Project: Backstage (App Definition and Development / Application Definition & Image Build)
File: url-reader.md

id: url-reader title: URL Reader sidebar_label: URL Reader description: URL Reader is a backend core API responsible for reading files from external locations. Some of the core plugins of Backstage have to read files from an external location. has to read the entity descriptor files to register and track an entity. have to download the template skeleton files before creating a new component. has to download the markdown source files before generating a documentation site. Since, the requirement for reading files is so essential for Backstage plugins, the package provides a dedicated API for reading from such URL based remote locations like GitHub, GitLab, Bitbucket, Google Cloud Storage, etc. This is commonly referred to as "URL Reader". It takes care of making authenticated requests to the remote host so that private files can be read securely. If users have set up, URL Reader even refreshes the token, to avoid reaching the GitHub API rate limit. As a result, plugin authors do not have to worry about any of these problems when trying to read files. When the Backstage backend starts, a new instance of URL Reader is created. You can see this in the index file of your Backstage backend i.e.`packages/backend/src/index.ts`. ```ts // File: packages/backend/src/index.ts import { UrlReaders } from '@backstage/backend-common'; function makeCreateEnv(config: Config) { // .... const reader = UrlReaders.default({ logger: root, config }); // } ``` This instance contains all in the backend-common package including GitHub, GitLab, Bitbucket, Azure, Google GCS. As the need arises, more URL Readers are being written to support different providers. The generic interface of a URL Reader instance looks like this. ```ts export type UrlReader = { / Used to read a single file and return its content. / readUrl(url: string, options?: ReadUrlOptions): Promise<ReadUrlResponse>; / Used to read a file tree and download as a directory. / readTree(url: string, options?: ReadTreeOptions): Promise<ReadTreeResponse>; / Used to search a file in a tree. / search(url: string, options?: SearchOptions): Promise<SearchResponse>; }; ``` The `reader` instance is available in the backend plugin environment and passed on to all the backend plugins. You can see an . When any of the methods on this instance is called with a URL, URL Reader extracts the host for that URL (e.g. `github.com`, `ghe.mycompany.com`, etc.). Using the package, it looks inside the config of the `app-config.yaml` to find out how to work with the host based on the configs provided like authentication token, API base URL, etc. Make sure your plugin-specific backend file at `packages/backend/src/plugins/<PLUGIN>.ts` is forwarding the `reader` instance passed on as the `PluginEnvironment` to the actual plugin's `createRouter` function. See how this is done in and backend

Once the reader instance is available inside the plugin, one of its methods can directly be used with a URL. Some example usages - - Catalog using the `readUrl` method to read the CODEOWNERS file in a repository. - TechDocs using the `readTree` method to download markdown files in order to generate the documentation site. - TechDocs using `NotModifiedError` to maintain cache and speed up and limit the number of requests. - Catalog using the `search` method to find files for a location URL containing a glob pattern. Note that URL Readers which target git-based version control systems may, under the hood, leverage the ability to create tar archives based on a specific git commit-ish. A consequence of this is that files and directories configured with via `.gitattributes` will not be visible to the URL reader when using the `readTree` or `search` methods. Be aware of this limitation and ensure that end-users of your plugin are also aware via, for example, documentation. If the available URL Readers are not sufficient for your use case and you want to add a new URL Reader for any other provider, you are most welcome to contribute one! Feel free to use the as a source of inspiration. The provider for your new URL Reader can also be called an "integration" in Backstage. The `integrations:` section of your Backstage `app-config.yaml` config file is supposed to be the place where a Backstage integrator defines the host URL for the integration, authentication details and other integration related configurations. The `@backstage/integration` package is where most of the integration specific code lives, so that it is shareable across Backstage. Functions like "read the integrations config and process it", "construct headers for authenticated requests to the host" or "convert a plain file URL into its API URL for downloading the file" would live in this package. Create a new class which implements the inside `@backstage/backend-common`. Create and export a static `factory` method which reads the integration config and returns a map of host URLs the new reader should be used for. See the for example. We want to make sure all URL Readers behave in the same way. Hence if possible, all the methods of the `UrlReader` interface should be implemented. However it is okay to start by implementing just one of them and create issues for the remaining. `readUrl` method expects a user-friendly URL, something which can be copied from the browser naturally when a person is browsing the provider in their browser. Valid URL : `https://github.com/backstage/backstage/blob/master/ADOPTERS.md` Not a valid URL : `https://raw.githubusercontent.com/backstage/backstage/master/ADOPTERS.md` Not a valid URL : `https://github.com/backstage/backstage/ADOPTERS.md` Upon receiving the URL, `readUrl` converts the user-friendly URL into an API URL which can be used to request the provider's

`readUrl` then makes an authenticated request to the provider API and returns the response containing the file's contents and ETag(if the provider supports it). `readTree` method also expects user-friendly URLs similar to `read` but the URL should point to a tree (could be the root of a repository or even a sub-directory). Valid URL : `https://github.com/backstage/backstage` Valid URL : `https://github.com/backstage/backstage/blob/master` Valid URL : `https://github.com/backstage/backstage/blob/master/docs` Using the provider's API documentation, find out an API endpoint which can be used to download either a zip or a tarball. You can download the entire tree (e.g. a repository) and filter out in case the user is expecting only a sub-tree. But some APIs are smart enough to accept a path and return only a sub-tree in the downloaded archive. `search` method expects a glob pattern of a URL and returns a list of files matching the query. Valid URL : `https://github.com/backstage/backstage/blob/master//catalog-info.yaml` Valid URL : `https://github.com/backstage/backstage/blob/master//*.md` Valid URL : `https://github.com/backstage/backstage/blob/master/*/package.json` Valid URL : `https://github.com/backstage/backstage/blob/master/READM` The core logic of `readTree` can be used here to extract all the files inside the tree and return the files matching the pattern in the `url`. There are two ways to make your new URL Reader available for use. You can choose to make it open source, by updating the method of URL Readers. But for something internal which you don't want to make open source, you can update your `packages/backend/src/index.ts` file and update how the `reader` instance is created. ```ts // File: packages/backend/src/index.ts const reader = UrlReaders.default({ logger: root, config, // This is where your internal URL Readers would go. factories: [myCustomReader.factory], }); ``` All of the methods above support an ETag based caching. If the method is called without an `etag`, the response contains an ETag of the resource (should ideally forward the ETag returned by the provider). If the method is called with an `etag`, it first compares the ETag and returns a `NotModifiedError` in case the resource has not been modified. This approach is very similar to the actual and HTTP headers. When debugging one of the URL Readers, you can straightforward use the when the backend starts and call one of the methods with your debugging URL. ```ts // File: packages/backend/src/index.ts async function main() { // ... const createEnv = makeCreateEnv(config); const testReader = createEnv('test-url-reader').reader; const response = await testReader.readUrl( 'https://github.com/backstage/backstage/blob/master/catalog-info.yaml', ); console.log((await response.buffer()).toString()); // ... } ``` This will be run every time you restart the backend. Note that after any change in the URL Reader code, you need to stop the backend and restart, since the `reader` instance is memoized and does not update on hot module reloading. Also, there are a lot of unit tests written for the URL Readers, which you can make use of.