Distributed Databases: what is next?

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• Massive distribution / replication

• Nested transactions

• Transactions and web services

• Summary and final remarks

New Challenges

DDB and transactions in the past

- few nodes ( < 10 ?)

- stable systems - stable net

Future

- Mobile systems

Q check out part of the DB and update "salesperson" application scenario

ÖMultimaster replication 2 – ~100 replicas Q Handheld devices

completely independent

- Sensor net data transactions?? - Web Service transactions

Replication schemes not scalable

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hs / FUB dbsII-03-21DDBNext-3

Nested Transactions

Flat transaction model

up to now:

begin r(x), ……w(x) commit

Why not recursive?

TA ( r(x) , TA1(…..) w(x) TA2(…..))

Generalize savepoints

- Savepoints allow sequential transactions to be aborted partially, work between two savepoints is a subtransaction

Allow transaction /subtransaction trees

Each subtransaction may be aborted

subtransaction

Nested Transactions

Nested transactions used to be an academic

exercise, become very important right now

- Object orientation

each method call is (may be) a subtransaction

- Business transactions between autonomous

systems (e.g. coordination of web services)

withdraw deposit

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^ ^ t2 Withdraw (x, 1000) r (r) r (l) r (p) w (p) Search (...) r (p) Fetch (x) Modify (x) r (s) r (t) Append (h, ...)

Fetch (a) Fetch (d) Store (e) Modify (d) Modify (a)

r (t) w (t) r (t) w (t) r (s)w (s) ^ ^ Deposit (y, 1000) r (r) r (l) r (q) w (q) Search (...) r (q) Fetch (y) Modify (y)

Nested Transaction rules (*)

Each transaction or subtransaction is bracketed by Start and Commit or Abort.

If a program is not executing a transaction, then Start creates a new top-level transaction If a program is executing inside a transaction,

then Start creates a subtransaction.

Commit and Abort by a top-level transaction have the usual semantics

If a subtransaction aborts, then all of its operations and

subtransactions are undone – top-level is notified, not aborted Until it commits, a subtransaction’s updated data items are only

visible to its subtransactions

After a subtransaction S commits, S’s updates are visible to other subtransactions of S’s parent.

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Nested Transactions

Features

Top-level transactions like flat transactions.

- They’re ACID and bracketed by Start, Commit, Abort.

Subtransaction abort is a new feature

Subtransactions of the same parent are isolated

from one another

Parent may decide to

compensate

a subtransaction

if another subtransaction aborts

(sometimes called

Multilevel TAs

)

Web service coordination

"

Webservice increasingly tie together a large number

of participants forming large distributed applications"

How to

coordinate

the activities?

Coordination types in the

web service transaction

specification

:

- Atomic transactions (AT) used to coordinate activities having a short duration

- Business activities (BA) used to coordinate activities that are long in duration and desire to apply business logic - Web service can include both ATs and BAs

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hs / FUB dbsII-03-21DDBNext-9 Example: Web service activities as nested transactions

Coordination framework of WS consists of: - activation service

- registration service - coordination service

Web service coordination

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Web service coordination

Registration service

Register and select protocol for the activity

Includes specification of transaction protocols and properties available for a particular web service

Coordination service

Controls activity completion processing for registered WS using selected coordination protocol

(until now: atomic transaction and business coordination) Context for created activity:

- Identifier

- expire (activity timeout) - coordination type - registration service

Transactional Web service: scenario of atomic TAs

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Step 1: Application creates transactional activity

using WS coordination framework's activation

service

Step 2: Application registers

(as responsible driving the completion protocol)

Step 3: Application performs operation on a Web service

Step 4: Register Resource with Reg. service for

processing

(only the first time, compare XA)

Step 5-7: Ack , application completes acitivty

Step 8-11: 2PC

Example: Business activity

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Example: Business activity abort

No 2 PC

Application decides to abort

(

Business Activity Failure

)

Coordinator sends compensation record

Compare nested transactions

Protocols defined in XML syntax with SOAP envelope

….. BUT

Specification for Web Service TA

TWO different specifications by mainplayers

- Microsoft / IBM et al.

http://www.ibm.com/developerworks/library/ws-tanspec/

- Sun, Oracle, Iona et al

(see reader)

Seems to be more elaborate, includes e.g. subprotocols like

- status of work - Restart - Checkpoint

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Business TA in large scale distributed, unstable

systems

Example: Buying electronic goods

n offerors, 1 client, bank

…

payment accept offer: contract

delivery

Trends for Transactions

Example (cont.):

Nested Transaction

(offer) ( (contract (payment) (delivery) ) Important:

- delivery must be successful if payment successful

(electronic good!)

- payment must precede delivery. Why?

- payment sub TA must guarantee correctness (“don’t create or destroy money”)