Ben Kraft
95 lines
4.9 KiB
Markdown
95 lines
4.9 KiB
Markdown
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# genqlient: a truly type-safe Go GraphQL client
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This is a proof-of-concept of using code-generation to create a truly type-safe GraphQL client in Go. It is certainly not ready for production use!
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## Why another GraphQL client?
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To understand the issue, consider the example from the documentation of [shurcooL/graphql](https://github.com/shurcooL/graphql/):
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```go
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// to send a query `{ me { name } }`:
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var query struct {
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Me struct {
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Name graphql.String
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}
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}
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// error handling omitted for brevity
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client.Query(context.Background(), &query, nil)
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fmt.Println(query.Me.Name)
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// Output: Luke Skywalker
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```
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While your code is in principle type-safe, there's nothing to check that the schema looks like you expect it to. In fact, perhaps here we're querying the GitHub API, in which the field is called `viewer`, not `me`, so this query will fail. Even more common than misusing the name of the field is mis-capitalizing it: the GraphQL convention is `myUrl` whereas the Go convention is `MyURL`, and it's easy to forget or mistype the struct tag. (Even if you get it right, it adds up to a lot of handwritten boilerplate!) Other clients, such as [machinebox/graphql](https://github.com/machinebox/graphql), have even fewer guardrails to help you make the right query and use the result correctly. This isn't a big deal in a small application, but for serious production-grade tools it's not ideal.
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These problems should be entirely avoidable: GraphQL and Go are both typed languages; and GraphQL servers expose their schema in a standard, machine-readable format. We should be able to simply write a query `{ viewer { name } }`, have that automatically validated against the schema and turned into a Go struct which we can use in our code. In fact, there's already good prior art to do this sort of thing: [99designs/gqlgen](https://github.com/99designs/gqlgen) is a popular server library that does exactly this: it generates type-safe GraphQL resolvers from a schema.
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This is a proof-of-concept of a GraphQL client that does the same sort of thing: you specify the query, and it generates type-safe helpers that make your query.
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## Usage
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```graphql
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# queries.graphql
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query getViewer {
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viewer {
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MyName: name
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}
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}
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```
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```go
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// generated.go (auto-generated):
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type getViewerResponse struct { ... }
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func getViewer(ctx context.Context, client *graphql.Client) (*getViewerResponse, error) { ... }
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// your code (error handling omitted for brevity)
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graphqlClient := graphql.NewClient("https://example.com/graphql", http.DefaultClient)
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viewerResp, _ := getViewer(context.Background(), graphqlClient)
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fmt.Println("you are", viewerResp.Viewer.MyName)
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//go:generate go run github.com/Khan/genqlient
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```
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For a complete working example, see `example/`.
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## Tests
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`go test ./...` tests code generation. (This is run by GitHub Actions.)
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Most of the tests are snapshot-based; they use the schema, queries, and snapshots in `generate/testdata`. The schema is in `schema.graphql`; the queries are in `TestName.graphql`. The test by default asserts that the output of the generator matches the snapshot `TestName.graphql.go`. To update the snapshots, run with `UPDATE_SNAPSHOTS=1`; it will fail the tests and print the diffs, but regenerate the snapshots. Make sure to check that the output is sensible!
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`make example` rebuilds the example, and tests that everything wires up to a real API correctly.
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TODO(benkraft): Figure out how to get GitHub Actions to run the example -- it needs a token.
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## Design
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See [DESIGN.md](DESIGN.md) for documentation of major design decisions in this library.
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## Major TODOs
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(*) denotes things we need to use this in prod at Khan
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(+) denotes things we further need before recommending anyone else use this in prod
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Generated code:
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- add flag(s) to make a field use a pointer (for optionality or perf; see DESIGN)
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- redo support for interfaces, unions, fragments (see DESIGN)
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- (optional) collapsing -- should be able to have `mutation { myMutation { error { code } } }` just return `(code string, err error)`
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Config options:
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- (+) proper config/arguments setup (e.g. with [viper](https://github.com/spf13/viper))
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- (+) improve client_getter to be more usable (and document it), or flag it out for now.
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- get schema via HTTP (perhaps even via GraphQL introspection)
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- send hash rather than full query
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- whether names should be exported
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- default handling for optional fields (pointers, HasFoo, etc.)
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- response/function-name format (e.g. force exported/unexported, change "Response" suffix, change how input objects work, etc.)
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- generate mocks?
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Other:
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- (+) improved validation and error checking
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- (+) more tests
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- (+) documentation
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- get a designer to fix my bad logo-thing
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- custom scalar types (or custom mappings for standard scalars, if you want a special ID type say)
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- allow mapping a custom type to a particular val (if you want to use a named type for some string, say)
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