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Ems
- 1. EMS 5.1 TIBCO’s Enterprise Messaging Offering
- 3. JMS Message Models • Point-to-Point (Queues) • Publish Subscribe (Topics) • Multicast (Topics)
- 4. Point-to-Point(Queues) TIBCO EMS Server Send Message Receive Message Acknowledge
- 5. Publish-Subscribe(Topics) TIBCO EMS Server Publish Message Deliver Message Acknowledge Subscribe to Topic
- 6. Multicast(Topics) TIBCO EMS Server Publish Message Broadcast Message Subscribe to Message
- 7. EMS : An extension to JMS
- 8. EMS Extensions to JMS Messages - I • JMS provides 2 delivery modes for messages • PERSISTENT • NON_PERSISTENT • EMS adds the 3rd delivery mode • RELIABLE_DELIVERY
- 9. EMS Extensions to JMS Messages - II • For restriction of acknowledge messages in JMS • NO_ACKNOWLEDGE mode • To restrict acknowledgement in EMS, there are also • EXPLICIT_CLIENT_ACKNOWLEDGE mode • EXPLICIT_CLIENT_DUPS_OK_ACKNOWLEDGE mode
- 10. EMS Extensions to JMS Messages - III • EMS extends MapMessage and StreamMessage body types of JMS which allow EMS to exchange messages with TIBCO RV and ActiveEnterprise formats. • The extensions are : • Can nest a MapMessage or StreamMessage • Can use arrays as well as primitive types for values
- 13. Message Headers • JMSDestination • Destination to which message is sent. • JMSDeliveryMode • Persistent, Non-persistent or Reliable. • Default is Persistent. • JMSExpiration • Length of time the message will live before expiry. • If the server expiration property is set for a destination, it will override the JMSExpiration value set by the message producer.
- 14. Message Headers • JMSPriority • Priority of the message. Larger numbers indicate higher priority. • JMSMessageID • Unique identifier for a message. • JMSTimeStamp • Timestamp of the time when the message was handed off to a provider to send. Message may actually be sent later than this timestamp.
- 15. Message Headers • JMSCorrelationID • This ID can be used to link messages, such as linking a response message to a request message. • JMSReplyTo • A destination to which a message reply should be sent. • JMSRedelivered • If this field is set, it is possible that the message has been delivered to the client earlier, but not acknowledged at that time.
- 16. Message Properties • JMS_TIBCO_CM_PUBLISHER • JMS_TIBCO_CM_SEQUENCE • JMS_TIBCO_COMPRESS • JMS_TIBCO_DISABLE_SENDER • JMS_TIBCO_IMPORTED • JMS_TIBCO_MSG_TRACE • JMS_TIBCO_MSG_EXT • JMS_TIBCO_SENDER • JMS_TIBCO_SS_SENDER • JMS_TIBCO_PRESERVE_UNDELIVERED
- 17. Undelivered Message Queue Message Expiry Queue 2 ORJMS_TIBCO_PRESERVE_UNDELIVERED = TRUE BODY PROPERTIES HEADER SERVER Undelivered message queue $sys.undelivered JMS_TIBCO_PRESERVE_UNDELIVERED = FALSE BODY PROPERTIES HEADER SERVER
- 18. Message Body MESSAGE TYPE CONTENTS OF BODY MESSAGE Message No Body Text Message Java.lang.String Map Message Name/Value pairs Bytes Message Stream of bytes Stream Message Stream of primitive data types Object Message Serializable object “EMS supports messages up to a maximum size of 512MB”
- 20. Persistent • When a producer sends a PERSISTENT message, the producer must wait for the server to reply with a confirmation. • The message is persisted on disk by the server. This delivery mode ensures delivery of messages to the destination on the server in almost all circumstances. • However, the cost is that this delivery mode incurs two-way network traffic for each message or committed transaction of a group of messages.
- 22. Non Persistent • Sending a NON_PERSISTENT message omits the overhead of persisting the message on disk to improve performance. • If authorization is disabled on the server, the server does not send a confirmation to the message producer. • If authorization is enabled on the server, the default condition is for the producer to wait for the server to reply with a confirmation in the same manner as when using PERSISTENT mode.
- 23. Non Persistent Message Producer EMS Server Message Depending on npsend_check_mode Regardless of whether authorization is enabled or disabled, you can use the npsend_check_mode parameter in the tibemsd.conf file to specify the conditions under which the server is to send confirmation of NON_PERSISTENT messages to the producer.
- 24. Reliable Delivery • EMS extends the JMS delivery modes to include reliable delivery. Sending a RELIABLE_DELIVERY message omits the server confirmation to improve performance regardless of the authorization setting. Message Producer EMS Server Message
- 25. Reliable Delivery • When using RELIABLE_DELIVERY mode, the server never sends the producer a receipt confirmation or access denial and the producer does not wait for it. • Reliable mode decreases the volume of message traffic, allowing higher message rates, which is useful for messages containing time-dependent data, such as stock price quotations. • When you use the reliable delivery mode, the client application does not receive any response from the server. Therefore, all publish calls will always succeed (not throw an exception) unless the connection to the server has been terminated.
- 26. Reliable Delivery • In some cases a message published in reliable mode may be disqualified and not handled by the server because the destination is not valid or access has been denied. • In this case, the message is not sent to any message consumer. However, unless the connection to the server has been terminated, the publishing application will not receive any exceptions, despite the fact that no consumer received the message.
- 27. EMS Delivery Modes Reviewed • NON_PERSISTENT and RELIABLE_DELIVERY messages are never written to persistent storage. • PERSISTENT messages are written to persistent storage when they are received by the EMS server.
- 29. EMS Persistent Mode Management • Persistent Messages Sent to Queues Persistent messages sent to a queue are always written to disk. Should the server fail before sending persistent messages to consumers, the server can be restarted and the persistent messages will be sent to the consumers when they reconnect to the server. TIBCO EMS Server Send Message Receive Message Acknowledge
- 30. EMS Persistent Mode Management • Persistent Messages Sent to Topics Persistent messages published to a topic are written to disk ONLY IF that topic has at least one durable subscriber or one subscriber with a fault-tolerant connection to the EMS server. • Non-durable subscribers that re-connect after a server failure are considered newly created subscribers and are not entitled to receive any messages created prior to the time they are created.
- 31. EMS Persistent Mode Management TIBCO EMS Server Publish Message Subscribe to Topic Subscribe to Topic Subscribe to Topic
- 32. • When using file storage, persistent messages received by the EMS server are by default written asynchronously to disk. • When a producer sends a persistent message, the server does not wait for the write-to-disk operation to complete before returning control to the producer. • This means that the producer has no way of detecting the failure of persisting the message and take corrective action if the server fails before completing the write-to-disk operation. Persistent Messages & Synchronous Storage
- 33. Persistent Messages & Synchronous Storage What do you do if you want to SYNCHRONOUSLY write to disk? • You can set the mode parameter to sync for a given file storage in the stores.conf file to specify that persistent messages for the topic or queue be synchronously written to disk. • When mode = sync, the persistent producer remains blocked until the server has completed the write-to-disk operation.
- 34. STORE
- 35. Store EMS Server File-based Store (Default) Database Store Pre allocate disk space for store file Truncate the file to relinquish disk space Mode : Sync or Async Store messages in DB or not Properties that allow the user to control how server manages the store file
- 36. Default Store Files • File based stores are enabled by default. • Server automatically defines 3 default stores • $sys.nonfailsafe Server writes messages using asynchronous I/O calls. • $sys.failsafe Server writes messages using synchronous I/O calls. • $sys.meta Server writes state information about durable subscribers & fault tolerant connections.
- 38. Message Compression • TIBCO Enterprise Message Service allows a client to compress the body of a message before sending the message to the server. • EMS supports message compression/decompression across client types (Java, C and C#). For example, a Java producer may compress a message and a C consumer may decompress the message. • Message compression is supported in .NET clients when using the install package for Visual C++ 8 / .NET 2.0 or above.
- 39. Message Compression • Less memory storage for PERSISTENT queue messages or DURABLE topic subscribers. • Compression option only compresses the BODY content. Headers and properties are NEVER compressed. • When messages are not stored, compression is not a good option. Why? • Because, Compression takes TIME…!
- 40. Message Compression • Compression specific for individual messages. • Not on a per-queue or per-topic basis. • To set message compression JMS_TIBCO_COMPRESS to TRUE
- 42. Message Acknowledgement TIBCO EMS Server Message Message Confirmation Acknowledgement Confirmation of Acknowledgement JMS EMS (JMS + Below mentioned modes) CLIENT_ACKNOWLEDGE NO_ACKNOWLEDGE AUTO_ACKNOWLEDGE EXPLICIT_CLIENT_ACKNOWLEDGE DUPS_OK_ACKNOWLEDGE EXPLICIT_CLIENT_DUPS_OK_ACKNOWLEDGE
- 43. JMS : CLIENT_ACKNOWLEDGE • Consumer is to acknowledge all messages that have been delivered so far by the session. • Possible for the consumer to fall behind in its message processing and build up a large number of unacknowledged messages Message 1 Message 1 Acknowledgement # 1,2,3 Message 2Message 3 Message 3Message 2
- 44. JMS : AUTO_ACKNOWLEDGE • Session is to automatically acknowledge consumer receipt of messages when message processing has finished Message 1 Message 1 Acknowledgement 1 Message 2Message 3 Acknowledgement 3Acknowledgement 2 Message 3Message 2
- 45. JMS : DUPS_OK_ACKNOWLEDGE • Session is to "lazily" acknowledge the delivery of messages to the consumer. • "Lazy" means that the consumer can delay acknowledgement of messages to the server until a convenient time; meanwhile the server might redeliver messages. Message 1 Message 1 Acknowledgement #1,2 Message 2Message 3 Acknowledgement #3 Message 3Message 2
- 46. EMS : NO_ACKNOWLEDGE • NO_ACKNOWLEDGE mode suppresses the acknowledgement of received messages. • After the server sends a message to the client, all information regarding that message for that consumer is eliminated from the server. • Therefore, there is no need for the client application to send an acknowledgement to the server about the received message.
- 47. EMS : EXPLICIT_CLIENT_ACKNOWLEDGE • EXPLICIT_CLIENT_ACKNOWLEDGE is like CLIENT_ACKNOWLEDGE except it acknowledges only the individual message, rather than all messages received so far on the session. Message 1 Message 1 Acknowledgement 1 Message 2Message 3 Acknowledgement 3Acknowledgement 2 Message 3Message 2
- 48. EMS : EXPLICIT_CLIENT_ACKNOWLEDGE • When EXPLICIT_CLIENT_ACKNOWLEDGE would be used ? • When we receive the messages and put their information in a database and if the database insert operation is slow, you may want to use multiple application threads all doing simultaneous inserts. • As each thread finishes its insert, it can use this mode to acknowledge only the message that it is currently working on.
- 49. EMS : EXPLICIT_CLIENT_DUPS_OK_ACKNOWLEDGE • EXPLICIT_CLIENT_DUPS_OK_ACKNOWLEDGE is like DUPS_OK_ACKNOWLEDGE except it ‘lazily’ acknowledges only the individual message, rather than all messages received so far on the session.
- 51. Message Selectors • A message selector is a string that lets a client program specify a set of messages, based on the values of message headers and properties. • A selector matches a message if, after substituting header and property values from the message into the selector string, the string evaluates to true. • Consumers can request that the server deliver only those messages that match a selector.
- 52. Destinations
- 53. Types of Destinations • Static Destinations • Dynamic Destinations • Temporary Destinations
- 54. Static Destinations • Purpose • Allows administrators to configure EMS behavior at enterprise level • Scope of delivery • Supports concurrent use • Creation • Using config files, tibemsadmin or by API’s by administrator • Duration • Until explicitly deleted by the administrator
- 55. Dynamic Destinations • Purpose • Provide flexibility to define them as needed for short term use • Scope of delivery • Supports concurrent use • Creation • Client programs create it if permitted by server configuration • Duration • As long as at least 1 client actively uses it
- 56. Temporary Destinations • Purpose • Ideal for limited scope usage, like reply subjects (in routing) • Scope of delivery • Supports local use • Creation • Client programs create it • Duration • Explicit deletion by the client or disconnection from the server
- 57. Mixed Bag - Destinations • Clients can obtain references to static destinations through a naming service such as JNDI or LDAP • But they cannot obtain the references to dynamic or temporary destinations • Dynamic topics and queues have an asterisk (*) marked in front of them, when you use the commands show queues or show topics in tibemsadmin • If a property of a queue or topic has an asterisk (*) character in front of its name, it means that the property was inherited from the parent queue or topic
- 58. tibemsd – TIBCO EMS Server
- 59. Starting the EMS Server “Running this service starts the EMS Server” This service starts tibemsd.exe located in <EMS HOME>/bin folder tibemsd.exe reads tibemsd.conf for server settings
- 60. Using tibemsd to start EMS server
- 61. tibemsadmin – TIBCO EMS Admin
- 63. Using connect
- 64. Using help
- 65. Using disconnect
- 66. Using exit
- 67. Using shutdown
- 68. Using show
- 69. Using create
- 70. Using delete
- 71. Using addprop
- 72. Using removeprop
- 73. Using setprop
- 74. Using purge
- 75. Using whoami & info show server
- 76. Using set Parameters in tibemsd.conf
- 77. Using time & timeout
- 78. Using commit & autocommit
- 79. Using compact
- 80. Using add
- 81. Using remove
- 82. Using grant & revoke
- 83. Using echo
- 85. channel
- 86. channel • Channel property determines the multicast channel over which messages sent to the topic are broadcast • Configure multicast channels in channels.conf file and enable this feature in tibemsd.conf • Cannot create channel by any command in tibemsadmin • Only 1 channel allowed for each topic • Available only for topics
- 88. channel
- 89. exclusive
- 90. exclusive • Available only for queues • Set the exclusive property using addprop or setprop • When exclusive is set for the queue, the server sends all the messages on that queue to one consumer. No other consumer can receive messages from the queue. • Instead, these additional consumers act in a standby role; if the primary consumer fails, the server selects one of the standby consumers as the new primary and begins delivering messages to it.
- 91. exclusive
- 92. expiration
- 93. expiration • If an expiration property is set for a destination, the server honors the overridden expiration period • If expiration property for the server is set, the server overrides the JMSExpiration value set by the producer in the message header • expiration=time[msec|sec|min|hour|day]
- 94. expiration
- 95. maxbytes
- 96. maxbytes Topics & queues can specify the maxbytes property in the form maxbytes=value[KB|MB|GB] FOR QUEUES • maxbytes: maximum size (in bytes) that the queue can store, summed over all the messages in the queue • If this limit is exceeded, the messages will be rejected by the server and the producer send calls will return an error
- 97. maxbytes FOR TOPICS • maxbytes: maximum size(in bytes) that the topic can store for delivery to each durable or non-durable online subscriber on that topic • The limit applies separately to each subscriber on the topic • Example : offline durable subscriber messages accumulate until they exceed maxbytes non durable subscriber maxbytes limits the number of pending messages that can accumulate while the subscriber is online
- 98. maxbytes
- 99. maxmsgs
- 100. maxmsgs Topics & queues can specify the maxmsgs property in the form maxmsgs=value • maxmsgs: maximum number of messages that can be waiting in a queue • When adding a message into a queue/topic would exceed this limit, the server would reject the message and the producer’s send call returns an error • Can set both maxmsgs and maxbytes properties on the same queue. Exceeding either limit causes server to reject new messages until consumers reduce the queue size to below these limits.
- 101. maxmsgs
- 102. maxRedelivery
- 103. maxRedelivery • The number of attempts the server should make to deliver a message sent to a destination maxRedelivery=count count is between 2 & 255 • For messages that have been redelivered, JMSRedelivered header property is set to true JMSXDeliveryCount property is set to the number of times the message has been delivered to the destination
- 104. maxRedelivery
- 105. overflowPolicy
- 106. overflowPolicy • To change the effect of exceeding the message capacity established by maxbytes or maxmsgs overflowpolicy=default|discardOld|rejectIncoming • default For TOPICS maxbytes or maxmsgs exceed for a subscriber, that subscriber does not receive message No error returned to producer For QUEUES New messages are rejected by the server if maxbytes or maxmsgs are exceeded Error returned to producer
- 107. overflowPolicy • discardOld For TOPICS Oldest messages are discarded before they are delivered to the subscriber Impacts subscribers individually. 3 subscribers, 1 exceeds the message limit. So, only the oldest messages for the one subscriber are discarded, while other two continue to receive all messages No error returned to producer, as message can be delivered to some and discarded for others For QUEUES Error returned to producer if maxbytes or maxmsgs are exceeded Oldest messages are discarded from the queue by the server
- 108. overflowPolicy • rejectIncoming For TOPICS If ANY of the subscribers have an outstanding number of undelivered messages on the server that are over the message limit, all new messages are rejected Error is returned to producer For QUEUES Error returned to producer if maxbytes or maxmsgs are exceeded Newest messages are rejected from the queue by the server
- 109. overflowPolicy
- 110. Quick Quiz • How do I discard messages on myQueue when the number of queued messages exceeds 2500 ? setprop queue myQueue maxmsgs=2500, overflowpolicy=discardOld • How do I reject all new messages published to myTopic when the memory used by undelivered messages for any of the topic subscribers exceeds 3 MB? setprop topic myTopic maxbytes=3MB,overflowPolicy=rejectIncoming
- 111. flowControl
- 112. flowControl • Specifies the target maximum size the server can use to store pending messages for the destination • If number of messages > flowControl then slow down the producers to the rate required by the message consumers • Useful when message producers send messages more quickly than message consumers can consume them
- 114. prefetch
- 115. prefetch • Consumer and the EMS Server cooperate to regulate message fetching through this property prefetch=value Value Description 2 or more Never fetches more than specified number (Auto Fetch) 1 Fetch only if it has no message (Auto Fetch) none Disable Auto Fetch. Cannot be used with topics or global queues 0 (default) Destination inherits the prefetch value. Default value for queues = 5 & topics = 64
- 116. prefetch 2 15 4 3
- 117. prefetch
- 118. prefetch • Improves performance by decreasing or eliminating client idle time while the server transfers a message • When a queue consumer prefetches a group of messages, the server does not deliver them to other queue consumers (unless the first queue consumer’s connection to the server is broken)
- 119. prefetch • Even when prefetch = none, a queue consumer can hold a message Example A receive call initiates a fetch, but its timeout elapses before the server finishes transferring the message This leaves a fetched message waiting in the message consumer A second receive call does not fetch another message; instead it accepts the message already waiting The third receive call initiates another fetch
- 120. prefetch • A waiting message still belongs to the queue consumer, and the server does not deliver it to another queue(unless the first queue consumer’s connection to the server is broken) • To prevent messages from waiting in this state for long periods • Call receive with no timeout • Call receive with timeout repeatedly and shorten the interval PARENT CHILD all parents : none none any parent : non-zero numeric value highest does not set any value default (5 : queues, 64 : topics)
- 121. prefetch
- 122. secure
- 123. secure If secure is enabled for a destination, it instructs the server to check the user permissions whenever a user attempts to perform an operation on that destination tibemsd.conf
- 124. sender_name
- 125. sender_name • Server may include the sender’s username for messages sent to this destination • When connection between producer and server is established, server takes the username supplied by the producer and places it in the JMS_TIBCO_SENDER property of the message • But if producer sets the JMS_TIBCO_DISABLE_SENDER to true for a message, server will not add the sender name to the message
- 126. sender_name
- 127. sender_name_enforced
- 128. sender_name_enforced • Specifies that the messages sent to this destination MUST include the sender’s username • Unlike sender_name property, there is no way for message producers to override this property • This property overrides sender_name if already set.
- 129. sender_name_enforced
- 130. store
- 131. store • Specifies where the messages sent to this destination are stored • Configure stores in stores.conf
- 132. Destination BRIDGES
- 133. Bridges • Some applications require the same message to be sent to more than one destination, possibly of different types. Example An application can publish messages to several topics. All messages however, must also be sent to a database for backup and for data mining. A queue is used to collect all messages and send them to the database.
- 134. Bridges • Bridges are created between one destination and one or more other destinations of the same or of different types. • That is, you can create a bridge from a topic to a queue or from a queue to a topic. You can also create a bridge between one destination and multiple destinations. • For example, you can create a bridge from topic a.b to queue q.b and topic a.c.
- 135. Bridging Topic to Queue
- 136. Bridging Topic to Multiple Destinations
- 137. Bridging Queue to Multiple Destinations
- 138. Bridges • When a bridge exists between two queues, the message is delivered to both queues. The queues operate independently; if the message is retrieved from one queue, that has no effect on the status of the message in the second queue. • Bridges are not transitive Topic A.B has a bridge to queue Q.B. Queue Q.B has a bridge to topic B.C. Messages delivered to A.B are also delivered to Q.B, but not to B.C.
- 139. Creating a bridge • Configured in bridges.conf file • Use of Selector All messages sent to a destination with a bridge are sent to all bridged destinations. This can cause unnecessary network traffic if each bridged destination is only interested in a subset of the messages sent to the original destination.
- 140. ROUTING
- 141. Routing • EMS servers can route messages to other servers • Topic messages can travel one hop or multiple hops from the first server • Queue messages can travel ONLY ONE hop to and from the home queue
- 142. Routing : Basic Operation • Each route connects two TIBCO EMS servers. • Each route forwards messages between corresponding destinations (that is, global topics with the same name, or explicitly routed queues) on its two servers. • Routes are bidirectional; that is, each server in the pair forwards messages along the route to the other server.
- 143. Routing : Hawk Eye View Server : A Server : B
- 144. Global Destinations : Topics “Routes forward messages only between global destinations” “For TOPICS, the global property must be set on both servers”
- 145. Unique Routing Path • It is illegal to define a set of routes that permit a message to reach a server by more than one path. TIBCO EMS servers detect illegal duplicate routes and report them as configuration errors. A B C ED P Q R TS
- 146. Zones • A zone is a named set of routes. • Every route belongs to a zone. • Zones restrict the behavior of routes, so you can configure complex routing paths. • Zones affect topic messages, but NOT queue messages.
- 147. Zones : Basic Operation • In a multi-hop (mhop) zone, topic messages travel along all applicable routes to all servers connected by routes within the zone. • In a one-hop (1hop) zone, topic messages travel only one hop (from the first server). • Queue messages travel only one hop, even within multi-hop zones.
- 148. Eliminating Redundant Paths with One-Hop Zone B1 B2 RM GOAL : Forward messages from B1 and B2 to both M and R
- 149. Overlapping Zones M3 M2 M1M4 B2 B3 B4B1 D1 D2 D3D4
- 150. Creating Routes Syntax create route EMS-SERVER url=tcp://ipserver:7222 zone_name=zoneName zone_type=1hop|mhop route name MUST be the name of the EMS server which is specified in tibemsd.conf url indicates the other server by the URL
- 151. Active & Passive Routes • A route connects two servers. • You may configure a route at either or both of the servers. • A route is active from the perspective of the server where it is configured. This server actively initiates the connection to the other server, so we refer to it as the active server, or initiating server. • A route is passive from the perspective of the other server. This server passively accepts connection requests from the active server, so we refer to it as the passive server.
- 152. Active – Active Routes • Two servers can both configure an active route one to the other. This arrangement is called an active-active configuration. For example, server A specifies a route to server B, and B specifies a route to A. Either server can attempt to initiate the connection. This configuration results in only one connection; it does not result in redundant routes. • You can promote an active-passive route to an active-active route by using this command on the passive server create route name url=url • The url argument is required, so that the server (where the route is being promoted) can connect to the other server if the route becomes disconnected
- 153. Routed Topic Messages • A server forwards the message along the routes only when the global property is defined by the topic
- 154. Routed Topic Messages • Topic messages can traverse multiple hops. • When a route becomes disconnected (for example, because of network problems), the forwarding server stores topic messages. When the route reconnects, the server forwards the stored messages. • Servers connected by routes do exchange messages sent to temporary topics.
- 155. Routing : Propagating Subscribers
- 156. Routed Queues How is routing in queues different from that in topics? • Servers route queue messages between the queue owner and adjacent servers. • The concept of zones and hops does not apply to queue messages (only to topic messages).
- 157. Routed Queues • In routing topics, the declaration of the topic is identical on all servers • Queue declarations make a distinction between the server that owns the queue and other servers with routed queues that reference both the queue name and the owning server
- 158. Routed Queues
- 159. Routed Queues • Routed queues serve as proxies for the real queue • Messages published to the proxy queue are forwarded to the real queue, and are not eligible for delivery until they reach the real queue
Editor's Notes
- The timeout value is the number of seconds the Administration Tool will wait for a response from the server after sending a command.
- The file will not be compacted unless the store_truncate parameter is enabled in the tibemsd.conf file (This is given in the EMS documentation, but it works fine for me even when I don’t have a store_truncate parameter set) If truncation is not enabled for the store file, the compact command will not reduce the file size. Truncation is enabled using the file_truncate parameter in the stores.conf file. (This is given in the EMS documentation, but it works fine for me even when I don’t have a file_truncate parameter set)