Middleware

Command Middleware

TypeScript Support

A command middleware function has an associated TypeScript type:

• Type Name - CommandMiddleware
• Package - @resolve-js/core

A command middleware function has the following structure:

const commandMiddleware = (next) => (
  middlewareContext,
  state,
  command,
  context
) => {
  ...
  state,
  return next(middlewareContext, state, command, context)
}

The middleware handler function receives the following arguments:

Parameter Name	Description
middlewareContext	Contains data that describes the currently processed operation.
state	The state object built by the aggregate projection.
command	An object that contains data about the incoming command.
context	The command context object.

middlewareContext

A command middleware handler's middlewareContext argument is an object with the following fields:

Field Name	Description
req	Stores data that describes the currently processed HTTP request.
res	Contains the function used to configure the server's response.

Both req and res fields are included only if the client sends the command. If the command is generated on the server (for example, by a saga or API handler), these fields are omitted.

Read Model Projection Middleware

TypeScript Support

A read model projection middleware function has an associated TypeScript type:

• Type Name - ReadModelProjectionMiddleware
• Package - @resolve-js/core

A read model projection middleware function has the following structure:

const projectionMiddleware = (next) => (
  middlewareContext,
  store,
  event,
  context
) => {
  ...
  return next(middlewareContext, store, event, context)
}

The middleware handler function receives the following arguments:

Parameter Name	Description
middlewareContext	Contains data that describes the currently processed operation.
store	Exposes API used to communicate with the read model's persistent data storage.
event	The incoming event object.
context	The read model projection context object.

middlewareContext

A projection middleware handler's middlewareContext argument is an object with the following fields:

Field Name	Description
req	Stores data that describes the currently processed HTTP request.
res	Contains the function used to configure the server's response.
readModelName	The name of the processed read model.

Read Model Resolver Middleware

TypeScript Support

A read model resolver middleware function has an associated TypeScript type:

• Type Name - ReadModelResolverMiddleware
• Package - @resolve-js/core

A read model resolver middleware function has the following structure:

const resolverMiddleware = (next) => (
  middlewareContext,
  store,
  params,
  context
) => {
  ...
  return next(middlewareContext, store, params, context)
}

The middleware handler function receives the following arguments:

Parameter Name	Description
middlewareContext	Contains data that describes the currently processed operation.
store	Exposes API used to communicate with the read model's persistent data storage.
params	An object that contains the request parameters as key-value pairs.
context	The read model resolver context object.

middlewareContext

A projection middleware handler's middlewareContext argument is an object with the following fields:

Field Name	Description
req	Stores data that describes the currently processed HTTP request.
res	Contains the function used to configure the server's response.
readModelName	The name of the processed read model.
resolverName	The name of the queried resolver.