Before we dig in, here is a high-level overview of Cosmic.


The definition of REST is a controversial subject. For our purposes, these are the key elements:

  • Objects are assigned unique URLs.
  • These URLs are used to link objects together.
  • Objects are manipulated by a set of standard methods (CRUD).

It is fairly straightforward to create a REST API from a database table, but other kinds of information can be expressed with REST as well. In general it is a good idea to try, and fall back on RPC only if REST is clearly a bad fit.

RPC stands for “remote procedure call”. It is a simple interface where a client sends a request to call a remote function with certain parameters and gets the return value of the function in the response. You might find many discussions online that are framed as “REST vs RPC”, but in reality they are tools that complement each other.


Cosmic is a layer that sits between business code and HTTP. The client and server component are perfectly symmetrical, so similar that they are represented by the same interface, BaseAPI. On the server, an instance of its subclass, API, gets populated with user-defined handler functions, bits of documentation and other metadata. On the client, another subclass, APIClient gets created automatically from a spec, the handler functions being HTTP calls.

The API spec is represented in JSON and served from a standard location: /spec.json.

Cosmic’s simplicity comes from treating HTTP as nothing more than an elaborate serialization scheme for function inputs and outputs. Because different types of REST calls require different HTTP methods and return calls, Cosmic defines several endpoints, subclasses of Endpoint.

Each endpoint defines the methods build_request(), parse_request(), build_response() and parse_response(). These methods abstract away all HTTP nonsense so the rest of Cosmic (and, of course, your code) deals with purely native data.

Built on Teleport


This version of Cosmic relies on an out-of-date version of Teleport. Until Cosmic is ported to Teleport 0.3, it includes a copy of Teleport in the cosmic.legacy_teleport, so you can install it side-by-side with an up-to-date version of Teleport.

See also

The Teleport documentation is worth a look if you are getting started with Cosmic.

Teleport is our very own tiny library that is used for JSON serialization, validation, and generating documentation. At first this might seem like an odd set of features for a library, but they come quite naturally from the fact that Teleport is essentially a very simple static type system. All information that gets carried between Cosmic clients and servers is statically typed with the help of Teleport.

Teleport is implemented as a collection of composable type objects. The composition of these objects mirrors the data it is meant to serialize and validate. One important feature of Teleport is that these type definitions, the schema, is also serializable. This makes it possible to use Teleport to serialize model properties, function definitions, and indeed, the whole API spec.

Teleport makes it easy to define custom types, a feature used by Cosmic.

The Teleport docs will teach you to import from the teleport module:

from teleport import *

In Cosmic, you should import from cosmic.types:

from cosmic.types import *

Hypermedia with JSON HAL

JSON HAL is a compact specification for linking REST-ful resources as well as returning multiple embedded resources in one call (this is used by the get_list endpoint). Note that HAL recommends application/hal+json for the Content-Type header, but currently Cosmic responds only to application/json.