2009-12-21

# Decentralized datatypes

Over the past year or so there have been quite a few blog postings in the REST world about MIME types, and their role in the REST architecture. A lot of the discussion seems to be prompted by WADL, which is an attempt to define a WSDL-style interface description language for REST services. Joe Gregorio argues that MIME types are more useful for describing the semantics of a service than a WADL document, since there are parts of the service’s semantics that just can’t be encoded in a machine-readable format. MIME types acknowledge this, providing a standard way of identifying a data format and pointing to the human- and machine-readable documents (such as RFCs and XSDs) that define the syntax and accompanying semantics.

Following this idea, several people have begun debating whether or not the centralized assignment of MIME types is the right way to handle the variety of data formats that REST-based systems produce and consume. Mark Baker comes in on the side of centralized assignment, whereas Stefan Tilkov, Dan Diephouse, and James Strachan argue in favor of decentralized types. Bill Burke and Benjamin Carlyle have good summaries of the different proposed technical solutions that would enable decentralized types.

## “Extended” types

One of the arguments in favor of centralized assignment is that allowing everyone to invent their own MIME types would ruin interoperability. And for certain cases, this seems pretty obviously true. It’s a good thing that we have a standardized image/png MIME type; this allows your browser to correctly display the website logo you see up in the upper left corner. If I were daft, I could decide to serve that logo using a MIME type of image/x-dcreager-png (or similar) to indicate that I’ve included some particular set of metadata in an ancillary chunk of the PNG.

Why would I want to do this? Maybe I’m writing an application that knows how to process this metadata, and I’d like to easily determine whether a particular resource I’m accessing has this metadata or not. The Open Microscopy Environment does exactly this; they’ve defined an XML schema that allows biology researchers to provide additional scientific metadata about an image or movie captured from a high-end microscope. One way to encode an image and its metadata is as an “OME-TIFF”, a data format that includes the metadata in an optional TIFF section. OME-TIFF files are also perfectly valid as regular TIFF files. This has the benefit that an OME-aware application can access the scientific metadata, whereas a “regular” image processing application can read the image using its normal TIFF decoder.

Of course, now we have competing goals that we have to reconcile. On the one hand, we need to ensure that OME-aware applications can see that a particular image is an OME-TIFF. On the other, we need non-OME-aware applications to see the image as a regular TIFF. One of the decentralized proposals — MIME type parameters — tries to address this. For instance, a MIME type for an OME-TIFF might be image/tiff; ome=xml. By using the standard image/tiff as the base MIME type, non-OME-aware applications correctly treat it as a simple TIFF. OME-aware applications would know that the ome=xml parameter indicates that the OME-specific metadata is present.

## The multitude of XML types

Another example given is that of an XML document. Most applications will generate XML documents that conform to a particular schema (for instance, a company-specific purchase order), which they might encode as an XSD. Now, the XSD on its own doesn’t give you the full story on how to process that data, but it does provide some detail on how the data is structured. If you’re writing an application that consumes this data, having the XSD available would be helpful. More interesting is an application that can consume any XML document — and which might use an XSD or RelaxNG schema to customize the UI used to display the document.

In both cases, the schema is necessary to process the document, but for different reasons. In the first case, the consuming application was built with advance knowledge of how the data should be handled, and the schema is used to direct a particular document to the code that implements this knowledge. In the second case, the particular datatype is unimportant, and the application-specific semantics aren’t used; the data is only consumed as a “generic XML document”, and the schema is used to describe the specific structure of the elements.

## Data doesn’t have a single type

The common theme in both of these examples is that a single datatype isn’t enough to describe the data we’re dealing with. As Roy Fielding points out, “all data formats correspond to multiple media types”. It’s tempting to think of a datatype as just “the syntax and structure of the data”. But it must also include some intuition about how the data will be used.

From this point of view, the generic XML processing application does not handle a multitude of datatypes. Instead, it handles exactly one: “generic XML document with associated schema”. The application that knows how to process this particular schema will handle a different, completely distinct, datatype: “company-specific purchase order XML document”. And the particular XML document in question — a single sequence of bytes that is a single representation of a single resource — is an instance of both types.

Why shift things around like this? Doesn’t it just move the complexity from the consumer (who used to consume multiple types) to the producer (who must now publish the XML document under different types)? Not necessarily. The key idea is that we can use transformation graphs to encode the relationships between the datatypes:

In this specific example, the transformation is simple — since the same sequence of bytes is a valid instance of both types, we don’t have to modify the data itself. The decentralized MIME types (especially the MIME parameter proposal) already support these kinds of “no-op” transformations: the more generic type is the “base” MIME type, and the more specific extensions are encoded as MIME parameters. However, by modeling the type relationships as an arbitrary graph, we open up the possibility of more complex sets of types, which might require actual code to transform between them, but which can be defined in a decentralized manner.

Even though the model is more complex, we haven’t required the producer or consumer to be more complex. A transformation graph is necessary to translate between the two different (but compatible) types, but the graph doesn’t have to specifically live at the producer or the consumer. The producer can publish the data using the only type it knows about (the “company-specific” type), and a consumer can request the data using the only type it knows about (such as the “generic XML” type). Anywhere along the path from the producer to the consumer, we can use the transformation graph to automatically transform the data from one type to the other.

More details on transformation graphs, including more complex examples, can be found in my DPhil thesis.