kandi X-RAY | Paxos Summary
kandi X-RAY | Paxos Summary
Paxos
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Top functions reviewed by kandi - BETA
- Propagate a given value to the destination
- Resets all accepted proposals
- Set the proposal number
- Generates a unique number
- Process a learner message
- Gets the accepted value
- Process a proposed value
- Gets the source id
- Receives a message
- Process a prepare request
- Handles an accept request
- Returns the value
- Register a component
- Compares this Acceptor
- The main method
- Runs the next proposal
- Create a 128 - bit hash code
- The main loop
- Send a new learnt value to all learners
- Make a message delivery
- Get a set of components for the given type
Paxos Key Features
Paxos Examples and Code Snippets
Community Discussions
Trending Discussions on Paxos
QUESTION
I have read the paper Paxos made simple.
And after hard thinking, I come to this conclusion:
The Paxos protocol always guarantees the majority of servers accept the same value at the same turn of the proposal and so that finally, we can derive the consensus from the majority of servers.
For example, table 1 shows that there are three servers(S1, S2, S3), and the N1, N2... denotes the proposal, there are many gaps in the proposals but for every proposal, we can derive the consensus by the majority servers.
...ANSWER
Answered 2021-Feb-22 at 23:22From what you have said I think you are seeing whether or not Consensus on a sequence of values is obtained from looking at the values accepted in increasing proposals. Follow this, the log is obtained from the proposal numbers ((N1, A), (N2, B), (N3, C), (N4, B)). I think this is the case because you refer to gaps in the sequence of proposals.
If this is the case, then I think there might be some confusion.
In Paxos, there are two things to worry about:
Proposals (or ballots, attempts to choose a value)
Log values (values agreed on by the acceptors and then executed by a state machine)
For the log, there must be fault-tolerant agreement on each value. A value can be executed only if all previous values have also been executed, therefore there must be no gaps in the log if you wish to execute all values. This is a liveness issue though and has nothing to do with safety. You can safely learn of all the values in the log and see them as "committed". If it were a machine executing commands issued by clients, you could tell a client their request for a command will occur but you couldn't tell them the result of executing their command as other commands that you do not know of will be executed before it.
The log uses the basic Paxos protocol to provide the fault-tolerant agreement on a single value. This is where the idea of committing a value is useful. The basic Paxos protocol outlined in Paxos Made Simple allows for agreement on only a single value and it uses a totally ordered sequence of proposals (ballots)to agree on that single value.
The main thing here is that there are can be multiple proposals in deciding on one value to be at a single index in the log.
Following that, there might be gaps in the sequence of proposals promised or accepted by acceptors in Paxos but this is okay and part of the algorithm's operation. The Paxos algorithm ensures that if a value is agreed upon in one proposal, all higher-numbered proposals will see the same value proposed and therefore agreed upon. Please note this is different from the example in Tables 1 and 2 of the question. In that example, if a majority of servers (acceptors) had accepted A in proposal N1, then all later proposals would also accept value N1.
This ability to agree on multiple proposals but with the same is necessary for fault tolerance, in particular for liveness. Imagine if a proposal Nl was accepted by a majority of acceptors, and then all but one acceptor crashed before any notification of acceptances of Nl could be made. A value has been agreed upon but no one knows that. Now, to recover from this, another proposer should issue a higher-numbered proposal. Let's call this proposal Nh.
Making a proposal has two phases: Promise and Accept. In the first phase, the proposer must obtain promises from a majority (or quorum) of acceptors on the proposal Nh. The purpose of this is to determine whether or not a value was (possibly) agreed upon in a lower-numbered proposal. In this case, we will learn that one acceptor had accepted a value in a lower-numbered proposal, Nl. (The other acceptors who responded will report they've accepted nothing.) Despite only one acceptor reporting they've accepted a value in Nl, the proposer must (for agreement-safety reasons) assume it was possibly chosen. In the second phase, then, the proposer will issue acceptance requests to the acceptors for the proposal Nh, and will include the value of the earlier proposal Nl. If at least a majority (quorum) of acceptors accept this, then the proposal is chosen and all things being well, learners will learn that value to the value committed for a single index in the log. Otherwise, the process of issuing proposals for that to decide on the value at the log index will continue again.
Finally, the last point to raise is there is also no requirement that a proposal has a majority of acceptance and this is important because there might not be a majority of acceptors non-faulty at the time a proposal is issued. To motivate this consider if a proper, or too many acceptors fail during a proposal.
Imagine if in an instance of the Paxos algorithm there are there acceptors, and one of them accepts a value in proposal Na. Then before Na can be accepted by a majority, the proposer of Na fails and any messages in transit over the network are lost. In this case, proposers need to be able to make new proposals and see a value chosen. In the scenario, another proposer will begin a higher numbered proposal Nb. It will go through the promise phase as before and determine no value could possibly be chosen after talking to the two acceptors who didn't accept anything. Now the proposer of Nb can propose any value and see it accepted by a majority and agreed upon. Proposing any value here is safe because the proposer of the Na, should it come back online, will not have their value accepted by a majority because a majority of acceptors had promised and accepted on a higher-numbered proposal.
Michael :)
QUESTION
So, I’m studying Paxos and the Professor made this question:
Assume that acceptors do not change their vote. In other words, if they vote for value v in round i, they will not send learn messages with value different from v in larger rounds. Show that Paxos does not work any more (it can reach a livelock).
I’ve reasoned about this for the entire day, but I’m not understanding how can the livelock arises and so my colleagues.
Does anyone have a clue?
...ANSWER
Answered 2021-Jan-14 at 21:25Assume there were network failures such that every acceptor accepted a different value. Without the ability to change their value in future rounds, no progress could ever be made and you have a "livelock".
QUESTION
Consider the following flow :
Proposer prepares a message, gets a promise, sends a proposal with some value, gets it accepted. All fine.
After acceptance, another proposer comes along and prepares a message with a higer id, and the same flow continues.
Is this a valid flow of a single round of the paxos algorithm? Or is this actually multi paxos?
...ANSWER
Answered 2020-Dec-11 at 20:01After acceptance if another proposer comes along and prepares a message, it will receive at least one reply containing the previously accepted value. The Paxos rules then require that the second proposer MUST propose the previously accepted value. The value it wants to propose is overridden by the first value. This ensures that only a single value can be chosen for a single instance of the Paxos algorithm.
QUESTION
I am reading Lamport's Paxos Mode Simple, and I get confused with the meaning "value" here.
For example, Lamport says:
If a proposal with value v is chosen, then every higher-numbered proposal that is chosen has value v
I don't know what value v means here:
- Does it mean different value of a certain variable, such as variable x's value can be 1 or 42?
- Or is it something like one log entry in Raft, such as
x=1ory=42?
I think the first interpretion is right, and basic Paxos can't determine multiple values, it just Propose-Accept-Chosen, and the whole basic Paxos instance finishes its mission.
However, I am not for sure.
...ANSWER
Answered 2020-Dec-08 at 17:32Your second interpretation is correct ("It's like one log entry in Raft").
You are also correct that Basic Paxos can't choose multiple values, it just chooses one, like a single log entry in Raft. To choose a series of values you need to chain multiple Basic Paxos instances together, like in Multi-Paxos or Raft.
QUESTION
I have a cluster of cassandra (v3.11.4) with 14 nodes and I wanna to add a new node. The machine has 256GB memory and I set heap size (max and min) to 64GB . But I cannot add a new node due to memory error! What is the exact problem and What I need to do?
The last line of logs are as follows:
...ANSWER
Answered 2020-Jun-14 at 13:49The error message says that the JVM failed to commit memory, because mmap syscall returned with the error code 12 (ENOMEM).
This typically happens when the process reaches one of the OS memory limits:
The number of memory mappings exceeded
vm.max_map_countsysctl limit. This is a quite common problem with Cassandra, since Cassandra tends to mmap thousands files, and the default limit is rather low - around 65K.How to check:
wc -l /proc//mapswhereis Java process ID.How to fix:
sudo sysctl vm.max_map_count=1000000Total amount of the process' virtual memory exceeded
RLIMIT_ASHow to check:
ulimit -v,cat /proc//status | grep VmHow to fix:
ulimit -v unlimitedbefore starting Cassandra in the same shell.Overcommit is disabled, and the overcommit ratio is too low.
How to check:
sysctl vm.overcommit_memory,sysctl vm.overcommit_ratio,cat /proc/meminfoHow to fix:
sudo sysctl vm.overcommit_memory=0
QUESTION
I've quite understood what the Raft is and implemented it in MIT6.824 distributed system. I also know what's the basic Paxos, I've not implemented this yet, so I can't grab all details of it. For Multi-Paoxs, I'm even more confused, i.e., WHY it can eliminate lots of Prepare RPC? I know the answer should be Multi-Paxos can have a fixed leader along with noMoreAccepted response from other peers to determine if reduce Prepare RPC. But, I can't get it in detailed level, why and how it works
I want to get more some recommendations, articles, sample code or anything that can help for Multi-Paxos,
- I've read
Paxos made liveandPaxos made simple, those two papers can give me a basic idea about what's Paxos and how it works - I've watched https://www.youtube.com/watch?v=YbZ3zDzDnrw&t=3035s&ab_channel=DiegoOngaro several times as well, it's a great talk, but it does not involve too many details
ANSWER
Answered 2020-Nov-14 at 07:12To answer your specific question:
WHY it can eliminate lots of Prepare RPC?
In the paper Paxos Made Simple page 10 it says:
A newly chosen leader executes phase 1 for infinitely many instances of the consensus algorithm—in the scenario above, for instances 135–137 and all instances greater than 139.
That is saying that if a leader broadcasts Prepare(135,n) which is a prepare for instance 135 using ballot number n then it is valid that this can be defined as applying to all instances >=135 that are not yet fixed. We can reason that it is safe for any node to be "spamming" out prepare messages for an infinite number of the unfixed positions in our log stream. This is because for each position each acceptor uses the rules of Paxos for that position. We can compress that infinite set of prepare messages down to a single one that applies to all higher unfixed positions. We then eliminate all but one prepare message for the term of a stable leader. So it is fantastic optimisation.
You asked about any example code. I wrote an implementation of multi-paxos using functional programming in Scala that aims to be true to the paper Paxos Made Simple over at https://github.com/trex-paxos/trex. The core state is PaxosData, the message protocol is at the bottom of PaxosProtcol and the algorithm is a set of message matching functions in PaxosAlgorithm. The algorithm takes the current immutable state and an immutable message as input and outputs the next immutable state for the node. Common behaviours are written as partial functions that have full unit tests. These partial functions are composed into complete functions used by leaders, followers and candidate leaders. There is a write up at this blog.
It adds additional messages to the basic set as optimisations speed up log replication. Those involve some implementation details that Lamport does not get into in his paper. An example is that negative acknowledgements are used to pass information between nodes to try to avoid interrupting a stable leader due to only one failed network link between a node and the leader. TRex tries to keep those features to a minimum to create a basic but complete solution.
An answer that you might find helpful about Multi-Paxos is this one that discusses why Multi-Paxos is called that https://stackoverflow.com/a/26619261/329496
There is also this one about how the original Part-Time Parliament paper uses a leader and also describes a stable leader running multi-Paxos https://stackoverflow.com/a/46012211/329496
Finally, you might enjoy my defence of Paxos post The Trial Of Paxos Algorithm.
QUESTION
The Spanner paper makes it clear that commit timestamps are chosen for read-write transactions by the coordinator leader. However, I'm not sure of where the timestamps are chosen for read-only transactions.
The documentation here says:
API Layer will pick the read timestamp by using the current TrueTime.
But where is that API layer situated? Does that refer to the location proxy that the paper says clients use to locate the relevant spanservers? The paper says it uses TT.now().latest but I can't tell where that gets invoked.
I had assumed that the timestamp could be chosen by any of the Paxos leaders involved in the multi-site read, but apparently not. Can someone please help clarify?
...ANSWER
Answered 2020-Sep-03 at 22:08But where is that API layer situated? Does that refer to the location proxy that the paper says clients use to locate the relevant spanservers?
That's correct, it's in the API proxy.
I had assumed that the timestamp could be chosen by any of the Paxos leaders involved in the multi-site read, but apparently not.
To negotiate a timestamp, strong read requests must contact the Paxos leader for each Spanner group involved in that read.
QUESTION
Service definition
...ANSWER
Answered 2020-Jul-13 at 14:34Based on the dig output you shared zevrant-oauth2-service-db is resolving to 92.242.140.2 but it looks like the IP address of your K8s service is 10.97.75.171 (ClusterIP) (based on the output you shared too).
If you hit 10.97.75.171 5432 you should be able to access your Postgres database, provided that you don't have any Kubernetes Network Policy and/or firewall blocking access. Make sure you that in your Postgres config you are binding the server to 0.0.0.0 otherwise if it's something like localhost you will only be able to get to it from the pod.
So the question is what is 92.242.140.2? Wny is coredns responding to a query to zevrant-oauth2-service-db with 92.242.140.2? Is there a DNS forwarder configured in coredns? Is there a default domain configured that is not part of svc.cluster.local?
QUESTION
I have this following table:
...ANSWER
Answered 2020-Apr-03 at 07:20The major reason why this query fails is that it's incorrect - Cassandra works fast only when you have full partition key, and then you can do the range query inside that partition. In your case, you have partition key consisting of 5 columns, but you're providing only one in the query, and Cassandra needs to perform scanning of the whole data to find where the corresponding rows are located. I really wondering that it worked before...
To solve your problem you need to change the table structure to have partition/primary key matching your queries - all data modeling for Cassandra starts with queries that should be executed. I recommend to take DS220 course at DataStax Academy on data modelling.
QUESTION
P.s. I have already had a look at SWIM protocol how does a new node get an address of an existing node in a cluster and my question asks a little bit more than that, which is why made a separate post.
I am trying to implement the SWIM protocol in c, and I have two questions: 1) How does a node get introduced into the group? From what I can think of, I'm guessing a standard "introducer" node sends the member list over to the new node and starts gossiping about the node joining the cluster? And 2) What happens if the "introducer" node fails? How is a new introducer elected? I'm thinking maybe Paxos but its too complicated for me to implement.
...ANSWER
Answered 2020-Mar-05 at 11:12The new node member selects a random subset of peers(seed members) of cluster as targets for gossip messages while joining the cluster based on the configuration provided to it. The address of at least one alive node member / seed member address of the cluster must be known to the new node member so that it can join the cluster. In case the seed member (introducer) gets killed at the initial stage, there is no other way for the new node to convey its existence and it shall become isolated unless there is alternate seed address in subset that is alive and known to the new node member. However, if the seed member (introducer) gets killed at a stage where the initial gossip of new node has been communicated to another node of the cluster, there shall not be much impact unless the new node is required for sure to reach a specific another node.
Once joined, the other nodes shall introduce it to gossips, this allows the new node member to gradually receive information about all the nodes of the cluster. The “gossip information” that new node is added is included in the packets such as (ping, ping-req, ack) that is used by the protocol to send messages to other nodes at regular intervals which is termed as “infection-style” dissemination mechanism. Accordingly, the membership list maintained by each node member shall get updated whenever a new node member joins or leaves the cluster.
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No vulnerabilities reported
Install Paxos
You can use Paxos like any standard Java library. Please include the the jar files in your classpath. You can also use any IDE and you can run and debug the Paxos component as you would do with any other Java program. Best practice is to use a build tool that supports dependency management such as Maven or Gradle. For Maven installation, please refer maven.apache.org. For Gradle installation, please refer gradle.org .
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