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asyncapi-codegen

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An AsyncAPI Golang Code generator that generates all Go code from the broker to the application/user. Just plug your application to your favorite message broker!

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# AsyncAPI Codegen

An AsyncAPI Golang Code generator that generates all Go code from the broker
to the application/user. Just plug your application to your favorite message broker!

![AsyncAPI Codegen Preview](https://raw.githubusercontent.com/lerenn/asyncapi-codegen/main/assets/codegen-preview.svg)

**⚠️ We do our best to progressively satisfy the entire AsyncAPI specification,**
**but some features may still be missing: please raise an issue on any bug or**
**missing feature.**

**❤️ Support is greatly appreciated and contributions are welcomed!**

*Inspired from popular [deepmap/oapi-codegen](https://github.com/deepmap/oapi-codegen)*

## Contents

* [Supported functionalities](#supported-functionalities)
* [Usage](#usage)
* [Concepts](#concepts)
* [Examples](#examples)
* [Supported Brokers](#supported-brokers)
  * [Kafka](#kafka)
  * [NATS](#nats) / [NATS JetStream](#nats-jetstream)
  * [Custom broker](#custom-broker)
* [CLI options](#cli-options)
* [Advanced topics](#advanced-topics)
  * [Middlewares](#middlewares)
  * [Context](#context)
  * [Logging](#logging)
  * [Versioning](#versioning)
  * [Extensions](#specification-extensions)
  * [ErrorHandler](#errorhandler)
  * [Validations](#validations)
* [Contributing and support](#contributing-and-support)

## Supported functionalities

* AsyncAPI versions:
  * 2.6.0
  * 3.0.0
* Brokers:
  * Kafka
  * NATS / NATS JetStream
  * Custom
* Formats:
  * JSON
* Logging:
  * Elastic Common Schema (JSON)
  * Text (Humand readable)
  * Custom
* Others:
  * Versioning support

## Usage

In order to use this library in your code, please execute the following lines:

```shell
# Install the tool
go install github.com/lerenn/asyncapi-codegen/cmd/asyncapi-codegen@latest

# Generate the code from the asyncapi file
asyncapi-codegen -i ./asyncapi.yaml -p <your-package> -o ./asyncapi.gen.go

# Install dependencies needed by the generated code
go get -u github.com/lerenn/asyncapi-codegen/pkg/extensions
```

You can also specify the generation part by adding a `go generate` instruction
at the beginning of your file:

```golang
//go:generate go run github.com/lerenn/asyncapi-codegen/cmd/asyncapi-codegen@<version> -i ./asyncapi.yaml -p <your-package> -o ./asyncapi.gen.go
```

### Docker image

You can also use the dockerized version of this tool:

```bash
docker run -v .:/code -w /code lerenn/asyncapi-codegen asyncapi-codegen -i ./asyncapi.yaml -p <your-package> -o ./asyncapi.gen.go
```

## Concepts

![basic schema](https://raw.githubusercontent.com/lerenn/asyncapi-codegen/main/assets/basic-schema.svg)

Let's imagine a message broker centric architecture: you have the application
that you are developing on the right and the potential user(s) on the left.

Being a two directional communication, both of them can communicate to each
other through the broker. They can even communicate with themselves, in case
of multiple users or application replication.

For more information about this, please refere to the [official AsyncAPI
concepts](https://www.asyncapi.com/docs/concepts).

### With Async API generated code

![with codegen schema](https://raw.githubusercontent.com/lerenn/asyncapi-codegen/main/assets/with-codegen-schema.svg)

* Yellow parts: when using the codegen tool,
you will generate the code that will act as an adapter (called **controller**)
between the user, the broker, and the application.
* Red parts: you will need to fill these parts
between user, broker and application. These will allow message production and
reception with the generated code.
* Orange parts: these parts will be available
in this repository if you use an already supported broker. However, you can also
use the implement it yourself if the broker is not supported yet.

## Examples

Here is a list of example, from basic to advanced ones.

**Please note that the examples are separated in different subdirectories per broker.**

It is strongly advised to read them in present order, following your AsyncAPI version.

* **HelloWorld**:
  * [AsyncAPI v2](./examples/helloworld/v2/README.md)
  * [AsyncAPI v3](./examples/helloworld/v3/README.md)
* **Ping**:
  * [AsyncAPI v2](./examples/ping/v2/README.md)
  * [AsyncAPI v3](./examples/ping/v3/README.md)

## Supported Brokers

In order to connect your broker to the autogenerated code, you will need to
create a controller that will be used to publish and subscribe to messages.

You can use one of the already supported brokers or implement your own.

### Kafka

In order to use Kafka as a broker, you can use the following code:

```golang
broker, _ := kafka.NewController([]string{"<host>:<port>", /* additional hosts */}, /* options */)
```

Here are the options that you can use with the Kafka controller:

* `WithGroupdID`: specify the group ID that will be used by the controller. If not specified, default queue name (`asyncapi`) will be used.
* `WithPartition`: specify the partition that will be used by the controller. If not specified, default partition (`0`) will be used.
* `WithMaxBytes`: specify the maximum size of a message that will be received. If not specified, default value (`10e6`, meaning `10MB`) will be used.
* `WithLogger`: specify the logger that will be used by the controller. If not specified, a silent logger is used that won't log anything.
* `WithAutoCommit`: specify if the broker should use auto-commit for incoming messages or manual commits. Note that commits are managed by the broker implementation regardless, with manual commits they are executed after the message is complete processed. Subscribers retain the option to manually handle errors via the ErrorHandler, to use mechanisms such as dead letter or retry topics. The default value is `true`
* `WithSasl`: specify sasl mechanism to connect to the broker. Per default no mechanism will be used.
* `WithTLS`: specify tls config to connect to the broker. Per default no tls config will be used.
* `WithConnectionTest`: specify if the controller should make a connection test on creation. The default value is `true`

#### Authentication and TLS

To use a TLS connection and or authentication for the connection to the kafka broker the following options can be used:

```golang
// Plain mechanism
kafkaController, err := kafka.NewController([]string{"<host>:<port>"},
 kafka.WithGroupID(queueGroupID),
 kafka.WithSasl(plain.Mechanism{Username: "<user>", Password: "<password>"}),
)

// Sha256 mechanism
sha256Mechanism, err := scram.Mechanism(scram.SHA256, "<user>", "<password>")
if err != nil {
 // handle error
}

kafkaController, err := kafka.NewController([]string{"<host>:<port>"},
 kafka.WithGroupID(queueGroupID),
 kafka.WithSasl(sha256Mechanism),
)

// Sha512 mechanism
sha512Mechanism, err := scram.Mechanism(scram.SHA512, "<user>", "<password>")
if err != nil {
 // handle error
}

kafkaController, err := kafka.NewController([]string{"<host>:<port>"},
 kafka.WithGroupID(queueGroupID),
 kafka.WithSasl(sha512Mechanism),
)

// TLS
// configure tls.config
myTLSConfig := &tls.Config{}

kafkaController, err := kafka.NewController([]string{"<host>:<port>"},
 kafka.WithGroupID(queueGroupID),
 kafka.WithTLS(myTLSConfig),
)
```

### NATS

In order to use NATS as a broker, you can use the following code:

```golang
// Create the NATS controller
broker, _ := nats.NewController("nats://<host>:<port>")
defer broker.Close()

// Add NATS controller to a new App controller
ctrl, err := NewAppController(broker, /* options */)

//...
```

Here are the options that you can use with the NATS controller:

* `WithLogger`: specify the logger that will be used by the controller. If not specified, a silent logger is used that won't log anything.
* `WithQueueGroup`: specify the queue group that will be used by the controller. If not specified, default queue name (`asyncapi`) will be used.
* `WithConnectionOpts`: specify connection Options for establishing connection with nats see [Nats Options](https://pkg.go.dev/github.com/nats-io/go-nats#Option) for more information. If not specified, no options will be used.

#### Authentication and TLS

To use a TLS connection and or authentication for the connection to the nats broker the following nats options can be used:

```golang
import (
"github.com/lerenn/asyncapi-codegen/pkg/extensions/brokers/nats"

// import natsio go client option
natsio "github.com/nats-io/nats.go"

)

func main(){

myTLSConfig := &tls.Config{}

natsController, err := nats.NewController("nats://<host>:<port>",
		nats.WithQueueGroup(queueGroupID),
		nats.WithConnectionOpts(natsio.UserCredentials("<user.jwt>", "<user.nk>"), natsio.Secure(myTLSConfig))
	)

}
```

### NATS JetStream

In order to use NATS JetStream as a broker, you can use the following code:

```golang
// Create the NATS controller
broker, _ := natsjetstream.NewController("nats://<host>:<port>", /* options */)
defer broker.Close()

// Add NATS controller to a new App controller
ctrl, err := NewAppController(broker)

//...
```

It is important to either create/update a stream with `WithStreamConfig` or to use `WithStream` to specify the stream that will be used by the broker.
Consumer for the user controller can be either created/updated with `WithConsumerConfig` or `WithConsumer`.

#### Limitations

* the messages will be ack'd from the consumer even though the subscription was not setup (this will be logged)

### Custom broker

In order to connect your application and your user to your broker, we need to
provide a controller to it. Here is the interface that you need to satisfy:

```go
import(
  "github.com/lerenn/asyncapi-codegen/pkg/extensions"
)

type BrokerController interface {
  // Publish a message to the broker
  Publish(ctx context.Context, channel string, mw extensions.BrokerMessage) error

// Subscribe to messages from the broker
  Subscribe(ctx context.Context, channel string) (msgs chan extensions.BrokerMessage, stop chan any, err error)
}
```

You can find that there is an `extensions.BrokerMessage` structure that is provided and
that aims to abstract the event broker technology.

By writing your own by satisfying this interface, you will be able to connect
your broker to the generated code.

## CLI options

### Generation parts (`-g, --generate`)

The default options for asyncapi-codegen will generate everything; user, application,
and type definitions but you can generate subsets of those via the -generate
flag. It defaults to user,application,types
but you can specify any combination of those.

Here are the universal parts that you can generate:

* `application`: generate the application boilerplate. `application` requires
  the types in the same package to compile.
* `user`: generate the user boilerplate. It, too, requires the types to be
  present in its package.
* `types`: all type definitions for all types in the AsyncAPI spec.
  This will be everything under `#components`, as well as request parameter,
  request body, and response type objects.

### Package name (`-p, --package`)

The package name is the name of the package that will be used in the generated
code. It is important to have the same package name for the user, application,
and types in order to compile the code.

### Input files (`-i, --input`)

The input file is the path to the AsyncAPI specification file that will be used
to generate the code. It can be either a JSON or a YAML file.

Also, you can specify dependencies by separating them with a comma:

```shell
asyncapi-codegen -i ./asyncapi.yaml,./dependency1.yaml,./dependency2.yaml -p <your-package> -o ./asyncapi.gen.go
```

A dependency can also be an **OpenAPI** document: only its `components` are kept,
so its schemas can be referenced (e.g. `$ref: './openapi.yaml#/components/schemas/Pet'`)
even when the document contains fields AsyncAPI does not allow, such as an
array-typed `servers`.

### Output file (`-o, --output`)

The output file is the path to the file that will be generated by the tool. It
will contain the generated code.

### Disable formatting (`-f, --disable-formatting`)

By default, the generated code will be formatted using `gofmt`. If you want to
disable this feature, you can use the `-f` flag.

### JSON keys conversion (`-c, --convert-keys`)

By default, the generation will use the key specified in the asyncapi codegen.
This is also called the `none` convertion. You can also convert your keys to
`snake`, `camel`, or `kebab`.

#### Example

Given this schema:

```yaml
Payload:
  type: object
  properties:
    This_is a-Property:
      type: string
```

Here are the generated JSON sent, given by the different options:
* No conversion (`none`): `{ "This_is a-Property": "value" }`
* Camel case (`camel`): `{ "thisIsAProperty": "value" }`
* Pascal case (`pascal`): `{ "ThisIsAProperty": "value" }`
* Kebab case (`kebab`): `{ "this-is-a-property": "value" }`
* Snake case (`snake`): `{ "this_is_a_property": "value" }`

## Advanced topics

### Middlewares

You can use middlewares that will be executing when receiving and publishing
messages. You can add one or multiple middlewares using the  `WithMiddlewares`
function in the initialization of the App or User controller:

```golang
// Create a new app controller with middlewares
ctrl, _ := NewAppController(/* Broker of your choice */, WithMiddlewares(myMiddleware1, myMiddleware2 /*, ... */))
```

Here the function signature that should be satisfied:

```golang
func(ctx context.Context, msg *extensions.BrokerMessage, next extensions.NextMiddleware) error
```

**Note:** the returned context will be the one that will be passed to following
middlewares, and finally to the generated code (and subscription callback).

#### Examples

##### Filtering messages

```golang
import(
  "github.com/lerenn/asyncapi-codegen/pkg/extensions"
  // ...
)

func myMiddleware(ctx context.Context, _ *extensions.BrokerMessage, _ middleware.Next) error {
  // Execute this middleware only if this is a received message
  extensions.IfContextValueEquals(ctx, extensions.ContextKeyIsDirection, "reception", func() {
    // Do specific stuff if message is received
  })

return nil
}
```

You can even discriminate on more specification. Please see the [Context section](#context).

##### Modify messages before sending/receiving

```golang
import(
  "github.com/lerenn/asyncapi-codegen/pkg/extensions"
  // ...
)

func myMiddleware(_ context.Context, msg *extensions.BrokerMessage, _ middleware.Next) error {
  msg.Headers["additional"] = "some-info"
  return nil
}
```

##### Stopping message processing

```golang
import(
  "github.com/lerenn/asyncapi-codegen/pkg/extensions"
  // ...
)

func myMiddleware(_ context.Context, msg *extensions.BrokerMessage, _ middleware.Next) error {
  if msg.Headers["author"] != "me" {
    return fmt.Errorf("this is not me, aborting...")
  }
  return nil
}
```

#### Executing code after receiving/publishing the message

By default, middlewares will be executed right before the operation. If there is
a need to execute code before and/or after the operation, you can call the `next`
argument that represents the next middleware that should be executed or the
operation corresponding code if this was the last middleware.

Here is an example:

```golang
import(
  "github.com/lerenn/asyncapi-codegen/pkg/extensions"
  // ...
)

func surroundingMiddleware(ctx context.Context, next extensions.NextMiddleware) error {
  // Pre-operation
  fmt.Println("This will be displayed BEFORE the reception/publication")

// Calling next middleware or reception/publication code
  // The given context will be the one propagated to other middlewares and operation source code
  err := next(ctx)

// Post-operation
  fmt.Println("This will be displayed AFTER the reception/publication")

return err
}
```

### Context

When receiving the context from generated code (either in subscription,
middleware, logging, etc), you can get some information embedded in context.

To get these information, please use the functions from
`github.com/lerenn/asyncapi-codegen/pkg/extensions`:

```golang
// Execute this middleware only if this is from "ping" channel
extensions.IfContextValueEquals(ctx, extensions.ContextKeyIsChannel, "ping", func() {
  // Do specific stuff if the channel is ping
})
```

You can find other keys in the package `pkg/extensions`.

### Logging

You can have 2 types of logging:
* **Controller logging**: logs the internal operations of the controller (subscription, malformed messages, etc);
* **Publication/Reception logging**: logs every publication or reception of messages.

#### Controller logging

To log internal operation of the controller, the only thing you have to do is
to initialize the controller with a logger, with the function `WithLogger()`:

```golang
import(
  "github.com/lerenn/asyncapi-codegen/pkg/extensions/brokers"
  // ...
)

func main() {
  // Create a new app controller with an Elastic Common Schema JSON compatible logger
  ctrl, _ := NewAppController(/* Broker of your choice */, WithLogger(log.NewECS()))

// ...
}
```

You can find all loggers in the directory `pkg/log`.

By default, the provided loggers (`NewText` and `NewECS`) output every message.
You can restrict them to a minimum severity level with the `WithLevel` option:

```golang
import(
  log "github.com/lerenn/asyncapi-codegen/pkg/extensions/loggers"
)

// Only warnings and errors will be printed, info messages are discarded.
logger := log.NewECS(log.WithLevel(log.LevelWarning))
```

Available levels are `LevelInfo` (default, logs everything), `LevelWarning`
(warnings and errors) and `LevelError` (errors only).

#### Publication/Reception logging

To log published and received messages, you'll have to pass a logger as a middleware
in order to execute it on every published and received messages:

```golang
import(
  "github.com/lerenn/asyncapi-codegen/pkg/extensions/brokers"
  // ...
)

func main() {
  // Create a new app controller with a middleware for logging incoming/outgoing messages
  loggingMiddleware := middleware.Logging(log.NewECS())
  ctrl, _ := NewAppController(/* Broker of your choice */, WithMiddlewares(loggingMiddleware))

// ...
}
```

#### Custom logging

It is possible to set your own logger to the generated code, all you have to do
is to fill the following interface:

```golang
type Logger interface {
    // Info logs information based on a message and key-value elements
    Info(ctx log.Context, msg string, info ...log.AdditionalInfo)

// Error logs error based on a message and key-value elements
    Error(ctx log.Context, msg string, info ...log.AdditionalInfo)
}
```

Here is a basic implementation example:

```golang
type SimpleLogger struct{}

func (logger SimpleLogger) formatLog(ctx log.Context, info ...log.AdditionalInfo) string {
 var formattedLogInfo string
 for i := 0; i < len(keyvals)-1; i += 2 {
 formattedLogInfo = fmt.Sprintf("%s, %s: %+v", formattedLogInfo, info.Key, info.Value)
 }
 return fmt.Sprintf("%s, context: %+v", formattedLogInfo, ctx)
}

func (logger SimpleLogger) Info(ctx log.Context, msg string, info ...log.AdditionalInfo) {
  log.Printf("INFO: %s%s", msg, logger.formatLog(ctx, info...))
}

func (logger SimpleLogger) Error(ctx log.Context, msg string, info ...log.AdditionalInfo) {
  log.Printf("ERROR: %s%s", msg, logger.formatLog(ctx, info...))
}
```

You can then create a controller with a logger using similar lines:

```golang
// Create a new app controller with the custom logger
ctrl, _ := NewAppController(
  /* Broker of your choice */,
  WithLogger(SimpleLogger{}),                         /* Use on as internal logger */
  WithMiddleware(middleware.Logging(SimpleLogger{})), /* Use to log incoming/outgoing messages */
)
```

The generated code passes the contextual information (channel, correlation ID,
direction, provider, broker message, version) directly to your logger as
explicit `LogInfo` arguments at every log call. Your logger therefore only has
to log the `msg` and the `info` it is given — there is no need to unwrap any
context key:

```golang
func (logger SimpleLogger) Info(ctx context.Context, msg string, info ...extensions.LogInfo) {
  // 'info' already contains the contextual values (channel, correlationID, …)
  // ... log msg with info
}
```

> **Migration note (from versions where loggers enriched from context):** the
> contextual values are no longer read from the `context.Context` by the
> built-in `Text`/`ECS` loggers — they are now supplied as explicit `LogInfo`
> arguments by the generated code, using the generic keys returned by
> `extensions.LogInfosFromContext` (`channel`, `correlationID`, `brokerMessage`,
> `direction`, `provider`, `version`). This means:
> * The built-in `Text` and `ECS` loggers now emit those generic keys instead of
>   their previous custom names (e.g. ECS no longer maps to `trace.id`,
>   `event.action`, `event.original`).
> * Custom loggers receive everything as arguments and no longer need to call
>   `extensions.LogInfosFromContext` themselves.
>
> The values are still stored in the `context.Context` (it stays useful for
> other purposes), so a custom logger that *wants* to keep reading from context
> can still call `extensions.LogInfosFromContext(ctx)`.

### Versioning

If you are in need to do a migration or support multiple versions of your
AsyncAPI specifications, you can use the `versioning` package:

```golang

import (
  "github.com/lerenn/asyncapi-codegen/pkg/extensions/brokers/nats"
  "github.com/lerenn/asyncapi-codegen/pkg/extensions/versioning"
  v1 "path/to/asyncapi/spec/version/1"
  v2 "path/to/asyncapi/spec/version/2"
)

func main() {
  // Create a broker (here from NATS)
  broker, _ := nats.NewController("nats://nats:4222"))
  defer broker.Close()

// Add a version wrapper to the broker
  vw := versioning.NewWrapper(broker)

// Create application for version 1
  appV1, _ := v1.NewAppController(vw, /* controller options */)
  defer appV1.Close(context.Background())

// Create v2 app
  appV2, _ := v2.NewAppController(vw, /* controller options */)
  defer appV2.Close(context.Background())

// ...
}
```

Then you can use each application independently:

```golang
err := appV1.SubscribeHello(context.Background(), func(ctx context.Context, msg v1.HelloMessage) {
  // Stuff for version 1
})

err := appV2.SubscribeHello(context.Background(), func(ctx context.Context, msg v2.HelloMessage) {
  // Stuff for version 2
})
```

That way, you can support multiple different versions with the same broker.

#### Version tagging

The versioning feature will add an `application-version` header to each
message in order to have the correct version of the application on each of
them.

##### Non-tagged messages

If messages can have no `application-version`, you can use the option `WithDefaultVersion`
to add a default version to non-tagged messages.

```golang
vw := versioning.NewWrapper(broker, versioning.WithDefaultVersion("1.1.4"))
```

##### Change header key for application version

Also, if you don't want to use this header as a recipient to the application version,
you can specify your own header with the option `WithVersionHeaderKey`.

```golang
vw := versioning.NewWrapper(broker, versioning.WithVersionHeaderKey("my-version-key"))
```

#### Knowing generated version

If you want to use the version of the AsyncAPI document used, you can access the constant
`AsyncAPIVersion` that is generated with the types. It is generated as followed:

```go
const AsyncAPIVersion = "{{ .Info.Version }}"
```

### Specification extensions

#### Operation Object extensions (AsyncAPI v3)

These extension properties apply to "Operation Objects" and override the Go name
of the functions generated for that operation. When omitted, the default derived
name is used.

* `x-go-subscribe-func`: overrides the `SubscribeTo<Operation>` method name.
* `x-go-received-func`: overrides the `<Operation>Received` subscriber callback name.
* `x-go-reply-func`: overrides the `ReplyTo<Operation>` method name.
* `x-go-send-func`: overrides the `Send{As,To}<Operation>` method name.
* `x-go-request-func`: overrides the `Request{As,To}<Operation>` method name.

For example,

```yaml
operations:
  ping:
    action: receive
    channel:
      $ref: '#/channels/ping'
    x-go-subscribe-func: ListenForPing
    x-go-received-func: OnPing
    x-go-send-func: PublishPing
    x-go-request-func: AskPing
```

#### Schema Object extensions

These extension properties apply to "Schema Objects" in AsyncAPI spec.

* `x-go-type`: Overrides the default Go type with the specified Go type name.

For example,

```yaml
  schemas:
    Object:
      properties:
        flag:
          type: integer
          x-go-type: uint8
  ```

will be generated as

```go
  type Object struct {
          Flag uint8 `json:"flag"`
  }
  ```

* `x-go-name`: Overrides the Go identifier generated for a type or a struct
  field, while keeping the original property key in the JSON tag.

For example,

```yaml
  schemas:
    event:
      type: object
      x-go-name: Event
      properties:
        event_id:
          type: integer
          x-go-name: EventID
  ```

will be generated as

```go
  type Event struct {
          EventID *int64 `json:"event_id,omitempty"`
  }
  ```

* `x-go-type-import`: Specifies the import package for `x-go-type`.
                      This has two properties `name` and `path`.
        `path` is the package import path, e.g. `github.com/google/uuid`.
        `name` is the package import name, which is optional.
  For example,

```yaml
  schemas:
    Object:
      properties:
        flag:
          type: integer
          x-go-type: mypackage.Flag
          x-go-type-import:
            path: abc.xyz/repo/mypackage
  ```

will be generated as

```go
  import (
          "abc.xyz/repo/mypackage"
  )

// ...

type Object struct {
          Flag mypackage.Flag `json:"flag"`
  }
  ```

while

```yaml
  schemas:
    Object:
      properties:
        flag:
          type: integer
          x-go-type: alias.Flag
          x-go-type-import:
            path: abc.xyz/repo/mypackage
            name: alias
  ```

will be generated as

```go
  import (
          alias "abc.xyz/repo/mypackage"
  )

// ...

type Object struct {
          Flag alias.Flag `json:"flag"`
  }
  ```

* `x-omitempty`: Controls the addition of the `omitempty` tag in JSON tags of generated Go structures.
  When set to `false`, the `omitempty` tag won't be added even for optional fields.

For example,

```yaml
  schemas:
    User:
      type: object
      properties:
        id:
          type: string
        name:
          type: string
          x-omitempty: false
  ```

will be generated as

```go
  type User struct {
      Id   *string `json:"id,omitempty"`
      Name *string `json:"name"`
  }
  ```

### Default values

When an optional property declares a `default` value, a `SetDefaults()` method is
generated on the corresponding struct. Calling it fills the fields that are still
unset (`nil`) with their default, leaving already-set fields untouched:

```yaml
schemas:
  Settings:
    type: object
    properties:
      retries:
        type: integer
        default: 3
```

generates:

```go
func (t *Settings) SetDefaults() {
    if t.Retries == nil {
        v := int64(3)
        t.Retries = &v
    }
}
```

Defaults are supported for scalar properties (string, boolean, integer, number).

### Multiple messages per operation (AsyncAPI v3)

When an operation (or its channel) declares more than one message, a dedicated
send function is generated for each one, named after the operation and the
message:

```yaml
channels:
  events:
    address: app.events
    messages:
      userCreated: { $ref: '#/components/messages/userCreated' }
      userDeleted: { $ref: '#/components/messages/userDeleted' }
operations:
  sendEvents:
    action: send
    channel: { $ref: '#/channels/events' }
```

generates `SendAsSendEventsOperationForUserCreated` and
`...ForUserDeleted`, each sending its own message type. Operations with a single
message are unchanged.

On the **receive** side, a subscribe function is likewise generated per message,
named after the operation and the message, with one callback per message type:

```yaml
operations:
  receiveEvents:
    action: receive
    channel: { $ref: '#/channels/events' }
```

generates `SubscribeToReceiveEventsOperationForUserCreated(ctx, fn)` and
`...ForUserDeleted(ctx, fn)` (plus the matching `UnsubscribeFrom...For...`
functions and per-message methods on the subscriber interface). A single broker
subscription is opened per channel and multiplexed to the registered handlers.

Each received message is discriminated with a **try-each-until-valid** strategy:
it is unmarshalled and validated against every expected message schema (using
[`go-playground/validator`](https://github.com/go-playground/validator), so this
dependency must be available in your module) and dispatched to the first one
that matches. This is heuristic and can be ambiguous when several message
schemas overlap — it works best when the messages have distinguishing required
or `enum`/`const` fields. A message that matches none of the expected types is
nacked and surfaced to the [error handler](#errorhandler) as
`extensions.ErrNoMatchingMessage`. Operations with a single message are
unchanged.

### Channel parameters from message location (AsyncAPI v3)

When a channel parameter declares a `location` runtime expression, the generated
send functions auto-fill that parameter from the outgoing message when the
caller leaves it empty. The parameter stays overridable: if you set it
explicitly, your value is used as-is.

```yaml
channels:
  user:
    address: app.user.{userId}
    parameters:
      userId:
        location: $message.payload#/userId
    messages:
      userSignedUp:
        $ref: '#/components/messages/userSignedUp'
```

With the spec above, `SendAs...` derives `userId` from `msg.Payload.UserId`, so
the message is published on `app.user.<userId>` without passing the parameter
manually. Supported locations are `$message.payload#/...` and
`$message.header#/...`, and the targeted field must be a string.

### ErrorHandler

You can use an error handler that will be executed when processing for messages
failed. To add a custom ErrorHandler to your controller use the  `WithErrorHandler`
function in the initialization of the App or User controller:

```golang
// Create a new app controller with ErrorHandler
ctrl, _ := NewAppController(/* Broker of your choice */, WithErrorHandler(myErrorHandler), ...)
```

Here the function signature that should be satisfied:

```golang
func(ctx context.Context, topic string, msg *AcknowledgeableBrokerMessage, err error)
```

**Note:** The default ErrorHandler is a Noop ErrorHandler doing nothing. By using a ErrorHandler you can add custom behavior for example to move messages to retry or dead letter topics/queues. 
Acks and Naks will be executed after the ErrorHandler, you can use the AcknowledgeableBrokerMessage in the handler to Ack/Nak the message manually.

#### Examples

##### Use the Logging ErrorHandler
```golang
// Create a new app controller with Logging ErrorHandler
ctrl, _ := NewAppController(/* Broker of your choice */, WithErrorHandler(errorhandlers.Logging(mylogger)), ...)
```

##### Build a custom ErrorHandler and handle Ack/Nak of the message
```golang
func(ctx context.Context, topic string, msg *extensions.AcknowledgeableBrokerMessage, err error) {
    // check error or move message to some other queue/topic
    handleTheErrorSomehow()

// Ack or Nak the message
    msg.Ack()
    msg.Nak()
}
```

### Validations

You can use [go-playground/validator](https://github.com/go-playground/validator) to validate the fields content against the contract.

The following tags are currently supported:

| Asyncapi         | Validator tag  | Comment                                                      |
|------------------|----------------|--------------------------------------------------------------|
| required         | required       | For a full support, the flag `--force-pointers` is necessary |
| minLength        | min            |                                                              |
| maxLength        | max            |                                                              |
| minimum          | gte            |                                                              |
| maximum          | lte            |                                                              |
| exclusiveMinimum | gt             |                                                              |
| exclusiveMaximum | lt             |                                                              |
| uniqueItems      | unique         | Only for arrays                                              |
| enum             | oneof          | Only string enum are supported                               |

You can validate manually wherever you hold a generated payload, or plug in the
provided opt-in middleware to reject invalid messages automatically:

```go
import (
    "github.com/go-playground/validator/v10"
    "github.com/lerenn/asyncapi-codegen/pkg/extensions/middlewares"
)

ctrl, _ := NewUserController(broker, WithMiddlewares(
    middlewares.Validation(validator.New(), func() any { return &UserMessagePayload{} }),
))
```

The middleware unmarshals the message payload and validates it against the
generated tags; if validation fails it returns an error, so the message is
neither delivered (on reception) nor sent (on publication).

## Contributing and support

If you find any bug or lacking a feature, please raise an issue on the Github repository!

Also please do not hesitate to propose any improvment or bug fix on PR.
Any contribution is warmly welcomed!

And if you find this project useful, please support it through the Support feature
on Github.

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