Distributed Computing. Using multiple sites to solve a problem

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Distributed Computing

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Distributed Computing

Using multiple sites to solve a problem

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Distributed Computing

Using multiple sites to solve a problem

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Distributed Computing

Using multiple sites to solve a problem

Network filesystem Cloud computing

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Distributed Computing

Using multiple sites to solve a problem

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Distributed Computing

Using multiple sites to solve a problem

Multi-player game Collaborative spreadsheet

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Distributed Computing

Using multiple sites to solve a problem

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Distributed Computing

Using multiple sites to solve a problem

E-commerce site Scientific simulation

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Distributed Computing

Using multiple sites to solve a problem

E-commerce site Scientific simulation

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Distributed Computing

Different applications ⇒ different levels of transparency:

• Hardware as a service: use of multiple computer is automatic for each application

• Volcano-monitoring sensors: distribution and communication explicit in the application

Different applications ⇒ same core problems:

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Happens Before

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Virtual Clocks

mumbling sleeping

muttering breathing fire

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Virtual Clocks

mumbling sleeping

muttering

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Virtual Clocks

mumbling sleeping

muttering sheilding

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Virtual Clocks

mumbling @ 0 sleeping @ 0

muttering @ 1 sheilding @ 2

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Virtual Clocks

mumbling @ 0 sleeping @ 0

muttering @ 1 sheilding @ 2

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Centralized Lock

One site manages a given lock Advantages:

• Simple to implement • Easy to make fair Disadvantages:

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Distributed Lock

Ask everyone else’s permission for lock Advantages:

Disadvantages:

• Lots of communication

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Majority Lock

Ask permission from more than 50% for lock Advantages:

• Handles failure of sites well

• Less communication than asking everyone Disadvantages:

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Token Ring

Wait for permission, then pass it on afterward Advantages:

• Relatively little communication • Clearly fair

• Makes sense when permission is needed often Disadvantages:

• Have to set up ring

• Single failure breaks the ring

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Elections

In case sites need to pick a new coordinator (e.g., for a lock)

• Bully algorithm • Ring algorithm

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Bully Algorithm

Rank all sites

• If a process i doesn’t hear back from a coordinator:

Send an elect message to every j > i

No responses? Then i considers itself elected and notifies everyone

Got response? Wait for notify from new coodinator...

• If a process i receives an elect message, treat it

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Bully Algorithm

1

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Bully Algorithm

1

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Bully Algorithm

2

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Bully Algorithm

2

wait on 4

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Bully Algorithm

2

elect 2!

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Bully Algorithm

2 elect 2? 4 3 elect 3! 4

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Bully Algorithm

2 elect 2? 4 3 inaug 3 4

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Bully Algorithm

2

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Bully Algorithm

1 2

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Bully Algorithm

1

elect 1!

2

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Bully Algorithm

1 elect 1? 2 elect 2! 3 3 3

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Bully Algorithm

1 elect 1? 2 elect 2? 3 3 elect 3! 3

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Bully Algorithm

1

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Bully Algorithm

1

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Bully Algorithm

1

3 2 3 3 3 4

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Bully Algorithm

1

3 2 3 3 3 4

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Bully Algorithm

1

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Ring Algorithm

When communication is always to the next live site • If a process i doesn’t hear back from a

coordinator:

Send an elect message to neighbor

Accumulate elect messages to learn about

live sites

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Ring Algorithm

1

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Ring Algorithm

1

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Ring Algorithm

2

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Ring Algorithm

2

wait on 4

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Ring Algorithm

2

[2,...] elect(2)

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Ring Algorithm

2 [2] 4 3 [2,3,...] elect(3) elect(2) 4

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Ring Algorithm

2 [2,3,...] elect(3) 4 3 [2,3,...] elect(2) 4

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Ring Algorithm

2 [2,3] elect(3) 4 3 [2,3,...] 4

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Ring Algorithm

2 [2,3] 4 3 [2,3] 4

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Ring Algorithm

2

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Multi-Site Atomicity

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Two-Phase Commit

T1:

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Two-Phase Commit

<buyer image> T1: <seller image> T1: <bank image> T1:

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Two-Phase Commit

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Two-Phase Commit

<buyer image> T1: <seller image> T1: give keys <bank image> T1: transfer money

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Two-Phase Commit

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Two-Phase Commit

<buyer image> T1: take keys <seller image> T1: give keys

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Two-Phase Commit

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Two-Phase Commit

<buyer image> T1: take keys <prepare> <seller image> T1: give keys <bank image> T1: transfer money

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Two-Phase Commit

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Two-Phase Commit

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Two-Phase Commit

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Two-Phase Commit

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Two-Phase Commit

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Two-Phase Commit

<buyer image> T1: take keys <prepare> <seller image> T1: give keys <ready> <bank image> T1: transfer money <ready>

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Two-Phase Commit

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Two-Phase Commit

<buyer image> T1: take keys <prepare> <commit> <seller image> T1: give keys <ready> <commit> <bank image> T1: transfer money <ready>

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Two-Phase Commit

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Two-Phase Commit

<buyer image> T1: take keys <prepare> <commit> <seller image> T1: give keys <ready> <commit> <bank image> T1: transfer money <ready>

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Two-Phase Commit

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Things You Can’t Do

• Two-way common knowledge

e.g., buyers commit only if they know that the bank received the message to finalize the

transaction

• Handle too many byzantine failures

Can use voting to detect when up to 1/3 of the sites are faulty, but not when more are faulty