2 - Modelling Flashcards
Explain what a distributed system is?
As a set of processes. Each process does local computations, and they’re interconnected with commincation channels. (links) A network is assumed to be at least connected (there is a path between each two nodes)
What components have to be defined for your network model?
- Type of Network
- Type of Process
- Type of Timing
What is a correct node?
A node that always follows the protocol
Explain the fault-model of crash-stop
A node is faulty if it crashes. After crashing, it stops executing forever.
Explain the fault-model of crash-recovery
A node might crash, lose its internal state, and might resume executing sometime later.
Explain the fault-model of byzantine
A node can deviate arbitrarily from a protocol
What is another name for the byzantine fault-model?
the fail-arbitrary fault model
What is the defenition of a complete network?
A direct link between any two processes.
The is the definition of the state of a process
the set of values of all its variables (includes initial and terminal states)
The is the definition of the state of a channel
messages in transmission on the channel (aka not received, initially empty)
The is the definition of the global state
set of the process states + set of the channel states
What is another term for global state?
Configuration of a Distributed System
What is a transition?
A state change in a DS
What triggers a transition?
events in processes (e1)
Explain what reliable links are.
A message is received iff it is sent. Messages
may be reordered.
Explain what Fair-loss links are.
Messages may be lost, duplicated or reordered. If you keep trying, eventually messages arrive.
Explain what Arbitrary links are.
a malicious adversary may interfere with messages. (eavesdrop, modify, drop, replay)
What is an synonym for reliable links?
perfect link
Explain how a system with Async. Com. can be simulated using a system with Sync. Com.
introduce for every link a process with a buffer that may delay messages
Explain how a system with Sync. Com. can be simulated using a system with Async. Com.
Use explicit acknowledgements to force the sender to wait untill the message is received.
Explain the basic properties of an Asynchronous network.
- message delays are finite but arbitrary
- the relative processor speeds may be unbounded
- there is no common clock
Explain the basic properties of an Synchronous network.
- message delays are bounded
- the relative processor speeds are bounded
- there is a common clock
What is the difference in the driver between sync and async networks?
Async networks are event driven. (sending receiving internal events trigger different code parts).
Sync networks are time driven; they loop over their tasks a finite number of time(s).
Explain what partial synchrony is.
The system is asynchronous for some unknown but finite amount
of time, and synchronous afterwards.
explain the expected message delay for partial synchronous systems.
There exist some time bound Δ and a special event GST (Global Stabilization Time) such that:
- The adversary must cause the GST event to eventually happen after some unknown finite time.
- Any message sent at time x must be delivered by time Δ + max(x,GST).
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