Modifier

A modifier component operates on an unformatted, unstructured stream of bytes as it passes through vhs. Modifier components are well suited to implementing compression/decompression algorithms, as an example. Modifiers may exist in either the input or output chains of the vhs data flow. Input modifiers and output modifiers must be implemented separately. Information on the input and output modifiers available in the current release of vhs is available here: input modifiers and output modifiers

Developing a Modifier

Modifiers are implemented in a slightly different fashion when compared to other vhs components. They do not have an Init function, and are instead implemented as wrappers around an InputReader or OutputWriter. Input modifiers must conform to the InputModifier interface defined in core/input_modifier.go, which is shown below:

// InputModifier wraps an InputReader.
type InputModifier interface {
     Wrap(InputReader) (InputReader, error)
}

Output modifiers must conform to the OutputModifier interface defined in core/output_modifier.go, which is shown below:

// OutputModifier wraps an OtputWriter.
type OutputModifier interface {
     Wrap(OutputWriter) (OutputWriter, error)
}

Internally, the implementation of a modifier usually entails the creation of a new type that conforms to the requisite interface (InputReader or OutputWriter). This the compression or encoding performed by the modifier will be implemented in the methods of this new type, and the Wrap function required by the modifier interfaces seen above will return this new type.

Other considerations when developing a modifier include:

• A modifier must provide a constructor. By convention this constructor should take an argument of type session.Context. This constructor may be trivial.
• Modifiers may utilize command line arguments if necessary. These flags must be declared in newRootCmd in cmd/vhs/main.go. The values of these flags should be stored in FlowConfig in the session package at core/config.go. These values will then be available within your package as part of the core.Context.
• If a modifier uses any internal goroutines, these must clean up and exit upon receiving a signal on the core.Context.Done() channel.

Example

The gzip input and output modifiers provided by vhs are useful examples of the implementation of modifiers. They are implemented as wrappers around the gzip.Reader and gzip.Writer packages from the Go standard library package compress/gzip. Many useful modifiers can be implemented in a similar fashion by wrapping other libraries. The implementation of the gzip modifiers is in gzipx/gzipx.go