Is there any benefit between requesting data via DAP4 over DAP2?

There are various differences between the DAP2 and the DAP4 protocol.

The DAP4 is broader in scope

Figure 1. Comparison between DAP2 and DAP4 data models and responses.

The OPeNDAP team strongly recommends to begin using DAP4. Despite the Grid no longer being part of the DAP4 data model, a coverage consisting of Maps, dimensions and Array provide the same functionality in DAP4 that Grid did in DAP2. That said, a dataset described by the DAP2 protocol is described by the DAP4.

Warning

A DAP4 OPeNDAP data server aims to provide backwards compatibility with DAP2. However, because there exists dataset data types that do not appear in the DAP2 data model from 1993, DAP-backwards compatibility is not guaranteed.

DAP4 offers improved HTTP transport

The DAP2 protocol was first envisioned in Cornillon, et al, 1993 and described in Gallagher J. and Flierl G., 1996, thus predating the HTTP/1.1 protocol. This means that in DAP2 the binary data (.dods) is not chunked. Moreover, DAP2 has no implementation for checksums, a standarized transfer practice nowadays. DAP4, in contrast, dap responses are chunked, embracing HTTP/1.1. Moreover, both Hyrax and Thredds implement checksums, with checksums being always implemented by Hyrax and only when requested (in the URL) by Thredds.

A word of caution

When a Dataset possess variables or attributes of a type described in DAP4 but not in DAP2 (e.g. int64 value), the DAP4 implementing server will return an error (see this dataset, and append a .dds to the Data URL):

Error { 
    code = 400;
    message = "Your request was for a response that uses the DAP2 data model. This dataset contains variables whose data type ( Int64 ) is not compatible with that data model, causing this request to FAIL. To access this dataset ask for the DAP4 binary response encoding.";
}

You can reproduce this error when attempting to inspect the DAP2 metadata (dds) for this ATLAS03 dataset. This is, replace the dmr.html with .dds .

However, if the only DAP4 type that appears on the dataset is a Group, which is a container type rather than a data type, a Hyrax DAP4 server can return a DAP2 response. It accomplishes this by flattening the dataset. You can inspect this by looking at the (flattened) metadata of this SimpleGroup test file.

DAP4 is the default

In the case of Hyrax and NASA OPenDAP servers, DAP4 is the default exposed DAP protocol. A DAP2 generated metadata / response sent to the user is then first translated from a DAP4 response, which can result in slower responses. This is significant for NASA Earthdata users.

Final Recommendation

The OPeNDAP team considers DAP4 a much more modern, faster, safer, and broader DAP protocol, and strongly recommends data producers and users to migrate to DAP4 servers.