Conditional queries

Conditional Requests in the S3 API allow you to perform operations only if certain conditions are met by using headers.You can use them:

perform read and write requests with a preliminary check of the object state;
manage updates and downloads of objects based on their current state or metadata;
conform to the familiar scenarios of working to S3 standards;
protect data from accidental overwriting;
optimize traffic costs.

Conditional queries can be used for different scenarios.

Headings

Operation	Supported headers
PutObject	`If-Match` `If-None-Match`
CopyObject	`x-amz-copy-source-if-match` `x-amz-copy-source-if-none-match` `x-amz-copy-source-if-modified-since` `x-amz-copy-source-if-unmodified-since`
UploadPartCopy	`x-amz-copy-source-if-match` `x-amz-copy-source-if-none-match` `x-amz-copy-source-if-modified-since` `x-amz-copy-source-if-unmodified-since`
DeleteObject	`If-Match`
CompleteMultipartUpload	`If-Match` `If-None-Match`

Caption	Conditional fulfillment	Error on violation
`If-Match`	The object is written if an active version exists and its ETag matches the value specified in the header	`412 Precondition Failed`
`If-None-Match`	The object is written if the key has no active version, including delete marker	`412 Precondition Failed`

Caption	Conditional fulfillment	Error on violation
`x-amz-copy-source-if-match`	The object is copied if the ETag of the source object matches the value specified in the header	`412 Precondition Failed`
`x-amz-copy-source-if-none-match`	The object is copied if the ETag of the source object does not match the value specified in the header	`412 Precondition Failed`
`x-amz-copy-source-if-modified-since`	The object is copied if the object is modified later than the date specified in the header	`412 Precondition Failed`
`x-amz-copy-source-if-unmodified-since`	The object is copied if the object has not been modified after the date specified in the header	`412 Precondition Failed`

Caption	Conditional fulfillment	Error on violation
`If-Match`	The object is deleted if an active version exists and its ETag matches the value specified in the header	`412 Precondition Failed`

Caption	Conditional fulfillment	Error on violation
`If-Match`	Segmented loading is completed if the ETag of the object matches the value specified in the header	`412 Precondition Failed`
`If-None-Match`	Segmented boot completion is performed if the object does not exist	`412 Precondition Failed`

`404 Not Found`	Object not found (e.g. with `If-Match` header)
`409 Conflict`	The object was modified or deleted before the conditional operation was executed
`412 Precondition Failed`	The condition in the title is not met

update conflict protection — the object will be updated only if it has not changed since the last invocation ( If-Match header);
data synchronization — the object will be loaded only if it was changed after the specified time ( If-Modified-Since header);
read optimization — the object will be retrieved only if it does not match the local version ( If-None-Match header);
overwriting protection — the object will not be written by key if there is already another object with the same key ( If-None-Match header).