It seems that more and more are beginning to grasp the hypermedia constraint of REST. This is an unmitigated Good Thing. However, once you get hypermedia the idea of a client persisting links that it has found starts to seem a little odd. For example, Kirk Wylie describes clients that store links as “not well behaved” in his excellent presentation on REST in financial systems. Even on the rest-discuss mailing list there is no consensus on the matter.
The idea of an application as a set of states (read: representations) with transitions (read: links) to other states seems to go against the idea of storing links. Transitions from one application state to another are surely transient. Any change in the application state, either by this client or some completely unrelated client, could easily invalidate those transitions. In that context a client that stored links for later use would surely be doomed dereferencing dead links for the rest of it’s days.
Further, the idea that clients might store links is a frightening specter for maintainers of services. If clients store links, and you prefer not to break those clients, you must continue supporting any links you have ever included in any representation in perpetuity. Talk about limiting your design freedom. Such a strict requirement would surely raise the cost of maintaining the service over time.
Reality sets in
Those are scary thoughts. Some of these issues are even real. But end the end it doesn’t matter. Almost all non-trivial systems are going to require that URIs be stored in places other than the origin server. Sometimes these stored URIs will merely be caches. Other times they will be data that cannot be recalculated mechanically.
For example, say you have an order taking system and an inventory system. When placing an order the user goes to the web site, searches for “coffee”, selects the third item in the results and places an order 1 of that item. An order is a set of line items each of which references a product. Once payment is received the order system is going to need to be able to tell the shipping department which items from inventory to send to the customer.
The inventory system has, of course, a URI for every type of product that is for sale. So the simplest and most effective way for an order to reference a product is to use that the inventory URI for that product. URIs are called universal resource identifiers for a reason, we might as well use them as such.
In this example, we have a situation where the product references in the order are not merely caches of URIs. Many things may change the ordering of search results – a new product being added, an old one being discontinued, even a small change to a description of some product. So at any moment the the third item in the search results for “coffee” might be different. Once the user has made their selection no automata can reliably retrace those steps.
The implications of this are significant. The inventory must continue to support the product URIs used in orders until such time as the order system would never care to dereference those URIs again. If a month from now the user comes back and wants to see their order history, those product URIs had better still work.
Fortunately, HTTP provides us with a ready solution. Behold the awesomeness that is HTTP redirection. HTTP redirection is your best friend when it comes to gracefully changing REST/HTTP applications. Clients get what they need – URIs continue to work as identifiers indefinitely – and servers get what they need – a lot of freedom to change the names and dispositions of resources.
We are still faced with this issue of the transient nature of links. Certainly, many links encode transitions which may be transient. The client has no general way of distinguishing between links which represent transiently available state transitions, and those that represent more permanent transitions.
In our example, immediately after creating an order, it probably provides some links to pay for the order. After the user has provided payment those transition would no longer be valid. However, the link to the inventory product is a more permanent part of the order resource.
The only tractable way i see to deal with this issue is to document the lifespan the various link found in a representation. Once the client implementer understand the semantics of the links they well often be able to infer the likely lifespan of the links without further input. However, guidance can be provided in situations where precision is required or the lifespan is ambiguous. A transient link is, by definition, an option part of the representation so documenting the conditions that cause it to be present is likely to be required anyway.
Best practices: Server
REST/HTTP application developers should assume that clients will store links and dereference them after indeterminate periods of time. When resources are relocated or renamed requests to the resource’s obsolete URI should be redirected to the canonical URI using a
301 Moved<br />
For links whose validity has a bounded lifespan the documentation of the representations (the media type) should explicitly layout that the link is transient and optional. If possible the documentation should also describe the conditions of the links existence.
Remind client developers early and often that client must follow any and all redirects from the server.
Best practices: Client
Clients should follow redirects. Fastidiously.
Clients should update it’s internal storage upon receiving a
Moved Permanently response by replace the URI it requested with newly provided location.
Client developers should be aware of transient links in the representations being dealt with. Either do not store these URIs or ensure that attempts so use these URIs handle failure in ways that make sense for the application.
Believe and follow the redirections the server sends to you. Seriously.