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Git-backed not-so-schema-less data store
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README.md

Ark

Ark is a breakout of the ideas around the persistence mechanism for Noah. You can read some of the discussions around these ideas here:

I am VERY thankful for the work done by @mattsears on Gaga. I've wanted to do something Git-backed for a while but Gaga prompted me to consider it again. Noah, however, has some different requirements so I've hijacked his original code to roll Ark.

Schemas

Unlike most key/value systems, objects in Ark are defined with a schema. This fits better into Noah.

Schemas are defined in the following format:

{"id":"<String>",
  "key":"<optional String>"
  "attributes":"<Array>",
  "validations":{
    "required":"<optional Array>",
    "unique":"<optional Array>",
    "member":{
      "<attribute>":"<Array>"
    }
  }
}

The only requirements are that your schema must define

  • an id
  • either an attribute called name or a key

This would look like

{"id":"host","key":"name"}

OR

{"id":"host","attributes":["name"]}

Everything else is optional. The name attribute, should it exist, will automatically become the unique identifier for records using the schema. You can override this with the key attribute.

Validations

Validations are, again, optional but should you choose to use them there are further requirements.

  • Any validation must contain a valid attribute
  • Attribute for validation must be an array (excluding member which is a valid attribute and array)

Using our previous example, if you wanted to ensure that every host had a status:

{"id":"host",
  "attributes":["name", "status"],
  "validations":{
    "required":["name","status"],
    "unique":["name"]
  }
}

If you wanted to ensure that every host had a status and that it was one of up,down or pending you would also add a member validation:

{"id":"host",
  "attributes":["name", "status"],
  "validations":{
    "required":["name","status"],
    "unique":["name"],
    "member":{
      "status":["up","down","pending"]
    }
  }
}

Git layout

The actual on-disk persistence is handled as a bare Git repo via Grit. It contains the following structure:

<root>
     _schema
            /<id>.json
     _objects
             /<id>
                  /<key>.json

Each schema is stored as a JSON file (<id>.json) in the _schema directory. Each object created with a given schema is stored as a JSON file (<key>.json) in the _objects/<id> directory.

The reason for storing as a bare repo is to eliminate the temptation and confusion around it being a working repo. You could check out the repo elsewhere and make changes but this currently would bypass all validation (of both schemas and created objects).

Object creation

Initial experiments around this idea were done in my person experiments repo. The general flow would work something like this:

  • Schemas are loaded from disk
  • Classes are dynamically created from schemas
  • Classes subclass a base class that defines validation and persistence
  • Classes gain various git-friendly abilities (versioning, checksum and the like)

So if you use our host example from above, you would get a new class called Host to work with.

TODO

I've still got more code to move over from my experiments. I'm trying to keep the Noah-specific stuff OUT of this code base. For instance, the sinatra application in the experiments won't be a part of this.

Other things:

  • Expose symlinks (while this is for Noah, it's still appropriate here)
  • Modify Ark::Repo to take the schema name on operations.
  • Flesh out pre/post CRUD hooks. (Either use git hooks or my own similar to ohm-contrib)
  • Figure out where cluster consistency fits. (Noah is the user but it's probably a separate repo - ark-cluster maybe?)

Contributions

I welcome any and all contributions. Wiki entries, bug reports, code...it's all valuable.

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