Replication and Group Communication

Replication and Group Communication

Management of Replicated Data Requests and replies C Clients

FE

Front ends C

FE

RM

RM

Service

RM

Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012

Replica managers

Replica Manager • Manages a single copy of the data • Interacts with other RMs • Behave as state machines – Operations are atomic – Deterministic – Ensures behaviour can be reproduced at replicas

• May be dynamic or static

Sequence of Events

RM Client

Request Response

Front End

Request, id

Coordination Execution

Response, id

Agreement RM

RM

Coordination Phase • Reach agreement on – Whether to execute or not – Ordering • FIFO • Causal • Total

Group Communication • Messages send to all members of a group – Reliable – Either all receive message or none do – Ordered

• Group membership – Static – Fixed number of processes – Dynamic – Processes can leave and join

• Implementation – Network support (IP Multicast) – Sending multiple messages

Group Types • • • •

Process vs Object groups Closed vs Open groups Overlapping vs Non-overlapping Synchronous vs Asynchronous

Reliability of Multicasts • Integrity – Message received same as message send – No message is delivered twice

• Validity – Message send is eventually delivered

• Agreement – If delivered to one, it is delivered to all

Managing Group Membership • • • •

Allow processes to be added and removed Detect failed/unreachable components Notify members of group changes Perform group address expansion – Processes may be part of multiple groups – Send message to group using group id

System Model RM Client

Request Response

Front End

Request, id

GM

GC

RM GC

Response, id RM GC

RM – Replication Manager GM – Group Manager GC – Group Communicator

GM

GM

Group Communicator • Assumptions – Reliable one-to-one channels – Processes only fail by crashing

• To send a message: – RM calls multicast(g,m) in GC – GC may communicate with GM to get group members

• To receive a message: – GC receives message over network – Processes message – Calls deliver(m) in RM

Basic Multicast • B-multicast(g,m): – For each p ∊ g, send (p,m)

• On receive(m) at p: – B-deliver(m) at p