DAP4: Chunked encoding

From OPeNDAP Documentation

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1 Background

One persistent (ahem) problem with DAP2 was the inability of client applications to recognize when a data transmission failed. If an error happened before the initial set of headers for the response were sent, then DAP2's error reporting scheme worked just fine. However, if the server encountered an error once the serialization of data started, for example, it was essentially impossible for a client to detect the error (essentially being the operative word; the Ocapi library did detect these errors, but it is the only client I know that did so).

This proposal suggests that DAP4 use a simple variation on the HTTP/1.1 chunked transmission scheme to serialize the data Part of the response document so that errors are simple to detect. Furthermore, this scheme is independent of the form or content of that part of the response, so the same scheme can be used with different response forms or dropped when/if DAP is used with protocols that support out-of-band error signaling, simplifying our ongoing refinement of the protocol.

2 References

  1. HTTP/1.1

3 Problem Addressed

The DAP needs to format its data responses so that a server that interleaves transmission with data reads and serialization can signal errors to a receiver reliably.

4 Proposed Solution

4.1 Overview

The data part of a response document (from now on I will just say 'response document') will be 'chunked' in a fashion similar to that outlined in HTTP/1.1. However, in addition to a prefix indicating the size of the chunk, DAP4 will include a chunk-type code. This will provide a way for the receiver to know if the next chunk is part of the data response or if it contains an error response. In the latter case, the client should assume that the data response ended, even though the correct closing information was not provided.

4.2 More detail

Each chunk will be prefixed by a chunk header consisting of a chunk type and byte count, where the chunk type is a single byte and the byte count is an unsigned integer using three bytes, with the entire header in the server's byte order. Immediately following the chunk header will be byte count bytes followed by another (four byte) chunk header. By writing the chunk header in the server's byte order, it should be simple for the server to write these headers and easy for the clients to read them.

Three chunk-type types are defined in this proposal:

data
This chunk header prefixes the next chunk in the current data response
error
This chunk header prefixes an error message; the current data response has ended
end
This chunk header is the last one for the current data response
4.3 Grammar
response = chunk *chunk

chunk = chunk-header chunk-data

chunk-header = chunk-type chunk-size

chunk-type = b0 (OCTET)
                      ; 0 = data, 1 = error, 2 = end

chunk-size = b1 ... b3 3(OCTET)
                      ; 3 bytes, interpreted as an unsigned integer, in the server's byte order

chunk-data = chunk-size(OCTET)
                      ; exactly chunk-size bytes

5 Rationale

The chunk headers use a simple prefix with a binary count because it will be fast and easy to build without cumbersome binary to ascii conversions (e.g., HTTP uses ASCII...). It will be simple to en/decode the chunk-type and count. Having space for 256 chunk types is overkill, but enables clients to use byte masks which might be easier in some programming environments.

6 Discussion

Jimg 14:58, 28 August 2012 (PDT) We decided to drop the multipart MIME in favor of chunking the entire response. Dennis made the point that using both MIME and chunking was redundant; I agree and there seems to be little real benefit to using MIME since there's little way a non-DAP client will be able to do anything useful with the data part. Chunking the entire response is a fine way to transmit the data and ensure reliable error transmission. Also note that there's no limit on the size of a chunk, so any response can be made up of just one chunk.


JohnCaron

1) Perhaps "message" is a better name than "chunk' ? Jimg 14:51, 28 August 2012 (PDT) Since these are parts of a single message I think chunk is right word.

2) I would not limit the size of the chunk to 2^24. In fact, I would use a base-128 variable length encoding to let the size be as large as needed, without wasting space. Jimg 15:31, 4 September 2012 (PDT) Originally, I thought this was OK, but Ethan and Dennis convinced me that 2^24 (~16.7*10^6) was plenty big enough for a chunk.