add merge extension

protocol.md

@@ -151,11 +151,10 @@ Servers MUST accept `PATCH` requests against any tus resource and apply the
 bytes contained in the message at the given `Offset`. All `PATCH` requests
 MUST use `Content-Type: application/offset+octet-stream`.
 
-The `Offset` value SHOULD be equal, but MAY also be smaller than the current
-offset of the resource, and servers MUST handle `PATCH` operations containing
-the same data at the same absolute offsets idempotently. The behavior of using
-`Offset` values larger than the current upload offset is undefined, see the
-[Parallel Chunks](#parallel-chunks) extension.
+The `Offset` value MUST be equal to the current offset of the resource. In order
+to achieve parallel upload the Merge extension MAY be used. If the offsets
+do not match the server MUST return the `409 Conflict` status code without
+modifying the upload resource.
 
 Clients SHOULD send all remaining bytes of a resource in a single `PATCH`
 request, but MAY also use multiple small requests for scenarios where this is
@@ -386,11 +385,6 @@ Die Würde des Menschen ist unantastbar.
 TUS-Resumable: 1.0.0
 ```
 
-### Parallel Chunks
-
-This extension will define how to upload several chunks of a file in parallel in
-order to overcome the throughput limitations of individual tcp connections.
-
 ### Streams
 
 This extension defines how to upload finite streams of data that have an
@@ -512,6 +506,123 @@ HTTP/1.1 204 No Content
 TUS-Resumable: 1.0.0
 ```
 
+### Merge
+
+This extension can be used to merge multiple uploads into a single one enabling
+clients to perform parallel uploads and uploading non-contiguous chunks. If the
+server supports this extension it MUST be announced by adding the `merge`
+element to the values of the `TUS-Extension` header.
+
+A partial upload is an upload which represents a chunk of a file. It is
+constructed by including the `Merge: partial` header when creating a new
+resource using the file creation extension. Multiple partial uploads are merged
+into a final upload in a specific order. The server SHOULD NOT process these
+partial uploads until they are merged to form a final upload. The length of the
+final resource MUST be the sum of the length of all partial resources. A final
+upload is considered finished if all of its partial uploads are finished.
+
+In order to create a new final upload the client MUST omit the `Entity-Length`
+header and add the `Merge` header to the file creation request. The headers
+value is the string `final` followed by a semicolon and a space-separated list
+of the URLs of the partial uploads which will be merged. The order of this list
+MUST represent the order using which the partial uploads are concatenated
+without adding, modifying or removing any bytes. This merge request MAY even
+happen if all or some of the corresponding partial uploads are not finished.
+
+The server MAY delete partial uploads once they are merged but they MAY be used
+multiple times for forming a final resource.
+
+Any `PATCH` request against a final upload MUST be denied and MUST neither
+modify the final nor any of its partial resources. The response of a `HEAD`
+request MUST NOT contain the `Offset` header. The `Entity-Length` header MUST be
+included if the length of the final resource can be calculated at the time.
+Responses to `HEAD` requests against partial or final uploads MUST include the
+`Merge` header and its value as sent in the file creation request.
+
+#### Headers
+
+##### Merge
+
+The `Merge` header indicates whether the upload created by the request is either
+a partial or final upload. If a partial upload is to be built, the header value
+MUST be `partial`. In the case of creating a final resource its value is the
+string `final` followed by a semicolon and a space-separated list of the URLs of
+the partial uploads which will be merged and form the file. All of the URLs MUST
+NOT contain a space. The host and protocol scheme of the URLs MAY be omitted. In
+this case the value of the `Host` header MUST be used as the host and the scheme
+of the current request.
+
+#### Example
+
+In the following example the `Host` and `TUS-Resumable` headers are omitted for
+readability although they are required by the specification.
+In the beginning two partial uploads are created:
+
+```
+POST /files HTTP/1.1
+Merge: partial
+Entity-Length: 5
+
+HTTP/1.1 204 No Content
+Location: http://tus.example.org/files/a
+```
+```
+POST /files HTTP/1.1
+Merge: partial
+Entity-Length: 6
+
+HTTP/1.1 204 No Content
+Location: http://tus.example.org/files/b
+```
+
+The next step is to create the final upload consisting of the two earlier
+generated partial uploads. In following request no `Entity-Length` header is
+presented.
+
+```
+POST /files HTTP/1.1
+Merge: final; /files/a http://tus.example.org/files/b
+
+HTTP/1.1 204 No Content
+Location: http://tus.example.org/files/ab
+```
+
+The length of the final resource is now 11 bytes:
+
+```
+HEAD /files/ab HTTP/1.1
+
+HTTP/1.1 204 No Content
+Entity-Length: 11
+Merge: final; /files/a /files/b
+```
+
+You are now able to upload data to the two partial resources using `PATCH`
+requests:
+
+```
+PATCH /files/a HTTP/1.1
+Offset: 0
+Content-Length: 5
+
+hello
+
+HTTP/1.1 204 No Content
+```
+```
+PATCH /files/b HTTP/1.1
+Offset: 0
+Content-Length: 6
+
+ world
+
+HTTP/1.1 204 No Content
+```
+
+In the first request the string `hello` was uploaded while the second file now
+contains ` world` with a leading space. The data stored for the final upload
+`/files/ab` is `hello world`.
+
 ## FAQ
 
 ### Why is the protocol using custom headers?