Http Caching and Concurrency Examples

This project aims to provide examples of how to use the Http Caching and Concurrency Controls.
For an in-depth understanding of the topic, one should read the RFC 7232 - Conditional Requests.

The Service

This is a simple weather service with two resources:

Daylight (today at Stockholm)
The idea with this one is to demonstrate the simplest time-based Http Caching Controls.
Since the response contains information valid for the whole day, we make use of the headers Cache-Control with max-age of 1 day, combined with Expires until the end of the day.
Climate (now at Stockholm)
The idea with this one is to demonstrate Http Conditional Requests and Concurrency Controls based on the ETag header (Last-Modified can also be used with the same purpose).
- Conditional Requests were used when retrieving the resource.
  Given that the client has knowledge about the resource at a given point in time, it can send its corresponding ETag along with the request so that the server will either respond with the current resource information in case it has changed since, or with a Not Modified response, saving some bandwith and time.
- Concurrency Controls were used when updating the resource. It is a kind of Optimistic Locking on the Http protocol.
  Given that the client has knowledge about the resource at a given point in time, it can send its corresponding ETag along with the request so that the server can update the resource whether that ETag matches its current one or refuse to and let the client know that he's trying to update the resource with basis on outdated information.

Caveat

When using Http Concurrency Controls for Optimistic Locking, pay attention to your implementation not to fall for the Lost Update Problem the same way:
Make sure to execute the 'retrieve resource, verify ETag then update resource' operations in a rather atomic way. That can be done with either a Transaction or Optimistic Locking at the DB level.

Trying it out

Requirements

Java 14
Maven

Building the application

mvn clean verify

Starting the application

java -jar target/weather-service-1.0-SNAPSHOT.jar server src/environments/development.yml

Examples of use

Retrieving the daylight information

curl -vvv http://localhost:8080/daylights/stockholm/today

Note the Cache-Control and Expires headers in the response.

< HTTP/1.1 200 OK
< Date: Sun, 11 Jun 2017 19:00:55 GMT
< Expires: Sun, 11 Jun 2017 23:59:59 GMT
< Content-Type: application/json
< Cache-Control: no-transform, max-age=86400
< Vary: Accept-Encoding
< Content-Length: 48
< 
{"sunrise":"06:00+02:00","sunset":"18:00+02:00"}

Retrieving the climate information

curl -vvv http://localhost:8080/climates/stockholm/now

Note the ETag header in the response.

< HTTP/1.1 200 OK
< Date: Sun, 11 Jun 2017 19:03:35 GMT
< ETag: "23723"
< Content-Type: application/json
< Vary: Accept-Encoding
< Content-Length: 32
< 
{"temperature":10,"humidity":80}

Retrieving the climate information conditionally

curl -vvv http://localhost:8080/climates/stockholm/now -H 'If-None-Match: "23723"'

We will get a 200 OK response back whether the If-None-Match header value doesn't match the current corresponding ETag on the server, or a 304 Not Modified response on the contrary.

Updating the climate information conditionally

curl -vvv -X PUT http://localhost:8080/climates/stockholm/now -H 'If-Match: "23723"' -H 'Content-type: application/json' -d '{"temperature":20,"humidity":90}'

We will get a 204 No Content response back whether the If-Match header value matches the current corresponding ETag on the server, or a 412 Precondition Failed response on the contrary.

Contributing

If you would like to help making this project better, see the CONTRIBUTING.md.

Maintainers

Send any other comments, flowers and suggestions to André Schaffer.

License

This project is distributed under the MIT License.

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