• Object Storage Service

obs
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  2. Object Storage Service
  3. Developer Guide (Node.js SDK)
  4. Object Download
  5. Performing a Conditioned Download

Performing a Conditioned Download

When downloading an object, you can specify one or more conditions. Only when the conditions are met, the object will be downloaded. Otherwise, an error code will be returned and the download will fail.

You can set the following conditions:

Parameter

Description

Format

IfModifiedSince

Returns the object if it is modified after the time specified by this parameter; otherwise, an error code is returned.

This parameter must conform to the HTTP time format specified in http://www.ietf.org/rfc/rfc2616.txt.

IfUnmodifiedSince

Returns the object if it remains unchanged since the time specified by this parameter; otherwise, an error code is returned.

This parameter must conform to the HTTP time format specified in http://www.ietf.org/rfc/rfc2616.txt.

IfMatch

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

Character string

IfNoneMatch

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

Character string

NOTE:
  • The ETag of an object is the MD5 check value of the object.
  • If a request includes IfUnmodifiedSince or IfMatch and the specified condition is not met, the object download will fail with error code 412 Precondition Failed returned.
  • If a request includes IfModifiedSince or IfNoneMatch and the specified condition is not met, the object download will fail with error code 304 Not Modified returned.

Sample code is as follows:

// Import the OBS library.
var ObsClient = require('./lib/obs');

// Create an instance of ObsClient.
var obsClient = new ObsClient({
    access_key_id: '*** Provide your Access Key ***',       
    secret_access_key: '*** Provide your Secret Key ***',       
    server : 'yourdomainname'
});

obsClient.getObject({
       Bucket : 'bucketname',
       Key : 'objectkey',
       IfModifiedSince : 'Thu, 31 Dec 2015 16:00:00 GMT'
}, (err, result) => {
       if(err){
              console.error('Error-->' + err);
       }else{
              console.log('Status-->' + result.CommonMsg.Status);
              if(result.CommonMsg.Status < 300 && result.InterfaceResult){
                   // Obtain the object content.
                   console.log('Object Content:'); 
                   console.log(result.InterfaceResult.Content.toString()); 
              }
       }
});