Project: Open Policy Agent (OPA) (Provisioning / Security & Compliance)
File: extensions.md

title: Extending OPA kind: documentation weight: 70 OPA can be extended with custom built-in functions and plugins that implement functionality like support for new protocols. This page explains how to customize and extend OPA in different ways. Read this section if you want to extend OPA with custom built-in functions. {{< info >}} This section assumes you are embedding OPA as a library and executing policies via the `github.com/open-policy-agent/opa/rego` package. If you are NOT embedding OPA as a library and instead want to customize the OPA runtime, read this section anyway because it provides useful information on implementing built-in functions. For a complete example that shows how to add custom built-in functions to the OPA runtime, see the appendix. {{< /info >}} OPA supports built-in functions for simple operations like string manipulation and arithmetic as well as more complex operations like JWT verification and executing HTTP requests. If you need to to extend OPA with custom built-in functions for use cases or integrations that are not supported out-of-the-box you can supply the function definitions when you prepare queries. Using custom built-in functions involves providing a declaration and implementation. The declaration tells OPA the function's type signature and the implementation provides the callback that OPA can execute during query evaluation. To get started you need to import three packages: ``` import "github.com/open-policy-agent/opa/ast" import "github.com/open-policy-agent/opa/types" import "github.com/open-policy-agent/opa/rego" ``` The `ast` and `types` packages contain the types for declarations and runtime objects passed to your implementation. Here is a trivial example that shows the process: ```golang r := rego.New( rego.Query(`x = hello("bob")`), rego.Function1( &rego.Function{ Name: "hello", Decl: types.NewFunction(types.Args(types.S), types.S), }, func(_ rego.BuiltinContext, a ast.Term) (ast.Term, error) { if str, ok := a.Value.(ast.String); ok { return ast.StringTerm("hello, " + string(str)), nil } return nil, nil }), ) query, err := r.PrepareForEval(ctx) if err != nil { // handle error. } ``` At this point you can execute the `query`: ```golang rs, err := query.Eval(ctx) if err != nil { // handle error. } // Do something with result. fmt.Println(rs[0].Bindings["x"]) ``` If you executed this code you the output would be: ```live:trivial:query:read_only "hello, bob" ``` The example above highlights a few important points. The `rego` package includes variants of `rego.Function1` for accepting different numbers of operands (e.g., `rego.Function2`, `rego.Function3`, etc.) The `rego.Function#Name` struct field specifies the operator that queries can refer to. The `rego.Function#Decl` struct field specifies the function's type signature. In the example above the function accepts a string and returns a string. The function indicates it's undefined by returning `nil` for the first return argument. Let's look at another example. Imagine you want to expose GitHub repository metadata to your policies. One option is to implement a custom built-in function to fetch the data for specific repositories on-the-fly. ```golang r := rego.New( rego.Query(`github.repo("open-policy-agent", "opa")`), rego.Function2( &rego.Function{ Name: "github.repo", Decl: types.NewFunction(types.Args(types.S, types.S), types.A), Memoize: true, Nondeterministic: true, }, func(bctx rego.BuiltinContext, a, b ast.Term) (ast.Term, error) { // see implementation below. }, ), ) ``` Built-in function names can include `.` characters. Consider namespacing your built-in functions to avoid collisions. This declaration indicates the function accepts two strings and returns a value of type `any`. The `any` type is the union of all types in Rego. {{< info >}} `types.S` and `types.A` are shortcuts for constructing Rego types. If you need to define use-case specific types (e.g., a list of objects that have fields `foo`, `bar`, and `baz`, you will need to construct them using the `types` packages APIs.) {{< /info >}} The declaration also

`rego.Function#Memoize` to true to enable memoization across multiple calls in the same query. If your built-in function performs I/O, you should enable memoization as it ensures function evaluation is deterministic. Since this built-in could have non-deterministic results, depending on network conditions, the declaration also sets `rego.Function#Nondeterministic` to true. This provides basic safety information to the runtime, so that the function isn't accidentally run during bundle builds or partial evaluation. If your builtin can have non-deterministic results, you should mark it appropriately to avoid surprises. The implementation wraps the Go standard library to perform HTTP requests to GitHub's API: ```golang func(bctx rego.BuiltinContext, a, b ast.Term) (ast.Term, error) { var org, repo string if err := ast.As(a.Value, &org); err != nil { return nil, err } else if err := ast.As(b.Value, &repo); err != nil { return nil, err } req, err := http.NewRequest("GET", fmt.Sprintf("https://api.github.com/repos/%v/%v", org, repo), nil) if err != nil { return nil, err } resp, err := http.DefaultClient.Do(req.WithContext(bctx.Context)) if err != nil { return nil, err } defer resp.Body.Close() if resp.StatusCode != http.StatusOK { return nil, fmt.Errorf(resp.Status) } v, err := ast.ValueFromReader(resp.Body) if err != nil { return nil, err } return ast.NewTerm(v), nil } ``` The implementation is careful to use the context passed to the built-in function when executing the HTTP request. See the appendix at the end of this page for the complete example. {{< danger >}} Custom built-in functions must not be used for effecting changes in external systems as OPA does not guarantee that the statement will be executed due to automatic performance optimizations that are applied during policy evaluation. {{< /danger >}} Read this section if you want to customize or extend the OPA runtime/executable with custom behaviour. OPA defines a plugin interface that allows you to customize certain behaviour like decision logging or add new behaviour like different query APIs. To implement a custom plugin you must implement two interfaces: to instantiate your plugin. to provide your plugin behavior. You can register your factory with OPA by calling inside your main function. The plugin may (optionally) report its current status to the plugin Manager via the `plugins.Manager#UpdatePluginStatus` API. {{< info >}} If no status is provided the plugin is assumed to be working OK. {{< /info >}} Typically the plugin should report `StatusNotReady` at creation time and update to `StatusOK` (or `StatusErr`) when appropriate. The example below shows how you can implement a custom that writes events to a stream (e.g., stdout/stderr). ```golang import ( "encoding/json" "github.com/open-policy-agent/opa/plugins/logs" ) const PluginName = "printlndecisionlogger" type Config struct { Stderr bool `json:"stderr"` // false => stdout, true => stderr } type PrintlnLogger struct { manager *plugins.Manager mtx sync.Mutex config Config } func (p *PrintlnLogger) Start(ctx context.Context) error { p.manager.UpdatePluginStatus(PluginName, &plugins.Status{State: plugins.StateOK}) return nil } func (p *PrintlnLogger) Stop(ctx context.Context) { p.manager.UpdatePluginStatus(PluginName, &plugins.Status{State: plugins.StateNotReady}) } func (p *PrintlnLogger) Reconfigure(ctx context.Context, config interface{}) { p.mtx.Lock() defer p.mtx.Unlock() p.config = config.(Config) } // Log is called by the decision logger when a record (event) should be emitted. The logs.EventV1 fields // map 1:1 to those described in https://www.openpolicyagent.org/docs/latest/management-decision-logs func (p *PrintlnLogger) Log(ctx context.Context, event logs.EventV1) error { p.mtx.Lock() defer p.mtx.Unlock() w := os.Stdout if p.config.Stderr { w = os.Stderr } bs, err := json.Marshal(event) if err != nil { p.manager.UpdatePluginStatus(PluginName, &plugins.Status{State: plugins.StateErr}) return nil } _, err = fmt.Fprintln(w, string(bs)) if err != nil { p.manager.UpdatePluginStatus(PluginName, &plugins.Status{State: plugins.StateErr}) } return nil } ``` Next, implement a factory function that instantiates your plugin: ```golang import ( "github.com/open-policy-agent/opa/plugins" "github.com/open-policy-agent/opa/util" ) type Factory struct{} func (Factory) New(m *plugins.Manager, config interface{}) plugins.Plugin { m.UpdatePluginStatus(PluginName, &plugins.Status{State: plugins.StateNotReady}) return &PrintlnLogger{ manager: m, config:

} } func (Factory) Validate(_ *plugins.Manager, config []byte) (interface{}, error) { parsedConfig := Config{} return parsedConfig, util.Unmarshal(config, &parsedConfig) } ``` Finally, register your factory with OPA and call `cmd.RootCommand.Execute`. The latter starts OPA and does not return. ```golang import ( "github.com/open-policy-agent/opa/cmd" "github.com/open-policy-agent/opa/runtime" ) func main() { runtime.RegisterPlugin(PluginName, Factory{}) if err := cmd.RootCommand.Execute(); err != nil { fmt.Println(err) os.Exit(1) } } ``` At this point you can build an OPA executable including your plugin. ``` go build -o opa++ ``` Define an OPA configuration file that will use your plugin: config.yaml: ```yaml decision_logs: plugin: printlndecisionlogger plugins: printlndecisionlogger: stderr: false ``` Start OPA with the configuration file: ```bash ./opa++ run --server --config-file config.yaml ``` Exercise the plugin via the OPA API: ``` curl localhost:8181/v1/data ``` If everything worked you will see the Go struct representation of the decision log event written to stdout. The source code for this example can be found . {{< info >}} If there is a mask policy set (see for details) the `Event` received by the demo plugin will potentially be different than the example documented. {{< /info >}} The OPA runtime version is set statically at build-time. The following global variables are exported by the `github.com/open-policy-agent/opa/version` package and can be set at build-time: | Name | Description | | | | | `Version` | Human-readable/semantic version of the OPA runtime. | | `Vcs` | Git SHA that the OPA runtime was built from. | | `Timestamp` | Date/time when the OPA runtime was built. | | `Hostname` | Hostname of the system where the OPA runtime was built. | These values can be set on the command-line when building OPA from source: ``` go build -o opa++ -ldflags "-X github.com/open-policy-agent/opa/version.Version=MY_VERSION" main.go ``` ```golang package main import ( "context" "encoding/json" "fmt" "log" "net/http" "github.com/open-policy-agent/opa/ast" "github.com/open-policy-agent/opa/rego" "github.com/open-policy-agent/opa/types" ) func main() { r := rego.New( rego.Query(`github.repo("open-policy-agent", "opa")`), rego.Function2( &rego.Function{ Name: "github.repo", Decl: types.NewFunction(types.Args(types.S, types.S), types.A), Memoize: true, Nondeterministic: true, }, func(bctx rego.BuiltinContext, a, b ast.Term) (ast.Term, error) { var org, repo string if err := ast.As(a.Value, &org); err != nil { return nil, err } else if err := ast.As(b.Value, &repo); err != nil { return nil, err } req, err := http.NewRequest("GET", fmt.Sprintf("https://api.github.com/repos/%v/%v", org, repo), nil) if err != nil { return nil, err } resp, err := http.DefaultClient.Do(req.WithContext(bctx.Context)) if err != nil { return nil, err } defer resp.Body.Close() if resp.StatusCode != http.StatusOK { return nil, fmt.Errorf(resp.Status) } v, err := ast.ValueFromReader(resp.Body) if err != nil { return nil, err } return ast.NewTerm(v), nil }, ), ) rs, err := r.Eval(context.Background()) if err != nil { log.Fatal(err) } else if len(rs) == 0 { fmt.Println("undefined") } else { bs, _ := json.MarshalIndent(rs[0].Expressions[0].Value, "", " ") fmt.Println(string(bs)) } } ``` ```golang package main import ( "fmt" "net/http" "os" "github.com/open-policy-agent/opa/ast" "github.com/open-policy-agent/opa/cmd" "github.com/open-policy-agent/opa/rego" "github.com/open-policy-agent/opa/types" ) func main() { rego.RegisterBuiltin2( &rego.Function{ Name: "github.repo", Decl: types.NewFunction(types.Args(types.S, types.S), types.A), Memoize: true, Nondeterministic: true, }, func(bctx rego.BuiltinContext, a, b ast.Term) (ast.Term, error) { var org, repo string if err := ast.As(a.Value, &org); err != nil { return nil, err } else if err := ast.As(b.Value, &repo); err != nil { return nil, err } req, err := http.NewRequest("GET", fmt.Sprintf("https://api.github.com/repos/%v/%v", org, repo), nil) if err != nil { return nil, err } resp, err := http.DefaultClient.Do(req.WithContext(bctx.Context)) if err != nil { return nil, err } defer resp.Body.Close() if resp.StatusCode != http.StatusOK { return nil, fmt.Errorf(resp.Status) } v, err := ast.ValueFromReader(resp.Body) if err != nil { return nil, err } return ast.NewTerm(v), nil }, ) if err := cmd.RootCommand.Execute(); err != nil { fmt.Println(err) os.Exit(1) } } ```