Database Management for Smalltalk

Archive for April, 2008

Mon 14th Apr 2008   03:04 PM
posted by John Clapperton

‘Garbage’, in an object-oriented database system, means objects which have become unreachable because references from other objects have been removed by the application, and which are therefore to be removed to reclaim storage space. The garbage collector (’GC’) in VOSS is a Baker design, similar in concept to the garbage collector in Smalltalk-80, which can be run in foreground and/or background modes of which more later.

Unlike the mark-sweep design, which truly is a ‘garbage collector’, the Baker design would be better characterised as a ‘good object preserver’ which copies all reachable objects within a virtual space back and forth from one ’semi-space’ to another, deleting everything left behind on each flip. This has consequences for database design, administration, and the choice of GC settings, to tune for maximum transaction throughput and minimum downtime.

Know which space your objects are in.

In a database design using multiple virtual spaces, the most important thing to know is that when the VOSS GC flips it preserves only those objects in a virtual space which are (indirectly) reachable from the rootDictionary of that virtual space or from the image in which the GC is then running (in the case of uncommitted new objects). In other words, any object in virtual space ‘A’ which is referenced only by an object in virtual space ‘B’, though it will behave normally whilst present, will not be preserved when the GC flips virtual space ‘A’. Or in other words again, reference(s) from object(s) in another virtual space alone are not sufficient to preserve an object from the GC of the virtual space in which it exists. In its systematic copying, the GC does not follow references to objects outside the virtual space which it is scanning.

This situation must be avoided, as after such a flip, those objects would become instances of VOUndefinedObject, and later magically become some arbitrary new object when the id number of that apparently garbage object in space ‘A’ was re-allocated to a new object, having spent some days, weeks or months on space ‘A’s free id list.

The simplest way to keep this right is always to use one of the explicit variants of the message to virtualize an object, preferably on creation, which specify the location explicitly, for example:

  myObject := MyClass newVirtualIn: aVOManager.

The non-specific variants, for example:

  myObject := MyClass newVirtual.

virtualize the new instance in the current virtual space of the current process, i.e. the virtual space which hosts the last object to have received a message in the current process.
 

Why use multiple virtual spaces?

One reason for distributing a database across multiple virtual spaces may be that some parts of the database are static, whilst others are volatile - subject to frequent update and new object creation - and in this case unnecessary GC processing can be avoided by suitable partitioning of the database, so that the more static virtual space is GC’d less frequently, if at all.
 

Foreground or Background Garbage Collection?

Foreground GC scans and saves a specified number of reachable objects as an addendum to  each transaction commit, before that transaction’s changes are physically written to disk, to minimise disk activity. The effect, to the user, is that each transaction commit takes longer than it otherwise would, depending on the number of objects it is set to scan.

Background GC runs as one or more separate background processes, effectively dummy users committing null transactions, which each do some GC as above. Within each image, background GC processes commit their invisible dummy transactions once every user-specified time delay, by default 3000 milliseconds, and scan a user-specified number of objects each time (which may be different from the foreground scan-rate). Foreground and background GC may run concurrently.

The advantage of background GC is that it utilises CPU cycles in between application transaction commits; the disadvantage is that it adds to the total amount of disk flushing activity, whereas foreground GC-writes are flushed within the same transaction commit - which was going to happen anyway.

Foreground GC is preferable if the application consists mainly of frequent transactions which are a short time in the preparation before commit (i.e. whilst the application has control); background GC is preferable if transactions are less frequent and/or a long time in the preparation, especially open-ended interactive transactions, when background GC can go on whilst the user is thinking.

If the operational objective is continuous operation with no downtime for GC, then, on average, the GC should scan objects for preservation at a rate equal to the average rate of object creation or modification per transaction, so that there is no backlog of new and/or changed objects to be scanned when a GC flip is requested, to delete the garbage and start the reverse trek. This is most simply achieved by setting a high GC scan rate, so that the GC is at or nearly at the end of its task, ready to flip, after each transaction. The visual GC Progress Indicator shows how close to 100% the GC is at any time.

Foreground GC does not attempt to flip automatically, and so will not perform unnecessarily eager copying of the reachable objects - if there is nothing more to scan, before it has scanned its quota, it stops and lets the transaction commit. Background GC, however, may be requested to flip the virtual space every time it reaches 100%, and if so set, then it is possible for the GC to churn the entire contens of the virtual space back & forth at a high rate, even if there are no garabage objects at all, wasting CPU cycles and disk flushing time. The Database Administrator should set background GC run frequency (i.e. milliseconds delay between each run) and the number of objects to be scanned in each run, to meet operation requirements, considering the applications’ average transaction size and frequency.

Note that a GC flip request (explicit or automatic) will succeed only if the image which is running the GC is the only image logged-on to that virtual space; this is because the GC has no knowledge of, and therefore cannot preserve, uncommitted non-garbage objects in another image.
 

24×7 uptime?

The actual flip of the GC is done with exclusive access to the virtual space, blocking all other activity on it, but it takes less than a second, so the eager GC strategy described above will allow continuous service uptime, save only for the need to log-off all but one image from time to time to allow the flip to take place.

Downtime for backup and possible hot-backup enhancement of VOSS will be the subject of a future post.

In practice, an application may have a greater transaction rate at some times of the day and week than at others, and it may be that such is the load during busy times that foreground and/or background GC rate and frequency may need to be changed to match. This may be done at any time, either via the Control Panel or programmed message-sending to the VOManager concerned, by a time-scheduled process if desired.


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Thu 3rd Apr 2008   07:04 PM
posted by John Clapperton

VOSS 3.145.00.10 (beta) is now available for download.

New feature in this release is Persistent Continuation Transactions.

An open VOContinuationTransaction which has committed any number of alternative futures for its set of objects in continuation sub-transactions may now itself be persisted, or persisted & deactivated which removes the live transaction from the image, allowing log-off and shutdown.

Whilst the persisted continuation transaction exists, it and all its objects are persistently set read-only in the name of that transaction. Whilst deactivated it may be re-activated from the Control Panel or by program message sending, to re-create the live open transaction.

Persistent transactions are useful in design or what-if type of applications, where transactions may extend over days of revision and re-working of different saved possible final states, until a final commit is chosen.

Persisting a VOContinuationTransaction stores in the current virtual space (in a private dictionary in the stateDictionary) a virtual object complex which is a representation of the open VOContinuationTransaction, as a VOPersistedContinuationTransaction containing the same virtual objects. The same complex also includes a representation of its owner VOSession and its virtualObjectPark dictionary and contents, as a VOPersistedSession.

By default, every committing VOConSubTransaction persists its parent VOContinuationTransaction within the same commit; this default may be changed by the class method:

  VOContinuationTransaction persistConTxnOnSubTxnCommitDefault: aBoolean.

The public instance methods of VOContinuationTransaction offer further options. For example, VOContinuationTransaction>>persistAndDeactivate persists the receiver and removes it from the image, setting persistent read-locks on its VOPersistedContinuationTransaction and contents.

If all open VOContinuationTransactions in an image have been persisted and deactivated, the image may log-off and shutdown if desired.

Whilst a VOContinuationTransaction is active (i.e. exists, open, in the image), but has not yet been persisted, its contents are locked by volatile cache and object table file (.vot) record locks in the same way as in an ordinary  transaction. When persisted, the persistent read-locks are additional, but incidental from the point of view of other Process’s transactions (in the same or other images). However, when an open VOContinuationTransaction has been deactivated, it no longer exists in the image and its volatile locks are released; other Process’s transactions may then see such objects, but the persistent read-locks will block them from committing any changes to those objects (or the VOPersistedContinuationTransaction itself).

At the application level, the database may be considered to be in an inconsistent state (this is not something which any DBMS can determine, it is a matter for the application designer and/or user; consider, for example, work in progress in a CAD design for a gearbox or building), and objects which were write-locked in the open VOContinuationTransaction will now be only read-locked and therefore visible to other Processes in their deemed inconsistent state. However, this situation may be managed, since such locks may be tested by the method:

  VORefPublic>>isReadOnlyForPersistedTransaction

and/or by checking the startTimestamp (id) of each extant VOPersistedContinuationTransaction in each virtual space returned by the method:

  VOManager>>persistedContinuationTransactionsOrIDs

so that other Process’s transactions may choose to see the database as at a specified #dateAndTimeToRead (or equivalent integer #timestampToRead) pre-dating any or all extant persisted continuation transactions. If transaction versioning is globally enabled (set by VOManagerManager>>versioning: aBoolean, or from the Control Panel) then all historical states prior to the continuation transaction(s) may be seen.

Persisted instances of VOPersistedContinuationTransaction are shown in the Control Panel, and may be (re)activated by menu there, or by one of the following messages to the single LocalVOSSServer global instance of VOSSServer.

  VOSSServer>> activatePersistedContinuationTransaction: aPersistedContinuationTxn
or
  VOSSServer>> activatePersistedContinuationTransactionID: anInteger in: aVOManager

Download here and test-drive this in the tutorial.


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