Handling cross-cutting, generic functionality

Middleware is a way to write reusable code that runs before or after (or both) the handling of API requests, often across several (or all) API endpoints.

It's commonly used to implement cross-cutting concerns like request logging, authentication, tracing, and so on. One of the benefits of Encore is that all of these use cases are already handled out-of-the-box, so there's no need to use middleware for those things.

Nonetheless, there are several use cases where it can be useful to write reusable functionality that applies to multiple API endpoints, and middleware is a good solution in those cases.

Encore provides built-in support for middleware by defining a function with the //encore:middleware directive. The middleware directive takes a target parameter that specifies which API endpoints it applies to.

Middleware functions

A typical middleware implementation looks like this:

import ( "encore.dev/beta/errs" "encore.dev/middleware" ) //encore:middleware global target=all func ValidationMiddleware(req middleware.Request, next middleware.Next) middleware.Response { // If the payload has a Validate method, use it to validate the request. payload := req.Data().Payload if validator, ok := payload.(interface { Validate() error }); ok { if err := validator.Validate(); err != nil { // If the validation fails, return an InvalidArgument error. err = errs.WrapCode(err, errs.InvalidArgument, "validation failed") return middleware.Response{Err: err} } } return next(req) }

Middleware forms a chain, allowing each middleware to introspect and process the incoming request before handing it off to the next middleware by calling the next function that's passed in as an argument. For the last middleware in the chain, calling next results in the actual API handler being called.

The req parameter provides information about the incoming request (see package docs).

The next function returns a middleware.Response object which contains the response from the API, describing whether there was an error, and on success the actual response payload.

This enables middleware to also introspect and even modify the outgoing response, like this:

//encore:middleware target=tag:cache func CachingMiddleware(req middleware.Request, next middleware.Next) middleware.Response { data := req.Data() // Check if we have the response cached. Use the request path as the cache key. cacheKey := data.Path if cached, err := loadFromCache(cacheKey, data.API.ResponseType); err == nil && cached != nil { return middleware.Response{Payload: cached} } // Otherwise forward the request to the handler return next(req) }

This uses target=tag:cache to have the middleware only apply to APIs that have that tag. More on this below in Targeting APIs.

Take care

Middleware functions can also be defined as methods on a Dependency Injection struct declared with //encore:service. For example:

//encore:service type Service struct{} //encore:middleware target=all func (s *Service) MyMiddleware(req middleware.Request, next middleware.Next) middleware.Response { // ... }

See the Dependency Injection docs for more information.

Middleware ordering

Middleware can either be defined inside a service, in which case it only runs for APIs within that service, or it can be defined as a global middleware, in which case it applies to all services. For global middleware the target directive still applies and enables you to easily match a subset of APIs.

Take care

Global middleware always run before all service-specific middleware, and then run in the order they are defined in the source code based on file name lexicographic ordering.

To avoid surprises it's best to define all middleware in a file called middleware.go in each service, and to create a single top-level package to contain all global middleware.

Targeting APIs

The target directive can either be provided as target=all (meaning it applies to all APIs) or a list of tags, in the form target=tag:foo,tag:bar. Note that these tags are evaluated with OR, meaning the middleware applies to an API if the API has at least one of those tags.

APIs can be defined with tags by adding tag:foo at the end of the //encore:api directive:

//encore:api public method=GET path=/user/:id tag:cache func GetUser(ctx context.Context, id string) (*User, error) { // ... }