Updated on 2024-04-29 GMT+08:00

Performing a Conditioned Download

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When downloading an object, you can specify one or more conditions. Only when the conditions are met, the object will be downloaded. Otherwise, an exception indicating a download failure will be thrown.

You can set the following conditions:

Parameter

Description

obs_get_conditions.if_modified_since

Returns the object if it has been modified since the specified time; otherwise, an error is returned.

obs_get_conditions.if_not_modified_since

Returns the object if it has not been modified since the specified time; otherwise, an error is returned.

obs_get_conditions.if_match_etag

Returns the source object if its ETag is the same as the one specified by this parameter; otherwise, an exception is thrown.

obs_get_conditions.if_not_match_etag

Returns the source object if its ETag is different from the one specified by this parameter; otherwise, an exception is thrown.

  • The ETag of an object is the MD5 check value of the object.
  • If the download request includes if_not_modified_since or If-Match and the specified condition is not met, an exception will be thrown with HTTP status code 412 precondition failed returned.
  • If a request includes if_modified_since or If-None-Match, and the specified condition is not met, 304 Not Modified will be returned.

Sample code:

static void test_get_object_by_range()
{
    // Create and initialize option.
    obs_options option;
    init_obs_options(&option);
    option.bucket_options.host_name = "<your-endpoint>";
    option.bucket_options.bucket_name = "<Your bucketname>";

    // Hard-coded or plaintext AK/SK are risky. For security purposes, encrypt your AK/SK and store them in the configuration file or environment variables. In this example, the AK/SK are stored in environment variables for identity authentication. Before running this example, configure environment variables ACCESS_KEY_ID and SECRET_ACCESS_KEY.
    // Obtain an AK/SK pair on the management console. For details, see https://support.huaweicloud.com/intl/en-us/usermanual-ca/ca_01_0003.html.
    option.bucket_options.access_key = getenv("ACCESS_KEY_ID");
    option.bucket_options.secret_access_key = getenv("SECRET_ACCESS_KEY");
    // Range download parameters
    obs_get_conditions getcondition;
    memset(&getcondition, 0, sizeof(obs_get_conditions));
    init_get_properties(&getcondition);
    getcondition.if_match_etag = "<object etag>";
    getcondition.if_modified_since = "<time object modified>";
    getcondition.if_not_match_etag = "<not matched etag>";
    getcondition.if_not_modified_since = "<time object modified>";
    // Download object information.
    obs_object_info object_info;
    memset(&object_info, 0, sizeof(obs_object_info));
    object_info.key = "<object key>";
    object_info.version_id = "<object version ID>";
    // Local file information after download
    get_object_callback_data data;
    data.ret_status = OBS_STATUS_BUTT;
    data.outfile = write_to_file("<file path>");
     // Set response callback function.
    obs_get_object_handler getobjectHandler =
    { 
        { &response_properties_callback, &get_object_complete_callback},
        &get_object_data_callback
    };  
       
    // Download objects.
    get_object(&option,&object_info,&getcondition,0,&getobjectHandler,&data);
    fclose(data.outfile);
    if (OBS_STATUS_OK == data.ret_status) {
        printf("get object successfully. \n");
    }
    else
    {
        printf("get object faied(%s).\n", obs_get_status_name(data.ret_status));
    }
}