Middleware Plugins

wasmcloud middleware plugins are used to allow code you write to be invoked during every single invocation. An invocation is a call between two entities within wasmcloud, and so can include the following use cases:

  • Actor -> Actor
  • Actor -> Capability Provider
  • Capability Provider -> Actor

Note that capability providers are forbidden from talking directly to each other for a number of reasons, not the least of which is security and the prevention of privilege escalation attacks.

All wasmcloud middleware must conform to the Middleware trait, which is found in the wasmcloud-host crate. The required functions of this trait are:

  • actor_pre_invoke(&Invocation) -> Result<()>

Invoked prior to an actor receiving an invocation from any source. Return Ok to indicate successful processing.

  • actor_post_invoke(InvocationResponse) -> Result<InvocationResponse>

Invoked after an actor has handled an invocation. Your code is given a chance to return a new invocation response, note that the original response is consumed (moved) in this call.

  • capability_pre_invoke(&Invocation) -> Result<()>

Invoked prior to a capability provider receiving an invocation from an actor.

  • capability_post_invoke(InvocationResponse) -> Result<InvocationResponse>

Invoked after a capability provider has processed an invocation, giving your code a chance to alter or replace the original invocation response.

All of these middleware calls are chain-breaking calls. If your code ever returns anything other than an Ok<T>, then the middleware chain will be broken and the original initiator of the call will be given the error result.

⚠️ NOTE Middleware chains are processed in the order in which they were added to the HostBuilder. So if you require that middleware processing follow a specific order, you must ensure that your plugins are added to the host builder in the right order.