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Runtime Architecture

Generated Libraries

When using jsii-pacmak to generate libraries in different programming languages, the Javascript code is bundled within the generated library, so that it can be used during at runtime. This is the reason why a node runtime needs to be available in order to execute code that depends on jsii libraries.

The generated libraries have a dependency on a Runtime client library for the language, which contains the necessary logic to start a child node process with the jsii-runtime. The jsii-runtime manages JSON-based inter-process communication over its STDIN and STDOUT, and manages a @jsii/kernel instance that acts as a container for the Javascript code that backs the jsii libraries.

Architecture Overview

A simplified representation of the execution environment of an application using jsii libraries from a different language follows:

┌────────────────────────┐             ┌────────────┬────┬────┬───┐
│                        │             │            │    │    │   │
│   User's Application   │             │@jsii/kernel│LibA│LibB│...│
│                        │             │            │    │    │   │
│     ┌──────────────────┤             ├────────────┴────┴────┴───┤
│     │                  │             │                          │
│     │Generated Bindings│             │      @jsii/runtime       │
│     │                  │             │                          │
│     ├──────────────────┤             ├────────┬─────────────────┤
│     │                  ├────────────▶│ STDIN  │                 │
│     │Host jsii Runtime │    JSON     ├────────┤                 │
│     │                  │◀────────────┤ STDOUT │   node          │
├─────┴──────────────────┤             ├────────┘                 │
│                        │             │     (Child Process)      │
│    JVM / .NET / ...    │             │                          │
│                        │             │                          │
├────────────────────────┴─────────────┴──────────────────────────┤
│                                                                 │
│                        Operating System                         │
│                                                                 │
└─────────────────────────────────────────────────────────────────┘

The initialization workflow can be described as:

  1. The host (Java, .NET, ...) application starts on its own runtime (JVM, .NET Runtime, ...)
  2. When the host code encounters a jsii entity for the first time (creating an instance of a jsii type, loading a static constant, ...), the runtime client library creates a child node process, and loads the jsii-runtime library (specified by the JSII_RUNTIME environment variable, or the version that is bundled in the runtime client library)
  3. The runtime client library interacts with the child node process by exchanging JSON-encoded messages through the node process' STDIN and STDOUT
  4. The runtime client library automatically loads the Javascript modules bundled within the generated bindings (and their depedencies, bundled in other generated bindings) into the node process when needed.
  5. Calls into the Generated bindings are encoded into JSON requests and sent to the child node process, which will execute the corresponding Javascript code, then responds back.
  6. Upon exiting, the host process closes the communication channels with the child node process, causing it to exit.