Re: [firebird-support] Multi-Version Consistent Read Question
Morning Ann, On 13/12/11 18:18, Ann Harrison wrote: ... The method used by Oracle (and MySQL's InnoDB) keeps old versions in log files which are purged periodically. A long running transaction can get inconsistent data if the log file that contained the record version appropriate for it has been purged. If I may be so bold Oracle uses log files for REDO and has ROLLBACK_SEGMENTS or UNDO Segments (depending on Oracle version) for UNDO. It never uses log files for UNDO - and UNDO is what provides Read Consistency/MVCC in an Oracle database. Changes are written to the LOG_BUFFER (n memory) and periodically - on commit, every 3 seconds max, or when the buffer is 33% full - flushed to the REDO logs. These REDO logs might be archived to disc when they fill up. That Depends on the database archive log mode though. These logs are used when a database is restored and rolled forward (using the RECOVER DATABASE command, for example). In order to roll back changes and to ensure read consistency, UNDO is used. These do live on disc - as tablespace files - but remain in memory in the buffer cache alongside data blocks etc. When a SELECT is started, the data returned are the data from the data blocks. Each row in a block has an indicator that tells when it was last updated. If a pending update is taking place (currently uncommitted) or if a commit has taken place since this SELECT started then the data read from that data block has changed - and is not consistent with the start time of this SELECT transaction. When this is detected, Oracle rolls back the changes to the start time of the SELECT taking place by looking for the UNDO block(s) associated with the transaction that made the changes. If that results in the correct (consistent) data, that's what you get. If it turns out that there were other transactions that also changed the data, they too will be detected and undone. In this way you only ever see data that was consistent at the start of your own transaction. As long as the DBA correctly sizes the UNDO tablespace and correctly sets the UNDO_RETENTION parameter to a decent enough value, data changes are able to be rolled back happily all the time. If the DBA failed miserably in his/her duties, the ORA-01555 Snapshot too old errors are the result. And are most irritating. Long running SELECTS - batch reports for example - tend to show up this error mostly. This mechanism is also used to carry out a CONSISTENT=YES export of the data - any changes made since the start of the export are rolled back to get the data as it were when the export started. UNDO segments are in three states: Active - currently in use by a transaction. Unexpired - A transaction has committed, but the UNDO_RETENTION period has not yet expired. This segment should not be used. Expired - this segment is free for use. When a transaction needs a new UNDO segment, it attempts to use the expired ones, if there are none, it will try to create a new one in the free space, if any, of the undo tablespace. If it cannot, it will then re-use an unexpired segment. This renders the data in that segment unavailable and is the cause of the above snapshot too old errors. Note to self: next time, be brief! Cheers, Norm. -- Norman Dunbar Dunbar IT Consultants Ltd Registered address: Thorpe House 61 Richardshaw Lane Pudsey West Yorkshire United Kingdom LS28 7EL Company Number: 05132767
Re: [firebird-support] Multi-Version Consistent Read Question
And a different good morning to you Norman, If I may be so bold Of course, and thank you for the detailed and complete description of a mechanism I glossed over irresponsibly. As long as the DBA correctly sizes the UNDO tablespace and correctly sets the UNDO_RETENTION parameter to a decent enough value, data changes are able to be rolled back happily all the time. If the DBA failed miserably in his/her duties, the ORA-01555 Snapshot too old errors are the result. And are most irritating. Long running SELECTS - batch reports for example - tend to show up this error mostly. Ha ha ha! You don't get month end this month because your DBA screwed up! Try Firebird next time. Cheers, Ann
Re: [firebird-support] Multi-Version Consistent Read Question
Ann Harrison aharri...@ibphoenix.com wrote: And a different good morning to you Norman, And already it's evening. :-) Of course, and thank you for the detailed and complete description of a mechanism I glossed over irresponsibly. Welcome. You may be surprised at how many people think that Oracle uses log files for undo. ... Ha ha ha! You don't get month end this month because your DBA screwed up! Try Firebird next time. But I do! I looked after over 600 Oracle databases and 43 Firebird ones until recently. More problems with Oracle. Cheers, Norm. -- Sent from my Android phone with K-9 Mail. Please excuse my brevity.
Re: [firebird-support] Multi-Version Consistent Read Question
Hello Joey, Can you please tell in what university you are studying? Why you have chosen Firebird - was it some recommendation? Regards, Alexey Kovyazin Hello, I am researching methods to achieve high throughput for a learning chat application I am designing as a self-led student. I came across the Multiversion Control method of ACID and have a theoretical question: though it seems remote that a reader of data wouldn't get an old read what has been the experience and when this occurs what methods are available to alert the read user that data has been updated? Thank you, Joey [Non-text portions of this message have been removed] [Non-text portions of this message have been removed]
Re: [firebird-support] Multi-Version Consistent Read Question
At 03:39 AM 14/12/2011, Joey S. wrote: Hello, I am researching methods to achieve high throughput for a learning chat application I am designing as a self-led student. I came across the Multiversion Control method of ACID and have a theoretical question: though it seems remote that a reader of data wouldn't get an old read what has been the experience and when this occurs what methods are available to alert the read user that data has been updated? What the client can see depends on the isolation level of the transaction. At all levels, your client's transaction sees its own changes. Those will not be visible to any other transaction until your client's transaction commits. -- Read Committed allows a client to see any changes committed after his transaction started. It is the responsibility of the client application to re-query the database to bring the remote view up to date. -- Snapshot isolation (a.k.a. concurrency) keeps a persistent view of the database state as it was when your client's transaction started. Your client won't see changes committed by other transactions until his transaction commits. -- Snapshot Table Stability (a.k.a. consistency) is like concurrency, but it also prevents any other transactions from committing work on the tables it is reading. On the other side of the coin, it is difficult for the consistency transaction to get write access to the tables if other transactions already have it. You wouldn't use this level often, if ever. A transaction can *never* see pending writes for other transactions (Dirty Read). However, a pending write by one transaction will always cause a wait (if allowed by the lock resolution policy) or a lock conflict in any others that attempt to change the same rows. How you alert the user to changes is up to you. The methods available vary according to what you are using to wrap the API - some drivers have extreme smarts to detect changes. Commonly, any such methods are based on trapping lock conflict exceptions, the nature of which will depend on both the isolation level and the lock resolution policy (wait/no wait). Another approach is to use the Events mechanism in triggers, whereby an After Insert or Update or Delete trigger will issue a POST_EVENT when a state-changing operation is committed. Clients are set up to listen for these events and you write the application to respond in some appropriate way, e.g., re-query the tables (refresh) in Read Committed; or attempt to commit and refresh in Snapshot/concurrency. ./heLen
