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Brackets Node Process: Overview for Developers

Peter Flynn edited this page Jan 18, 2015 · 28 revisions
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Overview

Brackets has a Node.js process integrated into the shell. This is useful for adding functionality that requires deeper native OS access than provided by the Brackets shell - for example, running command-line processes.

This document provides a quickstart guide for extension implementers on how to work with Node in Brackets, and also discusses the architecture of the Node integration in Brackets.

For more information on why we've integrated Node in the way we have, take a look at Research: Node.JS Integration.

Creating and Calling a Basic Brackets Node Module

Here we walk through how to implement a simple Node module that can be accessed from Brackets. This module allows client code to retrieve system memory usage information.

The entire example is available as a Brackets extension at https://github.com/njx/brackets-simple-node

Step 0: Setting up the extension.

There's no special setup you have to do to make a Brackets extension call Node code, so you can create your extension as usual: create a folder for it and put in a "main.js" file that's the entry point to the Brackets-side code. By convention (but not by necessity), we put all Node code for the extension in a subfolder called 'node' inside the extension folder, including third-party Node modules.

Note on third-party modules: Convention recommends putting any third-party modules inside a "node_modules" directory that lives inside the "node" directory. For development, Node modules can be installed through npm by putting a "package.json" file in the "node" directory and running npm install from inside that directory. By doing this (and adding "node/node_modules" to your .gitignore) you can avoid checking third party code into your repo. For distribution, the actual bits of any third-party modules should be bundled in to the zip - Brackets doesn't run npm install when installing an extension. (This is recommended practice in the Node community to ensure that all end users get the same bits. See http://www.futurealoof.com/posts/nodemodules-in-git.html)

Step 1: Implementing the Node module.

We call Node modules that are accessible from Brackets "domains" (for historical reasons - note that this is a completely different usage from the term "domains" in Node itself). All of our Node code will live in a file called "node/SimpleDomain.js". The first thing we need to do in this file is require the built-in libraries we'll be using:

var os = require("os");

Next, we need to implement a command handler that will actually get the data we want. Command handlers can take standard data types (including anonymous objects) as parameters and return those same data types. This command takes a parameter to specify whether to get the total memory or the free memory, and returns it as a number. (We could obviously return both in an object, but this is just to illustrate how parameters are passed from Brackets to Node commands.)

/**
 * @private
 * Handler function for the simple.getMemory command.
 * @param {boolean} total If true, return total memory; if false, return free memory only.
 * @return {number} The amount of memory.
 */
function cmdGetMemory(total) {
    if (total) {
        return os.totalmem();
    } else {
        return os.freemem();
    }
}

Brackets-node contains a "DomainManager" module that handles loading and initializing domains. When a domain is loaded from Brackets (as shown in the next section), the init() method of the domain's Node module will be called with the DomainManager as an argument. That method should use the registerCommand() function to tell Brackets which functions should be exposed to the client side.

When we register a command, we can optionally pass in documentation. This documentation isn't actually used by Node in any way. But, it is output through the "/api" call and could be used to actually build human-readable documentation that is (hopefully) in sync with the code. The last three parameters to registerCommand are documentation parameters.

/**
 * Initializes the test domain with several test commands.
 * @param {DomainManager} domainManager The DomainManager for the server
 */
function init(domainManager) {
    if (!domainManager.hasDomain("simple")) {
        domainManager.registerDomain("simple", {major: 0, minor: 1});
    }
    domainManager.registerCommand(
        "simple",       // domain name
        "getMemory",    // command name
        cmdGetMemory,   // command handler function
        false,          // this command is synchronous in Node
        "Returns the total or free memory on the user's system in bytes",
        [{name: "total", // parameters
            type: "string",
            description: "True to return total memory, false to return free memory"}],
        [{name: "memory", // return values
            type: "number",
            description: "amount of memory in bytes"}]
    );
}

Finally, we need to actually export this init function so the DomainManager can call it.

exports.init = init;

That's it! The domain is now implemented. Note that in the full version in the github repo mentioned earlier, the entire file is wrapped in an anonymous function that is immediately called. This is so we can add the "use strict" pragma once. There's no other significance of that code.

Step 2: Implementing the client code

Our client code is simple, too: we can simply use the NodeDomain utility class to load the domain and access its commands. The exec() method calls the command, and any arguments passed to exec after the command name are passed as parameters to the command on the Node side.

Because all communication with Node is asynchronous, you don't get a direct return value from exec() - instead, you get a Promise that's resolved with the return value (or rejected if there was an error).

var NodeDomain = brackets.getModule("utils/NodeDomain"),
    ExtensionUtils = brackets.getModule("utils/ExtensionUtils");

var simpleDomain = new NodeDomain("simple", ExtensionUtils.getModulePath(module, "node/SimpleDomain"));

// Helper function that runs the simple.getMemory command and
// logs the result to the console
function logMemory() {
    simpleDomain.exec("getMemory", false)
        .done(function (memory) {
            console.log(
                "[brackets-simple-node] Memory: %d bytes free",
                memory
            );
        }).fail(function (err) {
            console.error("[brackets-simple-node] failed to run simple.getMemory", err);
        });
}

// Log memory when extension is loaded
logMemory();

As usual for Brackets extensions, you need to wrap this in AMD define() boiilerplate. Check out the full source at https://github.com/njx/brackets-simple-node

More Tips for Creating Brackets Node Modules

Asynchronous methods

This example showed a command that was registered as "synchronous" (isAsync was set to false in registerCommand). This terminology is a bit confusing, because all communication between Brackets and Node is actually asynchronous. When isAsync is set to false, it means that the command function is synchronous on the Node side, so when DomainManager calls it, it expects to get a return value from the command function directly, which it then sends (asynchronously) back to Brackets.

If your command is asynchronous on the Node side, you can pass true for the fourth parameter (isAsync) to registerCommand. If you do this, then the final parameter to your command will be a Node-style "errback" function. When your processing is done, call the errback with two arguments, err and result. If you encountered an error, then you should pass an error string back in err. If you succeeded, pass null for err and pass your result as the second argument to the errback.

In either case ("synchronous" or asynchronous), your result or error will be sent back to your Brackets code via a resolved or rejected promise, as mentioned above.

Events

Your Node module can also asynchronously trigger events that can be listened to on the Brackets side. Events are registered using the registerEvent command in the DomainManager, and emitted using the emitEvent. On the client side, events can be listened for using the standard jQuery event system on instances of the NodeDomain class, by simply doing myDomain.on("someEvent", someHandler).

Note that Brackets client code automatically registers as a listener for "log" events and forwards them to the client console, prefixed with "[node-(level) (timestamp)]".

Initialization:

/**
 * Initializes the test domain with several test commands.
 * @param {DomainManager} domainManager The DomainManager for the server
 */
function init(domainManager) {
    if (!domainManager.hasDomain("simple")) {
        domainManager.registerDomain("simple", {major: 0, minor: 1});
    }
    domainManager.registerEvent(
    "simple",     // domain name
    "log",         // event name
    [{
        name: "scope",
        type: "string",
        description: "message scope"
    }, {
        name: "message",
        type: "string",
        description: "message body"
    }, {
        name: "payload",
        type: "object",
        description: "log message payload"
    }]);
}

Then emit event. You can pass scalar or array as third argument. Scalar will be recognized as event's first argument.

_domainManager.emitEvent ("simple", "log", scope);
_domainManager.emitEvent ("simple", "log", [scope, message, payload]);

Reloading the Node module

Usually, when you make a change to code in a Brackets extension, you can just reload Brackets to see the effect. However, the Node process isn't restarted when you reload Brackets. In order to see the effect of changes you make in Node code, you can use Debug > Restart Node, or quit and restart Brackets entirely.

Other examples

The "StaticServer" extension in "src/extensions/default/StaticServer" in the main Brackets repo is a more complicated example.

How to Debug

Debugging domains is fairly straightforward. Debugging the Node core launching process is much more complicated.

Standard debugging

To debug, you will need to start by installing node and node-inspector. Node can be installed from http://nodejs.org and node-inspector can then be installed with npm install -g node-inspector.

Once these are installed, do the following:

  1. Launch Brackets
  2. Enable Node debugging from the "Debug" menu
  3. Launch node-inspector
  4. Node inspector will print out a URL to go to for debugging. On some machines, you may need to replace "0.0.0.0" with "localhost" in the url.
  5. Set your breakpoints and call your code.

There are some important caveats in debugging. Read the "Common 'Gotchas'" below!

Debugging the startup process

If you need to debug the launch process, start by opening "Launcher.js" in node-core. As a first pass, try setting the LOG_FILENAME_ON_LAUNCH variable to an absolute path that you have write-access to. See if you get the information you need in this log file.

If that doesn't work, set DEBUG_ON_LAUNCH to true. Then, run Brackets (which launches node-core). The debugger will automatically be enabled. Connect to it with node-inspector, set your breakpoints, and then call the globally-defined function debugLaunch from the console. This will start the actual launch routine asynchronously, and you'll hit your breakpoints.

Important: It is very likely that you will get abandoned Brackets-node processes when doing this. Read the "Common 'Gotchas'" below! Failure to pay attention to this will cause massive confusion.

Common "Gotchas"

  • Abandoned Node process during debugging -- In order to figure out when to end its process, Node pays attention to what's happening on it's stdin and stdout pipes. If you hit a breakpoint while debugging and then never resume, Node won't pay attention to its pipes. If you then quit Brackets, the Node process will get abandoned.

    If this happens, the only way to end the process is to use pkill Brackets-node / Activity Manager / Task Manager to kill the process. Note that this should only happen to developers, not to end users.

    Making matters worse, while this abandoned process sticks around, it will hold on to the debugging port. This means that if you don't realize you have an abandoned process and try to debug another Brackets-node process, you'll have a bad time.

  • You cannot debug code that is called synchronously from the console in node-inspector -- That is, if you set a breakpoint in your global function "foo" and call "foo" from the console, the debugger will not stop at the breakpoint.

    A simple workaround is to do process.nextTick(foo)

  • Must restart Node process if you make code changes -- Reloading the Brackets window does not restart the Node process. So, if you make changes to both client and Node code, you have to reload Brackets and restart Node. This can be done from the Debug menu.

  • All extensions share the same "domain" space in Node, so use unique names -- If one extension loads a domain called foo, every other extension will have access to that domain. This can be advantageous, but it means that unique names are necessary to avoid collisions.

Architecture Notes

On both Mac and Windows, Node runs as a separate child process of the main Brackets-shell process. The child process communicates with the shell via stdin/stdout (communication with the parent process should be kept to a minimum). The Node process runs a simple http and websocket server on localhost at a randomly-assigned port, which are used to communicate with the client JS code.

Communication between the Node process and client JS code is handled almost exclusively through websockets. The high-level protocol for communication is modeled after the Chromium Web Inspector remote debugging protocol (which is used by our Live Development code).

A major goal of the architecture of the Node system is to keep "core" Node code as small as possible. This code is responsible for launching the http and websocket servers, managing connections, and allowing extensions to register new commands. Because the core code is small and tied to specific APIs in the brackets shell, this code is in the brackets-shell repo. All "core" code lives in the "appshell/node-core" directory of the brackets-shell repo. An added benefit of this organization is that we get to use the phrase "node-core" a lot, which sounds like a great name for an 80s punk band.

The "useful" parts of the Node process are implemented as "domains". (Note: this usage of the word "domains" is modeled after the web inspector usage, not after the Node.js usage. Node.js domains are very different.) Domains define commands and events that can be executed by the client. When a client connects, it can retrieve a list of all registered domains by making an HTTP request to the "/api" path. (I.e., by requesting http://localhost:port/api). This returns a JSON representation of the current API.

On the client side, a connection to the Node server is handled by the NodeConnection class inside src/utils/NodeConnection.js. Upon connection, this class automatically requests the API and constructs wrapper functions that make calling commands, receiving events, and so-forth straightforward. Suppose we have a connection to Node stored in an object pointed to by the variable name _nodeConnection. This object will have a property named "domains" which contains functions that wrap commands. In the usage example below, we define a domain called "simple", and a command in that domain called "getMemory". This command can be accessed at _nodeConnection.domains.simple.getMemory. Calling this function will return a promise that will resolve with the result, or reject with an error message.

As with almost all things JS, everything here is asynchronous. Here, there are two different sources of asynchrony to consider. First, communication between the client and Node is always asynchronous. Asynchrony on the client side is handled entirely through jQuery promises. Behind the scenes, NodeConnection assigns message IDs to each message sent, maps responses to those original IDs, and resolves appropriate promises that were returned at command call time.

The second source of asynchrony is entirely on the Node side: any single command that Node executes could be asynchronous (or, it could be synchronous). The ConnectionManager and DomainManager on the Node side handle this asynchrony with the isAsync parameter described above. We use errbacks instead of Promises on the Node side because most core Node modules use errbacks.

Events from Node domains are also dispatched through NodeConnection. Under normal circumstances you shouldn't need to deal with this, but if you want to listen to them at this level (e.g. to catch events across multiple domains), the jQuery event types will be in the format domainName:eventName. For example, for the log event in the base domain, listen for the base:log event type:

_nodeConnection.on("base:log", function (evt, level, timestamp, message) {
    console.log("[node] %s %s %s", level, timestamp, message);
});

Future Work

  • Implement "safe mode" in Node core -- It is easy to write a domain that continually crashes Node. Right now, if we get two crashes within 5 seconds, we do not restart the Node process. The risk here is that the user will install an extension that repeatedly crashes Node, parts of Brackets will become unusable. For the things we have implemented now, this isn't a huge problem. But, if we start using Node for filesystem access, that means we could get into a state where the user's Node process crashes and he or she can't save their open documents.

    A fix for this would be to implement a "safe mode" in node-core that doesn't allow registering of new domains. We would only load known-good domains (e.g. a filesystem domain). On any unintended crash/restart, we would enter this safe mode.

  • Remove auto-reconnect / auto-reload-domains from NodeConnection -- This code seems to work great, and has unit tests that seem to consistently pass. But it's really complicated, which means it's more likely to have bugs. If we go to a safe mode, we might be able to remove this logic. (But maybe not: We want intended reloads that happen during the development process to work properly.)

  • Improve the event infrastructure -- Right now, we send all events to every connection. We could instead have connections register for events they care about so that we don't transfer unwanted data over the websocket.

  • Figure out a way to transfer binary data very quickly -- Right now, the safest way to transfer binary data over the websocket is to base64 encode it. Ugh. We don't actually use binary data in Brackets right now, so this isn't a problem. But it could be soon...

  • Use named pipes for communication with parent process -- Right now, we're using stdin/stdout, which was easy but is not ideal. This would also allow us to move to overlapped reads on Windows and would make it cleaner to capture crash debugging info in syslogs.

  • Add more infrastructure around preventing abandoned processes -- Upon successful launch/closing, the shell could check for abandoned Brackets-node processes and kill them.

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