updatable property; you cannot use the `imqcmd` `update` `dst` subcommand to change a queue to a topic or a topic to a queue.

Viewing Physical Destination Information

To list the physical destinations on a broker, use the imqcmd list dst subcommand:

imqcmd list dst  -b hostName:portNumber  [-t destType][-tmp]

This lists all physical destinations on the broker identified by hostName and portNumber of the type (queue or topic) specified by destType. If the -t option is omitted, both queues and topics are listed. For example, the following command lists all physical destinations on the broker running on host myHost at port number 4545:

imqcmd list dst  -b myHost:4545

Note	The list of queue destinations always includes the dead message queue (`mq.sys.dmq`) in addition to any other queue destinations currently existing on the broker.

If you specify the -tmp option, temporary destinations are listed as well. These are destinations created by clients, normally for the purpose of receiving replies to messages sent to other clients.

The imqcmd query dst subcommand displays information about a single physical destination:

imq query dst  -t destType  -n destName

For example, the following command displays information about the queue destination curlyQueue:

imqcmd query dst  -t q  -n curlyQueue  -u admin

Example 7-3 shows an example of the resulting output. You can use the imqcmd update dst subcommand (see Updating Physical Destination Properties) to change the value of any of the properties listed.

Example 7-3 Physical Destination Information Listing

------------------------------------
Destination Name    Destination Type
------------------------------------
large.green.circle    Topic

On the broker specified by:

-------------------------
Host         Primary Port
-------------------------
localhost    7676

Destination Name                        large.green.circle
Destination Type                        Topic
Destination State                       RUNNING
Created Administratively                true

Current Number of Messages
   Actual                               0
   Remote                               0
   In Delay Delivery                    0
   Held in Transaction                  0
Current Message Bytes
   Actual                               0
   Remote                               0
   Held in Transaction                  0
Current Number of Producers             0
Current Number of Producer Wildcards    0
Current Number of Consumers             1
Current Number of Consumer Wildcards    1
   large.*.circle  (1)

Max Number of Messages                  unlimited (-1)
Max Total Message Bytes                 unlimited (-1)
Max Bytes per Message                   unlimited (-1)
Max Number of Producers                 100

Limit Behavior                          REJECT_NEWEST
Consumer Flow Limit                     1000
Is Local Destination                    false
Use Dead Message Queue                  true
XML schema validation enabled           false
XML schema URI List                     -
Reload XML schema on failure            false

For destinations in a broker cluster, it is often helpful to know how many messages in a destination are local (produced to the local broker) and how many are remote (produced to a remote broker). Hence, imqcmd query dst reports, in addition to the number and total message bytes of messages in the destination, the number and total bytes of messages that are sent to the destination from remote brokers in the cluster.

For topic destinations, imqcmd query dst reports the number of publishers that are wildcard publishers (including their corresponding symbolic destination names) and the number of subscribers that are wildcard subscribers (including their symbolic destination names), if any.

To display metrics information about a physical destination, use the imqcmd metrics dst subcommand:

imqcmd metrics dst   -t destType  -n destName
[-m metricType]
[-int interval]
[-msp numSamples]

The -m option specifies the type of metric information to display:

ttl (default): Messages and packets flowing into and out of the destination and residing in memory
rts: Rate of flow of messages and packets into and out of the destination per second, along with other rate information
con: Metrics related to message consumers
dsk: Disk usage

The -int and -msp options specify, respectively, the interval (in seconds) at which to display the metrics and the number of samples to display in the output. The default values are 5 seconds and an unlimited number of samples.

For example, the following command displays cumulative totals for messages and packets handled by the queue destination curlyQueue:

imqcmd metrics dst  -t q  -n curlyQueue  -m ttl  -u admin

Example 7-4 shows an example of the resulting output.

Example 7-4 Physical Destination Metrics Listing

-----------------------------------------------------------------------------------
   Msgs        Msg Bytes          Msg Count         Total Msg Bytes (k)     Largest
 In   Out     In       Out    Current  Peak  Avg  Current   Peak     Avg    Msg (k)
-----------------------------------------------------------------------------------
3128  3066  1170102  1122340    128    409   29     46      145      10       < 1
4858  4225  1863159  1635458    144    201   33     53      181      42       < 1
2057  1763   820804   747200     84    377   16     71      122      79       < 1

For a more detailed description of the use of the Command utility to report physical destination metrics, see Physical Destination Metrics.

Managing Physical Destination Disk Utilization

Because of the way message storage is structured in a file-based persistent data store (see File-Based Persistence Properties), disk space can become fragmented over time, resulting in inefficient utilization of the available resources. Message Queue’s Command utility (imqcmd) provides subcommands for monitoring disk utilization by physical destinations and for reclaiming unused disk space when utilization drops.

To monitor a physical destination’s disk utilization, use the imqcmd metrics dst subcommand:

imqcmd metrics dst  -m dsk  -t destType  -n destMame

This displays the total number of bytes of disk space reserved for the destination’s use, the number of bytes currently in use to hold active messages, and the percentage of available space in use (the disk utilization ratio). For example, the following command displays disk utilization information for the queue destination curlyQueue:

imqcmd metrics dst  -m dsk  -t q  -n curlyQueue  -u admin

Example 7-5 shows an example of the resulting output.

Example 7-5 Destination Disk Utilization Listing

--------------------------------------
Reserved   Used      Utilization Ratio
--------------------------------------
804096     675533    84
1793024    1636222   91
2544640    2243808   88

The disk utilization pattern depends on the characteristics of the messaging application using a particular physical destination. Depending on the flow of messages into and out of the destination and their relative size, the amount of disk space reserved might grow over time. If messages are produced at a higher rate than they are consumed, free records should generally be reused and the utilization ratio should be on the high side. By contrast, if the rate of message production is comparable to or lower than the consumption rate, the utilization ratio will likely be low.

As a rule, you want the reserved disk space to stabilize and the utilization ratio to remain high. If the system reaches a steady state in which the amount of reserved disk space remains more or less constant with utilization above 75%, there is generally no need to reclaim unused disk space. If the reserved space stabilizes at a utilization rate below 50%, you can use the imqcmd compact dst subcommand to reclaim the disk space occupied by free records:

compact dst  [-t destType  -n destName]

This compacts the file-based data store for the designated physical destination. If no destination type and name are specified, all physical destinations are compacted.

You must pause a destination (with the imqcmd pause subcommand) before compacting it, and resume it (with imqcmd resume) afterward (see Pausing and Resuming a Physical Destination):

imqcmd pause dst    -t q  -n curlyQueue  -u admin
imqcmd compact dst  -t q  -n curlyQueue  -u admin
imqcmd resume dst   -t q  -n curlyQueue  -u admin

Tip	If a destination’s reserved disk space continues to increase over time, try reconfiguring its `maxNumMsgs`, `maxBytesPerMsg`, `maxTotalMsgBytes`, and `limitBehavior` properties (see Physical Destination Properties).

Using the Dead Message Queue

The dead message queue, mq.sys.dmq, is a system-created physical destination that holds the dead messages of a broker’s physical destinations. The dead message queue is a tool for monitoring, tuning system efficiency, and troubleshooting. For a definition of the term dead message and a more detailed introduction to the dead message queue, see the Open Message Queue Technical Overview.

The broker automatically creates a dead message queue when it starts. The broker places messages on the queue if it cannot process them or if their time-to-live has expired. In addition, other physical destinations can use the dead message queue to hold discarded messages. This can provide information that is useful for troubleshooting the system.

Managing the Dead Message Queue

The physical destination configuration property useDMQ controls a destination’s use of the dead message queue. Physical destinations are configured to use the dead message queue by default; to disable a destination from using it, set the destination’s useDMQ property to false:

imqcmd update dst  -t q  -n curlyQueue  -o useDMQ=false

You can enable or disable the use of the dead message queue for all auto-created physical destinations on a broker by setting the broker’s imq.autocreate.destination.useDMQ broker property:

imqcmd update bkr  -o imq.autocreate.destination.useDMQ=false

You can manage the dead message queue with the Message Queue Command utility (imqcmd) just as you manage other queues, but with some differences. For example, because the dead message queue is system-created, you cannot create, pause, or destroy it. Also, as shown in Table 7-2, default values for the dead message queue’s configuration properties sometimes differ from those of ordinary queues.

Table 7-2 Dead Message Queue Treatment of Physical Destination Properties

Property Variant Treatment by Dead Message Queue

Property	Variant Treatment by Dead Message Queue
`maxNumMsgs`	Default value is `1000`, rather than `-1` (unlimited) as for ordinary queues.
`maxTotalMsgBytes`	Default value is `10m` (10 megabytes), rather than `-1` (unlimited) as for ordinary queues.
`limitBehavior`	Default value is `REMOVE_OLDEST`, rather than `REJECT_NEWEST` as for ordinary queues. `FLOW_CONTROL` is not supported for the dead message queue.
`maxNumProducers`	Does not apply to the dead message queue.
`isLocalOnly`	Permanently set to `false` in broker clusters; the dead message queue in a cluster is always a global physical destination.
`localDeliveryPreferred`	Does not apply to the dead message queue.

maxNumMsgs

Default value is 1000, rather than -1 (unlimited) as for ordinary queues.

maxTotalMsgBytes

Default value is 10m (10 megabytes), rather than -1 (unlimited) as for ordinary queues.

limitBehavior

Default value is REMOVE_OLDEST, rather than REJECT_NEWEST as for ordinary queues.

FLOW_CONTROL is not supported for the dead message queue.

maxNumProducers

Does not apply to the dead message queue.

isLocalOnly

Permanently set to false in broker clusters; the dead message queue in a cluster is always a global physical destination.

localDeliveryPreferred

Does not apply to the dead message queue.

Tip

By default, the dead message queue stores entire messages. If you do not plan to restore dead messages, you can reduce the size of the dead message queue by setting the broker’s imq.destination.DMQ.truncateBody property to true:

imqcmd update bkr  -o imq.destination.DMQ.truncateBody=true

This will discard the body of all messages and retain only the headers and property data.

Enabling Dead Message Logging

The broker configuration property logDeadMsgs controls the logging of events related to the dead message queue. When dead message logging is enabled, the broker will log the following events:

A message is moved to the dead message queue.
A message is discarded from the dead message queue (or from any physical destination that does not use the dead message queue).
A physical destination reaches its limits.

Dead message logging is disabled by default. The following command enables it:

imqcmd update bkr  -o imq.destination.logDeadMsgs=true

Note	Dead message logging applies to all physical destinations that use the dead message queue. You cannot enable or disable logging for an individual physical destination.

Managing Broker System-Wide Memory

Once clients are connected to the broker, the routing and delivery of messages can proceed. In this phase, the broker is responsible for creating and managing different types of physical destinations, ensuring a smooth flow of messages, and using resources efficiently. You can use the broker configuration properties described under Routing and Delivery Properties to manage these tasks in a way that suits your application’s needs.

The performance and stability of a broker depend on the system resources (such as memory) available and how efficiently they are utilized. You can set configuration properties to prevent the broker from becoming overwhelmed by incoming messages or running out of memory. These properties function at three different levels to keep the message service operating as resources become scarce:

Systemwide message limits apply collectively to all physical destinations on the system. These include the maximum number of messages held by a broker (imq.system.max_count) and the maximum total number of bytes occupied by such messages (imq.system.max_size). If either of these limits is reached, the broker will reject any new messages until the pending messages fall below the limit. There is also a limit on the maximum size of an individual message (imq.message.max_size) and a time interval at which expired messages are removed (imq.message.expiration.interval).
Individual destination limits regulate the flow of messages to a specific physical destination. The configuration properties controlling these limits are described in Physical Destination Property Reference. They include limits on the number and size of messages the destination will hold, the number of message producers and consumers that can be created for it, and the number of messages that can be batched together for delivery to the destination.
The destination can be configured to respond to memory limits by slowing down the delivery of message by message producers, by rejecting new incoming messages, or by throwing out the oldest or lowest-priority existing messages. Messages deleted from the destination in this way may optionally be moved to the dead message queue rather than discarded outright; the broker property imq.destination.DMQ.truncateBody controls whether the entire message body is saved in the dead message queue, or only the header and property data.
As a convenience during application development and testing, you can configure a message broker to create new physical destinations automatically whenever a message producer or consumer attempts to access a nonexistent destination. The broker properties summarized in Table 17-3 parallel the ones just described, but apply to such auto-created destinations instead of administratively created ones.
System memory thresholds define levels of memory usage at which the broker takes increasingly serious action to prevent memory overload. Four such usage levels are defined:
- Green: Plenty of memory is available.
- Yellow: Broker memory is beginning to run low.
- Orange: The broker is low on memory.
- Red: The broker is out of memory.
  The memory utilization percentages defining these levels are specified by the broker properties imq.green.threshold, imq.yellow.threshold , imq.orange.threshold, and imq.red.threshold , respectively; the default values are 0% for green, 80% for yellow, 90% for orange, and 98% for red.
  As memory usage advances from one level to the next, the broker responds progressively, first by swapping messages out of active memory into persistent storage and then by throttling back producers of nonpersistent messages, eventually stopping the flow of messages into the broker. (Both of these measures degrade broker performance.) The throttling back of message production is done by limiting the size of each batch delivered to the number of messages specified by the properties imq.`resourceState `.count, where resourceState is green , yellow, orange, or red , respectively.

The triggering of these system memory thresholds is a sign that systemwide and destination message limits are set too high. Because the memory thresholds cannot always catch potential memory overloads in time, you should not rely on them to control memory usage, but rather reconfigure the system-wide and destination limits to optimize memory resources.

Managing Durable Subscriptions

Message Queue clients subscribing to a topic destination can register as durable subscribers. The corresponding durable subscription has a unique, persistent identity and requires the broker to retain messages addressed to it even when its message consumer (the durable subscriber) becomes inactive. Ordinarily, the broker may delete a message held for a durable subscriber only when the message expires.

The Message Queue Command utility provides subcommands for managing a broker’s durable subscriptions in the following ways:

Listing durable subscriptions
Purging all messages for a durable subscription
Destroying a durable subscription

To list durable subscriptions for a specified physical destination, use the imqcmd list dur subcommand:

imqcmd list dur  -d topicName

For example, the following command lists all durable subscriptions to the topic SPQuotes on the default broker (host localhost at port 7676):

imqcmd list dur  -d SPQuotes

The resulting output lists the name of each durable subscription to the topic, the client identifier to which it belongs, its current state (active or inactive), and the number of messages currently queued to it. Example 7-6 shows an example.

Example 7-6 Durable Subscription Information Listing

Name        Client ID       Number of   Durable Sub
                            Messages      State
----------------------------------------------------------------
myDurable   myClientID       1           INACTIVE

The imqcmd purge dur subcommand purges all messages for a specified durable subscriber and client identifier:

imqcmd purge dur  -n subscriberName
                     -c clientID

For example, the following command purges all messages for the durable subscription listed in Example 7-6:

imqcmd purge dur  -n myCurable  -c myClientID

The imqcmd destroy dur subcommand destroys a durable subscription, specified by its subscriber name and client identifier:

imqcmd destroy dur  -n subscriberName
                       -c clientID

For example, the following command destroys the durable subscription listed in Example 7-6:

imqcmd destroy dur  -n myCurable  -c myClientID

Managing Transactions

All transactions initiated by client applications are tracked by the broker. These can be local Message Queue transactions or distributed transactions managed by a distributed transaction manager.

Each transaction is identified by a unique 64-bit Message Queue transaction identifier. Distributed transactions also have a distributed transaction identifier (XID), up to 128 bytes long, assigned by the distributed transaction manager. Message Queue maintains the association between its own transaction identifiers and the corresponding XIDs.

The imqcmd list txn subcommand lists the transactions being tracked by a broker:

imqcmd list txn

This lists all transactions on the broker, both local and distributed. For each transaction, it shows the transaction ID, state, user name, number of messages and acknowledgments, and creation time. Example 7-7 shows an example of the resulting output.

Example 7-7 Broker Transactions Listing

---------------------------------------------------------------
Transaction ID  State    User name   # Msgs/   Creation time
                                     # Acks
---------------------------------------------------------------

64248349708800  PREPARED  guest      4/0      1/30/02 10:08:31 AM
64248371287808  PREPARED  guest      0/4      1/30/02 10:09:55 AM

To display detailed information about a single transaction, obtain the transaction identifier from imqcmd list txn and pass it to the imqcmd query txn subcommand:

imqcmd query txn  -n transactionID

This displays the same information as imqcmd list txn, along with the client identifier, connection identification, and distributed transaction identifier (XID). For example, the command

imqcmd query txn  -n 64248349708800

produces output like that shown in Example 7-8.

Example 7-8 Transaction Information Listing

Client ID
Connection                 guest@192.18.116.219:62209->jms:62195
Creation time              1/30/02 10:08:31 AM
Number of acknowledgments  0
Number of messages         4
State                      PREPARED
Transaction ID             64248349708800
User name                  guest
XID                        6469706F6C7369646577696E6465723130313234313431313030373230

If a broker fails, it is possible that a distributed transaction could be left in the PREPARED state without ever having been committed. Until such a transaction is committed, its messages will not be delivered and its acknowledgments will not be processed. Hence, as an administrator, you might need to monitor such transactions and commit them or roll them back manually. For example, if the broker’s imq.transaction.autorollback property (see Table 17-2) is set to false, you must manually commit or roll back non-distributed transactions and unrecoverable distributed transactions found in the PREPARED state at broker startup, using the Command utility’s commit txn or rollback txn subcommand:

imqcmd commit txn  -n transactionID
imqcmd rollback txn  -n transactionID

For example, the following command commits the transaction listed in Example 7-8:

imqcmd commit txn  -n64248349708800

Note

Only transactions in the PREPARED state can be committed . However, transaction in the STARTED, FAILED, INCOMPLETE, COMPLETE, and PREPARED states can be rolled back. You should do so only if you know that the transaction has been left in this state by a failure and is not in the process of being committed by the distributed transaction manager.