Midterm Revision Session

Midterm Revision SessionCS 5414

● Midterm : Monday, in class, closed notes

● Be clear/concise○ Symbols used in class are better than words/handwaving

● Be careful about assumptions○ it’s easy to assume the solution○ Sentences like “it is obvious that” are dangerous

● Definitions, definitions, definitions

● Do the required reading! ○ (yes, even if its 42 pages … )

Introductory Material

● Can you prove impossibility of two general problems?

● Can you prove that:○ All k muddy children will say “yes” at round k○ If father says nothing/says individually to children,

there is no solution to the problem?

● What is common knowledge?

● Can you draw the Kripke structures?

System Model (Communication/Failure)

● Synchronous system● Asynchronous system

○ No upper bound on message delivery time/on relative process speed

● Properties that can be assumed of channels

Terminology

● Event epi(ith event of p) / Local History (total order) /

History

● Space time diagrams

● Happens-Before relationship○ If ep

k,epl ∈ hi and k<l, then ep

k→epl

○ If ei=send(m) and ej=receive(m) then ei→ej○ If e →e’ and e’→e’’ then e’→e’’

Terminology

● Run: total ordering of events in the history

● Consistent run: a run that respects →

● Cut: subset of the global history (prefix of each local history)○ Can also write it as (c1,c2,c3)

● Frontier: set of events at the prefix

Terminology

● Local state: pi. Result of applying the first ith events to

state of p

● Global state: n-tuple of local states

● To each cut (c1, c2, cn) corresponds a global state ( p1

c1, p2c2, p3

cn)

Terminology

● Consistent Cut

Can you detect consistent cuts on space time diagrams?

● Consistent global state: state for which the corresponding cut is consistent

Snapshot Protocol ● Why did we want a snapshot protocol?

● Example question: Why is not necessary to listen on pj?

● Without this protocol, what is the weakest channel delivery property for external monitor to have consistent state?

Reachability

● Lattice: every node represents a consistent state.

● Represents all possible orders (runs) for a particular computation

● What are the properties of s? Why do we care?○ a global state for

snapshot start○ f snapshot finishes○ then a → s → f

Lamport Clocks

● Clock Condition: ○ e →e’ => LC(e) < LC(e’)

● How can Lamport clocks implement FIFO

● How can Lamport clocks implement Causal delivery?

● No gap detection

Vector Clocks (you need to be very comfortable with this)

● Strong Clock Condition: ○ e →e’ <=> VC(e) < V(e’)

● Update Rules:● On send (pi) On receive (pi)

● Vector Ordering:

● Concurrency:

Vector Clocks (Consistency)

Causal Delivery

● Weak Gap Detection

● Strong Gap Detection

● Delivery Rule

● How would you prove that this guarantees causal del?● Could you derive a weaker condition assuming FIFO

channels?

Safety/Liveness/Non-stable properties

● You should know how to define safety/liveness

● How can I compute on the lattice:○ Possibly( )○ Definitely( )

● When can local states be garbage collected when building the lattice?

Atomic Commit

● Impossibility result on communication/ total failures

● No independent recovery possible

Two phase locking

● Why can coordinator always vote abort?

Two phase locking (Termination)

● Cooperative termination protocol

● If all processes are uncertain (ak voted yes) but coordinator is down, what happens?

● 2PC can block even for non-total failures and no communication failures

Three-Phase Locking

● (Non-blocking property) If any process is uncertain, no process (including coordinator) can have decided to commit => it is therefore always safe to abort.

Three-Phase Locking (Termination Protocol)

● Why must pi run termination protocol in Step 6? It knows that the decision is commit ...

Three-Phase Locking (Termination/Recovery Protocol)

● Why can p not simply commit if it recovers from a failure after pre-commit ?

● (TR1) Why don’t we need to wait for all processes to reply?

● Does the upset tracking need to be accurate? (ex: pn failed first, just after voting abort. All other processes p0 to pn-1 fail in sequence but vote commit. What happens if pn recovers first?

State Machine Replication

● Agreement: Every non-faulty state machine receives every command

● Order: Every non-faulty state machine processes the commands it receives in the same order

(Order Implementation/Logical Clocks/Replica-Generated Ids)

Primary Backup / Chain Replication

● What are the benefits of primary backup vs chain replication?○ For queries○ For updates

● What are hot/cold backups?

Rollback Recovery - Checkpointing

● What is the goal of rollback-recovery?

● Uncoordinated checkpointing○ Cheap but domino effect

● Coordinated checkpointing○ Always restart from most recent checkpoint but high

latency● Communication induced checkpointing

○ local/forced snapshots. Z-Paths

Rollback Recovery - Logging

● Piecewise deterministic assumption: all nondeterministic events can be identified

● Log determinants

● Orphan process: includes delivery event but send event has been lost

● Pessimistic Logging / Sender Based Logging / Optimistic Logging / Causal Logging

Rollback Recovery - Consistency

Rollback Recovery - Consistency

● What are the pros/cons of each scheme? (Check table in paper)

Reliable Broadcast● What happens if

the sender fails before sending any message?

● Is it ok for everyone to output m’ if the sender proposed m but is faulty?

Terminating Reliable Broadcast

● If one process delivers SF, what must the other processes do?

Terminating Reliable Broadcast

● If one process delivers SF, what must the other processes do?

Consensus

● What happens if a faulty process proposes a value v’?

● What happens if a process does not propose?

Consensus

● What happens if a faulty process proposes a value v’?

● What happens if a process does not propose?

Properties of send/receive

● Validity● Uniform integrity● Integrity (for arbitrary failures)