functions.md

Functions

Glee relies on functions to execute your business logic. Functions are files that export a default async Node.js function:

/* onHello.js */

export default async function (event) {
  // Your business logic here...
}

Functions take a single argument, which is the event received from a broker or a client, depending which kind of API you're building. The event argument has the following shape:

Attribute	Description
payload	The payload/body of the received event.
headers	The headers/metadata of the received event.
channel	The name of the channel/topic from which the event was read.
serverName	The name of the server/broker from which the event was received.

Functions may return an object to tell Glee what to do next. For instance, the following example greets the user back:

/* onHello.js */

export default async function (event) {
  return {
    reply: [{
      payload: 'Greetings! How is your day going?'
    }]
  }
}

Attribute	Type	Description
send	array<OutboundMessage>	A list of outbound messages to send when the processing of the inbound event has finished. All clients subscribed to the given channel/topic will receive the message.
reply	array<OutboundMessage>	A list of outbound messages to send as a reply when the processing of the inbound event has finished. This is useful when the target of your message is the sender of the inbound event. Note, however, that this only works when you're running Glee as a server. For example, using reply when receiving a WebSocket message is fine and the reply will exclusively go to the client that sent the message. However, if you're receiving a message from an MQTT broker, reply will work exactly the same way as send above, and will send the message to all the clients subscribed to the given channel/topic.

Anatomy of an outbound message

Attribute	Type	Description
payload	string	The payload/body of the message you want to send.
headers	object<string,string>	The headers/metadata of the message you want to send.
channel	string	The channel/topic you want to send the message to. Defaults to event.channel, i.e., the same channel as the received event.
server	string	The server/broker you want to send the message to. Defaults to event.serverName, i.e., the same server as the received event.

How does Glee know which function it should execute?

Glee reads your asyncapi.yaml file and searches for all the publish operations containing an operationId attribute. The operationId serves as a mechanism to bind a given operation to a specific function file. For instance, given the folowing AsyncAPI definition:

...
channels:
  hello:
    publish:
      operationId: onHello
      ...

Glee maps the onHello operation to the functions/onHello.js file.