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ETag HTTP Header

Most API endpoints implement HTTP caching. This means most responses to GET requests contain an ETag header that identifies the specific version of a resource. Each time the corresponding resource changes, a new ETag is generated. As such, comparing ETags indicates whether the representation of a resource has changed. 
Etag: W/"<etag_value>"
Example: 
Etag: W/"45cdx4"
ETags are prefixed with the “W/” characters to indicate that a weak validator is used.

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Conditional Requests

HTTP Request Headers can give you the ability to issue conditional requests, depending on the current version of the resource. This gives you the ability to prevent “lost update” problems.
  • If-Match: <etag_value> || W/"<etag_value>: Requests only succeeds if the latest version exactly matches the given ETag value.
  • If-None-Match: <etag_value> || W/"<etag_value>: Request only succeeds if the latest version does not match the given Etag value.
If the condition is not met, then the server will respond with either a 304 Not Modified or a 412 Precondition Failed HTTP status code response (examples below). The If-Match HTTP Header can be used on write requests (POST, PUT, PATCH, DELETE) in order to only apply them if the resource was not changed by someone else since it was retrieved. 
> Request
PATCH /lucca-api/leaves/123 HTTPS/1.1
Host: example.ilucca.net
Authorization: Bearer <ACCESS_TOKEN>
Api-Version: <API_VERSION>
Accept: application/json
Content-Type: application/json
If-Match: W/"sd457dx1"

{ ... }

< Response
HTTPS/1.1 412 Precondition Failed
ETag: W/"sd457dx1"
Content-Length: 0
The If-None-Match HTTP Header can be used on read requests (GET, HEAD) in order to only retrieve the representation of the resource if it has changed since the last time it was retrieved. 
> Request
GET /lucca-api/leaves/123 HTTPS/1.1
Host: example.ilucca.net
Authorization: Bearer <ACCESS_TOKEN>
Api-Version: <API_VERSION>
Content-Type: application/json
If-None-Match: W/"sd457dx1"

< Response
HTTPS/1.1 304 Not Modified
ETag: W/"sd457dx1"
Content-Length: 0