DataTalks.Club

Designing Data-Intensive Applications

by Martin Kleppmann

The book of the week from 08 Mar 2021 to 12 Mar 2021

NoSQL… Big Data… Scalability… CAP Theorem… Eventual Consistency… Sharding…

Nice buzzwords, but how does the stuff actually work?

As software engineers, we need to build applications that are reliable, scalable and maintainable in the long run. We need to understand the range of available tools and their trade-offs. For that, we have to dig deeper than buzzwords.

This book will help you navigate the diverse and fast-changing landscape of technologies for storing and processing data. We compare a broad variety of tools and approaches, so that you can see the strengths and weaknesses of each, and decide what’s best for your application.

Questions and Answers

Tino

Hey Martin Kleppmann :) Thanks for taking the time! How important would you consider planning this process in different stages of a company? Should a start up immediately plan applications for data-intense use cases or should the focus be elswhere?

Martin Kleppmann

Hey :) I would say it depends on what the company does. If you’re in the business of processing large amounts of data (e.g. you’re crawling large numbers of websites), it’ll be worth thinking about architecture up-front.
But most startups (e.g. typical SaaS apps) don’t initially have a lot of data. For these early stage companies, I would specifically advise against investing in a scalable architecture until you actually need it. The priority of an early-stage business is to be flexible and quickly adapt to the needs of customers as they are discovered. A complicated architecture is actually harmful here, because it reduces flexibility. Personally I’d just stick with PostgreSQL in this situation, and keep everything as simple as possible.

pie_f11235

Hi Martin could you elaborate this (chapter 3) a bit more?
Counterintuitively, the performance advantage of in-memory databases is not due to the fact that they don’t need to read from disk. Even a disk-based storage engine may never need to read from disk if you have enough memory, because the operating system caches recently used disk blocks in memory anyway. Rather, they can be faster because they can avoid the overheads of encoding in-memory data structures in a form that can be written to disk

Martin Kleppmann

What exactly do you find unclear or do you want to know more about?

Martin Kleppmann

Data in memory typically has a different representation from data on disk. For example, in memory you can use pointers, but pointers to memory addresses don’t make sense on disk, since after you restart a computer all the data will be at different memory addresses.

Martin Kleppmann

Therefore, you need to convert between the disk-oriented representation and the in-memory representation. This conversion incurs a cost. A database that doesn’t maintain on-disk data structures can save the conversion, because it can work with the in-memory representation only.

pie_f11235

when reading this from the book it gave me the impression is disk-based db + cache would be just as equivalent good and therefore we do not need to have in-memory db at all

pie_f11235

or if we do then in-memory dbs essentially shall be used as cache.
I wonder if I miss any point if I think in that extreme?

Martin Kleppmann

For a more detailed argument I suggest reading the papers referenced in this section:
https://hstore.cs.brown.edu/papers/hstore-lookingglass.pdf
http://www.vldb.org/pvldb/vol6/p1942-debrabant.pdf

pie_f11235

ok thanks Martin. let me check them out. I read the lookingglass one but not yet the other one. I probably misunderstood the whole thing

Vladimir Finkelshtein

In different data teams (small startup, midsize, large internet company) which role is usually responsible for the design?

Martin Kleppmann

I’m afraid I don’t have enough first-hand experience of different companies to give a good answer.

Vladimir Finkelshtein

It seems like you cover how different tools fit your tasks and other tradeoffs, and that is supposed to help you make your tooling choices. I wonder if in reality the choices are made based mainly on unrelated criteria. For example, how the pricing scheme of these tools matches your usage/budget or whether your team is already familiar with the tool.

Martin Kleppmann

I think whether a team is already familiar with a tool is an important factor. The fewer new tools you have to learn, the better! But it shouldn’t be the only factor: if you try to bend a single tool to all possible applications, you may run into difficulties too. As always, it’s a judgement call.

Martin Kleppmann

In practice, I suspect a lot of decisions are also made based on whether a technology is currently fashionable or not (e.g. whether the team members recently heard a conference talk about it, or read a tweet about it). That is poor practice in my opinion, but I can’t deny that it happens!

Martin Mihal

I just check book quickly (chapters, topics etc) - just question before invest time into full reading . Are those topics (eg. Scalabality, Maintanance, Replication ..) in world of cloud (eg. ..Kinesis, S3 as Data Lake, Athena, Snowflake..) still worth to go deep into? All of those tools were created to handle those topic “in price”… (I mean it’s important to undrstand those topics, question is how deep in this complex world we want/need to go)

Martin Kleppmann

Cloud services reduce the amount of investment you need to make into operations, because someone else is providing the operations team for you. But in order to know how to use a cloud service effectively, you still need a good understanding of what it can and cannot do. And you often don’t get that understanding from reading the vendor’s documentation alone, because a vendor will always pretend that their product is great for all purposes, even if it has strengths and weaknesses.
The purpose of my book is to teach you the fundamentals so that you can figure out the strengths and weaknesses of different technologies. That applies equally, regardless of whether you’re self-hosting or using a cloud service.

Slim

Hello, thanks for writing a great book like designing data intensive applications. My question is; do you think that relational database is the suitable solutions for banking systems. According to some experiences I had with finance companies they prefer relational database because of the utility of transactions and it’s straightforward to use. But sometimes I feel like it needs a graph database due to huge number of join and relation between tables which makes things complex. What do you think about that?

Martin Kleppmann

Hi Slim! I have never worked at a bank, so I don’t have personal experience. But my impression is that it doesn’t make sense to talk about the suitability of a technology for banking as a whole, since banking is a complex business where different parts of the business may have very different requirements. The needs of the people doing fraud prevention or anti-money-laundering will likely be very different from the needs of the people processing mortgage applications. The needs of those dealing with investments or trading will be different again.

Martin Kleppmann

I understand if there is a preference of relational databases since they are quite versatile and can satisfy the needs of a fairly wide range of applications (though not everything), and sometimes it’s better to stick with a tool you are deeply familiar with (“the devil you know”), even if it’s not quite optimal for the use case, than to incur the cost of learning how to use and reliably operate a different tool.

Martin Kleppmann

Keep in mind that choosing to adopt a certain database is not just a matter of the developers choosing one API or query language over another. There is also the need to set up backups, disaster recovery, to have ETL to bring the data from operational systems into a data warehouse, maybe audit logs, etc. etc. — adopting a new data storage technology also requires figuring out all this infrastructure.

Martin Kleppmann

So I have a lot of sympathy for organisations that are hesitant to adopt new technologies, even if they are better suited to the problem than whatever they are currently using.

Slim

thanks for this detailed answer :relaxed::relaxed:

A McCauley

Hey Martin :wave::skin-tone-3: great book! What would you say from your experience, is one of the most overlooked principles when designing data-intensive applications?

Martin Kleppmann

I’m afraid I don’t have a good pithy answer for this. Different companies/situations have different needs and different problems. Part of the ethos of the book is that there isn’t one single technology or principle that is universally applicable; rather, there are trade-offs, and knowing what they are will let you choose the right tool for the job.

A McCauley

And what is the most apparent challenge for teams/companies, etc. when designing data-intensive applications?

Martin Kleppmann

I think the biggest challenge is perhaps that there is such a bewilderingly large number of tools out there, and it can be really difficult to figure out which is best for a given task. Hence a primary goal of the book is to help you figure out what questions you need to ask in order to choose an appropriate technology.

Slim

what’s the approach that you recommend in microservices in order to guarantee the ACID properties in a transactions. I think it is not possible to guarantee ACID in distributed systems especially for rollback or when there is an error during the transaction.

Martin Kleppmann

most microservices architecture use transactions only within the bounds of a single service, and don’t have distributed transactions across service boundaries. this can sometimes make it really difficult to keep data consistent. there have been attempts to allow cross-service transactions (such as WS-AtomicTransaction in the SOAP world) but I fear this is not a good answer, since it introduces tight coupling between services.

Martin Kleppmann

one approach I’ve seen recommended is to draw the boundaries between services such that cross-service transactions simply are not necessary. this means that if you have two services that do need atomic commit, you need to merge them into a single service — i.e. don’t make the services too fine-grained.

Martin Kleppmann

another idea I’ve seen is to use a persistent log such as Kafka for communicating between services (instead of just ephemeral HTTP/RPC requests). this doesn’t quite give you atomic transactions, but it does give you pretty strong assurance that if one consumer reads a message off the log, all of the consumers will read that message. you can build some pretty strong guarantees based on that.

Martin Kleppmann

I elaborate on that second idea in this article on Online Event Processing: https://martin.kleppmann.com/papers/olep-cacm.pdf
and Ben Stopford has also written about using Kafka for inter-service communication: https://www.confluent.io/designing-event-driven-systems/

Slim

thanks Martin. I have seen a similair approach mentioned in Microservices patterns book written by Chris Richardson.

André Duarte

Hey Martin! What would be the top 3 things you would add, remove or change from the book if you were to rewrite it?
Thanks for your time

Martin Kleppmann

Big question! I have been collecting ideas for a second edition but have not yet started on it. Some ideas for improvement are:

  • talk more about hosted cloud DB-as-a-service infrastructure, which has gained popularity rapidly (but which still requires a lot of technical knowledge to use well)
  • there are tons of new tools in areas such as time series, workflow engines, new transaction processing engines (“NewSQL”)
Martin Kleppmann
  • maybe a bit more on decentralised and local-first technologies, since interest in those has been growing
  • I was thinking it might be cool to interview people at a couple of major internet companies and get them to share examples of their architectures, as case studies to discuss in the book
Sal DiStefano

If you have not checked out Martin Kleppmann blog, you should. He is also on Patreon.

Uriah Stephenson-Ward

Hi Martin Kleppmann do you plan on updating or making any revisions soon? Is there any specific tech/concepts/etc coming out that you think is exciting or will likely become the dominant way of working?

Martin Kleppmann

I am collecting ideas for a second edition, but not planning to start work on it for at least another year, since the current content is still pretty up-to-date.
I replied about ideas for a second edition in this thread

Elias

Hello Martin, it’s a great honor, and thank you for your book! In the last chapter, you are writing about the future of data systems. And many of your ideas and approaches from this chapter are already common nowadays (derived state, unifying batch and stream, designing apps around dataflows, etc.). But from your point of view, in recent years in data systems:

  • what are the things you expected to happen that didn’t happen (or you wish happened, or should’ve happened faster/at a bigger scale)
  • and what are the things that happened but you were not thinking about them when writing the book?
    Thank you!
Martin Kleppmann

some of this has happened, but my sense is that the big idea — redesigning interfaces and APIs around dataflow rather than request/response interaction — is still a long way away. there are plenty of REST APIs for online services out there, but not many of them let you subscribe to a log of changes. the best you can usually hope for is a webhook, and this is really just a notification mechanism — you can’t use it to reliably maintain derived views onto the data in someone else’s API.

Elias

Thank you for the answer!
But do you still believe this can/will happen, or “application” and “data” worlds are doomed to live apart :) (similar to DBMS and OS evolved separately)?

Martin Kleppmann

hard to say without a crystal ball. there is a huge amount of existing investment in REST API, and hence a huge inertia preventing a move to anything else. on the other hand, change data capture in databases seems to be becoming mainstream, and when you combine that with stream processing and event subscriptions you’re not too far off.

Nikolay

Hello Martin. I read your article “Please stop calling databases CP or AP”
Could you please provide some clarification? Does it mean that when we are talking about a distributed queue we can not talk about strong consistency ( linearizability) at all. I mean that even if value X is written to queue it will not be immediately available for reading even for that client(read your own writes consistency model). So distributed queue can not provide strong consistency, can it? if so… what the strongest level of consistency can we have in distributed queue.

Martin Kleppmann

Linearizability is a very useful property and it makes sense to talk about it. However, it’s a property of a particular operation or set of operations; a system may well provide some operations that are linearizable and others that are not. For this reason it doesn’t really make sense to label the system as “consistent” or not in the sense of CAP. Rather, we should say, for example: the enqueue and dequeue operations are linearisable.

Martin Kleppmann

Defining the “strongest level of consistency we can have” is a bit tricky because there is no good formal definition of what “stronger” means. But I do think that in practice, linearizable enqueues/dequeues are probably the strongest useful consistency model you will find for a queue.

Nikolay

One more question from my side. if we have 2 nodes - A and B. A is a leader. B is a follower. client issue write(X,1) to a leader. if the leader uses 2PC in order to implement atomic commit. is it possible to have a case when A and B have different observable values before A sends ACK to client?

Martin Kleppmann

observable to whom? to another client that is querying A and B? atomic commit/2PC does not guarantee anything about concurrency (it’s about handling crashes cleanly, not about concurrency). it is entirely possible for a client to query A and B while the 2PC protocol is in progress, and to get different responses from the two nodes.

Nikolay

I have a question that I guess is related to chapter 12 of the DDIA book. Most databases have their own cache. For example, oracle, postgress has its own buffer cache ( its cache of blocks). Cassandra has a row cache. Oracle also has a row cache. On another side, we have solutions like Redis, Memcached. As far as I can understand database is moving in the direction to have a separate computation engine and separate storage engine. Why not have a separate cache engine? I mean it would be nice to have the ability to have cache “inside” database(integrated cache) but so that we would be able to scale this cache. as far as I can see it’s not a way of modern databases. So there is probably something wrong with this approach. I mean we can not have an integrated scalable cache, can we?

Martin Kleppmann

This would make for an interesting research project! I don’t think it’s possible to say for certain whether this approach makes sense without actually trying it and seeing whether it helps and where it breaks down.

Martin Kleppmann

There is a long-standing debate between implementors of database engines and implementors of operating systems about caching of disk pages in memory. The OS automatically uses memory that would otherwise be unused to cache recently accessed disk blocks. But then a DB may need its own buffer cache (e.g. because it needs to control when a dirty page is written back to disk, because it has to go to the WAL before the data page is written), so data ends up being cached twice, which wastes memory.

Martin Kleppmann

there is a difference between memcache/redis and these block-level caches, which is that application-level caches such as memcache/redis don’t just store disk blocks, but they store entire precomputed responses to complex queries, potentially including some business logic. serving that data from a cache doesn’t only save on I/O, but also on computation time, which might be significant.

Nikolay

Cool. Thank you very much for your reply. Just want to add that Oracle has its own ROW cache. “When a query executes, the database searches the cache memory to determine whether the result exists in the result cache. If the result exists, then the database retrieves the result from memory instead of executing the query. If the result is not cached, then the database executes the query, returns the result as output, and stores the result in the result cache.
When users execute queries and functions repeatedly, the database retrieves rows from the cache, decreasing response time. Cached results become invalid when data in dependent database objects is modified.”

Nikolay

Regarding Chapter 3 of the DDIA book. As I understand LSM is optimized for write compare to B+Tree because LST writes less to disks.
If I understand correctly in the case of B+Tree each block is mapped to a disk file and when we update even 1 byte in B+Tree we have to update the whole block. as I know in oracle we will not update each block when we change 1 byte. we will write the whole block to disk just in case of checkpoint (every 3 seconds in the background) and during update statement, we will only write to WAL. So when I do update idx_value = 1 where id = ? database will write only 1 byte to WAL and the whole block will be flushed to disk only during checkpoint ( for simplicity I skipped part related to oracle undo segment). So it looks like for a simple update we will have to write to disk the same amount of bytes in the case of LSM and B+Tree, does not it?

Martin Kleppmann

I wouldn’t necessarily say that “LST writes less to disk” (a statement about write amplification), but rather that LST writes tend to be a more sequential access pattern compared to B-tree writes, and so there can be a performance advantage on disks where random-access writes are slower than sequential ones.

Martin Kleppmann

as far as I know, a disk always writes data an entire block at a time, even if you only change 1 byte in a file. (that’s why linux calls a disk a “block device”.)

Martin Kleppmann

a relational DB will tend to have its own concept of pages that doesn’t map exactly to physical disk blocks, but is roughly related.

Martin Kleppmann

it’s not the case that every database block/page maps to a separate file (file systems would not cope well with such a large number of small files). I think DBs usually allocate one big file containing many blocks, and use offsets into that file to identify blocks.

Martin Kleppmann

if you update just one row, you’re right that the WAL write will happen first, but the page containing the row will also have to be written sooner or later, even if the transaction is allowed to commit before it is written.

Nikolay

In chapter 5 of the DDIA book, you described multi-leader replication. Does it mean that in the case of multi-leader replication particular client can connect only to 1 leader? i mean that if we have 2 DC and lots of clients (say 10K) … each of our clients connects to a particular leader and if that leader is crashed … half of our clients can not connect to another leader? This part is not clear to me. as an example, it works like that in app calendar .. because i can connect only to 1 leader( which is on my own machine) . I can not connect to other clients :-). but what about 2 Data Center ( 3 DC)?

Martin Kleppmann

it will depend on the specific system, but in general I don’t see any reason why you can’t have one client to connect to multiple leaders. one potential setup would be for a client to connect to a leader in the local DC by default, and fall back to connecting to a different DC in the case of problems.

Alexey Grigorev

I have two questions from ankush khanna who’s travelling now and can’t ask the questions himself.
So the first one:
What do you think about the future of Serializable Snapshot Isolation?

Martin Kleppmann

I think it’s excellent, and more systems besides PostgreSQL should use it!

Alexey Grigorev

Second:
Your book covers a lot of ground regarding Streaming, but with advancement and popularity in streaming will you write more material on topics like Kafka or Pulsar?

Martin Kleppmann

Perhaps, although other authors have already covered streaming systems in great detail, so I’m not sure there is much more for me to add.

Rishabh Bhargava

Just to follow-up on this: which authors would you recommend reading to dive deeper into streaming systems?

Martin Kleppmann

“Streaming Systems” by Akidau et al.; “Kafka - The definitive guide” by Narkhede et al.

Martin Kleppmann

(I think a second edition of the Kafka book is in the works)

Rishabh Bhargava

Thank you!

Alexey Grigorev

Hi Martin Kleppmann! You were working as a software engineer, but then went to academia and started working as a researcher. What motivated you to focus on research?

Martin Kleppmann

I got tired of the short-termism in industry, especially in startups, where everything has always got to happen “right now”. I wanted a setup where I would have the space to think, to take the time to really understand things, and to work to improve the foundations of how we write software. In research I can work on things that may not be practical for another 5–10 years, and that’s fine. In a company you can’t normally work with such a long time horizon.

Alexey Grigorev

I can totally relate to that. Thank you for your answer!

Jonathan Diaz

Hi Martin Kleppmann. It’s awesome to e-meet you and I actually just finished reading DDIA a few days ago! One quote that stuck out to me was in the last chapter from Maciej Ceglowski “Machine learning is like money laundering for bias” which led me down a rabbit hole of finding the source here. What, in your opinion, can we do further so that ML algorithms/technologies don’t strengthen existing biases? Both for data folks who develop these algorithms and those who use them.

Martin Kleppmann

I’m afraid this is not my area; others are working very actively on this, but it’s fast-moving and I am not up-to-date. I suggest looking up work by folks such as Timnit Gebru and Cathy O’Neil.

Manoj Agarwal

Hi Martin Kleppmann I watched your amazing video series on Distributed Systems. Are videos of any other of your courses available publicly?

Martin Kleppmann

This is the only multi-lecture course available so far. I also have this playlist of conference talks I have done: https://www.youtube.com/watch?v=fU9hR3kiOK0

Martin Kleppmann

it includes some course-like material, such as this 2-hour lecture on formally verifying distributed algorithms: https://www.youtube.com/watch?v=Uav5jWHNghY

RH

Hi Martin,
Background: I work in a startup with a small engineering team; I mostly work on building NLP based microservices (data analysis to devops) in Python to serve other parts of our product. We have a few models in production which are working quiet well, and en route to building a few more.
Question 1: When I joined, we did not have a lot of data (few Gigabytes), but data is starting to build up (Terabyte). We currently store most of our data in Postgres. Most of the data is large blobs of semi structured texts that are uploaded by our customers to be processed and saved (the data format is similar to resumes). I believe that Postgres was the right DB to start with because it was very versatile and was good for making this happen “right now”, but not sure what the right thing to do is next? The queries are becoming slower and slower. I have thought of saving all the larger blobs of texts in a S3 bucket, and saving a retrieval link for that object in Postgres, but that makes retrieval slower. We retrieve the data very often.
Question 2: When the data is sent to us by the customer, we do not process it in anyway. We just save it the way it is sent by the customer. Do you think it is a good idea to process the data, and save “features” from the data in Postgres instead of reprocessing the data over and over again. That is, we should save the data the way the customer sent it (maybe in a S3 bucket) but we should process and save a version of the data that is relevant to our business case (this is something the engineering lead does not see value in, and prefer to beef up the servers).
Question 3 What is a good way to structure a data engineering / systems engineering career i.e. what makes a really good data/system engineer?

Martin Kleppmann

q1: it’s difficult to make a concrete recommendation without knowing much more about the characteristics of your data, the access patterns and queries you use, etc. You might be able to continue using Postgres by splitting the database across multiple machines, perhaps using something like Citus. If you don’t do many joins in queries, you could consider using a key-value store/document DB such as Cassandra or MongoDB, but that would be a much bigger change.

Martin Kleppmann

a distributed filesystem or an object store like S3 would make sense if the data items are large. this would scale well, but would make it much harder to do any indexing (i.e. finding documents based on some value that occurs within a document).

Martin Kleppmann

q2: Saving data in raw form, and separately storing data derived from it, is a great pattern. If you want to change your processing logic, you can then re-run it on the raw data. The datastore to use for the derived data will again depend on the data format and your access patterns.

Martin Kleppmann

q3: my suggestion would be knowing how to reason about trade-offs. there is never one true answer, only various options that each have their pros and cons. being able to figure out those strengths and weaknesses, and communicating them to the rest of the team, seems very valuable to me.

RH

Thank you so much for responding to my questions.

Amr Alaa

hey Martin Kleppmann
thanks for having this week with us, I am really enjoying reading these excellent questions and your great answers here, I do not think I have an exact question actually,
just if you have a plan for another book, more specific or more advanced
I believe that in this DATA era that we live in, you should consider a series of books or even a bundle of courses to help data engineers acquire more advanced skills in theory and in practical also
thanks again

Martin Kleppmann

Thanks! I am considering ideas for a book that would go into more details of algorithms used in distributed systems. But work has not yet started, so there is no timeline for when this might happen.

Alexey Shvets

Martin Kleppmann first of all thank you so much for you amazing work and legacy. In you video course you mentioned Leslie Lamport as a legend in the field of distributed system, but for many people you became a legend who made the field structured and accessible.
My question. What do you think, which programming language will dominate distribute systems development in the future? Now it is mostly Java, will it move to Golang/Rust? What is your personal preference?

Martin Kleppmann

I think programming languages are a question on which people have very strong opinions because each language is like a tribe, and people couple their identity to the tribe they belong to. In my opinion, the language in which a system is written rarely makes a big difference; more important is the system architecture. Most mainstream languages are probably okay for building many types of systems.

Martin Kleppmann

In particular, I think questions of expressiveness and details of language features do not have a big effect when it comes to system operations. What does make a difference is a language’s runtime characteristics, such as whether it supports threads, and whether it uses a garbage collection runtime. No GC means no GC pauses, which can be important for low-latency systems.

Martin Kleppmann

Rust is interesting to me because it’s memory-safe, supports threads, has no GC runtime, and is very portable (you can use it to build mobile apps or servers or web apps by compiling to wasm). But in the end, it’s the needs of a particular system that matter. There is no one language that is perfect for all systems programming.

Pavel Bukhmatov

Hey Martin Kleppmann! Thanks for a book and all the awesome research you do!
What is your opinion on implementing distributed storages using conflict-free replicated data types in future? The ideas behind CRDTs seems to be really compelling for distributed systems but the sheer complexity might overwhelm practical implementations as far as my limited understanding goes. What research / papers / books could you suggest on the topic?

Martin Kleppmann

I am a big believer in CRDTs, and have been doing research on them for the last 6 years! I maintain a community website https://crdt.tech/ that has a lot of resources on CRDTs, and links to all the latest research (including my own).

Alisher

hi, Martin Kleppmann, thanks for the book and all your educational activity!
I’d like to ask you a bit broad question - what do you think are the most promising and perspective topics in distributed systems research in next 5 years? which topics are underestimated and require more
researchers there?
Thank you.

Martin Kleppmann

Well, this is going to be very subjective, since every person has their own priorities! Personally I am excited about the potential of local-first software, and moving away some of the current cloud-centric view of distributed systems. That’s what I’m going to be working on for the next 5 years, anyway!

Nikolay

Hello Martin Kleppmann. Could you please help to build intuition about Version Vectors and Vector Clocks. i read about them in chapter 5 but can not understand the difference. i am looking for some examples in order to build intuition.I know that it’s a long story and i read also some articles from Riak founders. But i’s not still clear for me ). Maybe some good example will let to understand it.

Martin Kleppmann

Ah yes, I have struggled with this as well. I’m afraid a proper explanation goes beyond what I can provide here in a few sentences. I should do a blog post on this at some point.

Martin Kleppmann

at a high level, the difference is in purpose. a vector clock is used to compare events in a distributed system, and figure out which happened before which, and which are concurrent. a version vector is used to compare states of replicas, and figure out whether one state supersedes the other. the mechanism in both cases is similar: a vector of numbers that are incremented.

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