Reference

The technical definition of a Harper component is fairly loose. In the absolute, simplest form, a component is any JavaScript module that is compatible with the default component configuration. For example, a module with a singular resources.js file is technically a valid component.

Harper provides many features as built-in components, these can be used directly without installing any other dependencies.

Other features are provided by custom components. These can be npm packages such as @harperdb/nextjs and @harperdb/apollo (which are maintained by Harper), or something maintained by the community. Custom components follow the same configuration rules and use the same APIs that Harper's built-in components do. The only difference is that they must be apart of the component's dependencies.

Documentation is available for all built-in and custom Harper components.

Component Configuration

Harper components are configured with a config.yaml file located in the root of the component module directory. This file is how a component configures other components it depends on. Each entry in the file starts with a component name, and then configuration values are indented below it.

name:
  option-1: value
  option-2: value

It is the entry's name that is used for component resolution. It can be one of the built-in components, or it must match a package dependency of the component as specified by package.json. The Custom Component Configuration section provides more details and examples.

For some built-in components they can be configured with as little as a top-level boolean; for example, the rest extension can be enabled with just:

rest: true

Other components (built-in or custom), will generally have more configuration options. Some options are ubiquitous to the Harper platform, such as the files and urlPath options for an Extension, or package for a custom component. Additionally, custom options can also be defined for Extensions.

Custom Component Configuration

Any custom component must be configured with the package option in order for Harper to load that component. When enabled, the name of package must match a dependency of the component. For example, to use the @harperdb/nextjs extension, it must first be included in package.json:

{
  "dependencies": {
    "@harperdb/nextjs": "1.0.0"
  }
}

Then, within config.yaml it can be enabled and configured using:

'@harperdb/nextjs':
  package: '@harperdb/nextjs'
  # ...

Since npm allows for a variety of dependency configurations, this can be used to create custom references. For example, to depend on a specific GitHub branch, first update the package.json:

{
  "dependencies": {
    "harper-nextjs-test-feature": "HarperDB/nextjs#test-feature"
  }
}

And now in config.yaml:

harper-nextjs-test-feature:
  package: '@harperdb/nextjs'
  files: './'
  # ...

Default Component Configuration

Harper components do not need to specify a config.yaml. Harper uses the following default configuration to load components.

rest: true
graphql: true
graphqlSchema:
  files: '*.graphql'
roles:
  files: 'roles.yaml'
jsResource:
  files: 'resources.js'
fastifyRoutes:
  files: 'routes/*.js'
  urlPath: '.'
static:
  files: 'web/**'

Refer to the built-in components documentation for more information on these fields.

If a config.yaml is defined, it will not be merged with the default config.

Extensions

A Harper Extension is a extensible component that is intended to be used by other components. The built-in components graphqlSchema and jsResource are both examples of extensions.

As of Harper v4.6, a new Extension API has been introduced as a major overhaul of the previous API. The new API is designed to be more flexible, extensible, performant, and easier to use. It is recommended that all new extensions use the new API, and that existing extensions are migrated to the new API as soon as possible. The documentation for the legacy API is still available below in the Legacy Extensions section.

Functionally, what makes an extension a component is the contents of config.yaml. Unlike the Application Template referenced earlier, which specified multiple components within the config.yaml, an extension must specify an extensionModule option. This must be a path to the extension module source code. The path must resolve from the root of the extension module directory.

For example, the Harper Next.js Extension config.yaml specifies extensionModule: ./extension.js.

If the extension is being written in something other than JavaScript (such as TypeScript), ensure that the path resolves to the built version, (i.e. extensionModule: ./dist/index.js)

It is also recommended that all extensions have a package.json that specifies JavaScript package metadata such as name, version, type, etc. Since extensions are just JavaScript packages, they can do anything a JavaScript package can normally do. It can be written in TypeScript, and compiled to JavaScript. It can export an executable (using the bin property). It can be published to npm. The possibilities are endless!

Furthermore, what defines an extension separately from a component is that it exports a handleComponent() method.

An extension cannot export both handleComponent() and any of the Legacy extension methods. The component loader will throw an error if both are defined.

The handleComponent() method is executed only on worker threads during the component loading sequence. It receives a single, scope argument that contains all of the relevant metadata and APIs for interacting with the associated component.

The method can be async and is awaited by the component loader.

However, it is highly recommended to avoid event-loop-blocking operations within the handleComponent() method. See the examples section for best practices on how to use the scope argument effectively.

Example: Statically hosting files

This is a functional example of how the handleComponent() method and scope argument can be used to create a simple static file server extension. This example assumes that the component has a config.yaml with the files option set to a glob pattern that matches the files to be served.

This is a simplified form of the static built-in component.

export function handleComponent(scope) {
  const staticFiles = new Map();

  scope.options.on('change', (key, value, config) => {
    if (key[0] === 'files' || key[0] === 'urlPath') {
      // If the files or urlPath options change, we need to reinitialize the static files map
      staticFiles.clear();
      logger.info(`Static files reinitialized due to change in ${key.join('.')}`);
    }
  });

  scope.handleEntry((entry) => {
    if (entry.entryType === 'directory') {
      logger.info(`Cannot serve directories. Update the files option to only match files.`);
      return;
    }

    switch (entry.eventType) {
      case 'add':
      case 'change':
        // Store / Update the file contents in memory for serving
        staticFiles.set(entry.urlPath, entry.contents);
        break;
      case 'unlink':
        // Remove the file from memory when it is deleted
        staticFiles.delete(entry.urlPath);
        break;
    }
  });

  scope.server.http((req, next) => {
    if (req.method !== 'GET') return next(req);

    // Attempt to retrieve the requested static file from memory
    const staticFile = staticFiles.get(req.pathname);

    return staticFile ? {
      statusCode: 200,
      body: staticFile,
    } : {
      statusCode: 404,
      body: 'File not found',
    }
  }, { runFirst: true });
}

In this example, the entry handler method passed to handleEntry will manage the map of static files in memory using their computed urlPath and the contents. If the config file changes (and thus a new default file or url path is specified) the extension will clear the file map as well to remove artifacts. Furthermore, it uses the server.http() middleware to hook into the HTTP request handling.

This example is heavily simplified, but it demonstrates how the different key parts of scope can be used together to provide a performant and reactive application experience.

handleComponent(scope: Scope): void | Promise<void>

Parameters:

• scope - Scope - An instance of the Scope class that provides access to the component's configuration, resources, and other APIs.

Returns: void | Promise<void>

This is the only method an extension module must export. It can be async and is awaited by the component loader. The scope argument provides access to the component's configuration, resources, and other APIs.

Class: Scope

• Extends EventEmitter

Event: 'close'

Emitted after the scope is closed via the close() method.

Event: 'error'

• error - unknown - The error that occurred.

Event: 'ready'

Emitted when the Scope is ready to be used after loading the associated config file. It is awaited by the component loader, so it is not necessary to await it within the handleComponent() method.

scope.close()

Returns: this - The current Scope instance.

Closes all associated entry handlers, the associated scope.options instance, emits the 'close' event, and then removes all other listeners on the instance.

scope.handleEntry([files][, handler])

Parameters:

• files - FilesOptions | FileAndURLPathConfig | onEntryEventHandler - optional
• handler - onEntryEventHandler - optional

Returns: EntryHandler - An instance of the EntryHandler class that can be used to handle entries within the scope.

The handleEntry() method is the key to handling component entries. This method is used to register an entry event handler, specifically for the EntryHandler 'all' event. The method signature is very flexible, and allows for the following variations:

• scope.handleEntry() (with no arguments) Returns the default EntryHandler created by the files and urlPath options in the config.yaml.

• scope.handleEntry(handler) (where handler is an onEntryEventHandler) Returns the default EntryHandler instance (based on the options within config.yaml) and uses the provided handler for the 'all' event.

• scope.handleEntry(files) (where files is FilesOptions or FileAndURLPathConfig) Returns a new EntryHandler instance that handles the specified files configuration.

• scope.handleEntry(files, handler) (where files is FilesOptions or FileAndURLPathConfig, and handler is an onEntryEventHandler) Returns a new EntryHandler instance that handles the specified files configuration and uses the provided handler for the 'all' event.

For example,

export function handleComponent(scope) {
   // Get the default EntryHandler instance
  const defaultEntryHandler = scope.handleEntry();

  // Assign a handler for the 'all' event on the default EntryHandler
  scope.handleEntry((entry) => { /* ... */ });

  // Create a new EntryHandler for the 'src/**/*.js' files option with a custom `'all'` event handler.
  const customEntryHandler = scope.handleEntry({
    files: 'src/**/*.js',
  }, (entry) => { /* ... */ });

  // Create another custom EntryHandler for the 'src/**/*.ts' files option, but without a `'all'` event handler.
  const anotherCustomEntryHandler = scope.handleEntry({
    files: 'src/**/*.ts',
  });
}

And thus, if the previous code was used by a component with the following config.yaml:

customExtension:
  files: 'web/**/*'

Then the default EntryHandler instances would be created to handle all entries within the web directory.

scope.requestRestart()

Returns: void

Request a Harper restart. This does not restart the instance immediately, but rather indicates to the user that a restart is required. This should be called when the extension cannot handle the entry event and wants to indicate to the user that the Harper instance should be restarted.

This method is called automatically by the scope instance if the user has not defined an scope.options.on('change') handler or any event handlers for the default EntryHandler instance.

scope.resources

• Map<string, Resource> - A map of the currently loaded Resource instances.

scope.server

• server - A reference to the server global API.

scope.options

• OptionsWatcher

An OptionsWatcher instance associated with the component using the extension. Emits 'change' events when the respective extension part of the component's config file is modified.

Interface: FilesOption

• string | string[] | FilesOptionsObject

Interface: FilesOptionsObject

• source - string | string[] - required - The glob pattern string or array of strings.

• ignore - string | string[] - optional - An array of glob patterns to exclude from matches. This is an alternative way to use negative patterns. Defaults to [].

Interface: FileAndURLPathConfig

• files - FilesOptions - required - A glob pattern string, array of glob pattern strings, or a more expressive glob options object determining the set of files and directories to be resolved for the extension.

• urlPath - string - optional - A base URL path to prepend to the resolved files entries.

Class: OptionsWatcher

• Extends EventEmitter

Event: 'change'

• key - string[] - The key of the changed option split into parts (e.g. foo.bar becomes ['foo', 'bar']).

• value - ConfigValue - The new value of the option.
• config - ConfigValue - The entire configuration object of the extension.

The 'change' event is emitted whenever an configuration option is changed in the configuration file relative to the component and respective extension.

Given a component using the following config.yaml:

customExtension:
  files: 'web/**/*'
otherExtension:
  file: 'index.js'

The scope.options for the respective extensions customExtension and otherExtension would emit 'change' events when the files options relative to them are modified.

For example, if the files option for customExtension is changed to web/**/*.js, the following event would be emitted only within the customExtension scope:

scope.options.on('change', (key, value, config) => {
  key // ['files']
  value // 'web/**/*.js'
  config // { files: 'web/**/*.js' }
});

Event: 'close'

Emitted when the OptionsWatcher is closed via the close() method. The watcher is not usable after this event is emitted.

Event: 'error'

• error - unknown - The error that occurred.

Event: 'ready'

• config - ConfigValue | undefined - The configuration object of the extension, if present.

This event can be emitted multiple times. It is first emitted upon the initial load, but will also be emitted after restoring a configuration file or configuration object after a 'remove' event.

Event: 'remove'

The configuration was removed. This can happen if the configuration file was deleted, the configuration object within the file is deleted, or if the configuration file fails to parse. Once restored, the 'ready' event will be emitted again.

options.close()

Returns: this - The current OptionsWatcher instance.

Closes the options watcher, removing all listeners and preventing any further events from being emitted. The watcher is not usable after this method is called.

options.get(key)

Parameters:

• key - string[] - The key of the option to get, split into parts (e.g. foo.bar is represented as ['foo', 'bar']).

Returns: ConfigValue | undefined

If the config is defined it will attempt to retrieve the value of the option at the specified key. If the key does not exist, it will return undefined.

options.getAll()

Returns: ConfigValue | undefined

Returns the entire configuration object of the extension. If the config is not defined, it will return undefined.

options.getRoot()

Returns: Config | undefined

Returns the root configuration object of the component. This is the entire configuration object, basically the parsed form of the config.yaml. If the config is not defined, it will return undefined.

Interface: Config

• [key: string] ConfigValue

An object representing the configuration of the extension.

Interface: ConfigValue

• string | number | boolean | null | undefined | ConfigValue[] | Config

Any valid configuration value type. Essentially, the primitive types, an array of those types, or an object comprised of values of those types.

Class: EntryHandler

Extends: EventEmitter

Event: 'all'

• entry - FileEntry | DirectoryEntry - The entry that was added, changed, or removed.

The 'all' event is emitted for all entry events, including file and directory events. This is the event that the handler method in scope.handleEntry is registered for. The event handler receives an entry object that contains the entry metadata, such as the file contents, URL path, and absolute path.

An effective pattern for this event is:

async function handleComponent(scope) {
  scope.handleEntry((entry) => {
    switch(entry.eventType) {
      case 'add':
        // Handle file addition
        break;
      case 'change':
        // Handle file change
        break;
      case 'unlink':
        // Handle file deletion
        break;
      case 'addDir':
        // Handle directory addition
        break;
      case 'unlinkDir':
        // Handle directory deletion
        break;
    }
  });
}

Event: 'add'

• entry - AddFileEvent - The file entry that was added.

The 'add' event is emitted when a file is created (or the watcher sees it for the first time). The event handler receives an AddFileEvent object that contains the file contents, URL path, absolute path, and other metadata.

Event: 'addDir'

• entry - AddDirEvent - The directory entry that was added.

The 'addDir' event is emitted when a directory is created (or the watcher sees it for the first time). The event handler receives an AddDirEvent object that contains the URL path and absolute path of the directory.

Event: 'change'

• entry - ChangeFileEvent - The file entry that was changed.

The 'change' event is emitted when a file is modified. The event handler receives a ChangeFileEvent object that contains the updated file contents, URL path, absolute path, and other metadata.

Event: 'close'

Emitted when the entry handler is closed via the entryHandler.close() method.

Event: 'error'

• error - unknown - The error that occurred.

Event: 'ready'

Emitted when the entry handler is ready to be used.

Event: 'unlink'

• entry - UnlinkFileEvent - The file entry that was deleted.

The 'unlink' event is emitted when a file is deleted. The event handler receives an UnlinkFileEvent object that contains the URL path and absolute path of the deleted file.

Event: 'unlinkDir'

• entry - UnlinkDirEvent - The directory entry that was deleted.

The 'unlinkDir' event is emitted when a directory is deleted. The event handler receives an UnlinkDirEvent object that contains the URL path and absolute path of the deleted directory.

entryHandler.name

Returns: string

The name of the associated component.

entryHandler.directory

Returns: string

The directory of the associated component. This is the root directory of the component where the config.yaml file is located.

entryHandler.close()

Returns: this - The current EntryHandler instance.

Closes the entry handler, removing all listeners and preventing any further events from being emitted. The handler can be started again using the entryHandler.update() method.

entryHandler.update(config)

Parameters:

• config - FilesOption | FileAndURLPathConfig - The configuration object for the entry handler.

This method will update an existing entry handler to watch new entries. It will close the underlying watcher and create a new one, but will maintain any existing listeners on the EntryHandler instance itself.

This method returns a promise associated with the ready event of the updated handler.

Interface: BaseEntry

• stats - fs.Stats | undefined - The file system stats for the entry.
• urlPath - string - The recommended URL path of the entry.

• absolutePath - string - The absolute path of the entry.

The foundational entry handle event object. The stats may or may not be present depending on the event, entry type, and platform.

The urlPath is resolved based on the configured pattern (files: option) combined with the optional urlPath option. This path is generally useful for uniquely representing the entry. It is used in the built-in components such as jsResource and static.

The absolutePath is the file system path for the entry.

Interface: FileEntry

Extends BaseEntry

• contents - Buffer - The contents of the file.

A specific extension of the BaseEntry interface representing a file entry. We automatically read the contents of the file so the user doesn't have to bother with FS operations.

There is no DirectoryEntry since there is no other important metadata aside from the BaseEntry properties. If a user wants the contents of a directory, they should adjust the pattern to resolve files instead.

Interface: EntryEvent

Extends BaseEntry

• eventType - string - The type of entry event.

• entryType - string - The type of entry, either a file or a directory.

A general interface representing the entry handle event objects.

Interface: AddFileEvent

Extends EntryEvent, FileEntry

• eventType - 'add'

• entryType - 'file'

Event object emitted when a file is created (or the watcher sees it for the first time).

Interface: ChangeFileEvent

Extends EntryEvent, FileEntry

• eventType - 'change'

• entryType - 'file'

Event object emitted when a file is modified.

Interface: UnlinkFileEvent

Extends EntryEvent, FileEntry

• eventType - 'unlink'

• entryType - 'file'

Event object emitted when a file is deleted.

Interface: FileEntryEvent

• AddFileEvent | ChangeFileEvent | UnlinkFileEvent

A union type representing the file entry events. These events are emitted when a file is created, modified, or deleted. The FileEntry interface provides the file contents and other metadata.

Interface: AddDirEvent

Extends EntryEvent

• eventType - 'addDir'

• entryType - 'directory'

Event object emitted when a directory is created (or the watcher sees it for the first time).

Interface: UnlinkDirEvent

Extends EntryEvent

• eventType - 'unlinkDir'

• entryType - 'directory'

Event object emitted when a directory is deleted.

Interface: DirectoryEntryEvent

• AddDirEvent | UnlinkDirEvent

A union type representing the directory entry events. There are no change events for directories since they are not modified in the same way as files.

Legacy Extensions

As of Harper v4.6, the previous extension API has been marked as legacy. The legacy extension API is still supported, but will likely be removed in a future major release.

There are two key types of Legacy Extensions: Resource Extension and Protocol Extensions. The key difference is a Protocol Extensions can return a Resource Extension.

Furthermore, what defines an extension separately from a component is that it leverages any of the Resource Extension or Protocol Extension APIs.

Resource Extension

A Resource Extension is for processing a certain type of file or directory. For example, the built-in jsResource extension handles executing JavaScript files.

Resource Extensions are comprised of four distinct function exports, handleFile(), handleDirectory(), setupFile(), and setupDirectory(). The handleFile() and handleDirectory() methods are executed on all worker threads, and are executed again during restarts. The setupFile() and setupDirectory() methods are only executed once on the main thread during the initial system start sequence.

Keep in mind that the CLI command harperdb restart or CLI argument restart=true only restarts the worker threads. If a component is deployed using harperdb deploy, the code within the setupFile() and setupDirectory() methods will not be executed until the system is completely shutdown and turned back on.

Other than their execution behavior, the handleFile() and setupFile() methods, and handleDirectory() and setupDirectory() methods have identical function definitions (arguments and return value behavior).

Resource Extension Configuration

Any Resource Extension can be configured with the files and path options. These options control how files and directories are resolved in order to be passed to the extension's handleFile(), setupFile(), handleDirectory(), and setupDirectory() methods.

Harper relies on the fast-glob library for glob pattern matching.

• files - string | string[] | Object - required - A glob pattern string, array of glob pattern strings, or a more expressive glob options object determining the set of files and directories to be resolved for the extension. If specified as an object, the source property is required. By default, Harper matches files and directories; this is configurable using the only option.

  • source - string | string[] - required - The glob pattern string or array of strings.

  • only - 'all' | 'files' | 'directories' - optional - The glob pattern will match only the specified entry type. Defaults to 'all'.

  • ignore - string[] - optional - An array of glob patterns to exclude from matches. This is an alternative way to use negative patterns. Defaults to [].

• urlPath - string - optional - A base URL path to prepend to the resolved files entries.

  • If the value starts with ./, such as './static/', the component name will be included in the base url path

  • If the value is ., then the component name will be the base url path

    • Note: .. is an invalid pattern and will result in an error

  • Otherwise, the value here will be base url path. Leading and trailing / characters will be handled automatically (/static/, /static, and static/ are all equivalent to static)

For example, to configure the static component to serve all HTML files from the web source directory on the static URL endpoint:

static:
  files: 'web/*.html'
  urlPath: 'static'

If there are files such as web/index.html and web/blog.html, they would be available at localhost/static/index.html and localhost/static/blog.html respectively.

Furthermore, if the component is located in the test-component directory, and the urlPath was set to './static/' instead, then the files would be served from localhost/test-component/static/* instead.

The urlPath is optional, for example to configure the graphqlSchema component to load all schemas within the src/schema directory, only specifying a files glob pattern is required:

graphqlSchema:
  files: 'src/schema/*.schema'

The files option also supports a more complex options object. These additional fields enable finer control of the glob pattern matching.

For example, to match files within web, and omit any within the web/images directory, the configuration could be:

static:
  files:
    source: 'web/**/*'
    ignore: ['web/images']

In order to match only files:

test-component:
  files:
    source: 'dir/**/*'
    only: 'files'

Resource Extension API

In order for an extension to be classified as a Resource Extension it must implement at least one of the handleFile(), handleDirectory(), setupFile(), or setupDirectory() methods. As a standalone extension, these methods should be named and exported directly. For example:

// ESM
export function handleFile() {}
export function setupDirectory() {}

// or CJS
function handleDirectory() {}
function setupFile() {}

module.exports = { handleDirectory, setupFile }

When returned by a Protocol Extension, these methods should be defined on the object instead:

export function start() {
  return {
    handleFile () {}
  }
}

handleFile(contents, urlPath, absolutePath, resources): void | Promise<void>

setupFile(contents, urlPath, absolutePath, resources): void | Promise<void>

These methods are for processing individual files. They can be async.

Remember!

setupFile() is executed once on the main thread during the main start sequence.

handleFile() is executed on worker threads and is executed again during restarts.

Parameters:

• contents - Buffer - The contents of the file

• urlPath - string - The recommended URL path of the file

• absolutePath - string - The absolute path of the file

• resources - Object - A collection of the currently loaded resources

Returns: void | Promise<void>

handleDirectory(urlPath, absolutePath, resources): boolean | void | Promise<boolean | void>

setupDirectory(urlPath, absolutePath, resources): boolean | void | Promise<boolean | void>

These methods are for processing directories. They can be async.

If the function returns or resolves a truthy value, then the component loading sequence will end and no other entries within the directory will be processed.

Remember!

setupFile() is executed once on the main thread during the main start sequence.

handleFile() is executed on worker threads and is executed again during restarts.

Parameters:

• urlPath - string - The recommended URL path of the directory

• absolutePath - string - The absolute path of the directory

• resources - Object - A collection of the currently loaded resources

Returns: boolean | void | Promise<boolean | void>

Protocol Extension

A Protocol Extension is a more advanced form of a Resource Extension and is mainly used for implementing higher level protocols. For example, the Harper Next.js Extension handles building and running a Next.js project. A Protocol Extension is particularly useful for adding custom networking handlers (see the server global API documentation for more information).

Protocol Extension Configuration

In addition to the files and urlPath Resource Extension configuration options, and the package Custom Component configuration option, Protocol Extensions can also specify additional configuration options. Any options added to the extension configuration (in config.yaml), will be passed through to the options object of the start() and startOnMainThread() methods.

For example, the Harper Next.js Extension specifies multiple option that can be included in its configuration. For example, a Next.js app using @harperdb/nextjs may specify the following config.yaml:

'@harperdb/nextjs':
  package: '@harperdb/nextjs'
  files: './'
  prebuilt: true
  dev: false

Many protocol extensions will use the port and securePort options for configuring networking handlers. Many of the server global APIs accept port and securePort options, so components replicated this for simpler pass-through.

Protocol Extension API

A Protocol Extension is made up of two distinct methods, start() and startOnMainThread(). Similar to a Resource Extension, the start() method is executed on all worker threads, and executed again on restarts. The startOnMainThread() method is only executed once during the initial system start sequence. These methods have identical options object parameter, and can both return a Resource Extension (i.e. an object containing one or more of the methods listed above).

start(options): ResourceExtension | Promise<ResourceExtension>

startOnMainThread(options): ResourceExtension | Promise<ResourceExtension>

Parameters:

• options - Object - An object representation of the extension's configuration options.

Returns: Object - An object that implements any of the Resource Extension APIs