13
votes
External consistency vs linearizability
External consistency doesn't have a fixed meaning. In this context, it has the meaning appearing in the very next sentence in the document:
For any two transactions, $T_1$ and $T_2$ (even if on ...
- 275k
13
votes
Difference between Lamport timestamps and Vector clocks
Summary:
Lamport timestamps and vector clocks are both logical clocks, and both provide a total ordering of events consistent with causality.
Vector clocks allow you to determine if any two ...
- 261
11
votes
Parallel vs Distributed Algorithms
An algorithm is parallel if there are several processes (tasks, threads, processors) working on it at the same time. Often the tasks run in the same address space, and can communicate/reference ...
- 13.9k
9
votes
How do Functional Reactive Programming and the Actor model relate to each other?
I wanna point out how they are different from a practical point of view:
1) actors send messages to other actors, this message passing is described explicitly and imperatively.
For example:
...
- 255
9
votes
Accepted
Confused between 2 phase locking and 2 phase commit
These are two different things that have two different goals.
The two-phase locking protocol is designed to guarantee serializability for transactions that access concurrently a single, centralized ...
- 809
7
votes
Distributed vs parallel computing
Here is a recent paper that is worth reading:
Michel Raynal: "Parallel Computing vs. Distributed Computing: A Great Confusion?", Proc. Euro-Par 2015, doi:10.1007/978-3-319-27308-2_4
Abstract:
...
- 1,996
7
votes
Parallel vs Distributed Algorithms
One important quantitative distinction is that communication often costs more in distributed computing than in parallel computing.
An important qualitative distinction is that distributed algorithms ...
D.W.♦
- 156k
7
votes
Accepted
Who are the legislators of Paxos?
This is an educated guess of the transliterated names I could find in the Paxos paper. Most of these are people mentioned in the paper's references.
Λ˘ινχ∂: Lynch, N. - Legislator
Φισ∂ερ: Fischer, M. ...
- 829
7
votes
Accepted
What is the consensus algorithm that requires an odd number of nodes?
To my knowledge there is no quorum-based consensus algorithm that requires an odd number of nodes (processes). That's because such algorithms don't require a majority in the sense that a higher number ...
- 86
7
votes
Accepted
Why $e(C_i) = D_i$ is correct assumption? (FLP Impossibility 1985 - Lemma 3)
The paper says
By an easy induction, there exist neighbors $C_0, C_1 \in \mathscr{C}$ such that $D_i = e(C_i)$ is $i$-valent, $i = 0, 1$
Here is a proof:
The set of configurations forms the nodes ...
- 3,310
7
votes
Accepted
Why is Two-Phase Commit (2PC) blocking?
Is it because the cohorts don't employ timeout concept in 2PC?
Yes, in one case they can not use a timeout. It is described in the paper too (II.B.1):
The Two-Phase Commit Protocol goes to a
...
- 281
7
votes
Why are forks in the Blockchain eventually resolved?
If we simplify and assume that each miner randomly guesses a hash (as opposed to being more systematic) and we discretize time, say into minutes, then each minute each miner is hoping to "roll" the ...
7
votes
Difference between Lamport timestamps and Vector clocks
Although similar they have different purposes: version vectors can distinguish whether two operations are concurrent or one is causally dependent on the other; Lamport timestamps enforces total ...
- 71
6
votes
Parallel vs Distributed Algorithms
The terms can mean almost anything, but I will try to present here one way in which the terms "parallel algorithms" and "distributed algorithms" are understood. Here we interpret &...
- 1,996
6
votes
Accepted
Are vector clocks useful in centralized systems?
No, there's no need for a vector clock in a centralized system.
A vector clock uses a $N$-vector of timestamps, where $N$ is the number of computers in the distributed system and the $i$th component ...
D.W.♦
- 156k
6
votes
Accepted
What is the difference between Consensus and Leader Election problems?
This is not a matter of terminology: they're related, but different concepts.
A consensus algorithm is one that allows all the participants in a distributed system to choose a value from a set in ...
6
votes
Accepted
Why aren’t distributed computing and/or GPU considered non-deterministic Turing machines if they can run multiple jobs at once?
In parallel computing, the threads can talk to each other and exchange information during the computation. In nondeterminism, the only "communication" between threads is that we compute the OR of all ...
- 3,491
6
votes
Accepted
Algorithm notation
⋃ is the n-ary union operator, similar to how ∑ is the n-ary addition operator.
So, in the same way that ∑j someExpressionDependingOnJ means "add the values of all the different instances of ...
- 5,830
5
votes
Accepted
How to resize a large, distributed hash table?
Yes, after storing many items in a distributed hash table spread over a hundred computers, if hypothetically we used the sort of hash function popular for in-RAM hash table, adding another computer ...
- 886
5
votes
Why is the commit phase in PBFT necessary?
PBFT is a master piece, for its technical breakthrough and exquisitely precise language. Many descriptions on the protocol details worth reading multiple times to grasp all the nuances.
I will:
quote ...
- 151
5
votes
How do you compute the time complexity of distributed algorithms?
Time complexity is always measured relative to some model. For example, the $\Theta(n \log n)$ bound on sorting is the number of comparisons performed. If comparisons are not constant time, then the ...
- 29.7k
5
votes
Accepted
Lamport logical clock: what does partial mean in the concept of `Partial ordering`?
The "partial ordering" in the papar means partial order as in standard mathemtics theory. To be more rigorous, the "partial ordering" in that paper, also called "irreflexive parital ordering" in that ...
- 38.6k
4
votes
Accepted
Is there a difference between total-ordering consistency and linearizability?
Total, FIFO and causal are all different sequential approaches for ordering the events. Now, consistency applies to all these types ordering and at different levels: local and global.
Sequential ...
- 362
4
votes
Vector clocks: Why is it necessary to increment my clock on receiving a message?
Look at the definition of $<_H$.
We say that $e_1<_H e_2$ (event 1 happened before event 2) if:
$e_1,e_2$ took place in the same process, and $e_1$ happened first (events within the same ...
- 13.3k
4
votes
What is the consensus algorithm that requires an odd number of nodes?
"Always have an odd number of replica set members" is a common simplification of the MongoDB replica set election process and best practices for deployment, but certainly not a strict requirement.
...
- 141
4
votes
blockchain database - why so redundant
See https://en.wikipedia.org/wiki/Blockchain_(database)#Decentralization, which states that "Every node in a decentralized system has a copy of the blockchain. This avoids the need to have a ...
- 263
4
votes
Accepted
Synchronous model: is taking steps simultaneously equivalent to having fixed upper bounds for communication/processing delays?
All of these are, generally speaking, called "synchronous models", they are pretty similar to each other, and especially in the context of fault-tolerant algorithms they are very different from "...
- 1,996
4
votes
Where does the FLP impossibility proof depend on allowing a single process failure?
The part of the original paper's proof that requires node failure to prove the impossibility is case 2 of lemma 3. This case assumes that there is a "finite deciding run from C0 in which p takes no ...
- 41
4
votes
Accepted
Can I prove that I have x such that f(x) < c without revealing x?
If the function $f$ is publicly known and is efficiently computable, it is possible for Bob to prove that he knows a value $x$ such that $f(x)<c$. This is known as a zero-knowledge proof of ...
D.W.♦
- 156k
4
votes
Accepted
How do you compute the time complexity of distributed algorithms?
You understood it right. The standard models of distributed computing typically assume that local computation is free. It follows that in the LOCAL model of distributed computing, you can solve any ...
- 1,996
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