2.2.5 Zookeeper and Paxos: Introduction

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Del curso dictado por University of Illinois at Urbana-Champaign

Cloud Computing Applications, Part 2: Big Data and Applications in the Cloud

164 calificaciones

University of Illinois at Urbana-Champaign

Cloud Computing Applications, Part 2: Big Data and Applications in the Cloud

164 calificaciones

Curso 4 de 6 en SpecializationCloud Computing

Welcome to the Cloud Computing Applications course, the second part of a two-course series designed to give you a comprehensive view on the world of Cloud Computing and Big Data! In this second course we continue Cloud Computing Applications by exploring how the Cloud opens up data analytics of huge volumes of data that are static or streamed at high velocity and represent an enormous variety of information. Cloud applications and data analytics represent a disruptive change in the ways that society is informed by, and uses information. We start the first week by introducing some major systems for data analysis including Spark and the major frameworks and distributions of analytics applications including Hortonworks, Cloudera, and MapR. By the middle of week one we introduce the HDFS distributed and robust file system that is used in many applications like Hadoop and finish week one by exploring the powerful MapReduce programming model and how distributed operating systems like YARN and Mesos support a flexible and scalable environment for Big Data analytics. In week two, our course introduces large scale data storage and the difficulties and problems of consensus in enormous stores that use quantities of processors, memories and disks. We discuss eventual consistency, ACID, and BASE and the consensus algorithms used in data centers including Paxos and Zookeeper. Our course presents Distributed Key-Value Stores and in memory databases like Redis used in data centers for performance. Next we present NOSQL Databases. We visit HBase, the scalable, low latency database that supports database operations in applications that use Hadoop. Then again we show how Spark SQL can program SQL queries on huge data. We finish up week two with a presentation on Distributed Publish/Subscribe systems using Kafka, a distributed log messaging system that is finding wide use in connecting Big Data and streaming applications together to form complex systems. Week three moves to fast data real-time streaming and introduces Storm technology that is used widely in industries such as Yahoo. We continue with Spark Streaming, Lambda and Kappa architectures, and a presentation of the Streaming Ecosystem. Week four focuses on Graph Processing, Machine Learning, and Deep Learning. We introduce the ideas of graph processing and present Pregel, Giraph, and Spark GraphX. Then we move to machine learning with examples from Mahout and Spark. Kmeans, Naive Bayes, and fpm are given as examples. Spark ML and Mllib continue the theme of programmability and application construction. The last topic we cover in week four introduces Deep Learning technologies including Theano, Tensor Flow, CNTK, MXnet, and Caffe on Spark.

De la lección

Module 2: Large Scale Data Storage

In this module, you will learn about large scale data storage technologies and frameworks. We start by exploring the challenges of storing large data in distributed systems. We then discuss in-memory key/value storage systems, NoSQL distributed databases, and distributed publish/subscribe queues.

2.2.1 Eventual Consistency – Part 110:51

2.2.2 Eventual Consistency – Part 220:02

2.2.3 Consistency Trade-Offs4:46

2.2.4 ACID and BASE19:28

2.2.5 Zookeeper and Paxos: Introduction10:46

2.2.6 Paxos17:56

2.2.7 Zookeeper16:11

Conoce a los instructores

Reza Farivar
Data Engineering Manager at Capital One, Adjunct Research Assistant Professor of Computer Science
Department of Computer Science
Roy H. Campbell
Professor of Computer Science
Department of Computer Science

[SOUND] So, we've motivated both asset and

base and we've talked about eventual

consistency right at the beginning.

Now, what we want to do is see how you would take eventual consistency and

actually build a system to provide it in a cloud.

So, we are going to do a centralized service.

It's going to coordinate, it's coordination code you might say,

it's going to coordinate everything inside the system.

So that all of the system, whenever it wants to get to eventual consistency,

can use this particular algorithm to achieve that.

We're going to look at PackSource.

Packsource acts in a very distributive manner.

Lempel created it.

It's based off a quite interesting discussion of

a Greek island state operating in a parliamentary style.

But we're going to address that, we're just look at the way the algorithm works.

It, in general, it gets implemented as something called

Zookeeper In many of the Apache projects.

And ZooKeeper's a simplified version of Paxos and

operates perhaps not quite to such scale as Paxos could.

But it operates sufficiently well that to all intents and

purposes, Zookeeper is the module you use.

So we want to centralize service coordination.

It's going to maintain configuration information.

It'll allow you to do naming, get the consistency for naming right.

It'll allow you to do distributed synchronization.

It allows you to build group services,

any sort of function inside a cloud where you need consistency.

It avoids synchronization.

It avoids races, so that if you have a race,

you don't know who's going to win you may have to keep repeating the race.

It's going to avoid all that and

make sure that you'll have eventually a result you can use.

The file system based API allows you to manipulate small data nodes and

the state is a hierarchy of nodes.

The system uses that sort of file system based API

to keep track of everything so it remembers what the state of everything is.

And we will now look at the actual algorithm employed.

So If you were to describe this Paxos to people, what you would say is well

let's suppose there's somebody wanting to make a change to the system, a proposer.

The Proposer is going to request the Paxos system

accepts some sort of command or change its state.

And Paxos is going to act like a postal system.

It's going to make sure that the state changes.

It's going to think about the whole state of the system look at the letter,

what letter needs to be done, or what needs to be done in the letter.

It'll replicate data making sure that there's enough copies of

things to guarantee as far as it can see that it will all occur.

And then once all this is decided that whole process of how to do

things is decided it then goes ahead and

executes the proposer's command using its algorithm to make sure

that there's lots of redundancy and you're going to achieve eventual consistency.

And in the diagram you see the proposer sending mail to an acceptor.

The job of the acceptor is to keep

track that the proposer's asked this computation to be done.

They're replicated.

The acceptors are all going to sort of work out how to operate and

how to conduct the command and then when they've all sort of sufficiently

learned how to do that they will let their learners know what that command was.

Now we have enough acceptors, and each one of them is responsible for

trying to do this command, that the acceptors can actually form a quorum,

and the learners will learn from the quorum of acceptors what was successful.

So, here, you've got a system where

you know that you're going to get an answer because of the quorum mechanism.

You know that what you're going to achieve is a sort of an eventual consistency

because eventually some quorum of those accepters is going to

decide on some value that is how the mechanism is going to work.

So, more detail, the client issues a request to the distributed system,

waits for a response, and for instance might

request a right on a file on some sort of distributed file server.

The acceptors act as a fault-tolerant memory Reliable, durable memory.

This is what's going on, and somebody's acceptors could die in the meantime,

but it's still going to drive forward and actually produce a consistent result.

Acceptors, there's enough of them that if one or two of them died,

it doesn't matter.

In fact, we have a lot of acceptors, they can even have 40 operation.

What we're going to do is assume that the majority of the acceptors will come to

a decision or a way of writing to the file,

that is acceptable and that

Quorum once it's formed it will let the learners know they've achieved their goal.

Any message sent to an acceptor is duplicated to that quorum, and

any message received from an acceptor is ignored unless you

get enough of the acceptors sending the same copy of the message.

Paxos is based on very sort of weak assumptions.

The processors can operate at different arbitrary speeds.

The processors can all fail.

The processors with stable storage can rejoin the protocol after failures and

it doesn't hurt what's going on.

I remember the quorums, you can have people leave the quorum and

join the quorum reporting to a leaner, it's okay.

You can use crash,

recovery fault tolerance techniques in the middle of this.

And it doesn't affect any of the computations.

There are some assumptions that the processes don't collude or lie or

otherwise attempt to subvert the protocol.

So, this isn't a sort of mechanism for presenting failures

malicious code in there, sort of trying to cause trouble.

This is that everybody's trying to do their best.

And this protocol is trying to ensure that if they do their best,

you're going to get an eventually consistent result.

The network itself,

well processors will have to send messages over a network to other processors.

Messages can be sent asynchronisely, they can take arbitrarily long to deliver,

and this is the key nuts and bolts of things that

because they can take arbitrarily long, but you need to have some decisions made.

You're going to have set of acceptors in the middle are going to make sure that.

There is a reasonable confidence that the messages have been sent and received

enough that you can move forward even if some of the messages have got lost.

So messages are delivered without corruption,

you can obviously put check sums and other things in the messages and that avoids

some types of byzantine failures, and makes things a little bit more reliable.

In general, how many processes do you need to actually do this?

Well, there are consensus algorithms,

that show that there are actually limitations on

how well you can proceed in an asynchronized system.

In general, this particular consensus algorithm can make progress if it

has 2F+1 processors despite the simultaneous failure of any F processors.

So you do need a lot of machines, but what it will do is to guarantee

from one up to F that those failures can occur and it will still provide consensus.

And this is why you're using the quorum.

The quorum's going to guarantee that you vote amongst all of

those duplicate processes in order to establish the results.

If you're using reconfiguration, a protocols must be used and

it makes survival of any number of total failures

okay as long as you don't have more than F.

Fail, at the same time.

You can have F fail, or F minus one, fail.

Then restart and that will be okay, but what you can't do is the total number.

[MUSIC]

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