1. Fault Tolerant Services

2. Components
Replication
Group Management
Group Communication

3. Group Communication Reliable Consistent
Using reliable 1-to-1 channels (eg TCP/IP)
Using unreliable 1-to-1 channels (eg UDP/IP)
Using unreliable multicast (eg IP-Multicast)
Consistent

4. Group Management Tracks who is in the group
Allows processes to join
Allows processes to leave
Detects unavailable processes
Able to provide list of processes in group

5. Replication Passive (primary-backup) replication Active replication
Gossip architecture
Anti-antropy protocol
Coda architecture

6. Passive Replication Primary RM C FE Backup
Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn © Pearson Education 2012

7. Passive Replication – Event Sequence
Request – FE sends request with unique ID to primary
Coordination – Primary queues request locally (if duplicate, skip to response)
Execution – Primary executes request
Agreement – Reliably multicast request to all backups
Response – Primary responds back to FE

8. Passive Replication – Failure Mgmt
If Backup fails – remove it from the group
If Primary fails
remove primary from group and elect a leader to be new primary
If fails before agreement:
FE resends request after timeout to new primary
Primary treats it as new request
If fails after agreement, before response:
Primary sends stored response

9. Active Replication FE C RM
Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn © Pearson Education 2012

10. Active Replication – Event Sequence
Request – FE reliably multicasts request with unique ID to all replicas.
Coordination – Agree on ordering of requests
Execution – Each executes request
Agreement - No further action needed
Response – Each replica sends response with ID to FE

11. Active Replication – Failure Mgmt
If replica fails
Remove it from the group
Ensure group communication can lose member in middle of communication/computation

12. Gossip Architecture Service gossip RM Vector FE timestamps Clients
Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn © Pearson Education 2012

13. Gossip Architecture Guarantees
View from each client is consistent
Each query shows at least the updates the client has already seen.
Relaxed consistency between replicas
All replicas eventually receive all updates
May not receive and apply in the same order

14. Gossip Architecture – Event Sequence
Request – FE sends to one replica
Client will be blocked on query
Client may be blocked on update after sufficient replicas have received update
Update response – replica responds immediately on update request
Coordination – ordering of request
Execution – replica executes request
Query response – by replica to FE
Agreement - Using gossip messages

15. Gossip Messages Gossip messages contain log and timestamp
On receipt of gossip message
Merge with own log
Apply stable updates
Eliminate records executed by all

16. Implementing Gossip Architecture
Frequency and duration of network partitions
Frequency of sending gossip messages
Which processes exchange gossip messages