zookeeper-dev mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From Jordan Zimmerman <jor...@jordanzimmerman.com>
Subject Re: Criticism on ZK
Date Tue, 13 Feb 2018 14:47:05 GMT
> • 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 It’s a bit outdated, compared say with Raft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 <fpj@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


Mime
  • Unnamed multipart/alternative (inline, None, 0 bytes)
View raw message