Preflight Check

The Pre-flight check API allows an application to verify that a file will be accepted by Box before it uploads any bytes. It can both be used for new files, as well as uploading new versions of existing files.

Checklist

Preflight checks perform all the same checks as if the file was actually uploaded including:

• The permission of the application and the user to upload to the folder
• Any file name collisions
• Any file size caps and limits
• Any folder and file name restrictions
• Any folder and account storage quotas

Check for new file

To perform a check for a new file, call the OPTIONS /files/content API with the same parameters (except for the binary content) as if uploading an actual file.

curl -X OPTIONS https://api.box.com/2.0/files/content \
     -H 'Authorization: Bearer <ACCESS_TOKEN>'
     -H "Content-Type: multipart/form-data" \
     -F attributes='{"name":"Photo.jpg", "parent":{"id":"11446498"}}'

String fileName = "My Doc.pdf";
BoxFolder rootFolder = BoxFolder.getRootFolder(api);
try {
    folder.canUpload(fileName, 98734576);

    // If the file upload would not have succeeded, it will not be attempted
    folder.uploadFile(fileContents, fileName);
} catch (BoxAPIException ex) (

)

// Verify that uploading a 200MB file named "Preso.ppt" to folder 12345 would succeed
client.files.preflightUploadFile(
		'12345',
		{
			name: 'Preso.ppt',
			size: 200000000
		},
		null,
		callback
	);

Check for new file version

To perform a check for a new version of a file, call the OPTIONS /files/:id/content API with the same parameters (except for the binary content) as if uploading an actual file.

curl -X OPTIONS https://api.box.com/2.0/files/content \
     -H 'Authorization: Bearer <ACCESS_TOKEN>'
     -H "Content-Type: multipart/form-data" \
     -F attributes='{"name":"Photo.jpg", "parent":{"id":"11446498"}}'

// Check if uploading a larger version of this file will succeed
client.files.preflightUploadNewFileVersion('87646', {size: 300000000}, null, callback);

Checks & Chunk Uploads

When performing a chunked upload, performing a preflight check is not required as creating an Upload Session also performs a preflight check.

Response codes

When the API call detects any problems, a HTTP 409 Conflict status code is returned with a message to describe the possible conflict. If no problems were discovered, it returns a HTTP 200 OK status code and the upload can proceed.

A 200 OK response does not guarantee that the upload call will actually succeed. Pre-flight checks have show to reduce failed uploads by over 99%, yet concurrency issues still come into play when uploading a file.

Highly active folders, common filenames, and accounts near their quota limits may get a 200 OK for the preflight check, and then have a real conflict during the actual upload.

Response body

In many cases, the preflight check will return valuable data in the API response when a conflict has been detected. For example, when a name collision has been detected, the application can use the SHA-1 that is returned in the error response to check if the existing file is identical to the one it is trying to upload.