But how HTTP could prevent that?

HTTP provides a simple but powerful mechanism for aligning resource states by making use of entity tag or ETag and conditional request headers. An ETag is anything that uniquely identifies an entity, such as the ID associated with a persisted resource, a checksum of the entity headers and body, etc. If this resource changes—that is, when one or more of its headers, or its entity body, changes—then the entity tag changes accordingly, reflecting this new resource state.

When a response contains an ETag associated to a resource state and you want to continue working with this same resource, it's recommended to use this tag in subsequent requests (called conditional requests), otherwise the resource state might eventually become out of sync with service one, returning something like a 409 Conflict.

Conditional requests happens when the current ETag is supplied to a conditional request header, such as If-Match or If-None-Match, when user is requesting to update a resource for example. The service will then check the precondition, by comparing the current resource ETag with the one provided in the request. If it's satisfied than the server proceeds and process the request, otherwise it concludes that the resource has changed and responds with a 412 Precondition Failed.

Example

Given an online shop for home goods, where two people— Admin1 and Admin2 —are responsible for administrating its contents. In our scenario both administrators are trying to change the state of the same product (the Weber BBQ), around the same time. Admin1 wants to lower the product price down to $300.00 and Admin2 wants to change its state to "Not Available". Firstly, both administrators GET the current product state independently of one another by doing the following request:

GET /product/1 HTTP/1.1
Host: myshop.com

Returning the following resource (product) as response. Note that the service's response contains an ETag header.

HTTP/1.1 200 OK
Content-Length: 265
Content-Type: application/xml
ETag: "686897696a7c876b7e"

<product>
  <name>Weber Family BBQ</name>
  <description>Great for parties and cooks a neat roast too.</description>
  <price>$399.00</price>
  <status>In Stock</status>
</product>

When Admin1 does a conditional PUT, including an If-Match header with the ETag value from the previous GET.

PUT /product/1 HTTP/1.1
Host: myshop.com
If-Match: "686897696a7c876b7e"

<product>
  <name>Weber Family BBQ</name>
  <description>Great for parties and cooks a neat roast too.</description>
  <price>$300.00</price>
  <status>In Stock</status>
</product>

And as the product state hasn't changed since the last request, then the request is thus successful! Notice that the response returns an updated ETag value, reflecting the new product state.

HTTP/1.1 204 No Content
ETag: "616898r96a8cy86b8eee11"

Little time after Admin1 has updated the product, Admin2 does another PUT to the same product, including the same If-Match header with the ETag value from the GET request.

PUT /product/1 HTTP/1.1
Host: myshop.com
If-Match: "686897696a7c876b7e"

<product>
  <name>Weber Family BBQ</name>
  <description>Great for parties and cooks a neat roast too.</description>
  <price>$399.00</price>
  <status>Not Available</status>
</product>

The service then determines that someone is trying to change the same product, using an out-of-date resource representation (ETags are different!), and responds with a 412 Precondition Failed code. No race conditions whatsoever!

HTTP/1.1 412 Precondition Failed

Conclusion

Although ETags and conditional request headers make up a powerful mechanism for dealing with concurrency, one thing to keep in mind is that, depending on the amount of computation performed by the server to generate an ETag, response times might increase considerably. So use it only if you need it!

Special thanks to Jim Webber for helping me with this post. For more information on RESTful Web Services, check out his latest book (written together with Savas Parastatidis and Ian Robinson)— REST in Practice: Hypermedia and Systems Architecture.