> • Unlike Kafka it does not have a vibrant and huge community (merge those 
> PR’s please, anyone?)

This is clearly true. The community was active 5 or so years ago but in the 
past few years it's almost non-existent. Patrick is the only active committer. 
It can take years (!!) and numerous cajoling emails to get engagement on pull 
requests. Releases happen only once or twice a year. The worst culprit has been 
the so-called alpha/beta of 3.5.x. Whatever the beliefs of the ZooKeeper team 
are, 3.5.x has been in production at major tech companies for _years_ yet it's 
still treated as a non-released version. Even if we were to accept the 
alpha/beta label, the original 3.5.0 alpha was 3 and half years ago! That's 
crazy and has contributed dramatically to the negative perception of ZK.

> It uses a protocol which is hard to understand and it’s hard to maintain a 
> large Zookeeper cluster

This is a red herring. Raft may be easy to understand from the whitepaper but 
any distributed protocol is difficult in practice. Further, no user of a tool 
such as etcd or ZooKeeper remotely cares about the protocol. That's an 
implementation detail.

> It’s a bit outdated, compared say with Raft

Another red herring. Raft and ZAB are, essentially, the same protocol.

> It’s written in Java (yes, it’s opinionated but this is a problem for us as 
> ZK is an infrastructure component)

There is a current bias against Java. The reasons for this are beyond the scope 
of what we can discuss here. But, in my view, it's ludicrous. That said, the 
non-Java clients for ZooKeeper are lacking and this is a problem. I don't 
believe there is a good Go client for ZooKeeper for example.

> We run everything in Kubernetes and k8s by default has an in-built Raft 
> implementation, etcd

etcd is a good key/store system. However, I'm not sure how well it does for 
leaders/locks/etc. at scale. Also, is there a good Java/JVM client for it? I 
know they've been working on one but what is it's status? We are working 
against trends in the DevOps world here. DevOps has moved almost entirely to Go 
and the Hashicorp borg. If it's not in Go they're not really interested. This 
is not a problem for ZooKeeper as it addresses a different space - 
applications. But, the Ops people IMO confuse the two products and think "we 
already have etcd why do we need another system to support." A good white paper 
detailing the real differences between etcd/consul and ZooKeeper is needed.

> Linearizability (if there is a word like this) - check this comparison chart

This is just wrong. All operations in ZooKeeper are ordered. This, I think, 
comes up when using etcd as a k/v store. These two use cases, 
locks/leaders/register vs k/v store keep coming up. ZooKeeper is not a 
database. etcd _can_ be used as a k/v store.

> Performance and inherent scalability issues

ZK's performance is better than etcd AFAIK for the uses cases it was designed 
for. However, operating ZooKeeper can be a bear. I know that it's very 
difficult to find qualified ops engineers who can manage ZK ensembles at high 
scale. In particular, if ZK is used as a quasi-database it can be very 
difficult to operate (we're having that problem at Elasticsearch Cloud).

> Client side complexity and thick clients

Well, as the author of Apache Curator, I don't see why this is a problem. What 
does it matter if the client does a lot of the work or the server. It's opaque 
to application writers. In any event, most of the "recipes" in Curator are not 
in-the-box with consul/etcd. These need to be written and then you have a thick 
client again. Most of the things you want to do with ZooKeeper are already 
implemented in Curator. However, if you're not on the JVM you don't get those. 

> Lack of service discovery

Curator has had Service Discovery since its beginning: 
http://curator.apache.org/curator-x-discovery/index.html 
<http://curator.apache.org/curator-x-discovery/index.html> 

-Jordan

> On Feb 13, 2018, at 6:02 AM, Flavio Junqueira <f...@apache.org> wrote:
> 
> Hello community,
> 
> I came across this blog post:
> 
>      https://banzaicloud.com/blog/kafka-on-etcd/
> 
> And I thought it would be a good idea to discuss the criticism as a 
> community. Let me copy the points here and add some notes:
> 
>       • Unlike Kafka it does not have a vibrant and huge community (merge 
> those PR’s please, anyone?)
> I have personally met and worked with a lot of great people in this community 
> over the years, and as such, I probably have a pretty biased view. But, it is 
> a common concern that we are not fast enough at responding. We also don't 
> have conferences and large meetups compared to other communities. Are those 
> really necessary, though? What can we do to be a better community?
> 
>       • It uses a protocol which is hard to understand and it’s hard to 
> maintain a large Zookeeper cluster
> I can't really speak for the hard to understand part, and I don't understand 
> what "maintain a large ZooKeeper cluster" is referring to. How large is it 
> and why do we need it to be large? We have features like observers that 
> enable large clusters, but whether it solves the problem depends on what they 
> are after.
> 
>       • It’s a bit outdated, compared say with Raft
> When we wrote about Zab years back, we had as a goal to explain the protocol 
> in a way that could be reproduced. We had other goals too, like explaining 
> how we had been successful in implementing a system like ZooKeeper with that 
> protocol, the properties it guaranteed and so on. Raft focused on the 
> simplicity of understanding, which makes a lot of sense given that there was 
> interest in reproducing it. Given its focus, and clearly the quality of the 
> people behind it, Raft has been more successful in popularizing the 
> implementation of replicated state machines. At a protocol level, however, I 
> don't think there is anything that makes Zab outdated with respect to Raft.
> 
>       • It’s written in Java (yes, it’s opinionated but this is a problem for 
> us as ZK is an infrastructure component)
> This is arguable, there are pros and cons both ways.
> 
>       • We run everything in Kubernetes and k8s by default has an in-built 
> Raft implementation, etcd
> I can totally understand this point. No one wants to have to operate two 
> systems doing similar things. To consolidate operations, it clearly makes 
> sense to pick one. Ironically, this post talks about plugability, but 
> Kubernetes does not really give the option of using zk rather than etcd if 
> that's what I want to use.  
> 
>       • Linearizability (if there is a word like this) - check this 
> comparison chart
> We do provide linearizable reads with sync(), although I understand that it 
> is arguable whether that is truly linearizable. There has been a long running 
> discussion about whether we should make sync() truly linearizable by making 
> it a first-class txn. Back in the day, we haven't done it because we wanted 
> reads to be fast, so we implemented it in a way that it didn't have to go 
> through the whole pipeline of request processors, but it still reaches out to 
> the leader. See the issue for more detail: 
> https://issues.apache.org/jira/browse/ZOOKEEPER-2136
> 
>       • Performance and inherent scalability issues
> I don't know if those experiments were done using a dedicated device to the 
> txn log, which is a well-known fact about zk's performance. Incremental 
> snapshotting is clearly a good way to reduce the amount of disk load for 
> snapshots, but I wonder whether that's really a primary concern given that 
> servers these days often have multiple devices.
> 
> I don't understand that max CPU utilization for zk 
> (https://coreos.com/blog/performance-of-etcd.html). Perhaps this is something 
> to be investigated.
> 
>       • Client side complexity and thick clients
> Due to the set of features we wanted to offer, we have indeed chosen this 
> path. 
> 
>       • Lack of service discovery
> I don't have a good sense of how many users are actually bothered by this. I 
> have heard complaints over time about service discovery with ZooKeeper, but 
> I'm not sure there was any conclusion about whether service discovery is a 
> good use case for such coordination systems, including etcd for that matter.
> 
> Any feedback?
> 
> Thanks,
> -Flavio

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