FAQ: Why LinkML
Why should I use LinkML at all?
All data follows some kind of schema or data model, whether it is explicitly articulated, or implicit / in the background. In our experience it is always beneficial to explicitly articulate that schema. This holds true for a range of situations, including:
you have simple tabular data in TSVs or a spreadsheet
you have a highly interlinked relational model
you are working with JSON documents in a document store like MongoDB
you have a knowledge graph in Neo4J
you are working with linked data in RDF
LinkML is designed to be flexible enough to cover all these use cases, allowing for lighweight semantic data dictionaries for tabular data, through rich interlinked schemas for knowledge graphs and triplestores
My data is a simple spreadsheet/TSV, why should I use LinkML?
If your data is a simple CSV then using a framework like LinkML may seem daunting. It may be tempting to include a simple README alongside your TSV describing the column headers. While this is certainly better than not having any data description at all, consider some of the drawbacks:
the README isn’t a computer-digestible representation of your data
you do not have a mechanism for validating the TSV
you lack a computable way of mapping your column headers to a standard data dictionary, vocabulary
LinkML provides an easy way of making your data computable. You can start with a very simple schema, with a single class, and one slot per column in your TSV; for example, for a dataset where the rows represent people:
this is already useful as it states which column headers are expected in your data.
you can later expand that to provide constraints on the values each column:
identifier: true ## this ensures the id field is unique
pattern: "\\S+@[\\S\\.]+\\S+" ## regular expression
range: job_code ## enumeration
- description: email is unique
You can also provide textual descriptions and additional metadata on the columns that may be useful for human users
description: unique identifier
description: the full name of the person
description: the persons email address
description: the age of the person in years
description: the kind of job the person has
If you like, you can provide both linked data IRIs to your schema elements, or mappings to existing systems, for example:
This has a number of advantages:
you make the intended meaning of your columns more transparent and explicit
your TSVs can be automatically translated to JSON-LD and RDF via the LinkML framework
it faciliates automated and semi-automated mapping between your data and other representations
There are a number of proposed frameworks for providing lightweight data dictionaries for your TSVs/spreadsheets:
One advantage of LinkML is that it is not only for TSVs
Why should I use LinkML over JSON-Schema?
JSON-Schema is a fantastic framework for validating JSON documents. If your primary use case is validating JSON documents then by all means keep using it!
However, if any of the following apply to you, you may want to consider LinkML - and remember, you can always
compile your LinkML schema down to JSON-Schema!
You want to make use inheritance/polymorphism
you want a language with a simple core based around familiar concepts of classes and fields (slots)
you want to make your data more FAIR (Findable Accessible Interoperable Reusable), for example, by annotating schema elements with IRIs
you want to use JSON-LD but don’t want to coordinate separate JSON-Schema and JSON-LD context documents - LinkML is an all-in-one-solution!
you want a more advanced ontology-aware enumerations or dynamic enumerations
you want your datamodel to be used in other frameworks than JSON - e.g. TSVs, SQL databases, Triplestores, graph databases
When making your decision, you should weigh factors such as the fact that things that can be expressed in one framework may not be expressible in the other.
See also FAQ entries in modeling which compare some similar constructs.
Why should I use LinkML over ShEx/SHACL?
ShEx and SHACL are both excellent “shape” languages for providing
closed-world constraints on top of RDF/triples. If your universe
consists entirely of RDF then you may want to consider adopting one of
these as your primary means of expressing your data!
However, if any of the following apply to you, you may want to
consider LinkML - and remember, you can always compile your LinkML
schema to ShEx, with a prototype SHACL generator now available.
you want your datamodel to work for non-RDF representations such as JSON, YAML, and relational databases
your user based and developer base is not entirely semantic web / linked data enthusiasts
your emphasis is more on data modeling rather than validation
Note: LinkML lead developer Harold Solbrig is also one of the authors of the ShEx specification.
Why should I use LinkML over SQL DDL?
SQL Data Description Language (DDL; or simply “CREATE TABLE”
statements) is a means of describing the structure of a relational
If you are using a relational database and have minimal lightweight
application code that performs direct SQL queries over data, there may
be no compelling use case for LinkML.
However, most information architectures that use a SQL database also
involve some alternative representation of the data - for example, a
JSON representation in an API, or an object representation – or even
an RDF representation. It can be challenging to keep these different
representations in sync. There are frequently excellent products that
solve “one part” of this mapping problem – e.g. an ORM (Object
Relational Mapping) tool. LinkML is intended to allow you to give a
“bigger picture” view of your model that is as independent as possible
from underlying storage or exchange technologies, and at the same time
can be compiled down.
LinkML also allows you to specify URIs and CURIE/URI mappings for each
element of your schema, which provides a declarative specification of
how your data should be mapped. For example, if you are trying to
bring together two schemas (whether via federation or via
warehousing), if both schemas annotate the same field with the same
URI then we know these can be merged.
Even if you are not interested in mapping your SQL data model to
anything else, it can still be a great idea to use LinkML as a data
definition language for your schema, especially if you have a schema
with many fields that a domain scientist or stakeholder needs to
In our opinion, SQL DDL is not a great language for data
dictionaries. There is a lack of standard ways to even add comments or
descriptions to fields, and it can be challenging to introspect
these. LinkML provides a simple easy to use way to provide rich
metadata about your fields.
CREATE TABLE sample (
id TEXT PRIMARY KEY, -- unique sample id
individual_id TEXT FOREIGN KEY(person.id),
collec_location TEXT FOREIGN KEY(geoloc.id),
description: A unique identifier for the sample
title: sample name
description: A human-readable name for the sample
description: the disease with which the patient was diagnosed
title: tissue source
description: the anatomical location from which the tissue was sampled
- model this using an ontology term instead
title: collection location
description: the geocoordinates of the site where the sampling was obtained
- this should NOT be the site of processing
Here we have a standard way of providing human-readable names for our
columns (via title). We have a
human-friendly textual description. Both of these could be used to
drive dynamic tooltips in an application that collects or displays data.
Why should I use LinkML over UML?
UML is a powerful language for software engineer diagramming. UML
Class Diagrams provide a
standard way to draw a class hierarchy for a program, that has some
similarities to LinkML.
There are a number of reasons to use LinkML over UML
The UML standard is large and complex
UML is more geared towards software engineering and includes features not needed for data modeling, such as operations
UML tools are complex and expensive (both financially and in terms of learning curves)
In contrast LinkML makes it easy to author schemas using nothing more than a text editor
All the tooling for LinkML is free and open
LinkML has the ability to compile to other frameworks
LinkML has facilities for semantic modeling, not anticipated in UML
Currently there is no way to generate complete UML from a LinkML schema.
However, the yUML generator (used in the markdown generator) can be used to make yUML diagrams for any class or schema.
Why should I use LinkML over OWL?
LinkML is in a very different class of languages from OWL. LinkML is a
schema language, with features in common with JSON-Schema, XML Schema,
UML, SQL DDL, shape languages such as ShEx/SHACL.
In our experience OWL is best for “open model” direct representations
of domain knowledge, and should not be used as a schema
language. Many LinkML contributors and users also work with large biomedical
terminological-style ontologies in OWL. However, we appreciate this is nuanced, and we welcome any
questions or discussions via our GitHub issue tracker.
If you do have a schema expressed in OWL, you can use the linkml
toolkit to infer/bootstrap a LinkML schema from it. But note this is a
heuristic procedure, and will not give you sensible results from a
large “terminological” ontology (such as an OBO ontology); it is best used with schema-style “ontologies”,
such as common semantic web vocabularies.
It is possible to use LinkML to help you structure an OWL ontology
by using LinkML as a metaclass authoring system. See
an example, and see also the
Why should I use LinkML over Custom spreadsheets?
It is common in many projects for metadata elements and data dictionaries to be maintained as ad-hoc spreadsheets.
Doing this in the absence of any over-arching framework can be
problematic. It is easy for mistakes to creep into these spreadsheets
unnoticed, often the semantics are unclear, and when developers are
asked to write validators or UIs driven by these spreadsheets the lack
of clear semantics can lead to repetitive error-prone code that is
costly to maintain.
LinkML provides a systematic way to manage data dictionaries and metadata elements.
Historically this has required use of YAML files to maintain a schema,
which can be offputting to non-technical metadata curators. A new framework allows the best of both worlds:
This framework allows schemas to be maintained using Excel, google sheets, or TSVs, providing flexibility to non-technical modelers, with all the benefits of linkml.
Why should I use LinkML over CSV-on-the-web?
The W3C CSV on the Web
(CSVW) Working Group have developed standard ways to express metadata about
CSVs and other kinds of tabular data.
An example use of CSVW is to semantically describe the columns in a
CSV/TSV, for example a CSV about country locations and nomenculature. The following in Example 10 from the CSVW primer:
"dc:description": "The ISO two-letter code for a country, in lowercase."
"titles": "country group",
"dc:description": "A lowercase two-letter code for a group of countries."
"titles": "name (en)",
"dc:description": "The official name of the country in English."
"titles": "name (fr)",
"dc:description": "The official name of the country in French."
"titles": "name (de)",
"dc:description": "The official name of the country in German."
"dc:description": "The latitude of an indicative point in the country."
"dc:description": "The longitude of an indicative point in the country."
CSVW provides a number of advantages for making data FAIR over the use
of CSVs alone, or CSVs in combination with non-machine readable documentation files.
Most of the things that can be said using CSVW can also be said with
LinkML, and LinkML is not restricted solely to CSVs - e.g. you can
semantically describe JSON files with LinkML too.
We are currently planning on writing a generator for CSVW JSON-LD.
Why should I use LinkML over ISO-11179?
ISO-11179 is an ISO standard for Metadata and Metadata Registries. It
provides a rich standardized way of exchanging data dictionaries. For
example, an ISO-11179 registry can be used to register all of the
columns in a data file or tags in an XML file, and to map semantics
onto these, making integration across datasets more automatable.
Currently ISO-11179 and LinkML serve different purposes, but there are
overlaps in goals, and we are currently working on documentation
clarifying the connection.