# Access within AWS Compute

If you have access to machine running inside Amazon `us-west-2` (e.g. a NASA Openscapes JupyterHub or the ability to create a VM in EC2 using this compute region), you will be able to access NOAA EarthData using the S3 interface. This may offer slightly better performance than using the universal https interface which routes data through a proxy, but is rarely necessary. This approach is documented only for the sake of completeness. Use the http interface whenever possible!

(In future, this mechanism may also work with private S3 credentials in a ‘requester-pays’ model, charging for data egress rather than compute. For more context on access mechanisms for NASA EarthData see the motivations vignette).

Authentication via the S3 mechanism is an alternative to authentication via Bearer tokens over the http interface. Do not use `edl_set_token()`, you may want to use `edl_unset_token()` to ensure only one mechanism is in use.

There are three important issues to keep in mind if you ever chose to use the S3 token mechanism to access EarthData. (Please note that again these issues are specific to EarthData due to concerns about billing, and do not represent general statements about working with spatial data on the S3 protocol more generally!) Unlike the https mechanism, EarthData S3 authentication tokens:

- Can only be used from a machine inside `us-west-2` data center
- Expire after 1 hr and must be renewed
- Are specific to each DAAC

(Please note that these restrictions are more-or-less specific to how NASA has configured access to their AWS buckets. S3 tokens are widely used to access all kinds of data in various workflows, but these restrictions are rarely put in place.)

    library(earthdatalogin)
    library(rstac)
    library(gdalcubes)
    gdalcubes_options(parallel = TRUE)
    gdal_cloud_config()

### Earth Data Authentication

To create our S3 tokens, we use `edl_s3_token`. Note that we must specify the endpoint of the DAAC we are trying to access – e.g. the LP DAAC in this case. Remember, this token will only work from within the `us-west-2` hub, only for LP DAAC data, and will expire 1 hr after it is issued.

    edl_s3_token(daac = "https://data.lpdaac.earthdatacloud.nasa.gov")

We can search the STAC API as in any other case. This step does not require any authentication.

    bbox <- c(xmin=-122.5, ymin=37.5, xmax=-122.0, ymax=38)
    start <- "2022-01-01"
    end <- "2022-06-30"

    # Find all assets from the desired catalog:
    items <- stac("https://cmr.earthdata.nasa.gov:443/search/stac/LPCLOUD") |>
      stac_search(collections = "HLSL30.v2.0",
                  bbox = bbox,
                  datetime = paste(start,end, sep = "/")) |>
      post_request() |>
      items_fetch() |>
      items_filter(filter_fn = \(x) {x[["eo:cloud_cover"]] < 20})

The STAC search returns item metadata including links to the data object as https URLs. While these are ideal for the bearer token access, we want to convert these to S3-style addresses. The function `gdalcubes::stac_image_collection()` actually takes an optional argument for how URLs should be handled. The helper function `edl_as_s3()` provides a convenient way to do this (a trivial step, we just remove the domain name and replace it with GDAL’s S3 prefix, `/vsis3`):

    col <-
      stac_image_collection(items$features,
                            asset_names = c("B02", "B03", "B04", "Fmask"),
                            url_fun = \(url) edl_as_s3(url, prefix = "/vsis3/"))

    # Desired data cube shape & resolution
    v = cube_view(srs = "EPSG:4326",
                  extent = list(t0 = as.character(start),
                                t1 = as.character(end),
                                left = bbox[1], right = bbox[3],
                                top = bbox[4], bottom = bbox[2]),
                    nx = 2048, ny = 2048, dt = "P1M")

    cloud_mask <- image_mask("Fmask", values=1) # mask clouds and cloud shadows
    rgb_bands <- c("B04","B03", "B02")

    # Here we go! note eval is lazy
    raster_cube(col, v, mask=cloud_mask) |>
      select_bands(rgb_bands) |>
      plot(rgb=1:3)