Command

A command is a system-level building block of sbt, often used to capture user interaction or IDE interaction.

We can think of each command as a State => State function. In sbt, the state represents the following:

1. Build structure (build.sbt etc)
2. Your disk (source code, JAR outputs, etc)

Thus, a command would typically modify either the build structure or the disk. For example, the set command can apply a setting to modify the build strcuture:

> set name := "foo"

The act command can lift a task such as compile into a command:

> compile

The compilation would read from the disk and write outputs, or display error messages on the screen.

Commands are sequentially processed

Because there is only one state, a characteristic of commands is that they are executed one at a time.

There are some execptions to this rule, but generally commands run sequentially. One mental image that might be useful is that a command is similar to a cashier taking an order in a cafe, and it will be processed in the sequence it was received.

Tasks run in parallel

As mentioned above, the act command translates tasks into the command level. While doing so, the act command will broadcast the task across the aggregated subprojects and run independent tasks in parallel.

Similarly, the reload command that runs during the startup of a session will initialize the settings in parallel.

The role of sbt server

sbt server is a service that accepts commands from either the command line or a network API called Build Server Protocol. This mechanism allows both the build user and IDEs to share the same sbt session.