Web Console remoting

Introduction

This document describes the way Web Console remoting works. The Web Console is split between a client with its user interface, and the server which has listeners for all the things that happen in the tab. For communication between the server and the client we use the Remote Debugging Protocol. This architecture allows you to connect a Web Console client instance to a server running on B2G, Fennec or some other Firefox instance.

To better understand the architecture of the Web Console we recommend learning about the debugger architecture.

Note

The remote Web Console is a feature introduced in Firefox 18. This document describes the latest protocol, with changes that have been made since then.

The `WebConsoleActor` and the `WebConsoleClient`

The WebConsoleActor lives in dbg-webconsole-actors.js, in the toolkit/devtools/webconsole folder.

The WebConsoleClient lives in WebConsoleClient.jsm (in toolkit/devtools/webconsole) and it is used by the Web Console when working with the Web Console actor.

To see how the debugger is used in the Web Console code, look in browser/devtools/webconsole/webconsole.js, and search for WebConsoleConnectionProxy.

The new Web Console actors are:

The WebConsoleActor allows JS evaluation, autocomplete, start/stop listeners, etc.
The NetworkEventActor is used for each new network request. The client can request further network event details - like response body or request headers.

To attach to the WebConsoleActor, follow these steps:

connectToServer() // the usual
listTabs()
pickTheTabYouWant()
debuggerClient.attachConsole(tab.consoleActor, listeners, onAttachConsole)

The listeners argument is an array which specifies listeners you want to start in the web console. These can be: page errors, window.console API messages, network activity, and file activity. For example:

["PageError", "ConsoleAPI", "FileActivity"]

Note

The Web Console actor does not start any listeners by default. The client has the option to start each listener when needed. This approach allows for lower resource usage on the server - this is a potential issue if the server runs on devices with fewer resources.

The onAttachConsole callback receives a new instance of the WebConsoleClient object. This object provides methods that abstract away protocol packets, things like startListeners(), stopListeners(), etc.

Protocol packets look as follows:

{
  "to": "root",
  "type": "listTabs"
}
{
  "from": "root",
  "consoleActor": "conn0.console9",
  "selected": 2,
  "tabs": [
    {
      "actor": "conn0.tab2",
      "consoleActor": "conn0.console7",
      "title": "",
      "url": "https://tbpl.mozilla.org/?tree=Fx-Team"
    },
// ...
  ]
}

Notice that the consoleActor is also available as a global actor. When you attach to the global consoleActor you receive all of the network requests, page errors, and the other events from all of the tabs and windows, including chrome errors and network events. This actor is used for the Browser Console implementation and for debugging remote Firefox/B2G instances.

`startListeners(listeners, onResponse)`

The new startListeners packet:

{
  "to": "conn0.console9",
  "type": "startListeners",
  "listeners": [
    "PageError",
    "ConsoleAPI",
    "FileActivity"
  ]
}

The reply is:

{
  "startedListeners": [
    "PageError",
    "ConsoleAPI",
    "FileActivity"
  ],
  "from": "conn0.console9"
}

The reply tells which listeners were started.

Tab navigation

To listen to the tab navigation events you also need to attach to the tab actor for the given tab. The tabNavigated notification comes from tab actors.

Warning

Prior to Firefox 20 the Web Console actor provided a LocationChange listener, with an associated locationChanged notification. This is no longer the case: we have made changes to allow the Web Console client to reuse the tabNavigated notification (bug 792062).

When page navigation starts the following packet is sent from the tab actor:

{
  "from": tabActor,
  "type": "tabNavigated",
  "state": "start",
  "url": newURL,
}

When navigation stops the following packet is sent:

{
  "from": tabActor,
  "type": "tabNavigated",
  "state": "stop",
  "url": newURL,
  "title": newTitle,
}

`getCachedMessages(types, onResponse)`

The webConsoleClient.getCachedMessages(types, onResponse) method sends the following packet to the server:

{
  "to": "conn0.console9",
  "type": "getCachedMessages",
  "messageTypes": [
    "PageError",
    "ConsoleAPI"
  ]
}

The getCachedMessages packet allows one to retrieve the cached messages from before the Web Console was open. You can only get cached messages for page errors and console API calls. The reply looks like this:

{
  "messages": [ ... ],
  "from": "conn0.console9"
}

Each message in the array is of the same type as when we send typical page errors and console API calls. These will be explained in the following sections of this document.

Page errors

Page errors come from the nsIConsoleService. Each allowed page error is an nsIScriptError object.

The pageError packet is:

{
  "from": "conn0.console9",
  "type": "pageError",
  "pageError": {
    "errorMessage": "ReferenceError: foo is not defined",
    "sourceName": "http://localhost/~mihai/mozilla/test.js",
    "lineText": "",
    "lineNumber": 6,
    "columnNumber": 0,
    "category": "content javascript",
    "timeStamp": 1347294508210,
    "error": false,
    "warning": false,
    "exception": true,
    "strict": false,
    "private": false,
  }
}

The packet is similar to nsIScriptError - for simplicity. We only removed several unneeded properties and changed how flags work.

The private flag tells if the error comes from a private window/tab (added in Firefox 24).

Starting with Firefox 24 the errorMessage and lineText properties can be long string actor grips if the string is very long.

Console API messages

The window.console API calls send internal messages throughout Gecko which allow us to do whatever we want for each call. The Web Console actor sends these messages to the remote debugging client.

We use the ObjectActor from dbg-script-actors.js without a ThreadActor, to avoid slowing down the page scripts - the debugger deoptimizes JavaScript execution in the target page. The lifetime of object actors in the Web Console is different than the lifetime of these objects in the debugger - which is usually per pause or per thread. The Web Console manages the lifetime of ObjectActors manually.

Warning

Prior to Firefox 23 we used a different actor (WebConsoleObjectActor) for working with JavaScript objects through the protocol. In bug 783499 we did a number of changes that allowed us to reuse the ObjectActor from the debugger.

Console API messages come through the nsIObserverService - the console object implementation lives in dom/base/ConsoleAPI.js.

For each console message we receive in the server, we send the following consoleAPICall packet to the client:

{
  "from": "conn0.console9",
  "type": "consoleAPICall",
  "message": {
    "level": "error",
    "filename": "http://localhost/~mihai/mozilla/test.html",
    "lineNumber": 149,
    "functionName": "",
    "timeStamp": 1347302713771,
    "private": false,
    "arguments": [
      "error omg aloha ",
      {
        "type": "object",
        "className": "HTMLBodyElement",
        "actor": "conn0.consoleObj20"
      },
      " 960 739 3.141592653589793 %a",
      "zuzu",
      { "type": "null" },
      { "type": "undefined" }
    ]
  }
}

Similar to how we send the page errors, here we send the actual console event received from the nsIObserverService. We change the arguments array - we create ObjectActor instances for each object passed as an argument - and, lastly, we remove some unneeded properties (like window IDs). In the case of long strings we use the LongStringActor. The Web Console can then inspect the arguments.

The private flag tells if the console API call comes from a private window/tab (added in Firefox 24).

We have small variations for the object, depending on the console API call method - just like there are small differences in the console event object received from the observer service. To see these differences please look in the Console API implementation: dom/base/ConsoleAPI.js.

JavaScript evaluation

The `evaluateJS` request and response packets

The Web Console client provides the evaluateJS(requestId, string, onResponse) method which sends the following packet:

{
  "to": "conn0.console9",
  "type": "evaluateJS",
  "text": "document",
  "bindObjectActor": null,
  "frameActor": null,
  "url": null,
  "selectedNodeActor": null,
}

The bindObjectActor property is an optional ObjectActor ID that points to a Debugger.Object. This option allows you to bind _self to the Debugger.Object of the given object actor, during string evaluation. See evalInGlobalWithBindings() for information about bindings.

Note

The variable view needs to update objects and it does so by binding _self to the Debugger.Object of the ObjectActor that is being viewed. As such, variable view sends strings like these for evaluation:

_self["prop"] = value;

The frameActor property is an optional FrameActor ID. The FA holds a reference to a Debugger.Frame. This option allows you to evaluate the string in the frame of the given FA.

The url property is an optional URL to evaluate the script as (new in Firefox 25). The default source URL for evaluation is “debugger eval code”.

The selectedNodeActor property is an optional NodeActor ID, which is used to indicate which node is currently selected in the Inspector, if any. This NodeActor can then be referred to by the $0 JSTerm helper.

The response packet:

{
  "from": "conn0.console9",
  "input": "document",
  "result": {
    "type": "object",
    "className": "HTMLDocument",
    "actor": "conn0.consoleObj20"
    "extensible": true,
    "frozen": false,
    "sealed": false
  },
  "timestamp": 1347306273605,
  "exception": null,
  "exceptionMessage": null,
  "helperResult": null
}

exception holds the JSON-ification of the exception thrown during evaluation.
exceptionMessage holds the exception.toString() result.
result has the result ObjectActor instance.
helperResult is anything that might come from a JSTerm helper result, JSON stuff (not content objects!).

Warning

In Firefox 23: we renamed the error and errorMessage properties to exception and exceptionMessage respectively, to avoid conflict with the default properties used when protocol errors occur.

Autocomplete and more

The autocomplete request packet:

{
  "to": "conn0.console9",
  "type": "autocomplete",
  "text": "d",
  "cursor": 1
}

The response packet:

{
  "from": "conn0.console9",
  "matches": [
    "decodeURI",
    "decodeURIComponent",
    "defaultStatus",
    "devicePixelRatio",
    "disableExternalCapture",
    "dispatchEvent",
    "doMyXHR",
    "document",
    "dump"
  ],
  "matchProp": "d"
}

There’s also the clearMessagesCache request packet that has no response. This clears the console API calls cache and should clear the page errors cache - see bug 717611. An alternate version was added in Firefox 104, clearMessagesCacheAsync, which does exactly the same thing but resolves when the cache was actually cleared.

Network logging

The `networkEvent` packet

Whenever a new network request starts being logged the networkEvent packet is sent:

{
  "from": "conn0.console10",
  "type": "networkEvent",
  "eventActor": {
    "actor": "conn0.netEvent14",
    "startedDateTime": "2012-09-17T19:50:03.699Z",
    "url": "http://localhost/~mihai/mozilla/test2.css",
    "method": "GET"
    "isXHR": false,
    "private": false
  }
}

This packet is used to inform the Web Console of a new network event. For each request a new NetworkEventActor instance is created. The isXHR flag indicates if the request was initiated via an XMLHttpRequest instance, that is: the nsIHttpChannel’s notification is of an nsIXMLHttpRequest interface.

The private flag tells if the network request comes from a private window/tab (added in Firefox 24).

The `NetworkEventActor`

The new network event actor stores further request and response information.

The `networkEventUpdate` packet

The Web Console UI needs to be kept up-to-date when changes happen, when new stuff is added. The new networkEventUpdate packet is sent for this purpose. Examples:

{
  "from": "conn0.netEvent14",
  "type": "networkEventUpdate",
  "updateType": "requestHeaders",
  "headers": 10,
  "headersSize": 425
},
{
  "from": "conn0.netEvent14",
  "type": "networkEventUpdate",
  "updateType": "requestCookies",
  "cookies": 0
},
{
  "from": "conn0.netEvent14",
  "type": "networkEventUpdate",
  "updateType": "requestPostData",
  "dataSize": 1024,
  "discardRequestBody": false
},
{
  "from": "conn0.netEvent14",
  "type": "networkEventUpdate",
  "updateType": "responseStart",
  "response": {
    "httpVersion": "HTTP/1.1",
    "status": "304",
    "statusText": "Not Modified",
    "headersSize": 194,
    "discardResponseBody": true
  }
},
{
  "from": "conn0.netEvent14",
  "type": "networkEventUpdate",
  "updateType": "eventTimings",
  "totalTime": 1
},
{
  "from": "conn0.netEvent14",
  "type": "networkEventUpdate",
  "updateType": "responseHeaders",
  "headers": 6,
  "headersSize": 194
},
{
  "from": "conn0.netEvent14",
  "type": "networkEventUpdate",
  "updateType": "responseCookies",
  "cookies": 0
},
{
  "from": "conn0.netEvent14",
  "type": "networkEventUpdate",
  "updateType": "responseContent",
  "mimeType": "text/css",
  "contentSize": 0,
  "discardResponseBody": true
}

Actual headers, cookies, and bodies are not sent.

The `getRequestHeaders` and other packets

To get more details about a network event you can use the following packet requests (and replies).

The getRequestHeaders packet:

{
  "to": "conn0.netEvent15",
  "type": "getRequestHeaders"
}
{
  "from": "conn0.netEvent15",
  "headers": [
    {
      "name": "Host",
      "value": "localhost"
    }, ...
  ],
  "headersSize": 350
}

The getRequestCookies packet:

{
  "to": "conn0.netEvent15",
  "type": "getRequestCookies"
}
{
  "from": "conn0.netEvent15",
  "cookies": []
}

The getResponseHeaders packet:

{
  "to": "conn0.netEvent15",
  "type": "getResponseHeaders"
}
{
  "from": "conn0.netEvent15",
  "headers": [
    {
      "name": "Date",
      "value": "Mon, 17 Sep 2012 20:05:27 GMT"
    }, ...
  ],
  "headersSize": 320
}

The getResponseCookies packet:

{
  "to": "conn0.netEvent15",
  "type": "getResponseCookies"
}
{
  "from": "conn0.netEvent15",
  "cookies": []
}

Note

Starting with Firefox 19: for all of the header and cookie values in the above packets we use LongStringActor grips when the value is very long. This helps us avoid using too much of the network bandwidth.

The getRequestPostData packet:

{
  "to": "conn0.netEvent15",
  "type": "getRequestPostData"
}
{
  "from": "conn0.netEvent15",
  "postData": { text: "foobar" },
  "postDataDiscarded": false
}

The getResponseContent packet:

{
  "to": "conn0.netEvent15",
  "type": "getResponseContent"
}
{
  "from": "conn0.netEvent15",
  "content": {
    "mimeType": "text/css",
    "text": "\n@import \"test.css\";\n\n.foobar { color: green }\n\n"
  },
  "contentDiscarded": false
}

The request and response content text value is most commonly sent using a LongStringActor grip. For very short request/response bodies we send the raw text.

Note

Starting with Firefox 19: for non-text response types we send the content in base64 encoding (again, most likely a LongStringActor grip). To tell the difference just check if response.content.encoding == "base64".

The getEventTimings packet:

{
  "to": "conn0.netEvent15",
  "type": "getEventTimings"
}
{
  "from": "conn0.netEvent15",
  "timings": {
    "blocked": 0,
    "dns": 0,
    "connect": 0,
    "send": 0,
    "wait": 16,
    "receive": 0
  },
  "totalTime": 16
}

The `fileActivity` packet

When a file load is observed the following fileActivity packet is sent to the client:

{
  "from": "conn0.console9",
  "type": "fileActivity",
  "uri": "file:///home/mihai/public_html/mozilla/test2.css"
}

History

Protocol changes by Firefox version:

Firefox 18: initial version.
Firefox 19: bug - added LongStringActor usage in several places.
Firefox 20: bug - removed locationChanged packet and updated the tabNavigated packet for tab actors.
Firefox 23: bug - removed the WebConsoleObjectActor. Now the Web Console uses the JavaScript debugger API and the ObjectActor.
Firefox 23: added the bindObjectActor and frameActor options to the evaluateJS request packet.
Firefox 24: new private flags for the console actor notifications, bug. Also added the lastPrivateContextExited notification for the global console actor.
Firefox 24: new isXHR flag for the networkEvent notification, bug.
Firefox 24: removed the message property from the pageError packet notification, bug. The lineText and errorMessage properties can be long string actors now.
Firefox 25: added the url option to the evaluateJS request packet.

Conclusions

As of this writing, this document is a dense summary of the work we did in bug 768096 and subsequent changes. We try to keep this document up-to-date. We hope this is helpful for you.

If you make changes to the Web Console server please update this document. Thank you!

Web Console remoting

Introduction

The WebConsoleActor and the WebConsoleClient

startListeners(listeners, onResponse)

Tab navigation

getCachedMessages(types, onResponse)

Page errors

Console API messages

JavaScript evaluation

The evaluateJS request and response packets

Autocomplete and more

Network logging

The networkEvent packet

The NetworkEventActor

The networkEventUpdate packet

The getRequestHeaders and other packets

The fileActivity packet

History

Conclusions

The `WebConsoleActor` and the `WebConsoleClient`

`startListeners(listeners, onResponse)`

`getCachedMessages(types, onResponse)`

The `evaluateJS` request and response packets

The `networkEvent` packet

The `NetworkEventActor`

The `networkEventUpdate` packet

The `getRequestHeaders` and other packets

The `fileActivity` packet