The DBMS_ALERT package provides a facility to broadcast notification of database events (alerts) to multiple users who have previously registered their interest in receiving those alerts. You will use the DBMS_ALERT package to implement applications that respond immediately to data modifications of interest to the application. In this way, you can avoid the need to do regular polling on the data to determine if changes have taken place. This is typically accomplished by having the application register to receive an alert on the specific data of interest, querying the data to establish a baseline, and then waiting for the alert to be signaled, which indicates the need to requery the data. Alerts can be automatically signaled using database triggers on the tables of interest, so that all modifications to the data will signal the alert, regardless of which application or user modified the data. Alerts are asynchronous and transaction-based, meaning that users can wait for and receive notification after the signaling event and that only committed transactions (usually involving data changes) will signal the alert.
Here are two good examples of applications that could be implemented using DBMS_ALERT:
The DBMS_ALERT package is created when the Oracle database is installed. The dbmsalrt.sql script (found in the built-in packages source code directory, as described in Chapter 1 ) contains the source code for this package's specification. This script is called by catproc.sql , which is normally run immediately after database creation. Under Oracle7, no privileges are automatically granted on DBMS_ALERT. Under Oracle8, the EXECUTE_CATALOG_ROLE role is granted EXECUTE privilege on DBMS_ALERT. Thus the DBMS_ALERT programs are not generally available to users. Access to DBMS_ALERT is obtained by granting EXECUTE privilege explicitly to users or roles that require use of the package.
Note also that a public synonym for DBMS_ALERT is not created automatically by dbmsalrt.sql , so references to the package's programs must be qualified by the owning schema (SYS), unless synonyms have been created. To create a public synonym for DBMS_ALERT, issue the following SQL command:
CREATE PUBLIC SYNONYM DBMS_ALERT FOR SYS.DBMS_ALERT;
Table 3.2 lists the programs included in the DBMS_ALERT package.
The DBMS_ALERT package contains one nonprogram element, maxwait. It is defined as follows:
maxwait CONSTANT INTEGER := 86400000;
The maxwait constant is the maximum time to wait for an alert. It is used as the default value for the timeout parameter in the WAITONE and WAITANY procedures. The value of 86400000 seconds corresponds to 1000 days.
This section describes the programs available through the DBMS_ALERT package.
188.8.131.52 The DBMS_ALERT. REGISTER procedure
The REGISTER procedure registers interest in a specific alert by a database session. Once registered, the session will be notified of any occurrences of the alert. The header for this procedure is,
PROCEDURE DBMS_ALERT.REGISTER (name IN VARCHAR2);
where name is the name of the alert to register for notification.
The REGISTER procedure does not raise any package exceptions. It will raise an ORA-20000 exception for specific error conditions, with message text indicating the error as follows:
Note the following restrictions on calling REGISTER:
In this example, the session will be registered to be notified of the EMP_INSERT alert, which is raised whenever INSERTs are performed on the EMP table:
PROCEDURE DBMS_ALERT.REMOVE (name IN VARCHAR2);
where name is the name of the alert to unregister from notification.
The REMOVE procedure does not raise any package exceptions, nor does it assert a purity level with the RESTRICT_REFERENCES pragma.
The following example unregisters for the EMP_INSERT alert:
BEGIN DBMS_ALERT.REMOVE('EMP_INSERT'); END;
The REMOVEALL procedure does not raise package exceptions, nor does it assert a purity level with the RESTRICT_REFERENCES pragma.
This example stops all alert notifications to the session:
184.108.40.206 The DBMS_ALERT. SET_DEFAULTS procedure
The SET_DEFAULTS procedure is used to set session configurable settings used by the DBMS_ALERT package. Currently, the polling loop interval sleep time is the only session setting that can be modified using this procedure. The header for this procedure is,
PROCEDURE DBMS_ALERT.SET_DEFAULTS (sensitivity IN NUMBER);
where sensitivity is the polling interval sleep time in seconds.
The SET_DEFAULTS procedure does not raise any package exceptions.
The following example sets the polling interval to one second:
SQL> execute DBMS_ALERT.SET_DEFAULTS(600);
Setting the polling interval is relevant primarily to users of DBMS_ALERT under Oracle Parallel Server (OPS), since under OPS a polling loop is required to check for alerts issued from another Oracle instance.
220.127.116.11 The DBMS_ALERT. SIGNAL procedure
The SIGNAL procedure posts notification of the occurrence of an alert, which is then propagated to all sessions registered for the alert. Alert notification happens only if and when the signaling transaction COMMITs. Here's the header:
PROCEDURE DBMS_ALERT.SIGNAL (name IN VARCHAR2 ,message IN VARCHAR2);
Parameters are summarized in the following table.
When you are signaling alerts using SIGNAL, it is important to COMMIT (or ROLLBACK) the signaling transaction as soon as possible. Several problems can develop when signaling transactions are held open too long, including the following:
Multiple sessions can signal the same alert. Note that this process is serialized using DBMS_LOCK (described in Chapter 4, User Lock and Transaction Management ) and can add significant wait times unless transactions are closed quickly (as noted earlier).
This program does not raise any package exceptions. It will raise an ORA-20000 exception for specific error conditions, with message text indicating the error as follows:
Note the following restrictions on calling SIGNAL:
This trigger will signal the EMP_INSERT alert when rows are inserted into the EMP table. The empid column is passed as the alert message for receivers of the alert to use:
The WAITANY procedure waits for notification of any alerts for which the session is registered. The procedure call will complete when the first alert is signaled or when the wait timeout is reached. Here's the header:
PROCEDURE DBMS_ALERT.WAITANY (name OUT VARCHAR2 ,message OUT VARCHAR2 ,status OUT INTEGER ,timeout IN NUMBER DEFAULT MAXWAIT);
Parameters are summarized in the following table.
When multiple alerts for which the session is registered have been signaled, the call to WAITANY will return the most recent alert that has occurred.
If a session waits on an alert that it has also signaled, a lock request exception will occur unless a COMMIT has taken place between the calls to the SIGNAL and WAITANY procedures.
The WAITANY call uses a polling loop to detect alerts. This avoids notification problems that could otherwise occur when signaled, but uncommitted alerts mask notification of subsequent committed alerts. The polling loop begins with a 1-second interval that increases exponentially to 30 seconds.
The program does not raise any package exceptions. The program will raise an ORA-20000 exception for specific error conditions, with message text indicating the error as follows:
Note the following restrictions on WAITANY:
This example waits five minutes to receive the next alert for which the session is registered. If an alert is received, it is displayed. If the EMP_INSERT alert is received, the employee id should be the message, and the employee status is changed without displaying the alert.
DECLARE alert_msg VARCHAR2(1800); alert_status INTEGER; alert_name; BEGIN DBMS_ALERT.WAITANY(alert_name, alert_msg, alert_status, 300); IF alert_status = 1 THEN DBMS_OUTPUT.PUT_LINE('timed out'); ELSIF alert_name = 'EMP_INSERT' THEN UPDATE emp SET status = 'REGISTERED' WHERE empid := alert_msg; ELSE DBMS_OUTPUT.PUT_LINE('received alert: '||alert_name); END IF; END; /
PROCEDURE DBMS_ALERT.WAITONE (name IN VARCHAR2 ,message OUT VARCHAR2 ,status OUT INTEGER ,timeout IN NUMBER DEFAULT MAXWAIT);
Parameters are summarized in the following table.
Note the following special cases:
This program does not raise any package exceptions. The program will raise an ORA-20000 exception for specific error conditions, with message text indicating the error as follows:
Note the following restrictions on WAITONE:
This example waits specifically on the EMP_INSERT alert and updates the status when it is signaled:
DECLARE alert_msg VARCHAR2(1800); alert_status INTEGER; BEGIN DBMS_ALERT.WAITONE('EMP_INSERT', alert_msg, alert_status, 300); IF alert_status = 1 THEN DBMS_OUTPUT.PUT_LINE('timed out'); ELSE UPDATE emp SET status = 'REGISTERED' WHERE empid := alert_msg; DBMS_OUTPUT.PUT_LINE('employee registered'); END IF; END; /
The DBMS_ALERT package is a good example of how you can build higher-level functionality out of lower-level built-ins. Both the DBMS_LOCK and DBMS_PIPE packages are used extensively in the implementation of DBMS_ALERT.
An important feature of the alerting mechanism in DBMS_ALERT is that it is transaction-based. This means that alerts will be sent to registered sessions only if and when the signaling session issues a COMMIT. If the signaler issues a ROLLBACK instead, the alerts will not be sent. Applications that are interested only in real changes to data in the database will benefit from using transaction-based alerts. Applications that need to signal other sessions regardless of transaction boundaries or data modifications (like debuggers or auditing monitors) will probably need to use DBMS_PIPE instead of DBMS_ALERT.
What kind of application might actually need to be alerted to changes in data? The classic example given in the Oracle documentation is a continuous graphical display of data extracted from some table. Pulling data from the table at set intervals using a polling mechanism can be very inefficient. For one thing, the data may not have changed since the last pull, so a refresh is not really necessary. Also, if the application is separated from the database by a network (as it most likely would be), then the overhead of redundant data extraction is multiplied. In this example, the application could use DBMS_ALERT to suspend itself and wait for a signal to awaken and pull new data for the display. The signal will be received only when data in the table has actually been modified (i.e., a new pull is truly necessary).
Well, I wanted to do something new and original. I spent some time thinking about other examples for using DBMS_ALERT. Finally, I realized that I had participated in a perfect application for this technology many times already: an online auction. During an auction (especially a virtual one over a computer network), it is important to know when an item you have a bidding interest in has been bid upon. In a traditional auction, this happens because items are auctioned off serially, so bids can only be placed on the current item. In an online auction, the participants are not in a room together, and the auction itself typically takes longer than a traditional auction. Also, it is desirable to auction multiple items simultaneously, taking advantage of the virtual nature of the auction. An auction application that notifies participants of bidding activity relevant (to them) would relieve them of having to constantly monitor their screens to stay abreast of the auction. Bidding could take place simultaneously on multiple items since users interested in those items would automatically be notified of new bids.
The online auction was perfect for DBMS_ALERT, so I set about to prove the concept. First, I needed a basic schema. Professional data modelers may wince, but I came up with the following:
These objects are created by the auction.ddl script, reproduced as follows:
/* Filename on companion disk: auction.ddl */* rem ********************************************************* rem AUCTION.DDL rem rem Creates objects used in the "online auction" example rem for the DBMS_ALERT package. rem rem Auction_items -- table of items being auctioned rem Bids -- table of bids placed on items rem High_bids -- view showing the current high bids on rem items and who placed them rem rem Author: John Beresniewicz, Savant Corp rem rem rem 12/07/97: created rem ********************************************************* DROP VIEW high_bids; DROP TABLE bids; DROP TABLE auction_items; CREATE TABLE auction_items (id VARCHAR2(20) NOT NULL PRIMARY KEY ,description VARCHAR2(200) NOT NULL ,min_bid NUMBER NOT NULL ,curr_bid NUMBER ,status VARCHAR2(10) CONSTRAINT valid_status CHECK (status IN ('OPEN','CLOSED') ) ); CREATE TABLE bids (bidder VARCHAR2(30) ,item_id VARCHAR2(20) REFERENCES auction_items(id) ON DELETE CASCADE ,bid NUMBER NOT NULL ); CREATE OR REPLACE VIEW high_bids (item_id ,item_desc ,bidder ,high_bid) AS SELECT BID.item_id ,AI.description ,BID.bidder ,BID.bid FROM bids BID ,auction_items AI WHERE BID.item_id = AI.id AND BID.bid = (SELECT MAX(bid) FROM bids B2 WHERE BID.item_id = B2.item_id) /
The AUCTION_ITEMS table contains an identifier and a description of each auction item. There are also columns for the minimum bid, status, and current high bid. This latter is really redundant with information derived in the HIGH_BIDS view, but this denormalization makes for a more interesting example.
The BIDS table holds the bidding activity. Each bid is a bid on an auction_item by a user for a specified amount. Originally, I had a BIDDERS table to track the auction participants, and this would likely be necessary for a real-world application. However, to simplify the example I decided to use the Oracle session username to identify bidders. Thus, there is an assumption that the online auction users will all be connected using unique usernames. The BIDS table also has a complex integrity constraint, which states that all bids must exceed the previous high bid for the same item (this is, after all, how an auction works). An additional constraint is that no bids may be updated or deleted from the table. These constraints are enforced by database triggers discussed later.
The HIGH_BIDS view selects the highest bid for each item along with the item's description and the bidder who made the bid. The auction application's GUI component can make use of this view to display current bidding levels for all items.
Some basic requirements of the online auction application are as follows:
There is certainly more than one way to satisfy these requirements, especially the data integrity constraints on the two tables. I decided to implement a combination of database triggers and a package called auction. The database triggers enforce some data integrity constraints and signal changes to interested bidders using DBMS_ALERT.SIGNAL. A procedure called place_bid is responsible for placing bids on items, making sure that the complex integrity constraint is satisfied, and that the bidder is registered to receive notice of any bidding or status changes on the item. Another packaged procedure, called watch_bidding, demonstrates how an application might use DBMS_ALERT.WAITANY to be alerted for any bidding activity of interest to the user.
One immediate issue to address is what the alert names should be. The auction_items.id column seems a natural option since all alerts will concern a specific item.
Here are the triggers for the auction_items and bids tables:
/* Filename on companion disk: auction2.sql */* CREATE OR REPLACE TRIGGER auction_items_ARU AFTER UPDATE ON auction_items FOR EACH ROW BEGIN /* || trigger enforces no update of item_id and also || signals an alert when status changes */ IF UPDATING ('ITEM_ID') THEN RAISE_APPLICATION_ERROR(-20000, 'Cannot update item id'); ELSIF UPDATING ('STATUS') AND (:NEW.status != :OLD.status) THEN /* send new status on as the alert message */ DBMS_ALERT.SIGNAL(:NEW.id, :NEW.status); END IF; END auction_items_ARU; / CREATE OR REPLACE TRIGGER bids_ARIUD AFTER INSERT OR UPDATE OR DELETE ON bids FOR EACH ROW BEGIN /* || enforce all bids are final rule */ IF UPDATING OR DELETING THEN RAISE_APPLICATION_ERROR (-20001, 'Cannot update or delete, all bids final!'); ELSE /* || signal alert on item, send bidder name as message */ DBMS_ALERT.SIGNAL(:NEW.item_id, :NEW.bidder); END IF; END bids_ARIUD; /
The triggers enforce the basic integrity rules that auction_items.id is a non-updatable column and that rows in the BIDS table cannot be updated or deleted. More importantly, they signal database alerts to registered sessions that auction data has changed using DBMS_ALERT.SIGNAL. The trigger on auction items signals status changes for items. Note the additional check requiring that :NEW.status be different from :OLD.status in order for the alert to be signaled. Also note that the item id is used as the alert name and that the new item status is passed as the alert's message. The trigger on BIDS signals the alert named by the item id and passes the bidder's name as the message. The use of the message parameter with the alerts allows the alert receiver to implement a context-sensitive response to the alert.
By the way, my naming convention for triggers has the table name suffixed by a string like [ A|B ][ R|S ][ I|U|D ] where:
The rest of the online auction requirements are implemented in the auction package. Here is the package specification:
/* Filename on companion disk: auction1.sql */* CREATE OR REPLACE PACKAGE auction /* || Implements a simple interactive bidding system || using DBMS_ALERT to keep bidders informed || of activity in items they are interested in. || || The item_id is used as the ALERT name for the || item. || || Author: John Beresniewicz, Savant Corp || || 12/07/97: created || || Compilation Requirements: || || EXECUTE on DBMS_ALERT || SELECT, UPDATE on AUCTION_ITEMS || INSERT on BIDS || SELECT on HIGH_BIDS || || Execution Requirements: || */ AS /* || exceptions raised and handled in PLACE_BID || procedure */ invalid_item EXCEPTION; bid_too_low EXCEPTION; item_is_closed EXCEPTION; /* || place a bid on an item, the bid must exceed any || other bids on the item (and the minimum bid) || || bidding on an item registers interest in the || item using DBMS_ALERT.REGISTER || || only this procedure should be used to add rows || to the bids table, since it also updates || auction_items.curr_bid column */ PROCEDURE place_bid (item_id_IN IN VARCHAR2 ,bid_IN IN NUMBER); /* || close bidding on an item */ PROCEDURE close_item(item_id_IN IN VARCHAR2); /* || watch for any alerts on items bid by the user || indicating other users have raised the bid */ PROCEDURE watch_bidding(timeout_secs_IN IN NUMBER:=300); END auction; /
The place_bid procedure is intended to be used by the GUI application to place all bids in the auction. No INSERTS or UPDATES to the BIDS table should be allowed except through this procedure, as it maintains the complex integrity constraint on the table, updates the curr_bid column of AUCTION_ITEMS, and registers the session for receiving alerts on the item. The body of place_bid looks like this:
/* Filename on companion disk: auction1.sql */* PROCEDURE place_bid (item_id_IN IN VARCHAR2 ,bid_IN IN NUMBER) IS temp_curr_bid auction_items.curr_bid%TYPE; temp_statusauction_items.status%TYPE; CURSOR auction_item_cur IS SELECT NVL(curr_bid,min_bid), status FROM auction_items WHERE id = item_id_IN FOR UPDATE OF curr_bid; BEGIN /* || lock row in auction_items */ OPEN auction_item_cur; FETCH auction_item_cur INTO temp_curr_bid, temp_status; /* || do some validity checks */ IF auction_item_cur%NOTFOUND THEN RAISE invalid_item; ELSIF temp_status = 'CLOSED' THEN RAISE item_is_closed; ELSIF bid_IN <= temp_curr_bid THEN RAISE bid_too_low; ELSE /* || insert to bids AND update auction_items, || bidders identified by session username */ INSERT INTO bids (bidder, item_id, bid) VALUES (USER, item_id_IN, bid_IN); UPDATE auction_items SET curr_bid = bid_IN WHERE CURRENT OF auction_item_cur; /* || commit is important because it will send || the alert notifications out on the item */ COMMIT; /* || register for alerts on item since bidding, || register after commit to avoid ORU-10002 */ DBMS_ALERT.REGISTER(item_id_IN); END IF; CLOSE auction_item_cur; EXCEPTION WHEN invalid_item THEN ROLLBACK WORK; RAISE_APPLICATION_ERROR (-20002,'PLACE_BID ERR: invalid item'); WHEN bid_too_low THEN ROLLBACK WORK; RAISE_APPLICATION_ERROR (-20003,'PLACE_BID ERR: bid too low'); WHEN item_is_closed THEN ROLLBACK WORK; RAISE_APPLICATION_ERROR (-20004,'PLACE_BID ERR: item is closed'); WHEN OTHERS THEN ROLLBACK WORK; RAISE; END place_bid;
There are a few things to notice about place_bid. First, the row in AUCTION_ITEMS is locked FOR UPDATE to begin the transaction. I chose not to use NOWAIT in the cursor, because the transaction is small and should be quite fast, minimizing contention problems. The COMMIT immediately follows the INSERT and UPDATE and precedes the call to DBMS_ALERT.REGISTER. Originally I had it the other way around, but kept getting ORU-10002 errors when calling DBMS_ALERT.WAITANY immediately after place_bid. What was happening was that the call to DBMS_ALERT.REGISTER was holding a user lock that the insert trigger to BIDS was also trying to get. By doing the COMMIT first, the trigger is able to acquire and release the lock, which can then be acquired by DBMS_ALERT.REGISTER.
Exception handling in place_bid is inelegant but useful. The package defines named exceptions that place_bid detects, raises, and then handles using RAISE_APPLICATION_ERROR. In practice, it may be better to pass these out from the procedure to the calling application and let it handle them. Since I was prototyping in SQL*Plus and wanted to see the exception and an error message immediately, I used RAISE_APPLICATION_ERROR. When using DBMS_ALERT, note also that it is very important to terminate transactions to avoid the locking problems mentioned earlier, so the EXCEPTION section makes sure to include ROLLBACK WORK statements.
18.104.22.168.3 The watch_bidding procedure
With the triggers and the place_bid procedure in place, the online auction system is basically ready to go. Since a real application would involve a GUI, but I was prototyping in SQL*Plus, I needed a way to simulate what the GUI should do to receive DBMS_ALERT signals and inform the user of auction activity. This is basically what the watch_bidding procedure does. It could be modified and called directly from the GUI or its logic could be adapted and embedded into the GUI. The watch_bidding procedure uses DBMS_OUTPUT to display bidding alerts received. It also demonstrates the use of the alert message to implement a context-sensitive response to alerts.
Here is the source code for watch_bidding:
/* Filename on companion disk: auction1.sql */* PROCEDURE watch_bidding(timeout_secs_IN IN NUMBER:=300) IS temp_nameVARCHAR2(30); temp_message VARCHAR2(1800); temp_statusINTEGER; BEGIN /* || enter a loop which will be exited explicitly || when a new bid from another user received or || DBMS_ALERT.WAITANY call times out */ LOOP /* || wait for up to timeout_secs_IN for any alert */ DBMS_ALERT.WAITANY (temp_name, temp_message, temp_status, timeout_secs_IN); IF temp_status = 1 THEN /* || timed out, return control to application || so it can do something here if necessary */ EXIT; ELSIF temp_message = 'CLOSED' THEN /* || unregister closed item, re-enter loop */ DBMS_ALERT.REMOVE(temp_name); DBMS_OUTPUT.PUT_LINE('Item '||temp_name|| ' has been closed.'); ELSIF temp_message = USER OR temp_message = 'OPEN' THEN /* || bid was posted by this user (no need to alert) || re-enter loop and wait for another */ NULL; ELSE /* || someone raised the bid on an item this user is bidding || on, application should refresh user's display with a || query on the high_bids view and/or alert visually || (we will just display a message) || || exit loop and return control to user so they can bid */ DBMS_OUTPUT.PUT_LINE ('Item '||temp_name||' has new bid: '|| TO_CHAR(curr_bid(temp_name),'$999,999.00')|| ' placed by: '||temp_message); EXIT; END IF; END LOOP; END watch_bidding;
The watch_bidding procedure uses DBMS_ALERT.WAITANY to wait for any alerts for which the session has registered. In the auction system, registering for alerts is done when a bid is placed using place_bid. When an alert is received, the name of the alert is the auction item that has been updated. The alert message is used to respond intelligently to the alert as follows:
So the question is, does it work? I inserted some rows into the AUCTION_ITEMS table as follows:
/* Filename on companion disk: auction3.sql */* INSERT INTO auction_items VALUES ('GB123','Antique gold bracelet',350.00,NULL,'OPEN'); INSERT INTO auction_items VALUES ('PS447','Paul Stankard paperweight',550.00,NULL,'OPEN'); INSERT INTO auction_items VALUES ('SC993','Schimmel print',750.00,NULL,'OPEN'); COMMIT;
I granted EXECUTE privilege on the AUCTION package to two users, USER01 and USER02, and connected two SQL*Plus sessions, one for each user. Then I initiated a bidding war using the following PL/SQL block:
/* Filename on companion disk: auction4.sql */* set serveroutput on size 100000 set verify off BEGIN opbip.auction.place_bid('GB123',&bid); opbip.auction.watch_bidding(300); END; /
On each execution of the previous block in each session, I raised the bid. Here are the results from the USER01 session:
SQL> @auction4 Enter value for bid: 1000 Item GB123 has new bid: $1,100.00 placed by: USER02 PL/SQL procedure successfully completed. SQL> / Enter value for bid: 1200 Item GB123 has new bid: $1,300.00 placed by: USER02 PL/SQL procedure successfully completed.
USER01 opened the bidding on GB123 with a $1000 bid, which was quickly upped by USER02 to $1100, so USER01 came back with $1200 only to be topped finally by the winning $1300 bid by USER02. USER02's log of events confirms this exciting back and forth bidding war:
SQL> @auction4 Enter value for bid: 1100 Item PS447 has been closed. Item GB123 has new bid: $1,200.00 placed by: USER01 PL/SQL procedure successfully completed. SQL> / Enter value for bid: 1300 PL/SQL procedure successfully completed.
Note that each user is informed only of bids placed by the other user, and not of their own bids. Note also that USER02 was alerted to the fact that item PS447 (the subject of the previous bidding by USER02) had been closed.
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