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On Noah - Part 2

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This is the second part in a series on Noah. Part 1 is available here

In part one of this series, I went over a little background about ZooKeeper and how the basic Zookeeper concepts are implemented in Noah. In this post, I want to go over a little bit about a few things that Noah does differently.

Noah Primitives

As mentioned in the previous post, Noah has 5 essential data types, four of which are what I’ve interchangeably refered to as either Primitives and Opinionated models. The four primitives are Host, Service, Application and Configuration. The idea was to map some common use cases for Zookeeper and Noah onto a set of objects that users would find familiar.

You might detect a bit of Nagios inspiration in the first two.

  • Host: Analogous to a traditional host or server. The machine or instance running the operating system. Unique by name.
  • Service: Typically mapped to something like HTTP or HTTPS. Think of this as the listening port on a Host. Services must be bound to Hosts. Unique by service name and host name.
  • Application: Apache, your application (rails, php, java, whatever). There’s a subtle difference here from Service. Unique by name.
  • Configuration: A distinct configuration element. Has a one-to-many relationship with Applications. Supports limited mime typing.

Hosts and Services have a unique attribute known as status. This is a required attribute and is one of up,down or pending. These primitives would work very well integrated into the OS init process. Since Noah is curl-friendly, you could add something globally to init scripts that updated Noah when your host is starting up or when some critical init script starts. If you were to imagine Noah primitives as part of the OSI model, these are analagous to Layers 2 and 3.

Applications and Configurations are intended to feel more like Layer 7 (again, using our OSI model analogy). The differentiation is that your application might be a Sinatra or Java application that has a set of Configurations associated with it. Interestingly enough, you might choose to have something like Tomcat act as both a Service AND an Application. The aspect of Tomcat as a Service is different than the Java applications running in the container or even Tomcat’s own configurations (such as logging).

One thing I’m trying to pull off with Configurations is limited mime-type support. When creating a Configuration in Noah, you can assign a format attribute. Currently 3 formats or types are understood:

  • string
  • json
  • yaml

The idea is that, if you provide a type, we will serve that content back to you in that format when you request it (assuming you request it that way via HTTP headers). This should allow you to skip parsing the JSON representation of the whole object and instead use it directly. Right now this list is hardcoded. I have a task to convert this.

Hosts and Services make a great “canned” structure for building a monitoring system on top of Noah. Applications and Configurations are a lightweight configuration management system. Obviously there are more uses than that but it’s a good way to look at it.

Ephemerals

Ephemerals, as mentioned previously, are closer to what Zookeeper provides. The way I like to describe Ephemerals to people is a ‘512 byte key/value store with triggers’ (via Watch callbacks). If none of the Primitives fit your use case, the Ephemerals make a good place to start. Simply send some data in the body of your post to the url and the data is stored there. No attempt is made to understand or interpret the data. The hierarchy of objects in the Ephemeral namespace is completely arbitrary. Data living at /ephemerals/foo has no relationship with data living at /ephemerals/foo/bar.

Ephemerals are also not browseable except via a Linking and Tagging.

Links and Tags

Links and Tags are, as far as I can tell, unique to Noah compared to Zookeeper. Because we namespace against Primitives and Ephemerals, there existed the need to visualize objects under a custom hierarchy. Currently Links and Tags are the only way to visualize Ephemerals in a JSON format.

Tags are pretty standard across the internet by now. You might choose to tag a bunch of items as production or perhaps group a set of Hosts and Services as out-of-service. Tagging an item is a simple process in the API. Simply PUT the name of the tag(s) to the url of a distinct named item appended by tag. For instance, the following JSON posted to /applications/my_kick_ass_app/tag with tag the Application my_kick_ass_app with the tags sinatra, production and foobar:

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   {"tags":["sinatra", "production", "foobar"]}

Links work similar to Tags (including the act of linking) except that the top level namespace is now replaced with the name of the Link. The top level namespace in Noah for the purposes of Watches is //noah. By linking a group of objects together, you will be able to (not yet implemented), perform operations such as Watches in bulk. For instance, if you wanted to be informed of all changes to your objects in Noah, you would create a Watch against //noah/*. This works fine for most people but imagine you wanted a more multi-tenant friendly system. By using links, you can group ONLY the objects you care about and create the watch against that link. So //noah/* becomes //my_organization/* and only those changes to items in that namespace will fire for that Watch.

The idea is also that other operations outside of setting Watches can be applied to the underlying object in the link as well. The name Link was inspired by the idea of symlinking.

Watches and Callbacks

In the first post, I mentioned that by nature of Noah being “disconnected”, Watches were persistent as opposed to one-shot. Additionally, because of the pluggable nature of Noah Watches and because Noah has no opinion regarding the destination of a fired Watch, it becomes very easy to use Noah as a broadcast mechanism. You don’t need to have watches for each interested party. Instead, you can create a callback plugin that could dump the messages on an ActiveMQ Fanout queue or AMQP broadcast exchange. You could even use multicast to notify multiple interested parties at once.

Again, the act of creating a watch and the destination for notifications is entirely disconnected from the final client that might use the information in that watch event.

Additionally, because of how changes are broadcast internally to Noah, you don’t even have to use the “official” Watch method. All actions in Noah are published post-commit to a pubsub queue in Redis. Any language that supports Redis pubsub can attach directly to the queue and PSUBSCRIBE to the entire namespace or a subset. You can write your own engine for listening, filtering and notifying clients.

This is exactly how the Watcher daemon works. It attaches to the Redis pubsub queue, makes a few API calls for the current registered set of watches and then uses the watches to filter messages. When a new watch is created, that message is like any other change in Noah. The watcher daemon sees that and immediately adds it to its internal filter. This means that you can create a new watch, immediately change the watched object and the callback will be made.

Wrap up - Part Two

So to wrap up:

  • Noah has 5 basic “objects” in the system. Four of those are opinionated and come with specific contracts. The other is a “dumb” key/value store of sorts.
  • Noah provides Links and Tags as a way to perform logical grouping of these objects. Links replace the top-level hierarchy.
  • Watches are persistent. The act of creating a watch and notifying on watched objects is disconnected from the final recipient of the message. System A can register a watch on behalf of System B.
  • Watches are nothing more than a set of filters applied to a Redis pubsub queue listener. Any language that supports Redis and its pubsub queue can be a processor for watches.
  • You don’t even have to register any Watches in Noah if you choose to attach and filter yourself.

Part three in this series will discuss the technology stack under Noah and the reasoning behind it. A bit of that was touched on in this post. Part four is the discussion about long-term goals and roadmaps.

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