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subscription-types.md

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Subscription Types

Event Gateway supports two subscription types: async and sync.

async subscription

async subscription implements lightweight pub/sub system. There can be many async subscriptions listening to the same event type, on the same path and HTTP method. The function is asynchronously invoked by the Event Gateway.

sync subscription

In case of sync subscription invoked function can control the HTTP response returned by the Event Gateway. Because of that, there can be only one sync subscription for a path, HTTP method, and event type tuple.

Status code, headers and response body can be controlled by returning JSON object with the following fields:

  • statusCode - int - response status code, default: 200
  • headers - object - response headers
  • body - string - response body. Currently, the Event Gateway supports only string responses.

If the function invocation failed or the backing function didn't return JSON object in the above format Event Gateway returns 500 Internal Server Error.

Path parameters

The Event Gateway allows creating sync subscription with parameterized paths. Every path segment prefixed with : is treated as a parameter, e.g. /users/:id.

The Event Gateway prevents from creating subscriptions in following conflicting situations:

  • registering static path when there is parameterized path registered already (/users/:id vs. /users/foo)
  • registering parameterized path with different parameter name (/users/:id vs. /users/:name)

Key and value of matched parameters are passed to a function in an HTTP Request Event under params field.

Wildcard parameters

Special type of path parameter is wildcard parameter. It's a path segment prefixed with *. Wildcard parameter can only be specified at the end of the path and will match every character till the end of the path. For examples parameter /users/*userpath for request path /users/group1/user1 will match group1/user1 as a userpath parameter.