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:
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.
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:
For provider independence and portability, client applications typically use destination administered objects to interact with physical destinations. Chapter 11, 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 Message Queue Technical Overview.
The Message Queue Command utility (imqcmd) enables you to manage physical destinations interactively from the command line. See Chapter 15, Command Line Reference for general reference information about the syntax, subcommands, and options of the imqcmd command, and Chapter 17, Physical Destination Property Reference for specific information on the configuration properties used to specify physical destination behavior.Table 7–1 Physical Destination Subcommands for the Command Utility
Create physical destination
Destroy physical destination
Pause message delivery for physical destination
Resume message delivery for physical destination
Purge all messages from physical destination
Compact physical destination
Set physical destination properties
List physical destinations
List physical destination property values
Display physical destination metrics
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.
Queue destination names must conform to the following rules:
It must contain only alphanumeric characters.
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
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:
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 successive segments
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 all destination names starting with small., for example:
small.blue.circle small.red.square ...
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, as shown in the following examples:
... String DEST_LOOKUP_NAME = "large.*.circle"; Topic t = (Destination) ctx.lookup(DEST_LOOKUP_NAME); 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
... String DEST_LOOKUP_NAME = "**.square"; Topic t = (Destination) ctx.lookup(DEST_LOOKUP_NAME); 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.
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 Chapter 17, 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 16–3 for information on these properties.)
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
You cannot destroy the dead message queue.
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:
Pause delivery from message producers to the destination
Pause delivery from the destination to message consumers
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
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 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
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.
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 17–1 (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
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.
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
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.
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
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.
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.
For a more detailed description of the use of the Command utility to report physical destination metrics, see Physical Destination Metrics.
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.
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
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).
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 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.
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
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
Variant Treatment by Dead Message Queue
Default value is 1000, rather than -1 (unlimited) as for ordinary queues.
Default value is 10m (10 megabytes), rather than -1 (unlimited) as for ordinary queues.
Default value is REMOVE_OLDEST, rather than REJECT_NEWEST as for ordinary queues.
FLOW_CONTROL is not supported for the dead message queue.
Does not apply to the dead message queue.
Permanently set to false in broker clusters; the dead message queue in a cluster is always a global physical destination.
Does not apply to the dead message queue.
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.
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
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.
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 reclaimed (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 Chapter 17, 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 16–3 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.
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
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.
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
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
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.
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.
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 16–2) 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
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.