The middleware pipeline is a type of chain of responsibilty pattern.
Middleware are components that register a delegate as a step in the pipeline. The delegate is called by the previous delegate and in turn calls the next delegate. With this pattern, a middleware can perform work before and after the call to the next delegate or skip the next delegate if appropriate. Take the Help middleware for example. When
-? been specified, the help middleware prints help for the target command and does not call the next middleware.
As the middleware executes, it enhances the CommandContext. The CommandContext contains the context relevate to the current command. For example... the tokenizer middleware populates the CommandContext.Tokens property. The parser middleware populates the CommandContext.ParseResults property. The CommandContext is part of the delegate signature and is therefore available for all middleware.
Why we use the middleware pattern#
- Ease of adding new features without touching existing code. Observing Single Resposibility & Open-Closed principals.
- Ease of extending the framework using external packages. Supports community enhancements while reducing the bloat of the framework code base.
- Feature tests can exclude middleware that's not relevant to the tests.
The core functionality of the framework and optional features have been implemented using middleware. One of the classic challenges of using the middleware pattern is understanding the order middleware components should be registered. To address this challenge, the pipeline has been split into eight distinct stages, four core stages (in green) and four extensibility stages (in yellow). The four core stages contain the core functionality of the framework. After each core stage is complete, the framework guarantees specific parts of the CommandContext are populated.
The other four stages are where other middleware will typically be registered, based on CommandContext properties the middleware requires to operate. For example, PromptForMissingOperands is registered in the PostParseInputPreBindValues stage because it depends on the ParseResults.ArgumentValues property being populated in the ParseInput stage. FluentValidation is registerd in the PostBindValuesPreInvoke stage because it relies on the IInvocation.ParameterValues being populated in the BindValues stage. Read more in the intellisense of MiddlewareStages and the properties of the CommandContext classes.
Note: DisplayVersion is the only optional middleware in a core stage. Any middleware can be registered in a core stage if needed. In the case of DisplayVersion, the middleware must run before DisplayHelp otherwise help would trigger for the root command when it is not executable.
Avoid registering middleware into the core stages. in future versions of the framework, middleware may be added or removed or the order within stage could change. These will not be considered breaking changes. Only add middleware in these stages if you understand the risk. Use the constants in
MiddlewareSteps to understand the order of existing core middleware and add unit tests for these values to identify if when they change.
The invocation pipeline is assembled during the ParseInput stage, captured in CommandContext.InvocationPipeline property and run during the InvokeInvocationPipeline stage. The pipeline consists of interceptor methods and the target command method.