Creating a Transformer Plugin

There are two types of plugins that work within Gatsby’s data system, “source” and “transformer” plugins.

Source plugins “source” data from remote or local locations into what Gatsby calls nodes.
Transformer plugins “transform” data provided by source plugins into new nodes and/or node fields.

The purpose of this doc is to:

Define what a Gatsby transformer plugin is, and
Walk through a simplified reimplementation of an existing plugin, to demonstrate how to create a transformer plugin.

What do transformer plugins do?

Transformer plugins “transform” data of one type into another type. You’ll often use both source plugins and transformer plugins in your Gatsby sites.

This loose coupling between the data source and transformer plugins allow Gatsby developers to quickly assemble complex data transformation pipelines with little work.

How do you create a transformer plugin?

Just like a source plugin, a transformer plugin is a normal NPM package. It has a package.json file with optional dependencies as well as a gatsby-node.js file where you implement Gatsby’s Node.js APIs.

gatsby-transformer-yaml is transformer plugin that looks for new nodes with a media type of text/yaml (e.g. a .yaml file) and creates new YAML child node(s) by parsing the YAML source into JavaScript objects.

As an example, let’s partially rebuild a simplified gatsby-transformer-yaml directly in an example site. Let’s say we have a default gatsby starter site which includes a src/data/example.yml file:

src/data/example.yml

- name: Jane Doe
  bio: Developer based in Somewhere, USA
- name: John Smith
  bio: Developer based in Maintown, USA

Make sure the data is sourced

First, in gatsby-config.js, use the gatsby-source-filesystem plugin to create File nodes.

gatsby-config.js

module.exports = {
  plugins: [
    {
      resolve: `gatsby-source-filesystem`,
      options: {
        path: `./src/data/`,
      },
    },
  ],
}

These are exposed in your graphql schema which you can query:

query {
  allFile {
    edges {
      node {
        internal {
          type
          mediaType
          description
          owner
        }
      }
    }
  }
}

Now you have a File node to work with:

{
  "data": {
    "allFile": {
      "edges": [
        {
          "node": {
            "internal": {
              "contentDigest": "c1644b03f380bc5508456ce91faf0c08",
              "type": "File",
              "mediaType": "text/yaml",
              "description": "File \"src/data/example.yml\"",
              "owner": "gatsby-source-filesystem"
            }
          }
        }
      ]
    }
  }
}

Transform nodes of type `text/yaml`

Now, transform the newly created File nodes by hooking into the onCreateNode API in gatsby-node.js.

If you’re following along in an example project, install the following packages:

npm install --save js-yaml lodash

Now, in gatsby-node.js:

gatsby-node.js

const jsYaml = require(`js-yaml`)

async function onCreateNode({ node, loadNodeContent }) {
  // only log for nodes of mediaType `text/yaml`
  if (node.internal.mediaType !== `text/yaml`) {
    return
  }

  const content = await loadNodeContent(node)
  const parsedContent = jsYaml.load(content)
}

exports.onCreateNode = onCreateNode

File content:

- id: Jane Doe
  bio: Developer based in Somewhere, USA
- id: John Smith
  bio: Developer based in Maintown, USA

Parsed YAML content:

;[
  {
    id: "Jane Doe",
    bio: "Developer based in Somewhere, USA",
  },
  {
    id: "John Smith",
    bio: "Developer based in Maintown, USA",
  },
]

Now we’ll write a helper function to transform the parsed YAML content into new Gatsby nodes:

function transformObject(obj, id, type) {
  const yamlNode = {
    ...obj,
    id,
    children: [],
    parent: node.id,
    internal: {
      contentDigest: createContentDigest(obj),
      type,
    },
  }
  createNode(yamlNode)
  createParentChildLink({ parent: node, child: yamlNode })
}

Above, we create a yamlNode object with the shape expected by the createNode action.

We then create a link between the parent node (file) and the child node (yaml content).

In our updated gatsby-node.js, we’ll then iterate through the parsed YAML content, using the helper function to transform each into a new node:

gatsby-node.js

const jsYaml = require(`js-yaml`)
const _ = require(`lodash`)

async function onCreateNode({
  node,
  actions,  loadNodeContent,
  createNodeId,  createContentDigest,}) {
  function transformObject(obj, id, type) {    const yamlNode = {      ...obj,      id,      children: [],      parent: node.id,      internal: {        contentDigest: createContentDigest(obj),        type,      },    }    createNode(yamlNode)    createParentChildLink({ parent: node, child: yamlNode })  }
  const { createNode, createParentChildLink } = actions

  if (node.internal.mediaType !== `text/yaml`) {
    return
  }

  const content = await loadNodeContent(node)
  const parsedContent = jsYaml.load(content)

  parsedContent.forEach((obj, i) => {    transformObject(      obj,      obj.id ? obj.id : createNodeId(`${node.id} [${i}] >>> YAML`),      _.upperFirst(_.camelCase(`${node.name} Yaml`))    )  })}

exports.onCreateNode = onCreateNode

Now we can query for our new nodes containing our transformed YAML data:

query {
  allExampleYaml {
    edges {
      node {
        id
        name
        bio
      }
    }
  }
}

{
  "data": {
    "allExampleYaml": {
      "edges": [
        {
          "node": {
            "id": "3baa5e64-ac2a-5234-ba35-7af86746713f",
            "name": "Jane Doe",
            "bio": "Developer based in Somewhere, USA"
          }
        },
        {
          "node": {
            "id": "2c733815-c342-5d85-aa3f-6795d0f25909",
            "name": "John Smith",
            "bio": "Developer based in Maintown, USA"
          }
        }
      ]
    }
  }
}

Check out the full source code of gatsby-transformer-yaml.

Using the cache

Sometimes transforming properties costs time and resources. In order to avoid recreating these properties at each run, you can profit from the global cache mechanism Gatsby provides.

Cache keys should at least contain the contentDigest of the concerned node. For example, the gatsby-transformer-remark uses the following cache key for the html node:

extend-node-type.js

const htmlCacheKey = node =>
  `transformer-remark-markdown-html-${
    node.internal.contentDigest
  }-${pluginsCacheStr}-${pathPrefixCacheStr}`

Accessing and setting content in the cache is as simple as: