ASP.NET Web PDF Document Viewer/Editor Control Library

The magic is on line 14 where we recursively call ourselves with a new primary key value to lock over and over If you run the procedure after populating the table with 148 character strings, you should observe: ops$tkyte%ORA11GR2> exec do_update(1); locked out trying to select row 38 PL/SQL procedure successfully completed This output shows that we were able to lock 37 rows but ran out of transaction slots for the 38th row For this given block, a maximum of 37 transactions can concurrently access it If we redo the example with a slightly smaller string ops$tkyte%ORA11GR2> truncate table t; Table truncated ops$tkyte%ORA11GR2> insert into t (x,y) 2 select rownum, rpad('*',147,'*') 3 from dual 4 connect by level <= 46; 46 rows created ops$tkyte%ORA11GR2> ops$tkyte%ORA11GR2> select length(y), 2 dbms_rowidrowid_block_number(rowid) blk, 3 count(*), min(x), max(x) 4 from t 5 group by length(y), dbms_rowid.

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Posts a message to a mailbox queue. Returns the next message in the mailbox queue. If no messages are present, performs an asynchronous wait until the message arrives. If a timeout occurs, then raises a TimeoutException. Scans the mailbox for a message where the function returns a Some(_) value. Returns the chosen result. If no messages are present, performs an asynchronous wait until more messages arrive. If a timeout occurs, then raises a TimeoutException. Like Receive, but if a timeout occurs, then returns None. Like Scan, but if a timeout occurs, then returns None.

rowid_block_number(rowid); LENGTH(Y) BLK COUNT(*) MIN(X) MAX(X) ---------- ---------- ---------- ---------- ---------147 4663 46 1 46 ops$tkyte%ORA11GR2> exec do_update(1); we finished - no problems PL/SQL procedure successfully completed. This time we completed successfully the difference a single byte makes! In this case, having the extra 46 bytes of space free on the block (each of the 46 strings was just one byte smaller) allowed us to have at least 9 more transactions active on the block This example demonstrates what happens when many transactions attempt to access the same block simultaneously a wait on the transaction table may occur if there is an extremely high number of concurrent transactions Blocking may occur if the INITRANS is set low and there is not enough space on a block to dynamically expand the transaction.

In most cases, the default of 2 for INITRANS is sufficient, as the transaction table will dynamically grow (space permitting), but in some environments you may need to increase this setting to increase concurrency and decrease waits An example of when you might need to increase the setting would be on a table or, even more frequently, on an index (since index blocks can get many more rows on them than a table can typically hold) that is frequently modified and has a lot of rows per block on average You may need to increase either PCTFREE (discussed in 10 Database Tables ) or INITRANS to set aside ahead of time sufficient space on the block for the number of expected concurrent transactions.

TryReceive : timeout:int -> Async<'msg option> TryScan : ('msg -> Async<'a> option) * timeout:int -> Async<'a option>

This is especially true if you anticipate the blocks will be nearly full to begin with, meaning there is no room for the dynamic expansion of the transaction structure on the block..

It is common for a message-processing agent to end up in a state where it s not interested in all messages that might appear in a mailbox but only a subset of them. For example, you may be awaiting a reply from another agent and aren t interested in serving new requests. In this case, it is essential you use MailboxProcessor.Scan rather than MailboxProcessor.Receive. Table 13-6 shows the signatures of both of these. The former lets you choose between available messages by processing them in order, while the latter forces you to process every message. Listing 13-12 shows an example of using Mailbox.Scan. Listing 13-12. Scanning a Mailbox for Relevant Messages open Microsoft.FSharp.Control.Mailboxes type msg = | Message1 | Message2 of int | Message3 of string let agent = MailboxProcessor.Start(fun inbox -> let rec loop() = inbox.Scan(function | Message1 -> Some (async { do printfn "message 1!" return! loop() }) | Message2 n -> Some (async { do printfn "message 2!" return! loop() }) | Message3 _ -> None) loop()) We can now post these agent messages, including messages of the ignored kind Message3: > agent.Post(Message1) ;; message 1! val it : unit = () > agent.Post(Message2(100));; message 2! val it : unit = () > agent.Post(Message3("abc"));; val it : unit = ()

TM locks are used to ensure that the structure of a table is not altered while you are modifying its contents. For example, if you have updated a table, you will acquire a TM lock on that table. This will prevent another user from executing DROP or ALTER commands on that table. If another user attempts to perform DDL on the table while you have a TM lock on it, he ll receive the following error message: drop table dept * ERROR at line 1: ORA-00054: resource busy and acquire with NOWAIT specified