Get an object from an Amazon S3 bucket using an Amazon SDK, specifying an If-Modified-Since header - Amazon Simple Storage Service
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Get an object from an Amazon S3 bucket using an Amazon SDK, specifying an If-Modified-Since header

The following code example shows how to read data from an object in an S3 bucket, but only if that bucket has not been modified since the last retrieval time.

Rust
SDK for Rust
Note

There's more on GitHub. Find the complete example and learn how to set up and run in the Amazon Code Examples Repository.

use aws_sdk_s3::{ error::SdkError, operation::head_object::HeadObjectError, primitives::{ByteStream, DateTime, DateTimeFormat}, Client, Error, }; use tracing::{error, warn}; const KEY: &str = "key"; const BODY: &str = "Hello, world!"; /// Demonstrate how `if-modified-since` reports that matching objects haven't /// changed. /// /// # Steps /// - Create a bucket. /// - Put an object in the bucket. /// - Get the bucket headers. /// - Get the bucket headers again but only if modified. /// - Delete the bucket. #[tokio::main] async fn main() -> Result<(), Error> { tracing_subscriber::fmt::init(); // Get a new UUID to use when creating a unique bucket name. let uuid = uuid::Uuid::new_v4(); // Load the AWS configuration from the environment. let client = Client::new(&aws_config::load_from_env().await); // Generate a unique bucket name using the previously generated UUID. // Then create a new bucket with that name. let bucket_name = format!("if-modified-since-{uuid}"); client .create_bucket() .bucket(bucket_name.clone()) .send() .await?; // Create a new object in the bucket whose name is `KEY` and whose // contents are `BODY`. let put_object_output = client .put_object() .bucket(bucket_name.as_str()) .key(KEY) .body(ByteStream::from_static(BODY.as_bytes())) .send() .await; // If the `PutObject` succeeded, get the eTag string from it. Otherwise, // report an error and return an empty string. let e_tag_1 = match put_object_output { Ok(put_object) => put_object.e_tag.unwrap(), Err(err) => { error!("{err:?}"); String::new() } }; // Request the object's headers. let head_object_output = client .head_object() .bucket(bucket_name.as_str()) .key(KEY) .send() .await; // If the `HeadObject` request succeeded, create a tuple containing the // values of the headers `last-modified` and `etag`. If the request // failed, return the error in a tuple instead. let (last_modified, e_tag_2) = match head_object_output { Ok(head_object) => ( Ok(head_object.last_modified().cloned().unwrap()), head_object.e_tag.unwrap(), ), Err(err) => (Err(err), String::new()), }; warn!("last modified: {last_modified:?}"); assert_eq!( e_tag_1, e_tag_2, "PutObject and first GetObject had differing eTags" ); println!("First value of last_modified: {last_modified:?}"); println!("First tag: {}\n", e_tag_1); // Send a second `HeadObject` request. This time, the `if_modified_since` // option is specified, giving the `last_modified` value returned by the // first call to `HeadObject`. // // Since the object hasn't been changed, and there are no other objects in // the bucket, there should be no matching objects. let head_object_output = client .head_object() .bucket(bucket_name.as_str()) .key(KEY) .if_modified_since(last_modified.unwrap()) .send() .await; // If the `HeadObject` request succeeded, the result is a typle containing // the `last_modified` and `e_tag_1` properties. This is _not_ the expected // result. // // The _expected_ result of the second call to `HeadObject` is an // `SdkError::ServiceError` containing the HTTP error response. If that's // the case and the HTTP status is 304 (not modified), the output is a // tuple containing the values of the HTTP `last-modified` and `etag` // headers. // // If any other HTTP error occurred, the error is returned as an // `SdkError::ServiceError`. let (last_modified, e_tag_2): (Result<DateTime, SdkError<HeadObjectError>>, String) = match head_object_output { Ok(head_object) => ( Ok(head_object.last_modified().cloned().unwrap()), head_object.e_tag.unwrap(), ), Err(err) => match err { SdkError::ServiceError(err) => { // Get the raw HTTP response. If its status is 304, the // object has not changed. This is the expected code path. let http = err.raw(); match http.status().as_u16() { // If the HTTP status is 304: Not Modified, return a // tuple containing the values of the HTTP // `last-modified` and `etag` headers. 304 => ( Ok(DateTime::from_str( http.headers().get("last-modified").unwrap(), DateTimeFormat::HttpDate, ) .unwrap()), http.headers().get("etag").map(|t| t.into()).unwrap(), ), // Any other HTTP status code is returned as an // `SdkError::ServiceError`. _ => (Err(SdkError::ServiceError(err)), String::new()), } } // Any other kind of error is returned in a tuple containing the // error and an empty string. _ => (Err(err), String::new()), }, }; warn!("last modified: {last_modified:?}"); assert_eq!( e_tag_1, e_tag_2, "PutObject and second HeadObject had different eTags" ); println!("Second value of last modified: {last_modified:?}"); println!("Second tag: {}", e_tag_2); // Clean up by deleting the object and the bucket. client .delete_object() .bucket(bucket_name.as_str()) .key(KEY) .send() .await?; client .delete_bucket() .bucket(bucket_name.as_str()) .send() .await?; Ok(()) }
  • For API details, see GetObject in Amazon SDK for Rust API reference.

For a complete list of Amazon SDK developer guides and code examples, see Using this service with an Amazon SDK. This topic also includes information about getting started and details about previous SDK versions.