etcd vs consul vs ???

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:

d4rkb1ue/gist:4da19a6e3a543d894a48e08ae6b41633