Prevayler

1
Prevayler

2
Agenda

• Introduction
• Object Prevalence
• Concepts
• Prevayler
• Concepts
• Transaction processing
• Performance
• Conclusion

3
Introduction

• Persistence layer of business applications
• Typical Usage of an RDBMS and O/R-Mapping
• Bridging between object model and relational
  model
• Mapping mostly not transparent
• Mapping expensive
• Ideal
• Transparent persistency
• Business objects...
• having ACID semantics
• implementation matchestransient implementation
• ?No mapping between data models
• ?rapid development
• ?quick prototyping possible

4
Object Prevalence

• Concept first published in 1987 by A. D. Birrell,
  M. B. Jones und E. P. Wobber in A Simple and
  Efficient Implementation for Small Databases
• Pattern for keeping in-memory data structures
  durable
• Later named Object Prevalenceby Klaus
  Wuestefeld
• Founder of the Prevayler project

5
Object Prevalence

• Core Concepts
• All data organized in single data structure
• Data structure held in memory
• Premises
• System has plenty of memory to hold all data
• Price of memory keeps dropping
• No shortage of address space due to VM and 64 bit
• ? Data available to owning process only
• Contrary to RDBMS RDBMS designed not to have
  all data in memory

6
Object Prevalence

• Core Concepts (2)
• Periodic snapshot of data structure
• Saved on durable medium (hard disk)
• Data structure accessed indirectly
• Command-Pattern
• Write Ahead Log for modifying accesses
• Log documents all modifications since last
  snapshot
• Saved on durable medium (hard disk)
• Startup/Recovery
• Read recent snapshot
• Playback log

7
Prevayler

• Project Prevayler
• Founded by Klaus Wuestefeld
• First public implementation of Object Prevalence
• First universal implementation
• Written in Java
• Open Source (BSD Lizense)
• First release in 2001 (LGPL)
• Meanwhile, further Object Prevalence
  implementations exist
• Gained publicity by performance claims
• 9000 times faster than Oracle (über JDBC)
• 3000 times faster than MySQL (über JDBC)
• ... but no TPC-C available

8
Prevayler Concepts

• Data structure
• Tree of Java objects
• Contains all business objects
• All objects must be accessible by a single root
• Root object named Prevalent System
• All objects must be serializable
• Implementation of java.io.Serializable or
  java.io.Externalizable
• ? required for taking snaphots

9
Prevayler Concepts

• Data access
• Indirect access thru command objects
• Readonly Query-object
• Modifications Transaction-object
• Only programmatic access
• No Query Language
• Commands are opaque to Prevayler
• Prevayler cannot determineactual modifcations
  made
• Synchronisation must be performedon Data
  structure level
• Much more coarse-grained than RDBMS (page level)

10
Prevayler Concepts

• Data access (2)
• Query/Transaction execution
• Before execution, lock on data structure must be
  aquired
• No reader/writer lock distinction
• Serial execution of all commands
• Readonly Access
• Implementation of Query-Interface
• Commands not written to log
• ? Query-objects must not perform any
  modifications

public interface Query public Object
query( Object prevalentSystem, Date
executionTime) throws Exception
11
Prevayler Concepts

• Data access (3)
• Modifying Access
• Implementation of Transaction/TransactionWithQuery
  -Interface
• Object must be serializable
• Will be serialized to command log before
  execution
• Deterministic behaviour
• Replay during Startup/Recovery phase
• External resources should not be used
• (historic) Prevayler-time must be used instead of
  system time

public interface Transaction extends
java.io.Serializable public void
executeOn( Object prevalentSystem, Date
executionTime)
12
Prevayler Data safety

• Snapshots
• Serialization of complete object tree
• Standard Java Object Serialization
• All objects are serialized, not only dirty
  objects
• Performed periodically
• Process
• Acquire lock on object tree
• Serialize tree into file
• Release lock
• Mark snapshot file as complete
• Truncate command log, delete old snapshot
  (optional)
• Process may take several minutes ? Downtime
• Can be avoided by using a replica

13
Prevayler Data safety

• Transaction processing
• Process
• Serializability-Test of command object
• Approval by Censor
• Adding entry to Transaction Log
• Call Transaction.executeOn()
• Execution of Transaction-Command considered
  atomic
• But Atomicity must be assured by developer
• On error, all modifications must be rolled back
  manually
• ? Complex for non-trivial commands
• Transaction.executeOn may throw an exception
• Result depends on Censor used

14
Prevayler Data safety

• Transaction processing Censors (2)
• Censors
• Approve Commands before their execution
• Are optional
• StrictTransactionCensor
• Command executed twice
• Execution on copy of data structure (Food
  Taster)
• If successful
• Execution on main data strcture
• If an error occured (Exception)
• Transaction is aborted
• Recovery of Food Taster
• ? Costly operation
• Consequences
• Failing commands may leave data structure in
  inconsistent state
• Inconsistencies less likely though
• Increased memory usage and execution time

15
Prevayler Data safety

• ACID
• Snapshots and logs ensure durability
• Serial command execution implies Isolation
• Developer is responsible of atomicity
• Commands must be implememnted in atomic fashion
• May be achived by using compensating actions
• Developer has to ensure consistency of data
  structure
• Object tree must be consistent before and after
  command execution, even if command failed
• ? A flawed command implementation can lead to
  inconsistency

16
Prevayler Performance

• Comparison to RDBMS problematic
• RDBMS mostly run out-of-process
• Usage of ODBC/JDBC/etc necessary
• Published performance results taken on single-CPU
  machine
• Synchonisation as bottleneck
• Serial execution of Queries and Transactions
• Can degrade performance on parallel systems
• Can degrade performance on SMP sytemes
• ? Commands still processed serially
• Long running transaction may stall system
• Waiting for lock on data structure

17
Prevayler Performance

• Reading
• No disk access neccessary
• All required data already in memory
• ? Quick access if data structure is organized
  well (using Hashtables etc)
• ? Minimal overhead
• ?Can be siginificantly faster than RDBMS
• Modifications
• Disk access neccessary
• WriteAhead-Logging
• If StrictTransactionCensor is used
• Additional overhead
• Rollbacks expensive
• About same order of maginitude as RDBMS

18
Conclusion

• Prevayler can substitute RDBMS...
• for moderate data volumes
• if only local (in-process) access to data is
  required
• If data accesses are mostly read-only
• Benefits
• No O/R-Mapping neccessary
• Rapid development
• High transparency for business objecs
• ? but low transparency in data access
  (Query/Transaction-Modell is propriatary)
• Performace
• High performance for readonly access
• Not designed for SMP systems
• Data safety
• Requires great care of developer
• Prevayler does not reach reliability and
  robustness of modern RDBMS

19
References

• RJW Birrell , Andrew Jones, Michael Wobber,
  Edward A Simple and Efficient Implementation for
  Small Databases, digital Systems Reseach Center,
  1987
• Carver Carver, Frank Thoughts about Prevayler
  and Databases http//radio.javaranch.com/frank/20
  04/12/27/1104152030000.html (11/10/2005)
• Evans Evans, Huw Why Object Serialization is
  Inappropriate for Providing Persistence in Java,
  Department of Computing Science, University of
  Glasgow
• Fowler Fowler, Martin Design
  Blikihttp//www.martinfowler.com/bliki/design.htm
  l (12 /19/2005)
• Melton Melton, Hayden An Evaluation of Object
  Prevalence, Dept. of Electrical and Electronic
  Engineering, University of Auckland
• ON Obermayer, Nathanael ACID gratis, iX
  Ausgabe 02/2004
• Prevayler Prevayler Homepage,
  http//www.prevayler.org
• Spille Spille, Mike Prevayler
  Revisitedhttp//www.pyrasun.com/mike/mt/archives/
  2004/12/25/15.02.00/index.html (11/10/2005)
• PT Printezis, Tony Garbage Collection in the
  Java HotSpot Virtual Machine, DevXhttp//www.devx
  .com/Java/Article/21977/0/ (01/10/2006)