Preconditions in the HTTP protocol are one of the aspects used during requests sent to the server to preserve data integrity and prevent unwanted updates. In fact, preconditions provide us with the capability to perform operations like updates or deletes only if certain conditions are met.
One of the most important use cases of this feature is to reduce concurrent modifications and avoid accidental overwrites. For instance, when multiple users simultaneously operate on a single resource, using these preconditions can prevent conflicts. For example, we can determine that if the data has not been modified since the last request, it should be updated.
In HTTP, preconditions are typically handled through special headers such as If-Match or If-None-Match for ETags, and also If-Modified-Since for the last modification date.
In precise work, these preconditions can provide a useful approach to improve the efficiency and reliability of backend applications. This work ensures that changes to shared resources only occur when specific conditions are met, minimizing the risk of conflicts.
Let's take a look at a code example that shows how these preconditions can be used in HTTP requests.
GET /resource HTTP/1.1
Host: example.com
If-None-Match: "etag12345"
In this simple code snippet, we send a GET request to the server. This request includes a header If-None-Match indicating that the request should only proceed if the current ETag of the resource does not match "etag12345".
If the ETag matches the value we have sent, the server may respond with a 304 Not Modified, allowing the client to use its cached version instead of re-downloading.