XML Store

Christian Theil Have, cth@itu.dkRené Kofoed, rkofoed@itu.dk

References: Kasper Pedersen & Jesper Pedersen, Value-oriented XML StoreMads Pultz, Garbage Collection in Distributed Value-Oriented Storage System

XML Store overview XML store

Stores values Name service

Like a DNS service

XML Store overview What’s in an XML store

Values Value references

XML store helps you to store values, and locating them again with references.

What’s a value ? A piece of data of some sort, maybe the

number 5 :) Are 5 and 5 and 5 the same values? Nope – there is only one value: 5 The other 5 and 5 are occurences of the

first 5.

What’s a value reference? A shortname for a value :) A shortname is shorter than the data being

stored.

Value references

to xml or not to xml

<speech><speaker>BBCD01</speaker>

<line>BBCD02</line></speech>

Hamlet

Values

Value references

Shared valuesDocument A<speech>

<speaker>BBCD01</speaker>

<line>BBCD02</line></speech>

Hamlet

Document B<speech>

<speaker>BBCD01</speaker>

<line>BBCD03</line></speech>

to be

or not to be

Shared value

And updates? By changing speaker: ”Hamlet” in

document A, will also change document B This will be problematic!

Therefore…. Perform non-destructional updates by:

Creating a new value in a new tree Replacing existing references to the old value

to point to the new value

Changing Hamlet to OpheliaDocument A<speech>

<speaker>BBCD01</speaker>

<line>BBCD02</line></speech>

Document A’<speech>

<speaker>BBCD04</speaker>

<line>BBCD03</line></speech>

Hamlet

Document B<speech>

<speaker>BBCD01</speaker>

<line>BBCD03</line></speech>

Ophelia

Put in another way

Node

Value

XML - format for representing tree-structured data XML models:

Document Object Model (DOM) - loads the entire document into memory in order to manipulate it.

XML Enabled Databases (XED) – stores XML data in an existing relational database.

Native XML Databases – internal model designed for persisting XML

Document Value Model DVM – a way to program XML centric applications

What’s DVM? An API consisting of only two methods:

Save Load

That’s it!

Implementations 1. Based on Chord 2. Based on IP-multicast

Uses locators and a reference server to map a value reference to a location independant value.

XMLStore architechture Overview

Layered architechture Application layer DVM layer Disk layer

A concrete XMLStore is build with plugable modules.

Layers in XMLStore

”SPEECH”Name server:

6FE02A

42.63.145.108:6949/XAA/42,8

<speaker>5A7012,87D311</speaker>

Value reference(hashed value)

Lookup (p2p routing)

load

Locator

Value

Lookup

Document

XMLStore

Modules (1)XMLStore is organized in a modular way.

Funtionality can be added using decorators:

LoadSave

9A01F0451234

Save Load

Spawnsthread

Modules (2)Example modules:

Buffering Caching Asynchronous operation Replication (peer-to-peer) distribution Garbage collection

Name service Binds a human readable name to a value reference. Must provide provide destructive updates

Lost update problem Pessimistic locking Optimistic concurrency control

Should be Atomic

Distributed name service

Garbage collection in XMLStore Problem: Since values cannot be deleted

from XMLStore, it could eventually be flooded.

The solution: Garbage collection!

Garbage collection in XMLStore is concerned with reclaiming values that cannot be reached using references.

Garbage collection Live values - can be reached using an existing value

reference. The existing value references comprise the ”Root set”

Mutator – A program that accesses memory Collector – The garbage collector Local vs. Global garbage collection Different approaches

Reference counting Reference listing Tracing

Garbage collection1234F0459A01

<a>F3B2</a>”foo””bar”

FF01 ”foobar”

Name server

9A01F0451234

……

F3B2FF01F3B2

……

111

11

F3B2 … 0

Stub Skeleton

Reference count

Garbage collection XMLStore needs extended API to to keep track

of value references.

Garbage collection Reference counting

Basic idea: count the number of references to a given object, and reclaim it when the count reaches zero.

A reference count is associated with each value occurence

Scales well Doesn’t prevent cycles

This is not a problem since XMLStore contains only acyclic data structures

No resilience to lost messages Low fault tolerance

Garbage collection Reference listing

Each value occurence has a separate skeleton for each client…

Mapping of peers to value references

F3B2

Client A

Client B

Peer

hamlet

Skeleton

Garbage collection Tracing

Live objects are recursively traced from the roots and unreachable objects are determined to be garbage

Stop-and-collect vs. Incremental collection Group based tracing Pros and cons

Can detect cycles Scalability issues

http://www.itu.dk/English/

Approach adapted for xmlstore

Local garbage collection Reference listing Tracing (copying collector)

.. So cycles can be collected (only relevant with cells/mutable data)

Global garbage collection Tracing

Summary