[webkit-dev] WebKit blog post proposal: Remote debugging with Web Inspector.

Pavel Feldman pfeldman at chromium.org
Fri Apr 29 13:00:44 PDT 2011

Surfin' Safari
<< Web Inspector: Understanding Stack

Remote Debugging with Web Inspector <http://www.webkit.org/blog/?p=1620>Posted
by *Pavel Feldman* on Friday, April 29th, 2011 at 12:59 pm

As you might know, WebKit Web Inspector (aka Chrome Developer Tools) is
implemented as an HTML + CSS + JavaScript web application. What you might
not know is that Web Inspector can run outside of the rendering engine
environment and still provide complete set of its features to the end user.

Running outside the page context (and browser) is interesting, because
mobile platforms do not often provide enough screen real estate for quality
debugging; they have network stack and CPU specifics that often affect page
load and runtime. Still, they are based on the WebCore rendering engine,
they have Web Inspector backend instrumentation running and they can expose
valuable debugging information to the user.

Another scenario for remote debugging is IDE integration. Web IDEs would
like to provide seamless debugging experience, integrated into their
environments to the user. Exposing unified WebKit remote debugging protocol
will allow them to use alternate front-ends for the WebKit debugging.

Here is an example of the remote debugging session using Chrome Developer
Debugging over the wire

1. Start your target browser (recent Chromium build or Google Chrome will
do) with the remote-debugging-port command line switch:

Chromium.app/Contents/MacOS/Chromium --remote-debugging-port=9222

2. Navigate to the given port from your client browser instance (WebKit
nightly or another Chrome instance will do) and it will list inspectable
pages as web links.
[image: Tab discovery page]
Follow any of these links to start remote debugging session for the
corresponding page.
[image: Tab attached page]

You will find remote debugging interface very similar to the Web Inspector /
Chrome Developer Tools and here is why:

   - Target Chrome browser acts as a web server bound to the port 9222 on
   the localhost.
   - Once you follow the link, your client browser fetches HTML, JavaScript
   and CSS files of the Developer Tools front-end over HTTP.
   - Upon load event, Developer Tools establishes Web Socket connection back
   to the target browser and starts interchanging JSON messages with it.

In fact, pretty much the same scenario takes place within any WebKit-based
browser when user opens Web Inspector. The only difference is that the
transports being used for the JSON message interchange may vary. Note, that
in case of mobile devices, front-end files can also be served from the
Remote Debugging Protocol

Under the hood, Web Inspector front-end is talking to the browser backend by
means of the Remote Debugging Protocol. This protocol is based on the JSON-RPC
2.0 <http://groups.google.com/group/json-rpc/web/json-rpc-2-0> specification.
It is bidirectional: clients send asynchronous requests to the server,
server serves responses to these requests and generates notifications. Since
API surface for general purpose web debugging is huge, we divided it into a
number of domains. Each domain contains requests and notifications specific
to some area. Here is the list of the domains supported at the moment:

   - *Browser Debugger* – allows setting breakpoints on particular DOM
   operations and events. JavaScript execution will stop on these operations as
   if there was a regular breakpoint set.
   - *Console* – defines methods and events for interaction with the
   JavaScript console.
   - *CSS* – exposes raw CSS read / write operations.
   - *Debugger* – exposes JavaScript debugging functions; allows setting and
   removing breakpoints, stepping through execution, exploring stack traces,
   - *DOM* – This domain exposes DOM read/write operations.
   - *Network* – allows tracking network activities of the page; exposes
   information about HTTP and WebSocket requests and responses, their headers,
   bodies, raw timing, etc.
   - *Page* – actions and events related to the inspected page.
   - *Runtime* – exposes JavaScript runtime by means of remote evaluation
   and mirror objects.
   - *Timeline* – provides its clients with instrumentation records that are
   generated during the page runtime.

You can find JSON schema defining the protocol
For your convenience, we generated documentation from this schema and
published it on the Chrome DevTools
Note that there are few unlisted domains such as Application Cache, DOM
Storage, and Database, but they are not ready for the prime time yet.
What’s next

   - We are now open to the feedback on the WebKit Remote Debugging
   Protocol. We will collect all the feedback in the form of the bug
reports<http://webkit.org/new-inspector-bug> and
   the Chrome DevTools
   - We will then address initial feedback, polish the protocol a bit and
   publish the first draft.
   - Once we have the protocol defined, developers can come up with the
   alternate front-ends (IDEs and such) that will interact with the WebKit
   instrumentation in various browsers.
   - We also expect all the WebKit ports to expose WebSocket interfaces
   similar to explained above or to come up with any other transport and bridge
   it to the Web Inspector front-end.

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On Fri, Apr 29, 2011 at 11:09 PM, David Kilzer <ddkilzer at webkit.org> wrote:

> Hi Pavel,
> You need to send the contents of the blog post via email.  Not everyone can
> read it (even after logging in).
> Dave
> On Apr 29, 2011, at 11:44 AM, Pavel Feldman wrote:
> Hi guys,
> I started drafting the blog post "Remote Debugging with Web Inspector":
> http://www.webkit.org/blog/?p=1620&preview=true.
> I'd like to cover following topics there:
>     - ability to use Web Inspector front-end with remote / embedded devices
>     - ability to implement alternate front-ends for IDEs
>     - share the update on the protocol work progress
> Calling for the early feedback.
> Thanks
> Pavel
> On Mon, Aug 9, 2010 at 11:48 PM, Pavel Feldman <pfeldman at chromium.org>wrote:
>> Hi guys,
>> As some of you know, we are working on a remote debugging feature in Web
>> Inspector. There are many good reasons behind the project including the
>> following:
>> - Debugging WebKit on embedded devices
>> - Shaping up a good protocol for ourselves
>> - Introducing external SDKs on top of the protocol for IDE integrations
>> and alternate front-ends
>> We've had serialized interaction with the out-of-process inspector for
>> quite a while in Chromium. We were upstreaming it into WebKit and have
>> reached an important milestone recently: all the interaction between the
>> inspected page and inspector is entirely serialized on the WebKit level. All
>> the embedder needs to do is to implement a socket that would serve the
>> inspector front-end files and provide our messaging with appropriate
>> transport.
>> Now this socket is likely to be platform-specific, implemented on the
>> WebKit and/or host browser levels. It also makes more sense to implement
>> socket on mobile platforms first. However, we've done a proof-of-concept
>> implementation in Chromium and it is now in a demoable state! See the
>> screencast at http://screencast.com/t/YTI2OTY4YTEt. It has Chromium
>> nightly to the left + WebKit nightly to the right. WebKit nightly connects
>> remotely to Chromium over HTTP on the port 9222 and does remote debugging
>> including DOM inspection, breakpoints and such. The communication is
>> established by means of a WebSocket. The interesting thing about the
>> implementation is that inspector front-end is fetched from the host browser,
>> so that there is no mess with protocol versioning and no need in exposing
>> the interaction protocol any time soon.
>> So I made the demo and it looked cool. I thought maybe we do a blog post
>> on it. The blog post would draw attention to the Web Inspector and its
>> progress, share the remote debugging vision with the interested parties and
>> would simply look cool. Front-end is working as a pure HTML5 application
>> (obviously full of WebKit-specific styles, but still) which is impressive.
>> Now the project is nowhere complete in terms of finalizing the message
>> format and the protocol itself, but there is no intention to expose it right
>> now. We'd like to let it live with fetchable front-end and mature before we
>> expose the protocol and commit to any level of interface support.
>> What do you think, is it ready for a blog post?
>> Thanks
>> Pavel
> _______________________________________________
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> webkit-dev at lists.webkit.org
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