Writing Event-based Clients and Servers

This topic includes the following sections:

•	Overview of Events

•	Defining the Unsolicited Message Handler

•	Sending Unsolicited Messages

•	Checking for Unsolicited Messages

•	Subscribing to Events

•	Unsubscribing from Events

•	Posting Events

•	Example of Event Subscription

Overview of Events

Event-based communication provides a method for an Oracle Tuxedo system process to be notified when a specific situation (event) occurs.

The Oracle Tuxedo system supports two types of event-based communication:

•	Unsolicited events

•	Brokered events

Unsolicited Events

Unsolicited events are messages used to communicate with client programs that are not waiting for and/or expecting a message.

Brokered Events

Brokered events enable a client and a server to communicate transparently with one another via an “anonymous” broker that receives and distributes messages. Such brokering is another client/server communication paradigm that is fundamental to the Oracle Tuxedo system.

The EventBroker is an Oracle Tuxedo subsystem that receives and filters event posting messages, and distributes them to subscribers. A poster is an Oracle Tuxedo system process that detects when a specific event has occurred and reports (posts) it to the EventBroker. A subscriber is an Oracle Tuxedo system process with a standing request to be notified whenever a specific event has been posted.

The Oracle Tuxedo system does not impose a fixed ratio of service requesters to service providers; an arbitrary number of posters can post a message buffer for an arbitrary number of subscribers. The posters simply post events, without knowing which processes receive the information or how the information is handled. Subscribers are notified of specified events, without knowing who posted the information. In this way, the EventBroker provides complete location transparency.

Typically, EventBroker applications are designed to handle exception events. An application designer must decide which events in the application constitute exception events and need to be monitored. In a banking application, for example, it might be useful to post an event whenever an unusually large amount of money is withdrawn, but it would not be particularly useful to post an event for every withdrawal transaction. In addition, not all users would need to subscribe to that event; perhaps only the branch manager would need to be notified.

Notification Actions

The EventBroker may be configured such that whenever an event is posted, the EventBroker invokes one or more notification actions for clients and/or servers that have subscribed. Table 8‑1 lists the types of notification actions that the EventBroker can take.

 

Table 8‑1 EventBroker Notification Actions

Notification Action	Description
Unsolicited notification message	Clients may receive event notification messages in their unsolicited message handling routine, just as if they were sent by the tpnotify() function.
Service call	Servers may receive event notification messages as input to service routines, just as if they were sent by the tpacall() function.
Reliable queue	Event notification messages may be stored in an Oracle Tuxedo system reliable queue, using the tpenqueue(3c) function. Event notification buffers are stored until requests for buffer contents are issued. An Oracle Tuxedo system client or server process may call the tpdequeue(3c) function to retrieve these notification buffers, or alternately TMQFORWARD(5) may be configured to automatically dispatch an Oracle Tuxedo system service routine that retrieves a notification buffer. For more information on /Q, see Using the ATMI /Q Component.

In addition, the application administrator may create an EVENT_MIB(5) entry (by using the Oracle Tuxedo administrative API) that performs the following notification actions:

•	Invokes a system command

•	Writes a message to the system’s log file on disk

Note:

Only the Oracle Tuxedo application administrator is allowed to create an EVENT_MIB(5) entry.

For information on the EVENT_MIB(5), refer to the File Formats, Data Descriptions, MIBs, and System Processes Reference.

EventBroker Servers

TMUSREVT is the Oracle Tuxedo system-supplied server that acts as an EventBroker for user events. TMUSREVT processes event report message buffers, and then filters and distributes them. The Oracle Tuxedo application administrator must boot one or more of these servers to activate event brokering.

TMSYSEVT is the Oracle Tuxedo system-supplied server that acts as an EventBroker for system-defined events. TMSYSEVT and TMUSREVT are similar, but separate servers are provided to allow the application administrator the ability to have different replication strategies for processing notifications of these two types of events. Refer to Setting Up an Oracle Tuxedo Application for additional information.

System-defined Events

The Oracle Tuxedo system itself detects and posts certain predefined events related to system warnings and failures. These tasks are performed by the EventBroker. For example, system-defined events include configuration changes, state changes, connection failures, and machine partitioning. For a complete list of system-defined events detected by the EventBroker, see EVENTS(5) in the File Formats, Data Descriptions, MIBs, and System Processes Reference.

System-defined events are defined in advance by the Oracle Tuxedo system code and do not require posting. The name of a system-defined event, unlike that of an application-defined event, always begins with a dot (“.”). Names of application-defined events may not begin with a leading dot.

Clients and servers can subscribe to system-defined events. These events, however, should be used mainly by application administrators, not by every client in the application.

When incorporating the EventBroker into your application, remember that it is not intended to provide a mechanism for high-volume distribution to many subscribers. Do not attempt to post an event for every activity that occurs, and do not expect all clients and servers to subscribe. If you overload the EventBroker, system performance may be adversely affected and notifications may be dropped. To minimize the possibility of overload, the application administrator should carefully tune the operating system IPC resources, as explained in Installing the Oracle Tuxedo System.

Programming Interface for the EventBroker

EventBroker programming interfaces are available for all Oracle Tuxedo system server and client processes, including Workstation, in both C and COBOL.

The programmer’s job is to code the following sequence:

1.	A client or server posts a buffer to an application-defined event name.

2.	The posted buffer is transmitted to any number of processes that have subscribed to the event.

Subscribers may be notified in a variety of ways (as discussed in “Notification Actions”), and events may be filtered. Notification and filtering are configured through the programming interface, as well as through the Oracle Tuxedo system administrative API.

Defining the Unsolicited Message Handler

To define the unsolicited message handler function, use the tpsetunsol(3c) function with the following signature:

int
tpsetunsol(*myfunc)

If you are running on Windows-based operating systems you must declare unsolicited message handler functions as:

void _TMDLLENTRY CustomerUnsolFunc(char *data, long len, long flags)

The _TMDLLENTRY macro is required for Windows-based operating systems to obtain the proper calling conventions between the Tuxedo libraries and your code.

On Unix systems, the _TMDLLENTRY macro is not required because it expands to the null string.

Table 8‑2 describes the single argument that can be passed to the tpsetunsol() function.

 

Table 8‑2 tpsetunsol( ) Function Argument

Argument	Description
myfunc	Pointer to a function that conforms to the prototype of a call-back function. In order to conform, the function must accept the following three parameters: • data—points to the typed buffer that contains the unsolicited message • len—length of the buffer • flags—currently not used

When a client receives an unsolicited notification, the system dispatches the call-back function with the message. To minimize task disruption, you should code the unsolicited message handler function to perform only minimal processing tasks, so it can return quickly to the waiting process.

Sending Unsolicited Messages

The Oracle Tuxedo system allows unsolicited messages to be sent to client processes without disturbing the processing of request/response calls or conversational communications.

Unsolicited messages can be sent to client processes by name, using tpbroadcast(3c), or by an identifier received with a previously processed message, using tpnotify(3c). Messages sent via tpbroadcast() can originate either in a service or in another client. Messages sent via tpnotify() can originate only in a service.

 

Broadcasting Messages by Name

The tpbroadcast(3c) function allows a message to be sent to registered clients of the application. It can be called by a service or another client. Registered clients are those that have successfully made a call to tpinit() and have not yet made a call to tpterm().

Use the following signature to call the tpbroadcast() function:

int
tpbroadcast(char *lmid, char *usrname, char *cltname, char *data, long len, long flags)

Table 8‑3 describes the arguments to the tpbroadcast() function.

 

Table 8‑3 tpbroadcast( ) Function Arguments

Argument	Description
lmid	Pointer to the logical machine identifier for the client. A value of NULL acts as a wildcard, so that a message can be directed to groups of clients.
usrname	Pointer to the username of the client process, if one exists. A value of NULL acts as a wildcard, so that a message can be directed to groups of clients.
cltname	Pointer to the client name of the client process, if one exists. A value of NULL acts as a wildcard, so that a message can be directed to groups of clients.
data	Pointer to the content of a message.
len	Size of the message buffer. If data points to a self-defining buffer type, for example, FML, then len can be set to 0.
flags	Flag options. Refer to tpbroadcast(3c) in the Oracle Tuxedo ATMI C Function Reference for information on available flags.

Listing 8‑1 illustrates a call to tpbroadcast() for which all clients are targeted. The message to be sent is contained in a STRING buffer.

Listing 8‑1 Using tpbroadcast( )

char *strbuf;

if ((strbuf = tpalloc("STRING", NULL, 0)) == NULL) {
          error routine
          }

(void) strcpy(strbuf, "hello, world");

if (tpbroadcast(NULL, NULL, NULL, strbuf, 0, TPSIGRSTRT) == -1)
              error routine

 

Broadcasting Messages by Identifier

The tpnotify(3c) function is used to broadcast a message using an identifier received with a previously processed message. It can be called only from a service.

Use the following signature to call the tpnotify() function:

int
tpnotify(CLIENTID *clientid, char *data, long len, long flags)

Table 8‑4 describes the arguments to the tpnotify() function.

 

Table 8‑4 tpnotify( ) Function Arguments

Argument	Description
clientid	Pointer to a CLIENTID structure that is saved from the TPSVCINFO structure that accompanied the request to this service.
data	Pointer to the content of the message.
len	Size of the message buffer. If data points to a self-defining buffer type, for example, FML, then len can be set to 0.
flags	Flag options. Refer to tpnotify(3c) in the Oracle Tuxedo ATMI C Function Reference for information on available flags.

Checking for Unsolicited Messages

To check for unsolicited messages while running the client in “dip-in” notification mode, use the tpchkunsol(3c) function with the following signature:

int
tpchkunsol()

The function takes no arguments.

If any messages are pending, the system invokes the unsolicited message handling function that was specified using tpsetunsol(). Upon completion, the function returns either the number of unsolicited messages that were processed or -1 on error.

If you issue this function when the client is running in SIGNAL-based, thread-based notification mode, or is ignoring unsolicited messages, the function has no impact and returns immediately.

Subscribing to Events

The tpsubscribe(3c) function enables an Oracle Tuxedo system ATMI client or server to subscribe to an event.

A subscriber can be notified through an unsolicited notification message, a service call, a reliable queue, or other notification methods configured by the application administrator. (For information about configuring alternative notification methods, refer to Setting Up an Oracle Tuxedo Application.)

Use the following signature to call the tpsubscribe() function:

long handle
tpsubscribe (char *eventexpr, char *filter, TPEVCTL *ctl, long flags)

Table 8‑5 describes the arguments to the tpsubscribe() function.

 

Table 8‑5 tpsubscribe( ) Function Arguments

Argument	Description
eventexpr	Pointer to a set of one or more events to which a process can subscribe. Consists of a NULL-terminated string of up to 255 characters containing a regular expression. Regular expressions are of the form specified in tpsubscribe(3c), as described in the Oracle Tuxedo ATMI C Function Reference). For example, if eventexpr is set to: • "\\..*"—the caller is subscribing to all system-defined events. • "\\.SysServer.*"—the caller is subscribing to all system-defined events related to servers. • "[A-Z].*"—the caller is subscribing to all user events starting with any uppercase letter between A and Z. • ".*(ERR|err).*"—the caller is subscribing to all user events with names that contain either err or ERR, such as the account_error and ERROR_STATE events, respectively.
filter	Pointer to a string containing a Boolean filter rule that must be evaluated successfully before the EventBroker posts the event. Upon receiving an event to be posted, the EventBroker applies the filter rule, if one exists, to the posted event’s data. If the data passes the filter rule, the EventBroker invokes the notification method specified; otherwise, the EventBroker ignores the notification method. The caller can subscribe to the same event multiple times with different filter rules. By using the event-filtering capability, subscribers can discriminate among the events about which they are notified. For example, a poster can post an event for withdrawals greater than $10,000, but a subscriber may want to specify a higher threshold for being notified, such as $50,000. Or, a subscriber may want to be notified of large withdrawals made by specific customers. Filter rules are specific to the typed buffers to which they are applied. For more information on filter rules, refer to tpsubscribe(3c) in the Oracle Tuxedo ATMI C Function Reference.
ctl	Pointer to a flag for controlling how a subscriber is notified of an event. Valid values include: • NULL—sends unsolicited messages. Refer to “Notification via Unsolicited Message” on page 8‑10 for more information. • Pointer to a valid TPEVCTL structure—sends information based on the TPEVCTL structure. Refer to “Notification via Service Call or Reliable Queue” on page 8‑10 for more information.
flags	Flag options. For more information on available flag options, refer to tpsubscribe(3c) in the Oracle Tuxedo ATMI C Function Reference.

You can subscribe to both system- and application-defined events using the tpsubscribe() function.

For purposes of subscriptions (and for MIB updates), service routines executed in an Oracle Tuxedo system server process are considered to be trusted code.

Notification via Unsolicited Message

If a subscriber is an Oracle Tuxedo system client process and ctl is NULL, when the event to which the client has subscribed is posted, the EventBroker sends an unsolicited message to the subscriber as follows. When an event name is posted that evaluates successfully against eventexpr, the EventBroker tests the posted data against the associated filter rule. If the data passes the filter rule (or if there is no filter rule for the event), then the subscriber receives an unsolicited notification along with any data posted with the event.

In order to receive unsolicited notifications, the client must register an unsolicited message handling routine using the tpsetunsol() function.

ATMI clients receiving event notification via unsolicited messages should remove their subscriptions from the EventBroker list of active subscriptions before exiting. This is done using the tpunsubscribe() function.

Notification via Service Call or Reliable Queue

Event notification via service call enables you to program actions that can be taken in response to specific conditions in your application without human intervention. Event notification via reliable queue ensures that event data is not lost. It also provides the subscriber the flexibility of retrieving the event data at any time.

If the subscriber (either a client or a server process) wants event notifications sent to service routines or to stable-storage queues, then the ctl parameter of tpsubscribe() must point to a valid TPEVCTL structure.

The TPEVCTL structure contains the following elements:

long flags;
char name1[127];
char name2[127];
TPQCTL qctl;

Table 8‑6 summarizes the TPEVCTL typed buffer data structure.

 

Table 8‑6 TPEVCTL Typed Buffer Format

Field	Description
flags	Flag options. For more information on flags, refer to tpsubscribe(3c) in the Oracle Tuxedo ATMI C Function Reference.
name1	Character string of 127 characters or fewer.
name2	Character string of 127 characters or fewer.
qctl	TPQCTL structure. For more information, refer to tpsubscribe(3c) in the Oracle Tuxedo ATMI C Function Reference.

Unsubscribing from Events

The tpunsubscribe(3c) function enables an Oracle Tuxedo system ATMI client or server to unsubscribe from an event.

Use the following signature to call the tpunsubscribe() function:

int
tpunsubscribe (long subscription, long flags)

Table 8‑7 describes the arguments to the tpunsubscribe() function.

 

Table 8‑7 tpunsubscribe( ) Function Arguments

Argument	Description
subscription	Subscription handle returned by a call to tpsubscribe().
flags	Flag options. For more information on available flag options, refer to tpunsubscribe(3c) in the Oracle Tuxedo ATMI C Function Reference.

Posting Events

The tppost(3c) function enables an Oracle Tuxedo ATMI client or server to post an event.

Use the following signature to call the tppost() function:

tppost(char *eventname, char *data, long len, long flags)

Table 8‑8 describes the arguments to the tppost() function.

 

Table 8‑8 tppost( ) Function Arguments

Argument	Description
eventname	Pointer to an event name containing up to 31 characters plus NULL. The first character cannot be a dot (“.”) because the dot is reserved as the first character in names of Oracle Tuxedo system-defined events. When defining event names, keep in mind that subscribers can use wildcard capabilities to subscribe to multiple events with a single function call. Using the same prefix for a category of related event names can be helpful.
data	Pointer to a buffer previously allocated using the tpalloc() function.
len	Size of data buffer that should be posted with the event. If data points to a buffer of a type that does not require a length to be specified (for example, an FML fielded buffer) or if you set it to NULL, the len argument is ignored and the event is posted with no data.
flags	Flag options. For more information on available flag options, refer to tppost(3c) in the Oracle Tuxedo ATMI C Function Reference.

Listing 8‑2 illustrates an event posting taken from the Oracle Tuxedo system sample application bankapp. This example is part of the WITHDRAWAL service. One of the functions of the WITHDRAWAL service is checking for withdrawals greater than $10,000 and posting an event called BANK_TLR_WITHDRAWAL.

Listing 8‑2 Posting an Event with tppost( )

.
.
.
/* Event logic related */
static float evt_thresh = 10000.00 ; /* default for event threshold */
static char emsg[200] ; /* used by event posting logic */
.
.
.
/* Post a BANK_TLR_WITHDRAWAL event ? */
if (amt < evt_thresh) {
     /* no event to post */
     tpreturn(TPSUCCESS, 0,transb->data , 0L, 0);
}
/* prepare to post the event */
if ((Fchg (transf, EVENT_NAME, 0, "BANK_TLR_WITHDRAWAL", (FLDLEN)0) == -1) ||
(Fchg (transf, EVENT_TIME, 0, gettime(), (FLDLEN)0) == -1) ||
(Fchg (transf, AMOUNT, 0, (char *)&amt, (FLDLEN)0) == -1)) {
     (void)sprintf (emsg, "Fchg failed for event fields: %s",
     Fstrerror(Ferror)) ;
}
/* post the event */
else if (tppost ("BANK_TLR_WITHDRAWAL", /* event name */
(char *)transf, /* data */
0L, /* len */
TPNOTRAN | TPSIGRSTRT) == -1) {
/* If event broker is not reachable, ignore the error */
     if (tperrno != TPENOENT)
     (void)sprintf (emsg, "tppost failed: %s", tpstrerror (tperrno));
}

 

This example simply posts the event to the EventBroker to indicate a noteworthy occurrence in the application. Subscription to the event by interested clients, who can then take action as required, is done independently.

Example of Event Subscription

The following example illustrates a portion of a bankapp application server that subscribes to BANK_TLR_.* events, which includes the BANK_TLR_WITHDRAWAL event shown in the previous example, as well as any other event names beginning with BANK_TLR_. When a matching event is posted, the application notifies the subscriber via a call to a service named WATCHDOG.

Listing 8‑3 Subscribing to an Event with tpsubscribe( )

.
.
.
/* Event Subscription handles */
static long sub_ev_largeamt = 0L ;
.
.
.
/* Preset default for option 'w' - watchdog threshold */
(void)strcpy (amt_expr, "AMOUNT > 10000.00") ;
.
.
.
/*
* Subscribe to the events generated
* when a "large" amount is transacted.
*/
evctl.flags = TPEVSERVICE ;
(void)strcpy (evctl.name1, "WATCHDOG") ;
/* Subscribe */
sub_ev_largeamt = tpsubscribe ("BANK_TLR_.*",amt_expr,&evctl,TPSIGRSTRT) ;
if (sub_ev_largeamt == -1L) {
     (void)userlog ("ERROR: tpsubscribe for event BANK_TLR_.* failed: %s",
     tpstrerror(tperrno)) ;
     return -1 ;
}
.
.
.
{
/* Unsubscribe to the subscribed events */
if (tpunsubscribe (sub_ev_largeamt, TPSIGRSTRT) == -1)
    (void)userlog ("ERROR: tpunsubscribe to event BANK_TLR_.* failed: %s",
    tpstrerror(tperrno)) ;
    return ;
}
/*
* Service called when a BANK_TLR_.* event is posted.
*/
void
#if defined(__STDC__) || defined(__cplusplus)
WATCHDOG(TPSVCINFO *transb)
#else
WATCHDOG(transb)
TPSVCINFO *transb;
#endif
{
FBFR *transf; /* fielded buffer of decoded message */
/* Set pointr to TPSVCINFO data buffer */
transf = (FBFR *)transb->data;
/* Print the log entry to stdout */
(void)fprintf (stdout, "%20s|%28s|%8ld|%10.2f\n",
Fvals (transf, EVENT_NAME, 0),
Fvals (transf, EVENT_TIME, 0),
Fvall (transf, ACCOUNT_ID, 0),
*( (float *)CFfind (transf, AMOUNT, 0, NULL, FLD_FLOAT)) );
/* No data should be returned by the event subscriber's svc routine */
tpreturn(TPSUCCESS, 0,NULL, 0L, 0);
}