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What would you like to do?
- What do Etcd, Consul, and Zookeeper do?
- Service Registration:
- Host, port number, and sometimes authentication credentials, protocols, versions
numbers, and/or environment details.
- Service Discovery:
- Ability for client application to query the central registry to learn of service location.
- Consistent and durable general-purpose K/V store across distributed system.
- Some solutions support this better than others.
- Based on Paxos or some derivative (i.e. Raft) algorithm to quickly converge to a consistent state.
- Centralized locking can be based on this K/V store.
- Leader Election:
- Not to be confused with leader election within the quorum of Etcd/Consul nodes. This is an
implementation detail that is transparent to the user. What we are talking about here is leader
election among the services that are registered against Etcd/Consul.
- Etcd tabled their leader election module until the API stabilizes.
- Other non-standard use cases:
- Distributed locking
- Atomic broadcast
- Sequence numbers
- Pointers to data in eventually consistent stores.
- How do they behave in a distributed system?
- All of the solutions under consideration are primarily CP systems in the CAP context.
That is, they favor consistency over availability. This means that all nodes have a
consistent view of written data but at the expense of availability in the event that
a network partitions occurs (i.e. loss of node).
- Some of these solutions will support "stale reads" in the event of node loss.
- Each solution can work with only one node. It is generally advised that we have one etcd/
consul per VM/physical host. We do not want to have an etcd/consul per container!
- Immediate problems that we are trying to solve:
- Get and set dynamic configuration across a distributed system (e.g. things in moc.config.json):
- This is perhaps the most pressing problem that we need to solve.
- An SCM tool like Puppet/Anisble are great for managing static configurations but
they are too heavy for dynamic changes.
- Service registration:
- We need to be able to spin up a track and have services make themselves visible
via DNS.
- This would be useful primarily outside of production where we would want to regularly
spin up and destroy tracks.
- That said, we don't have a highly-distributed and elastic architecture so we could get
by without this for a while.
- Service discovery:
- Services must be able to determine which host to talk to for a particular service.
- This may not be as important for production if we have a loadbalancer. In fact, a
loadbalancer would be more transparent to our existing apps as they work at the IP level.
- That said, we don't have a highly-distributed and elastic architecture so we could get
by without this for a while.
- Features that we don't need for now:
- Leader election. Many of our apps are currently not designed to scale horizontally.
However, it should be noted that Consul has the ability to select a leader based on
health checks.
- Problems that these tools are not designed to solve:
- Load-balancing.
- Things that I've explored:
- Etcd:
- Basic info:
- Service registration relies on using a key TTL along with heartbeating from the service
to ensure the key remains available. If a services fails to update the key’s TTL, Etcd
will expire it. If a service becomes unavailable, clients will need to handle the
connection failure and try another service instance.
- There would be a compelling reason to favor Etcd if we ever planned to use CoreOS
but I don't see this happening anytime soon.
- Pros:
- Service discovery involves listing the keys under a directory and then waiting for
changes on the directory. Since the API is HTTP based, the client application keeps a
long-polling connection open with the Etcd cluster.
- Has been around for longer than Consul. 150% more github watches/stars.
- 3 times as many contributors (i.e. more eyes) and forks on github.
- Cons:
- There are claims that the Raft implementation used by Etcd (go-raft) is not quite right (unverified).
- Immature, but by the time its use is under consideration in production, it should
have reached 1.0.
- Serving DNS records from Etcd may require a separate service/process (verify):
- http://probablyfine.co.uk/2014/03/02/serving-dns-records-from-etcd/
- SkyDNS is essentially DNS on top of Etcd
- Consul:
- Pros:
- Has more high-level features like service monitoring.
- There is another project out of Hashicorp that will read/set environment variable
for processes from Consul.
- https://github.com/hashicorp/envconsul
- Better documentation.
- I had an easier time installing and configuring this over Etcd, not that Etcd was
particularly hard. Docs make all the difference.
- Stuff like this makes me want to shed a tear. I commend the KIDS at Hashicorp.
- http://www.consul.io/docs/internals/index.html
- You can make DSN queries directly against Consul agent! Nice! No need for SkyDNS or Helix
- We can add arbitrary checks! Nice, if we are into that sort of thing.
- Understands the notion of a datacenter. Each cluster is confined to datacenter but the
cluster is able to communicate with other datacenters/clusters.
- At Skybox, we might use this feature to separate docker tracks, even if they live on same host.
- It has a rudimentary web UI:
- http://demo.consul.io/ui/
- Cons:
- There are claims that Consul's implementation of Raft is better (unverified).
- Immature. Even younger than Etcd (though there are no reason to believe that there are problems with it).
- Etcd and Consul similarities:
- HTTP+JSON based API. Curl-able.
- Docker containers can talk directly with Etcd/Consul over the docker0 interface (i.e. default gateway).
- Atomic look-before-you-set:
- Etcd: Compare-and-set by both value and version index.
- Consul: Check-and-set by sequence number (ModifyIndex)
- DNS TTLs can be set to something VERY low.
- Etcd: supports TTL (time-to-live) on both keys and directories, which will be honoured:
if a value has existed beyond its TTL
- Consul: By default, serves all DNS results with a 0 TTL value
- Has been tested with Jepsen (tool to simulate network partitions in distributed databases).
- Results were not 100% for either but still generally promising.
- https://news.ycombinator.com/item?id=7884640
- Both work with Confd by Kelsey Hightower.
- A tool that watches Etcd/Consul and modifies config files on disk.
- https://github.com/kelseyhightower/confd
- Long polling for changes:
- Etcd: Easily listen for changes to a prefix via HTTP long-polling.
- Consul: A blocking query against some endpoints will wait for a change to potentially
take place using long polling.
- Things that I have not explored:
- SkyDNS: Anyone have good input on this one?
- Zookeeper: It seems mature but it would take a lot more work to make it work for us.
- We would be have to configure and use it without high-level features.
- Provides only a primitive K/V store.
- Requires that application developers build their own system to provide service discovery.
- Java dependency (and Dan Streit hates Java)
- All clients must maintain active connections to the ZooKeeper servers, and perform keep-alives.
- Zookeeper not recommended for virtual environments? Why? I just read this somewhere.
- Corosync/Pacemaker (not sure if this is a viable solution, actually)
- Redis is not viable! It is an in-memory K/V that does not persist data. Nope.
- Smartstack + Synapse + Nerve from AirBnB (not viable as it only does TCP through HAproxy).
- Ruby dependencies and many moving parts.
- References:
http://www.hashicorp.com/blog/twelve-factor-consul.html (heroku's excellent 12-factor thing).
http://12factor.net/
http://www.consul.io/intro/vs/index.html
http://www.consul.io/docs/internals/index.html
https://news.ycombinator.com/item?id=7604787
https://news.ycombinator.com/item?id=7623317
https://news.ycombinator.com/item?id=7884640
http://www.activestate.com/blog/2014/03/brandon-philips-explains-etcd
http://jpmens.net/2013/10/24/a-key-value-store-for-shared-configuration-etcd-confd/
http://igor.moomers.org/smartstack-vs-consul/
http://jasonwilder.com/blog/2014/02/04/service-discovery-in-the-cloud/
http://nerds.airbnb.com/smartstack-service-discovery-cloud/
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