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What are events?
An event in RTR is a trigger that causes a notification (also called a "broadcast") to be sent to the application that subscribed to the event. RTR Events are created only by RTR and are used internally by RTR to help manage activities such as site failover. Application developers may subscribe to RTR Events to activate certain processing in their application. User Events are also available to enable application developers to send event notification or broadcast messages to other RTR applications. RTR provides the call rtr_broadcast_event to enable an application developer to trigger a User Event.
Events have special characteristics and restrictions:
The list below shows the RTR Events that are available for subscription. These events can be grouped in four basic categories:
RTR sends events to the server either inside or outside a transactional boundary. A transaction is considered to start on receipt of an rtr_mt_msg1 message, and to end when the transaction is accepted or rejected (receipt of an rtr_mt_accepted or rtr_mt_rejected message). Events containing information about primary, secondary, or standby servers can arrive ahead of a transactional boundary. Gain and loss events arrive inside transactional boundaries.
Table 2-5 lists the RTR events that can be received on a channel (associated with the rtr_mt_rtr_event message type). Events are listed in order of event number. See the description for rtr_open_channel in Chapter 3, RTR Call Reference, for further information.
| Event Number Name | No. | Description |
|---|---|---|
| RTR_EVTNUM_FACREADY | 96 | The facility has become operational. |
| RTR_EVTNUM_FACDEAD | 97 | The facility is no longer operational. |
| RTR_EVTNUM_FERTRGAIN | 98 | Frontend link to current router established. |
| RTR_EVTNUM_FERTRLOSS | 99 | Frontend link to current router lost. |
| RTR_EVTNUM_RTRBEGAIN | 100 | Current router established link to a backend. |
| RTR_EVTNUM_RTRBELOSS | 101 | Current router lost link to a backend. |
| RTR_EVTNUM_KEYRANGEGAIN | 102 | Server(s) for new routing key range are now available. |
| RTR_EVTNUM_KEYRANGELOSS | 103 | No more servers remain for a particular routing key range. |
| RTR_EVTNUM_BERTRGAIN | 104 | Backend established link to a router. |
| RTR_EVTNUM_BERTRLOSS | 105 | Backend lost link to a router. |
| RTR_EVTNUM_RTRFEGAIN | 106 | Router established link to a frontend. |
| RTR_EVTNUM_RTRFELOSS | 107 | Router lost link to a frontend. |
| RTR_EVTNUM_SRPRIMARY | 108 | Server has become primary. 1 |
| RTR_EVTNUM_SRSTANDBY | 109 | Server has become standby. |
| RTR_EVTNUM_SRSECONDARY | 110 | Server in a shadow pair has become secondary. 1 |
| RTR_EVTNUM_SRSHADOWLOST | 111 | Server in a shadow pair lost its shadow partner. 2 |
| RTR_EVTNUM_SRSHADOWGAIN | 112 | Server in a shadow pair acquired a shadow partner. |
| RTR_EVTNUM_SRRECOVERCMPL | 113 | Server completed recovery processing. |
Subscribing to Events
RTR Events can be used for triggering special application processing based on a change in RTR system status, or for sending notification to the system operator after certain application or RTR conditions that require intervention.
User Events can be used for actions such as broadcasting stock prices to update a price table, or triggering special application processing such as handling a failed transaction. User events can be used to send a message in a one-to-one or a one-to-many method.
Event subscription is established when the rtr_open_channel call is executed. See the RTR rtr_open_channel call description for details on this call. The rtr_open_channel call is as follows:
rtr_open_channel (channel,
flags,
facnam,
rcpnam,
pevtnum,
access,
numseg,
pkeyseg)
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Two parameters on the call are used to establish event subscription: rcpnam and pevtnum.
rcpnam is a pointer to an optional channel name for receiving event messages. If a User Event is sent to a particular channel name, only those subscribers that match both name AND event number are notified. For example, a client channel named "New York" and a client channel named "Hong Kong" could both subscribe to receive User Event number 999. If event 999 was triggered by the server using the channel named "Hong Kong," the event would be received only by the "Hong Kong" client. Specify RTR_NO_RCPNAM for this parameter if a name is not used. This parameter is case sensitive.
pevtnum is a pointer to lists of RTR and User event numbers to which the channel wants to subscribe. These lists use the numeric values of the events shown in Table 2-5. Use the special symbols in Table 2-6 to construct the event list.
| Symbol | Description |
|---|---|
| RTR_NO_PEVTNUM | No events selected. |
| RTR_EVTNUM_USERDEF | Begin User Event list. |
| RTR_EVTNUM_RTRDEF | Begin RTR Event list. |
| RTR_EVTNUM_ENDLIST | End of entire list. |
| RTR_EVTNUM_UP_TO | Specifies an event range in the form x RTR_EVTNUM_UP_TO y. |
| RTR_EVTNUM_USERBASE | Smallest User Event number (0). |
| RTR_EVTNUM_USERMAX | Largest User Event number (250). |
| RTR_EVTNUM_RTRBASE | Smallest RTR Event number. |
| RTR_EVTNUM_RTRMAX | Largest RTR Event number. |
Example 2-4 illustrates how to set up a list of all User Event numbers for the rtr_open_channel call.
| Example 2-4 User Event Example |
|---|
rtr_evtnum_t all_user_events[]={
RTR_EVTNUM_USERDEF,
RTR_EVTNUM_USERBASE,
RTR_EVTNUM_UP_TO,
RTR_EVTNUM_USERMAX,
RTR_EVTNUM_ENDLIST
} ;
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Example 2-5 illustrates how to set up a list of all RTR and User Event numbers for the rtr_open_channel call.
| Example 2-5 RTR and User Event Example |
|---|
rtr_evtnum_t all_events[]={
RTR_EVTNUM_USERDEF,
RTR_EVTNUM_USERBASE,
RTR_EVTNUM_UP_TO,
RTR_EVTNUM_USERMAX,
RTR_EVTNUM_RTRDEF,
RTR_EVTNUM_RTRBASE,
RTR_EVTNUM_UP_TO,
RTR_EVTNUM_RTRMAX,
RTR_EVTNUM_ENDLIST
} ;
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Sending Events
A broadcast event is triggered when the rtr_broadcast_event call is executed. See the rtr_broadcast_event call description for details on this call. The rtr_broadcast_event call syntax is as follows:
rtr_broadcast_event (channel,
flags,
pmsg,
msglen,
evtnum,
rcpspc,
msgfmt)
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The significant parameters on this call are:
channel is the channel identifier returned from the rtr_open_channel call.
pmsg is a pointer to the message to be broadcast.
msglen is the length in bytes of the message.
evtnum is the User Event number that the application developer has assigned to this event.
rcpspc is the optional recipient channel name that can be specified with the rcpnam parameter on the rtr_open_channel call.
Example 2-6 shows an example of an rtr_broadcast_event call.
| Example 2-6 Broadcast Event Example |
|---|
if ( bServerShutdown )
{
sts = rtr_broadcast_event (
/* channel */ BY_CHAN_CLIENT(cCurrentChannel,client)->chan,
/* flags */ RTR_NO_FLAGS,
/* pmsg */ &msgbuf,
/* msglen */ cbTotalSize,
/* evtnum */ USER_EVT_SHUTDOWN,
/* rcpnam */ "*",
/* msgfmt */ szMsgFmt);
exit_if_error ( "rtr_broadcast_event", sts );
}
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Receiving Events
Any RTR transaction, RTR Event, or User Event can be received when the application executes the rtr_receive_message call. See the RTR rtr_receive_message call description for details on this call. The rtr_receive_message call syntax is as follows:
rtr_receive_message (channel,
flags,
prcvchan,
pmsg,
maxlen,
timoutms,
pmsgsb)
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The significant parameters on this call are:
channel is the channel on which the message is received.
pmsg is a pointer to an application buffer where the message is written.
maxlen is the maximum length of the application buffer in bytes.
pmsgsb is a pointer to a message status block describing the received message.
If the application has subscribed to events, any call to rtr_receive_message can return an event notification, either an RTR Event notification or a User Event notification. The results are described in Table 2-7.
| If this notification is delivered: | the rtr_receive_message call returns a message of type: | and the user/application buffer contains the associated: |
|---|---|---|
| RTR Event | rtr_mt_rtr_event | event message |
| User Event | rtr_mt_user_event | user broadcast message |
Two RTR Events return key range data to the application:
RTR_EVTNUM_KEYRANGEGAIN
RTR_EVTNUM_KEYRANGELOSS
No other RTR Events return data.
The key range data are received in the message returned to the application, with the length of the message specified in the message status block ( msgsb ). For example, the following illustrates rtr_receive_message usage.
rtr_status_t rtr_receive_message ( rtr_channel_t *pchannel, rtr_rcv_flag_t flags, rtr_channel_t *p_rcvchan, rtr_msgbuf_t pmsg, rtr_msglen_t maxlen, rtr_timout_t timoutms, rtr_msgsb_t *p_msgsb ) |
The message status block pointed to by *p_msgsb has the following structure:
typedef struct {
rtr_msg_type_t msgtype;
rtr_usrhdl_t usrhdl;
rtr_msglen_t msglen;
rtr_tid_t tid;
rtr_evtnum_t evtnum;
}rtr_msgsb_t;
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When an event number is 102 or 103, RTR returns key range data (the low and high bounds) in the message, padded as required for data marshalling and interoperability. The key range data can be examined by the application. For more detail on data marshalling and formatting, see Section 2.15.
Bounds data are treated as if defined as a structure. For example, if there are two key segments defined as rtr_uns_8_t and rtr_uns_32_t , then the bounds data are copied to outbuf as if they were contained in the structure; that is, the 32-bit ints are correctly aligned in the structure and the structure size is a multiple of four. For example,
struct{
rtr_uns_8_t low_bound_1;
rtr_uns_32_t low_bound_2;
rtr_uns_8_t hi_bound_1;
rtr_uns_32_t hi_bound_2;
}
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The one four-byte-alignment-fits-all is required for interoperability; no padding is allowed.
Example 2-7, which can be run manually from the RTR CLI, illustrates the return of key range data with the RTR Event RTR_EVTNUM_KEYRANGELOSS . The RTR CLI interprets the format of this message as appropriate. In Example 2-7, the format is string or ASCII data, the default.
| Example 2-7 Returned Event Data Example |
|---|
RTR> crea fac jws/all=sucre %RTR-S-FACCREATED, facility jws created RTR> crea part ab/fac=jws/noshadow/nostandby- /key1=(type=string,length=2,offset=0,low="AB",high="CD") %RTR-I-PRTCREATE, partition created RTR> rtr_open/chan=s/server/noshadow/nostandby/part=ab/fac=jws %RTR-S-OK, normal successful completion RTR> rtr_rec/chan=s/time=10 %RTR-S-OK, normal successful completion channel name: S msgsb msgtype: rtr_mt_opened msglen: 8 message status: normal successful completion reason: 0x00000000 RTR> rtr_open/chan=c/client/event=(102,103)/fac=jws %RTR-S-OK, normal successful completion RTR> rtr_rec/chan=c/time=10 %RTR-S-OK, normal successful completion channel name: C msgsb msgtype: rtr_mt_opened msglen: 8 message status: normal successful completion reason: 0x00000000 RTR> rtr_close/chan=s %RTR-S-OK, normal successful completion RTR> rtr_rec/chan=c/time=10 %RTR-S-OK, normal successful completion channel name: C msgsb msgtype: rtr_mt_rtr_event msglen: 4 evtnum: 103 (RTR_EVTNUM_KEYRANGELOSS) message ks_lo_bound: 0x4241 ks_hi_bound: 0x4443 RTR> reca RTR> rtr_rec/chan=c/time=10 %RTR-E-TIMOUT, call to rtr_receive_message timed out |
Design consideration: When an RTR application executes an rtr_receive_message call, the programmer could incorrectly anticipate that a particular message type may be received and only write instructions to respond to the expected message. However, an RTR or User Event could be received on any instance of the rtr_receive_message call (as could other unanticipated RTR messages). Therefore, as a general application design guideline, the application developer should always program the application so that it can properly handle any type of message that could be received by the rtr_receive_message call.
Events are delivered in the order in which they are broadcast; therefore event serialization will be preserved for a particular user. However, RTR does not enforce any particular serialization across different subscribers, so different subscribers could receive event notifications in any order.
Example 2-8 shows an rtr_receive_message call in use.
| Example 2-8 Receive Message Example |
|---|
status = rtr_receive_message( &channel,
RTR_NO_FLAGS,
RTR_ANYCHAN,
&receive_msg,
RTR_ANYCHAN,
&receive_msg,
sizeof(receive_msg),
receive_time_out,
&msgsb);
check_status( "rtr_receive_message", status );
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RTR manages both event routing and event delivery.
Event Routing
When an event subscription is created with the rtr_open_channel call, the event details are stored in a subscriber database on all routers. When an event is triggered, notification is delivered to all routers connected to that system in that facility. The routers then check their subscriber database for any systems that have subscribed to that event. If one or more subscribers are located, and the subscribers are currently attached to this router, then the router broadcasts the message to the subscribers. If no subscriber is located, then the message is discarded.
Event Delivery
RTR will reliably deliver RTR transactions and RTR events. The delivery of User Events on a properly configured system is reliable; however, delivery of User Events is subject to RTR Flow Control if the subscriber cannot process events as quickly as they are delivered. Flow Control is a message traffic governor that helps the affected system to manage a spike in message traffic. We will discuss in more detail the interaction of Flow Control and RTR event processing.
When a User Event is triggered, a broadcast that includes message data is routed to the subscriber system. User Events, along with RTR Events and transactions, are placed into an incoming message queue on the destination system until the subscriber application executes an rtr_receive_message call to receive the message into the application. If too many messages are sent to the destination system, then the RTR Flow Control feature will be activated.
Flow Control may then force the sending application to wait awhile in the next RTR call that sends data, or it may discard broadcasts from the message queue, until the message queue length reduces and Flow Control allows new broadcasts to be sent to the destination system. Because User Event broadcasts are usually used for streaming information such as the periodic update of a price table, RTR does not store event messages that are impacted by Flow Control for later processing. This technique would cause the application to spend time viewing stale data. Instead, RTR Flow Control may discard the message to help relieve the messaging backlog, and will rely on a future message delivery to supply the updated information.
Design issue: Because of the possibility that a User Event message could become delayed or discarded due to Flow Control, User Events should not be used for delivering information that is of a business critical nature, including information that previously was, or later will be, used in a transaction. To compensate for the possibility of a discarded message, the application developer may consider adding a sequence number to the event message and providing a read-only transaction in the application to detect and request retransmission of any discarded broadcast data from the sender.
Overhead of Using Events
Delivery of User Events is based upon the registration databases that are kept on the routers. The event is delivered from the sender to all connected routers, which means each event triggers a message traffic load of 1 (for a FE sender) or the number of routers (for a BE sender). The event is then propagated by the routers to all subscribers, creating message traffic of 0 or the number of systems with subscribers to the event.
Design Issue: Processing event messages does consume some system resources and could impact overall performance. If system resources become constrained, RTR Flow Control may become active, thus reducing the RTR throughput on the affected systems. Care should be exercised to provide enough system resources to handle the message load.
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