Developing Spin applications

The Spin CLI offers a few commands to simplify developing applications.

Building Spin applications

A Spin application is made up of one or more components. When developing a multi-component application, it is very common to have multiple directories with source code for components — and when making changes to components, having to manually go into the each component directory, compile the component, then go back to the directory with spin.toml can be a very repetitive task.

This is why Spin has a top-level command that will execute the build command set by each component, spin up:

command = "cargo build --target wasm32-wasi --release --manifest-path http-rust/Cargo.toml"

Then, running spin build will execute, sequentially, each build command:

$ RUST_LOG=spin=trace spin build
2022-04-25T03:01:56.721630Z  INFO spin_build: Executing the build command for component rust-hello.
    Finished release [optimized] target(s) in 0.05s
2022-04-25T03:01:56.832360Z  INFO spin_build: Executing the build command for component rust-static-assets.
    Finished release [optimized] target(s) in 0.02s
2022-04-25T03:01:56.905424Z  INFO spin_build: Executing the build command for component rust-outbound-http.
    Finished release [optimized] target(s) in 0.02s

The spin build command is intended to offer a built-in way to build more complex Spin applications without needing a separate build process. It is not intended to replace complex build scripts — if you have existing automated ways for building source code, those can be used instead, or the build command can call that process.

spin build --up can be used to start the application after the build process finishes for all application components.

Component workdir

By default, the command to build a component is executed in the manifest’s directory. This can be changed. For example, assume a component is located in subdirectory deep:

├── deep
│   ├── Cargo.toml
│   └── src
│       └──
└── spin.toml

To run the build command in directory deep, set the component’s workdir:

command = "cargo build --target wasm32-wasi --release"
workdir = "deep"

Note that workdir must be a relative path and it operates relative to the spin.toml. Specifying an absolute path leads to an error.