Abstract:
Disclosed is a method, apparatus and computer program for sampling a data feed of messages at regular time intervals. A current sample point is identified. A time interval is also identified which is defined by the current sample point and at least one of the previous sample point and the subsequent sample point. At least one message is selected to associate with the current sample point. The selection is made from the identified time interval. The selected message(s) is then associated with the current sample point

Description:
FIELD OF INVENTION  
       [0001]     The present invention relates to apparatus, methods and computer programs for sampling a data feed within a data communications network.  
       BACKGROUND  
       [0002]     The invention is applicable to many data processing and communications systems, including publish/subscribe messaging systems in which subscribers can specify a subset of published messages that they wish to receive, and including publish/subscribe messaging systems in which transmitted messages are retained for retransmission or analysis.  
         [0003]     Publish/subscribe communications involve information producers publishing information or events to a publish/subscribe system, and information consumers subscribing to particular categories of information or events and receiving relevant publications from the system. The publish/subscribe system may comprise a message broker, located between publisher and subscriber applications, which delivers published information or events to all interested subscribers.  
         [0004]     The publish/subscribe communication paradigm supports many-to-many communications in which individual publishers and subscribers may be anonymous to each other (communicating via an intermediate broker) and can be easily added and removed from the network without disruption. An example message broker is the IBM® WebSphere® Business Integration Message Broker product available from IBM. (IBM and WebSphere are registered trademarks of International Business Machines Corporation.)  
         [0005]     Many publish/subscribe messaging systems are subject-based. In these systems, each message belongs to one of a predefined set of subjects (also known as channels, or topics). Publishers label each message with a subject, and consumers subscribe to all the messages having a particular subject label. For example, a subject-based publish/subscribe system for stock trading may use a defined topic name for each stock issue—publishers post information using the appropriate topic name and subscribers include topic names when specifying which stocks they wish to receive information about.  
         [0006]     An alternative to subject-based publish/subscribe messaging is content-based publish/subscribe messaging as described in “An Efficient Multicast Protocol for Content-Based Publish-Subscribe Systems” by G Banava, T Chandra, B Mukherjee, J Nagarajarao, R Strom and D Sturman of IBM T.J. Watson Research Center(and other articles published by IBM Corporation via a Web site at URL www.research.ibm.com/gryphon/). Compared with subject-based systems, content-based systems support greater flexibility for publishers and allow subscribers to express a “query” against the content of messages published. Thus, the limitation to predefined subjects that is a feature of subject-based systems can be avoided by more complex analysis of message content.  
         [0007]     Some messaging systems provide a replay feature, for example retaining publications for replay to new subscribers (and newly recovered subscribers) so that the new subscribers are able to receive some or all of an earlier message feed. One such system is the CodeStreet ReplayService for Tibco Rendezvous/Java™ Messaging Service (www.codestreet.com). (Java and all Java-based trademarks and logos are trademarks of Sun Microsystems, Inc. in the United States, other countries, or both.)  
         [0008]     In particular, some messaging systems enable subscribers to request a replay and to specify a sampling interval for the replay. For example, a subscriber may not require all previous messages and may specify a requirement to only receive every N th  (e.g. 10 th ) message or to only receive a message once every M (e.g. 10) seconds.  
         [0009]     Sampling is particularly useful in situations of data overload where the amount and frequency of data being transmitted (replayed) is such that it is near impossible to process the data quickly enough (either in terms of computation or human interpretation). One such example is a ticker tape of stock prices. By way of another example, the recipient may be interested in data variance of river height data or seismic activity (for instance, data that is above a certain value or where there are large changes in value).  
         [0010]     A potential problem with such sampling of messages is that the sample may be unrepresentative of the sampled message feed, and this problem is emphasised if the number of messages transmitted in a sample period fluctuates and there are sparse periods. For example, if the sampling method periodically transmits the ‘last received message’ and there is a gap in the message feed spanning multiple sampling intervals, a single ‘last received message’ will be repeatedly sampled. There may be no way for a user to determine whether the repeats are valid results or whether an error has occurred (such as a connection failure or sampling inaccuracy).  
         [0011]     This problem is best illustrated using  FIG. 1 . Arrow t shows the progression of time. A data feed  20  of messages is replayed. Each message  1 - 10  replayed as data feed  20  is shown above the arrow. A user however does not require all messages from the replay but only a pre-specified sample. For example, samples could be taken at 10 second intervals.  FIG. 1  shows each sample a-j of the data feed depicted below the line.  
         [0012]     Using a basic “last received message” algorithm, the sequence of messages returned as a result of such sampling would be  2 ,  3 ,  3 ,  6 ,  6 ,  6 ,  7 ,  7 ,  9  and  10 . It can be seen from this that some messages are sampled two or more times. This occurs when no new messages are received between sampling intervals. For example message  3  is the last message received when sample b is taken and because no new messages are received between sample b and sample c, message  3  is once again the last message received when sample c is taken.  
         [0013]     Consequently, it should be appreciated that this sample completely distorts the actual picture, that is unless the receiving application is configured to recognise and deal with duplicate data. Indeed, depending on the messaging transport, it may not even be possible to distinguish between two different messages that happen to have the same data, and the same message delivered twice.  
         [0014]     Distorted results are particularly a problem when a recipient application wishes to perform some analysis on the sample. If the sample is unrepresentative of the actual data feed, the results gathered by the recipient application may have very little meaning.  
         [0015]     As discussed, sampling of fluctuating data feeds having sparse periods is particularly problematic, but it can also be difficult to obtain meaningful data from a populous data feed.  
       SUMMARY  
       [0016]     According to a first aspect, the invention provides a method for sampling a data feed of messages at regular time intervals, the method comprising: identifying a current sample point; identifying a time interval defined by the current sample point and at least one of the previous sample point and the subsequent sample point; selecting at least one message to associate with the current sample point, the selection being made from the identified time interval; and associating at least one selected message with the current sample point.  
         [0017]     A method according to this first aspect can be used to achieve a more representative sampling of messages than was achieved using known solutions.  
         [0018]     A message may be selected from the time interval defined by the current sample point and the previous sample point. If however it is determined that the selected message has already been associated with another sample point, then a message is preferably selected from the time interval defined by the current sample point and the subsequent sample point. In one embodiment, if it is determined that there is no message to return when looking behind the current sample point, a message is selected from the time interval defined by the current sample point and its subsequent sample point.  
         [0019]     Preferably, in any case where a message selected from one identified time interval is not suitable (or none is returned), a new time interval is identified and a selection made from this newly identified interval. In other situations however, another message may be selected from the first identified interval.  
         [0020]     In a conflict resolution embodiment, two sweeps are performed. Messages are selected and associated with sample points during a first sweep of the data feed. Conflicts between messages selected for adjacent sample points are identified and resolved during a second sweep of the data feed.  
         [0021]     The first sweep preferably involves, for each sample point, identifying messages received during the time interval defined by the sample point previous to a current sample point and messages received during the time interval defined by the sample point subsequent to the current sample point. Preferably at least one identified message is then selected and associated with the current sample point.  
         [0022]     Preferably the message selected is the one that was received most recently by comparison to the current sample point. Note, this message may be previous to the current sample point or subsequent thereto.  
         [0023]     The second sweep preferably involves identifying two adjacent sample points each having the same message associated therewith. It is then determined whether an alternative message can be associated with either sample point.  
         [0024]     If it is determined that neither sample point has an alternative message for association therewith, the conflicting message is preferably associated with the sample point occurring closest in time to the time at which the message was received.  
         [0025]     If is determined that an alternative message can be associated with one of the two sample points, that alternative is preferably associated with the relevant sample point.  
         [0026]     Preferably the message on which there was conflict is associated with the other sample point.  
         [0027]     If both sample points have alternatives, then an alternative is preferably associated with one of the sample points. E.g. the sample point with the closest alternative.  
         [0028]     In one embodiment the position of messages in the data feed is normalised. In this embodiment a message is preferably selected from the time interval defined by the current sample point and the subsequent sample point.  
         [0029]     In one embodiment, a message is selected having a data value that is closest to the mean value of data messages in the time interval selected. In another embodiment, a message is selected having a data value that is closest to the mean value of data messages received as part of the data feed.  
         [0030]     In one embodiment, it is determined whether a selected message has already been associated with a sample point and if the selected message has already been associated, an alternative selection is preferably made. Examples of how an alternative selection is made, are discussed with reference to the first embodiment and the conflict resolution embodiments below. For example, if a message has been previously used, a selection may be made from the time interval defined by the current sample point and the subsequent sample point. Alternatively a conflict may be identified and resolved via the processes of  FIGS. 5 and 6 .  
         [0031]     According to a second aspect, the invention provides an apparatus for sampling a data feed of messages at regular time intervals, the apparatus comprising: means for identifying a current sample point; means for identifying a time interval defined by the current sample point and at least one of the previous sample point and the subsequent sample point; means for selecting at least one message to associate with the current sample point, the selection being made from the identified time interval; and means for associating at least one selected message with the current sample point.  
         [0032]     Methods as described above may be implemented using computer program code that is executable by a data processing apparatus and the program code may be available for delivery over a network or recorded on a recording medium. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0033]     Embodiments of the invention are described below in more detail, by way of example only, with reference to the accompanying drawings in which:  
         [0034]      FIG. 1  shows an exemplary data feed and sample points;  
         [0035]      FIG. 2  provides an overview of the components involved in embodiments of the present invention;  
         [0036]      FIG. 3  provides an overview of the sampling process in accordance with embodiments of the present invention;  
         [0037]      FIG. 4  shows the processing of a first embodiment;  
         [0038]      FIGS. 5 and 6  show the processing of a conflict resolution embodiment;  
         [0039]      FIG. 7  shows the processing of a normalised embodiment of the present invention; and  
         [0040]      FIGS. 8   a  and  8   b  show the processing of a mean embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF EMBODIMENTS  
       [0041]     As discussed above, obtaining a representative sample of a data feed replayed by a replay server can be difficult irrespective of whether the data feed has sparse periods or is populous. Both types of data feed will be dealt with herein.  
         [0042]      FIG. 2  illustrates the environment in which the present invention is set in accordance with preferred embodiments. A replay server  100  participates in a messaging environment (for example a pub/sub environment) to store a copy of all messages received by a message broker ( 125 ) in message repository  130 . Note the replay server may be integrated as part of the message broker or may be a separate component (as shown in the figure). In addition to message repository  130 , replay server  100  includes a replay component  119  and a sampling component  120 . Replay server  100  subscribes via network connection  162  to receive all messages transmitted to the message broker.  
         [0043]     A recipient application  140  connects to message broker  125  via network connection  160 . Recipient application comprises a replay requester  150  which is operable to make replay requests via message broker  125 . Upon receipt of such a request, message broker  125  informs replay server  100  of the requester&#39;s requirements. For example, replay requester  150  could request a replay of all messages received with a specified time period (e.g. the last hour). Such replay is achieved at the replay server  100  via replay component  110 .  
         [0044]     Replay requester  150  may not however require all messages from the replay. Rather requester  150  may be interested in a sample of the complete replay. Thus requester  150  may specify this to server  100  which uses sampling component  120  to respond to recipient application  140 &#39;s request. Note, replay requester  150  may request that sampling is provided as soon as possible (i.e. having no regard for the timing of the original message sequence). Another option is for replay requester  150  to request that sampling is provided having regard for the original message timing.  
         [0045]     Replay messages are provided to the requester via the message broker.  
         [0046]      FIG. 3  provides an overview of the sampling process (at the replay server) in accordance with a preferred embodiment. A sample is taken every n seconds (specified by recipient application  140 ). Thus step  200  involves initialising n to the specified interval. For example, if application wants a sample every 5 seconds, then n is set to 5. At step  210  a message replay sequence is identified. Replay server  100  holds details of the order in which messages were received and the time at which they were received. Thus a set of messages must be identified, but there is no need to actually enact the replay.  
         [0047]     A sample is taken at time n in order to identify a message to associate with the current sample point for return to the recipient (step  220 ). When this message is actually returned to the recipient application will of course be dependent upon the required replay speed (see above). At step  230  n is incremented by the specified interval and another sample is taken. The process repeats until the end of the replay sequence.  
       First Embodiment  
       [0048]      FIG. 4  expands upon the processing of step  220  ( FIG. 3 ) in accordance with one embodiment of the present invention. Sampling is performed to identify the last message received by the system relative to each sample point.  
         [0049]     A sample point (sp) is identified at step  300 . At step  310  a processing thread looks for messages received prior to the sample point (look-behind). It is determined whether any such messages exist (step  330 ). Note the scope of the look-behind is bounded by a previous sample point.  
         [0050]     Assuming such messages exist in the previous interval, the last message received before the current sample point is identified and it is then determined whether the message has been previously associated with another sample point (step  350 ). If the answer is no, then the message is associated with the current sample point (step  370 ). If either there are no messages returned during the look-behind or the last message has already been used, then a look-ahead is performed to identify any messages appearing between the current time interval and the subsequent time interval (step  340 ). If no message is identified at step  360  then no message is associated with the current sample point and the whole process loops round until the end of the replay sequence is reached. If on the other hand a message(s) is identified then the message closest to the sp is associated with the current sp (step  370 ). The process then loops round again.  
         [0051]     Note, whether a message has been previously used can be determined because each message is preferably uniquely identifiable.  
         [0052]     The processing is best illustrated using the example of  FIG. 1 . Using look-behind and look-ahead for message selection over the time interval, the sequence of returned messages is  2 ,  3 ,  4 ,  6 , &lt;blank&gt;,  7 , &lt;blank&gt;,  8 ,  9 ,  10 . This sequence is more representative of the data in the sparse feed, and is reached as follows. (Note look-behind and look-ahead only span over one time interval and the look-behind includes checking the sample point itself): 
    a. Look-behind (step  310 ) identifies message  2  at step  330  which has not been used (step  350 ) before and consequently  2  is selected (step  370 ).     b. Look-behind (step  310 ) identifies message  3  at step  330  which has not been used before (step  350 ).  3  is therefore selected (step  370 ).     c. Look-behind (step  310 ) returns nothing (step  330 ), look-ahead (step  340 ) identifies message  4  (step  360 ).  4  is therefore selected (step  370 ).     d. Look-behind (step  310 ) identifies message  6  as the last received message (step  330 ). This message is then associated with sample point d (step  370 ).     e. Look-behind returns nothing (step  330 ), look-ahead (step  340 ) also returns nothing (step  360 ). A message is therefore not associated with sample point e.     f. Look-behind returns nothing (step  330 ), look-ahead (step  340 ) returns message  7  (step  360 ,  370 ).     g. Look-behind returns nothing at step  330  since  7  is the only message in the time interval between f and g and  7  has already been associated with sample point f (i.e.  7  is used-step  350 )), look-ahead also returns nothing (steps  340 ,  360 ).     h. Look-behind returns nothing (step  310 ,  330 ), look-ahead selects message  8  (step  340 ,  360 ,  370 ).     i. Look-behind selects message  9  (steps  310 ,  330 ,  350 ,  370 ).     j. Look-behind selects message  10  (steps  310 ,  330 ,  350 ,  370 ).    
 
         [0063]     In this way a message is associated with each sample point as appropriate.  
       Conflict Resolution Embodiment  
       [0064]     A second algorithm is shown with respect to  FIGS. 5 and 6 . This uses intelligent look-behind and look-ahead with conflict resolution. The sequence of returned messages from the  FIG. 1  example is  2 ,  3 ,  4 ,  6 , &lt;blank&gt;, &lt;blank&gt;,  7 ,  8 ,  9 ,  10 . Again, this produces sampling representative of the entire replayed data feed and is reached in two sweeps ( FIG. 5 ,  FIG. 6 ) as follows:  
         [0065]      FIG. 5  depicts the first sweep. At step  400  a sample point is identified. A look-behind is performed to identify any messages in the previous time interval (step  410 ,  420 ,  490 ). A look-ahead is then also performed to identify any messages within the subsequent time interval (steps  430 ,  440 ,  450 ). A message (either from the look-behind or from the look-ahead) is then associated with the current sample point (step  460 ). The message chosen is the one closest to the sample point. Note, if both a message from the look-behind and a message from the look-ahead are equidistance from the sample point, then the look-behind message is preferably chosen—this slightly reduces the risk of future conflicts. The process then loops round until all sample points have a message associated with them (steps  470 ,  400 ). Identified messages are then examined to determine any conflicts (step  480 ). For example, that two adjacent sample points have the same message associated with them. Step  480  is performed in a second sweep and is discussed in more detail with reference to  FIG. 6 .  
         [0066]     Note, whether a message has been previously used can be determined because each message is preferably uniquely identifiable.  
         [0067]     At step  500 , a conflict is identified between two adjacent sample points. It is determined at step  510  whether either sample point has an alternative message that it can return instead. This means, for example, that whilst there may be a conflict on the last look-behind message, it may be able to use the last look-ahead message instead. If there is no alternative, then the sample point closest to the conflicting message is identified (step  520 ) and the conflicting message is associated with this sample point (step  530 ). The other sample point returns nothing (step  540 ).  
         [0068]     If on the other hand, the result from step  510  is yes, then it is determined whether both sample points have alternatives.  
         [0069]     Assuming for the moment that the answer is no, the sample point with the alternative returns that alternative (step  590 ) and the sample point without an alternative returns the message on which there was conflict (step  595 ).  
         [0070]     If on the other hand both sample points have alternatives, then the sample point which has an alternative closest is identified and associated with that sample point (steps  560 ,  570 ). For the other sample point, the message on which there is conflict is associated. In an alternative embodiment, it is determined whether that sample point&#39;s alternative or the message on which there was conflict is closest to that sample point and the closest is selected.  
         [0071]     Note, alternatives used during conflict resolution may be those messages selected by adjacent sample points without conflict initially. In order not to get into a cycle of continual resolution, in this case, once a message has been selected for a sample point in the initial sweep with no conflict, it cannot be used as an alternative in conflict resolution of an adjacent sample point.  
         [0072]     The process repeats for all conflicts until all are resolved.  
         [0073]     Note, intelligent processing is provided to always select the “best” alternative. Best is defined as “closest to the sample point” and must of course not have been already associated with a sample point. All messages within each interval are returned such that if the closest one is not available the next closest might be selected instead.  
         [0074]     The processing is best explained with reference to the example of  FIG. 1 . 
    a. Look-behind returns messages  2  and  1  (steps  400 ,  410 ,  420 ,  490 ); look-ahead returns message  3  (steps  430 ,  440 ,  450 ). Messages  2  and  3  are nearest and equidistant so message  2  is selected (step  460 ).     b. Look-behind returns message  3  (steps  400 ,  410 ,  420 ,  490 ); look-ahead returns nothing (steps  430 ,  440 ,  450 ). Message  3  is selected (step  460 ).     c. Look-behind returns nothing (steps  400 ,  410 ,  420 ,  490 ); look-ahead returns messages  4 ,  5  and  6  (steps  430 ,  440 ,  450 ). Message  4  is nearest so this is selected (step  460 ).     d. Look-behind returns messages  6 ,  5  and  4  (steps  400 ,  410 ,  420 ,  490 ); look-ahead returns nothing (steps  430 ,  440 ,  450 ). Message  6  is nearest so this is selected (step  460 ).     e. Look-behind returns nothing (steps  400 ,  410 ,  420 ,  490 ); look-ahead also returns nothing (steps  430 ,  440 ,  450 ).     f. Look-behind returns nothing (steps  400 ,  410 ,  420 ,  490 ); look-ahead returns message  7  (steps  430 ,  440 ,  450 ), so message  7  is selected (step  460 ).     g. Look-behind returns message  7  (steps  400 ,  410 ,  420 ,  490 ); look-ahead returns nothing (steps  430 ,  440 ,  450 ), so message  7  is selected (step  460 ).     h. Look-behind returns nothing (steps  400 ,  410 ,  420 ,  490 ); look-ahead returns messages  8  and  9  (steps  430 ,  440 ,  450 ). Message  8  is nearest so this is selected (step  460 ).     i. Look-behind returns messages  9  and  8  (steps  400 ,  410 ,  420 ,  490 ); look-ahead returns message  10  (step  430 ,  440 ,  450 ). Message  10  is nearest so this is selected (step  460 ).     j. Look-behind returns message  10  (steps  400 ,  410 ,  420 ,  490 ); look-ahead returns nothing (steps  430 ,  440 ,  450 ), so message  10  is selected (step  460 ).    
 
         [0085]     The first sweep is then complete, since there are no more sample points (step  480 , no branch). Each sample point now has a message associated therewith. At step  480 , conflicts are identified.  
         [0086]     Step  480  is performed during a second sweep discussed in more detail with reference to  FIG. 6 .  
         [0087]     Conflicts are first identified (step  500 ). With reference to  FIG. 1 , the first conflict identified is between sample points f and g. Both sample points have message  7  associated therewith. Neither f or g have an alternative (step  510 ), so the sample point to which message  7  is closest is identified (step  520 ). Consequently, message  7  is resolved to sample point g (step  530 ) and the other sample point f returns nothing (step  540 ).  
         [0088]     The next conflict exists between i and j (step  500 ). Only one of these sample points has an alternative (step  510 , step  550  no branch). i has the alternative of message  9  and so i is associated with message  9  (step  590 ). j has no alternative and so returns the message on which there was conflict (i.e.. 10)—step  595 .  
         [0089]     To summarise:  
         [0000]     (i)  
         [0090]     At a time cycle interval, look-behind for a message to select within a defined time interval. By default this is the same as the sampling interval, but can be configured to be greater or smaller. If nothing is available, or the first message located has been previously used, use the same interval on a look-ahead. Select the first located message or return nothing if nothing is available in the time interval.  
         [0000]     (ii)  
         [0091]     At a time cycle interval, look-behind and look-ahead to check for any messages available in the range of the defined time interval (see above). Select the message nearest to the time interval. To resolve conflicts, if one interval has an alternative, use that. If both do, then the interval with the nearest alternative should use that.  
       Normalised Embodiment  
       [0092]     Another solution is to normalise the data feed. This preferably involves dividing the time of the data feed into equal increments. The messages are then force-mapped onto these equal increments, and the sample is selected using simple look-ahead at each time interval.  
         [0093]     The processing will be discussed with reference to  FIG. 7 . Sample points are identified at step  600 . For example a sampling interval of 10 seconds over a replay period of 60 seconds will result in 6 sample points (duration divided by sampling interval—60/10).  
         [0094]     At step  610  normalised message points are created. In other words, each message (starting from time  0 ) is taken and moved to a predefined message point. For example if there is a replay duration of 60 seconds and there are 5 messages in the data feed, then there is a normalisation factor of 12 (60/5). Thus a message point occurs every 12 seconds—i.e. 12, 24, 36, 48, 60 (not at zero).  
         [0095]     At step  620  a look-ahead is performed from each sample point to identify the message point (and thereby message) falling closest to the sample point. Such a message is then associated with the current sample point (step  630 ).  
         [0096]     In this way messages are evenly spread across the full time interval. As indicated above, simple look-ahead is used to select the message to pass back to the user at each sample time segment. The benefit of the algorithm is that there is a defined and predictable time interval between messages. Also, it is known that if there are more messages than time segments then the messages delivered will be evenly spaced in the sample.  
         [0097]     As discussed above, sampling can also be problematic for populous feeds not having sparse periods since it is important to be able to retrieve meaningful data.  
         [0000]     Populous Data Feeds  
         [0098]     The solutions suggested for populous data feeds preferably require the replay server to have means to process internal message data. The recipient application preferably supplies a comparison algorithm which the replay server then applies to identify messages to return to the recipient.  
       Mean Embodiments  
       [0099]     The “Mean” solution selects one message from each time interval (segment) to pass back to the recipient application. The message selected has the closest value to the average value of all the messages in that interval, or all the messages in the entire replay sample.  
         [0100]     The processing of the mean solution will now be described with reference to  FIGS. 8   a  and  8   b.    
         [0101]      FIG. 8   a  uses the mean value of the time interval preceding the current sample point. At step  700 , and for each sample point, the average value of all messages in the preceding time interval is calculated. A look-behind is then performed to identify the message in the previous time interval, having the data value closest to the average value for that interval (step  710 . The closest message is then associated with the current sample point (step  720 ).  
         [0102]      FIG. 8   b  uses the mean value of all messages in the replay sample. At step  800 , the mean value is calculated of all the messages in the replay sample. For each sample point, a look-behind is performed to identify the message having the data value closest to this average (step  810 ). The identified message is then associated with the current sample point (step  820 ).  
         [0103]     Such solutions provide overview of the data in the replay sample. Consequently a large sample of data can be smoothed and trends will be easier to spot.  
       Delta Embodiment “Delta”, where only messages with data values greater than a given delta (plus or minus from a pre-defined data value) are used. The time segment interval is ignored in this case.  
       [0104]     The “Delta” algorithm ignores the time interval for the sample and only passes back messages whose delta is greater than the supplied minimum. This algorithm requires two values: the absolute value of the delta and the reference value this is measured from (this second value could be the average of all the data in the sample). The algorithm will pass back messages whose data is greater than the reference value plus the delta, and those whose data is less than the reference value minus the delta. This is another good overview tool, except instead of smoothing out the abnormal events, it highlights them.  
         [0105]     For both the mean and the delta embodiments, functionality is preferably provided at the replay server to interpret data in each message so that the mean or data value (as appropriate) can be calculated. For the mean embodiment, the recipient application preferably provides the replay server with a reference value and a change value.  
         [0106]     Whilst the invention has been described in terms of replay, it should nevertheless be appreciated that replay is only necessary for algorithms employing look-ahead. For algorithms employing look-behind only, a non-replay system is possible. For example, the last interval&#39;s messages could be buffered and then a mean calculated using these buffered messages (although this buffering could be seen as a form of replay). The Delta algorithm could be applied to each message as it passes through—i.e. without buffering, since it is worked out on a per-message basis.  
         [0107]     Put simply, whenever it is necessary to determine what to send out at a sample point based on all the messages in the previous interval, buffering or replay is used for at least that interval.  
         [0108]     It should also be appreciated that “last message received” sampling is just an example of where the invention is useful. The invention is in no way limited to such. The key to the invention is that the sampling period is well defined.