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I'm trying to design a REST API over HTTP. I am totally new to this, so please tell me if any of my assumptions or ideas are just plain wrong.

The domain is minimalistic. I have a database of Products and for each Product there is an associated Image. As I see it, I can design my API in one of two ways:

  1. I can bundle each image with its product and represent them as one resource. The cons of this api would be that every time you PUT or GET a product, you have to send the image over the wire, even if you don't specifically need to read or change the image. As of my understanding, it would not be RESTful to not PUT or GET a complete representation of a resource. Also, client-side caching of images would be of no use in this scenario.

  2. I can model Products and Images as two different resources. When you GET a Product, it will contain an image_id which can be used to GET an Image. This model would require two HTTP Requests. One to GET the Product and one to GET its corresponding Image. Not so bad maybe, but what if I want to display a list of all Products along with their Images? Then I suddenly have a bunch of HTTP Requests. While using SSL, I guess this could create a performance issue. Good thing though, is that the consumer of my API could choose to cache images client-side.

So, how can I model my API to be both RESTful and efficient?

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It's good that you're thinking about the data model.
Related to that, REST doesn't specify or imply that the data model must be completely de-normalized.

Typically, when GETing a resource, you'd receive a packet of information that also includes URL references to other related resources, like a product image. It could also include a reference to a product category, a product manufacturer, and so on. Each might be URLs, or IDs that you could derive URLs from. A message like this: 

{ "id": 123456, "description" : "Your basic paperweight", "category" : { id: 17717, "name" : "Home furnishings" }, "manufacturer": { id : 78783, "name" : "Boeing" }, "price" : 1.99, "imageId" : 109101 }

...might imply URLs like this: 

http://api.mycompany.com/product/123456 http://api.mycompany.com/category/17717 http://api.mycompany.com/manufacturer/78783 http://api.mycompany.com/image/109101

...and note that the full representation of the linked-to resources, like category, manufacturer and so on, is not transmitted with the original resource. This is a partially de-normalized data model.

In regard to your comments on PUT:

  1. This is a matter of opinion, but... for many developers it's completely acceptable to allow partial update via PUT. So you could update the resources without specifying everything; existing fields would remain unchanged. If you choose this behavior, it can complicate your (server-side) code when dealing with edge cases. For example, how does a client indicate that he wants to erase or delete a field? (Passing null may work, but for some data, null is a meaningful value.)

  2. Why worry about PUT? If you want partial update, it's easy to use POST, with a verb (eg, "partialUpdate") in the query params. Actually this is what Roy Fielding advocates, and it makes sense to me.

A partial update would then be something like this: 

POST /products/123456?action=partialUpdate *headers* { "description" : "A fabulous paperweight designed in Sweden, now at a new low price." }, "price" : 1.78 }
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Paperweight from Sweden, hell yea!
Another option for partial updates in a REST api is the PATCH http method. See the RFC here
Hooked on the partial update idea, would it be ok to parameterize my GETs like GET api/products/{id}?includeImage=true to avoid that extra HTTP Request
Try to avoid parameters in the URL when using REST, it make everything more clearer, and it's the RESTful way
@MattiasNordqvist - I don't mind including data shaping options in the list of query parameters. StackExchange actually does it in an interesting way. Rather than allowing "includeImage" in the query param, they have a general "Data shaping" resource. It gets its own ID. On that data shaping resource, it may have "includeImage" and it may include a bunch of other "includes" and "excludes". Then you pass "dataShape=11244" as a query parameter. I like the idea. IF your needs are simple I see no problem using includeImage in the query params.
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I would use option 2, but instead of image_id, store the image URL. Also, don't be afraid to use custom scripts to return what you need (for example, displaying ALL products and images). REST is a design GOAL, not necessarily an implementation truth. Your design would still be RESTful.

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I agree with the other 2 answers and I think that you should choose option number 2. But you also asked about getting a list of products, so here is my answer regarding it.

Think of using another resource that can be used with GET only, that resource will return a list of products. In this way there would be only one HTTP request for consuming the list. In case that there is a chance that the list can be very big, you would need to implement some kind of paging mechanism.

For example, lets say that you need to return 2500 products but you decided to return no more than 1000 products. The first GET request would return the first 1000 items and would also include in the answer the URL to consume the next "page", in this case the next 1000 products, then in the second request you would return products 1001-2000 with a URL to the next "page", in this case the last 500 products.

Then the consumer would be able to get the images as well if needed. You can use this list option for the images as well, but the bunch of images should be significantly smaller in each "page". I would not recommend of choosing the list mechanism to consume images.

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