Two-phase commit Doug Woos Logistics notes Problem Set 2 out - - - PowerPoint PPT Presentation

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Two-phase commit Doug Woos Logistics notes Problem Set 2 out - - - PowerPoint PPT Presentation

Nov 17, 2023 116 likes •647 views

Two-phase commit Doug Woos Logistics notes Problem Set 2 out - Due next Friday Next week: Paxos - Notoriously difficult topic - Please do the reading! Caching Discussion Complexity as a downside Do the scalability/performance issues

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Two-phase commit

Doug Woos

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SLIDE 2

Logistics notes

Problem Set 2 out

  • Due next Friday

Next week: Paxos

  • Notoriously difficult topic
  • Please do the reading!
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SLIDE 3

Caching Discussion

“Complexity” as a downside Do the scalability/performance issues mentioned in the paper exist today? Why do we use NFS? When is the recovery protocol executed?

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SLIDE 4

Review: Sharding

Sharding

  • Different data on different servers
  • Partitioned via some function on keys
  • Implement in Lab 4!

Another axis of scaling, cf. replication

  • Usually, shard then replicate
  • Each piece of data lives on one replicated shard
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Sharding and Transactions

Adve rule we discussed lets us avoid synchronizing between shards for simple reads and writes But what if we do need to synchronize across shards? Transactions: atomic updates to multiple items

  • Bank account transfer
  • Calendar event
  • Unix user creation
  • etc.
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Calendar event creation

I have three advisors (Tom, Zach, Mike) Want to schedule a meeting with all of them

  • Let’s try Tues at 11, people are usually free then

Calendars all live on different nodes! Other students also trying to schedule meetings Nodes can fail, messages can be dropped (of course)

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Calendar event creation (wrong)

Tom Mike Zach Doug

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Calendar event creation (wrong)

Tom Mike Zach Doug Meet at 11 on Tues

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SLIDE 9

Calendar event creation (wrong)

Tom Mike Zach Doug OK

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SLIDE 10

Calendar event creation (wrong)

Tom Mike Zach Doug Meeting Doug @ 11 on Tues

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SLIDE 11

Calendar event creation (wrong)

Tom Mike Zach Doug Meet at 11 on Tues Meeting Doug @ 11 on Tues

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SLIDE 12

Calendar event creation (wrong)

Tom Mike Zach Doug OK Meeting Doug @ 11 on Tues

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SLIDE 13

Calendar event creation (wrong)

Tom Mike Zach Doug Meeting Doug @ 11 on Tues Meeting Doug @ 11 on Tues

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SLIDE 14

Calendar event creation (wrong)

Tom Mike Zach Doug Meet at 11 on Tues Meeting Doug @ 11 on Tues Meeting Doug @ 11 on Tues

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SLIDE 15

Calendar event creation (wrong)

Tom Mike Zach Doug Busy! Meeting Doug @ 11 on Tues Meeting Doug @ 11 on Tues

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SLIDE 16

Calendar event creation (wrong)

Tom Mike Zach Doug Meeting Doug @ 11 on Tues Meeting Doug @ 11 on Tues

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SLIDE 17

Calendar event creation (wrong)

Tom Mike Zach Doug Meeting Doug @ 11 on Tues Meeting Doug @ 11 on Tues

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SLIDE 18

Calendar event creation (better)

Tom Mike Zach Doug

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SLIDE 19

Calendar event creation (better)

Tom Mike Zach Doug Meet at 11 on Tues

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SLIDE 20

Calendar event creation (better)

Tom Mike Zach Doug OK

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SLIDE 21

Calendar event creation (better)

Tom Mike Zach Doug Maybe Meeting Doug @ 11 on Tues

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SLIDE 22

Calendar event creation (better)

Tom Mike Zach Doug Maybe Meeting Doug @ 11 on Tues Meet at 11 on Tues

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SLIDE 23

Calendar event creation (better)

Tom Mike Zach Doug Maybe Meeting Doug @ 11 on Tues OK

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SLIDE 24

Calendar event creation (better)

Tom Mike Zach Doug Maybe Meeting Doug @ 11 on Tues Maybe Meeting Doug @ 11 on Tues

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SLIDE 25

Calendar event creation (better)

Tom Mike Zach Doug Maybe Meeting Doug @ 11 on Tues Maybe Meeting Doug @ 11 on Tues Meet at 11 on Tues

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SLIDE 26

Calendar event creation (better)

Tom Mike Zach Doug Maybe Meeting Doug @ 11 on Tues Maybe Meeting Doug @ 11 on Tues Busy!

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SLIDE 27

Calendar event creation (better)

Tom Mike Zach Doug Maybe Meeting Doug @ 11 on Tues Maybe Meeting Doug @ 11 on Tues

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SLIDE 28

Calendar event creation (better)

Tom Mike Zach Doug Maybe Meeting Doug @ 11 on Tues Maybe Meeting Doug @ 11 on Tues Never mind!

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SLIDE 29

Calendar event creation (better)

Tom Mike Zach Doug Maybe Meeting Doug @ 11 on Tues

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SLIDE 30

Calendar event creation (better)

Tom Mike Zach Doug Maybe Meeting Doug @ 11 on Tues Never mind!

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SLIDE 31

Calendar event creation (better)

Tom Mike Zach Doug

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Two-phase commit

Atomic commit protocol (ACP)

  • Every node arrives at the same decision
  • Once a node decides, it never changes
  • Transaction committed only if all nodes vote Yes
  • In normal operation, if all processes vote Yes the

transaction is committed

  • If all failures are eventually repaired, the

transaction is eventually either committed or aborted

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Two-phase commit

Roles:

  • Participants (Mike, Tom, Zach): nodes that must

update data relevant to the transaction

  • Coordinator (Doug): node responsible for executing

the protocol (might also be a participant) RPCs:

  • VOTE-REQ: Can you commit this transaction?
  • COMMIT: Commit this transaction
  • ABORT: Abort this transaction
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Failures

In the absence of failures, 2PC is pretty simple! When can interesting failures happen?

  • Participant failures?
  • Coordinator failures?
  • Message drops?
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SLIDE 35

Participant failures

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Participant failures: Before sending response?

Coordinator Participant Participant Prepare Prepare Yes No Abort Abort Decision?

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Participant failures: After sending vote?

Coordinator Participant Participant Prepare Prepare Yes Yes Commit Commit Yes

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SLIDE 38

Participant failures: Lost vote?

Coordinator Participant Participant Prepare Prepare Yes Yes No Abort Abort Decision?

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SLIDE 39

Coordinator failures

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Coodinator failures: Before sending prepare

Coordinator Participant Participant Prepare Prepare Yes Yes Yes Commit Commit

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Coordinator failures: After sending prepare

Coordinator Participant Participant Prepare Prepare Yes Yes Yes Commit Commit Prepare Prepare

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Coordinator failures: After receiving votes

Coordinator Participant Participant Prepare Prepare Yes Yes Yes Commit Commit Prepare Prepare Yes Yes

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Coordinator failures: After sending decision

Coordinator Participant Participant Prepare Prepare Yes Commit Yes Yes Commit Decision?

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Do we need the coordinator?

Coordinator Participant Participant Prepare Prepare Yes Commit Commit Yes Yes Decision?

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What if we do not have the coordinator’s decision?

Coordinator Participant Participant Prepare Prepare Yes

  • r

No? Commit? Decision? Yes Yes

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2PC vs. SMR

Is 2PC a state machine replication protocol?

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2PC vs. SMR

Is 2PC a state machine replication protocol?

  • Could be—but isn’t a very good one!

2PC is for coordinating an operation between multiple nodes, where every node has to agree in order for the system to make progress!

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Typical architecture

State machine State machine State machine 2PC 2PC 2PC

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Typical architecture

State machine State machine State machine 2PC 2PC 2PC

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Two generals

Does 2PC solve Two Generals? Why not?

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SLIDE 51