emld v0.5.1

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Ecological Metadata as Linked Data

This is a utility for transforming Ecological Metadata Language ('EML') files into 'JSON-LD' and back into 'EML.' Doing so creates a list-based representation of 'EML' in R, so that 'EML' data can easily be manipulated using standard 'R' tools. This makes this package an effective backend for other 'R'-based tools working with 'EML.' By abstracting away the complexity of 'XML' Schema, developers can build around native 'R' list objects and not have to worry about satisfying many of the additional constraints of set by the schema (such as element ordering, which is handled automatically). Additionally, the 'JSON-LD' representation enables the use of developer-friendly 'JSON' parsing and serialization that may facilitate the use of 'EML' in contexts outside of 'R,' as well as the informatics-friendly serializations such as 'RDF' and 'SPARQL' queries.

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emld

The goal of emld is to provide a way to work with EML metadata in the JSON-LD format. At it’s heart, the package is simply a way to translate an EML XML document into JSON-LD and be able to reverse this so that any semantically equivalent JSON-LD file can be serialized into EML-schema valid XML. The package has only three core functions:

  • as_emld() Convert EML’s xml files (or the json version created by this package) into a native R object (an S3 class called emld, essentially just a list).
  • as_xml() Convert the native R format, emld, back into XML-schema valid EML.
  • as_json() Convert the native R format, emld, into json(LD).

Installation

You can install emld from github with:

# install.packages("devtools")
devtools::install_github("ropensci/emld")

Motivation

In contrast to the existing EML package, this package aims to a very light-weight implementation that seeks to provide both an intuitive data format and make maximum use of existing technology to work with that format. In particular, this package emphasizes tools for working with linked data through the JSON-LD format. This package is not meant to replace EML, as it does not support the more complex operations found in that package. Rather, it provides a minimalist but powerful way of working with EML documents that can be used by itself or as a backend for those complex operations. Version 2.0 of the EML R package uses emld under the hood.

Note that the JSON-LD format is considerably less rigid than the EML schema. This means that there are many valid, semantically equivalent representations on the JSON-LD side that must all map into the same or nearly the same XML format. At the extreme end, the JSON-LD format can be serialized into RDF, where everything is flat set of triples (e.g. essentially a tabular representation), which we can query directly with semantic tools like SPARQL, and also automatically coerce back into the rigid nesting and ordering structure required by EML. This ability to “flatten” EML files can be particularly convenient for applications consuming and parsing large numbers of EML files. This package may also make it easier for other developers to build on the EML, since the S3/list and JSON formats used here have proven more appealing to many R developers than S4 and XML serializations.

library(emld)
library(jsonlite)
library(magrittr) # for pipes
library(jqr)      # for JQ examples only
library(rdflib)   # for RDf examples only

Reading EML

The EML package can get particularly cumbersome when it comes to extracting and manipulating existing metadata in highly nested EML files. The emld approach can leverage a rich array of tools for reading, extracting, and manipulating existing EML files.

We can parse a simple example and manipulate is as a familiar list object (S3 object):

f <- system.file("extdata/example.xml", package="emld")
eml <- as_emld(f)
eml$dataset$title
#> [1] "Data from Cedar Creek LTER on productivity and species richness\n  for use in a workshop titled \"An Analysis of the Relationship between\n  Productivity and Diversity using Experimental Results from the Long-Term\n  Ecological Research Network\" held at NCEAS in September 1996."

Writing EML

Because emld objects are just nested lists, we can create EML just by writing lists:


me <- list(individualName = list(givenName = "Carl", surName = "Boettiger"))

eml <- list(dataset = list(
              title = "dataset title",
              contact = me,
              creator = me),
              system = "doi",
              packageId = "10.xxx")

ex.xml <- tempfile("ex", fileext = ".xml") # use your preferred file path

as_xml(eml, ex.xml)
#> NULL
eml_validate(ex.xml)
#> [1] TRUE
#> attr(,"errors")
#> character(0)

Note that we don’t have to worry about the order of the elements here, as_xml will re-order if necessary to validate. (For instance, in valid EML the creator becomes listed before contact.) Of course this is a very low-level interface that does not help the user know what an EML looks like. Creating EML from scratch without knowledge of the schema is a job for the EML package and beyond the scope of the lightweight emld.

Working with EML as JSON-LD

For many applications, it is useful to merely treat EML as a list object, as seen above, allowing the R user to leverage a standard tools and intuition in working with these files. However, emld also opens the door to new possible directions by thinking of EML data in terms of a JSON-LD serialization rather than an XML serialization. First, owing to it’s comparative simplicity and native data typing (e.g. of Boolean/string/numeric data), JSON is often easier for many developers to work with than EML’s native XML format.

As JSON: Query with JQ

For example, JSON can be queried with with JQ, a simple and powerful query language that also gives us a lot of flexibility over the return structure of our results. JQ syntax is both intuitive and well documented, and often easier than the typical munging of JSON/list data using purrr. Here’s an example query that turns EML to JSON and then extracts the north and south bounding coordinates:

hf205 <- system.file("extdata/hf205.xml", package="emld")

as_emld(hf205) %>% 
  as_json() %>% 
  jq('.dataset.coverage.geographicCoverage.boundingCoordinates | 
       { northLat: .northBoundingCoordinate, 
         southLat: .southBoundingCoordinate }') %>%
  fromJSON()
#> $northLat
#> [1] "+42.55"
#> 
#> $southLat
#> [1] "+42.42"

Nice features of JQ include the ability to do recursive descent (common to XPATH but not possible in purrr) and specify the shape of the return object. Some prototype examples of how we can use this to translate between EML and https://schema.org/Dataset representations of the same metadata can be found in https://github.com/ropensci/emld/blob/master/notebook/jq_maps.md

As semantic data: SPARQL queries

Another side-effect of the JSON-LD representation is that we can treat EML as “semantic” data. This can provide a way to integrate EML records with other data sources, and means we can query the EML using semantic SPARQL queries. One nice thing about SPARQL queries is that, in contrast to XPATH, JQ, or other graph queries, SPARQL always returns a data.frame which is a particularly convenient format. SPARQL queries look like SQL queries in that we name the columns we want with a SELECT command. Unlike SQL, these names act as variables. We then use a WHERE block to define how these variables relate to each other.

f <- system.file("extdata/hf205.xml", package="emld")
hf205.json <- tempfile("hf205", fileext = ".json") # Use your preferred filepath

as_emld(f) %>%
  as_json(hf205.json)

prefix <- paste0("PREFIX eml: <eml://ecoinformatics.org/", eml_version(), "/>\n")
sparql <- paste0(prefix, '

  SELECT ?genus ?species ?northLat ?southLat ?eastLong ?westLong 

  WHERE { 
    ?y eml:taxonRankName "genus" .
    ?y eml:taxonRankValue ?genus .
    ?y eml:taxonomicClassification ?s .
    ?s eml:taxonRankName "species" .
    ?s eml:taxonRankValue ?species .
    ?x eml:northBoundingCoordinate ?northLat .
    ?x eml:southBoundingCoordinate ?southLat .
    ?x eml:eastBoundingCoordinate ?eastLong .
    ?x eml:westBoundingCoordinate ?westLong .
  }
')

rdf <- rdf_parse(hf205.json, "jsonld")
df <- rdf_query(rdf, sparql)
df
#> # A tibble: 0 x 0

Please note that the emld project is released with a Contributor Code of Conduct. By contributing to this project, you agree to abide by its terms.

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Functions in emld

Name Description
eml_validate eml_validate
as_xml Coerce an emld object into XML (EML's standard format)
eml_version Set or check the EML version default
template Create a template for an EML object
emld-package emld: Ecological Metadata as Linked Data
find_real_root_name Get the real QName for the root element, including its prefix
guess_schema_location Guess an appropriate schemaLocation value for a given version of the schema
as_json Coerce an emld object into JSON
as_emld Coerce an EML file or object into an emld object.
eml_ns Get the XML namespace for a version of EML
guess_root_schema Find the root schema module and version
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tutorial.Rmd
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Details

URL https://docs.ropensci.org/emld/, https://github.com/ropensci/emld
BugReports https://github.com/ropensci/emld/issues
License MIT + file LICENSE
Encoding UTF-8
LazyData true
RoxygenNote 7.1.1
VignetteBuilder knitr
Language en-US
NeedsCompilation no
Packaged 2020-09-27 02:57:18 UTC; cboettig
Repository CRAN
Date/Publication 2020-09-27 10:50:10 UTC

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