The Architecture of Open Source Applications (Volume 1)
SocialCalc

19.1. WikiCalc

The first version of WikiCalc (Figure 19.1) had several features that set it apart from other spreadsheets at the time:

• Plain text, HTML, and wiki-style markup rendering for text data.
• Wiki-style text that includes commands to insert links, images, and values from cell references.
• Formula cells may reference values of other WikiCalc pages hosted on other websites.
• Ability to create output to be embedded in other web pages, both static and live data.
• Cell formatting with access to CSS style attributes and CSS classes.
• Logging of all edit operations as an audit trail.
• Wiki-like retention of each new version of a page with roll-back capability.

Figure 19.1: WikiCalc 1.0 Interface

Figure 19.2: WikiCalc Components

Figure 19.3: WikiCalc Flow

WikiCalc 1.0's internal architecture (Figure 19.2) and information flow (Figure 19.3) were deliberately simple, but nevertheless powerful. The ability to compose a master spreadsheet from several smaller spreadsheets proved particularly handy. For example, imagine a scenario where each salesperson keeps numbers in a spreadsheet page. Each sales manager then rolls up their reps' numbers into a regional spreadsheet, and the VP of sales then rolls up the regional numbers into a top-level spreadsheet.

Each time one of the individual spreadsheets is updated, all the roll-up spreadsheets can reflect the update. If someone wants further detail, they simply click through to view the spreadsheet behind the spreadsheet. This roll-up capability eliminates the redundant and error-prone effort of updating numbers in multiple places, and ensures all views of the information stay fresh.

To ensure the recalculations are up-to-date, WikiCalc adopted a thin-client design, keeping all the state information on the server side. Each spreadsheet is represented on the browser as a <table> element; editing a cell will send an ajaxsetcell call to the server, and the server then tells the browser which cells need updating.

Unsurprisingly, this design depends on a fast connection between the browser and the server. When the latency is high, users will start to notice the frequent appearance of "Loading…" messages between updating a cell and seeing its new contents as shown in Figure 19.4. This is especially a problem for users interactively editing formulas by tweaking the input and expecting to see results in real time.

Figure 19.4: Loading Message

Moreover, because the <table> element had the same dimensions as the spreadsheet, a 100×100 grid would create 10,000 <td> DOM objects, which strains the memory resource of browsers, further limiting the size of pages.

Due to these shortcomings, while WikiCalc was useful as a stand-alone server running on localhost, it was not very practical to embed as part of web-based content management systems.

In 2006, Dan Bricklin teamed up with Socialtext to start developing SocialCalc, a ground-up rewrite of WikiCalc in Javascript based on some of the original Perl code.

This rewrite was aimed at large, distributed collaborations, and sought to deliver a look and feel more like that of a desktop app. Other design goals included:

• Capable of handling hundreds of thousands of cells.
• Fast turnaround time for edit operations.
• Client-side audit trail and undo/redo stack.
• Better use of Javascript and CSS to provide full-fledged layout functionality.
• Cross-browser support, despite the more extensive use of responsive Javascript.

After three years of development and various beta releases, Socialtext released SocialCalc 1.0 in 2009, successfully meeting the design goals. Let's now take a look at the architecture of the SocialCalc system.

19.2. SocialCalc

Figure 19.5: SocialCalc Interface

Figure 19.5 and Figure 19.6 show SocialCalc's interface and classes respectively. Compared to WikiCalc, the server's role has been greatly reduced. Its only responsibility is responding to HTTP GETs by serving entire spreadsheets serialized in the save format; once the browser receives the data, all calculations, change tracking and user interaction are now implemented in Javascript.

Figure 19.6: SocialCalc Class Diagram

The Javascript components were designed with a layered MVC (Model/View/Controller) style, with each class focusing on a single aspect:

• Sheet is the data model, representing an in-memory structure of a spreadsheet. It contains a dictionary from coordinates to Cell objects, each representing a single cell. Empty cells need no entries, and hence consume no memory at all.
• Cell represents a cell's content and formats. Some common properties are shown in Table 19.1.
• RenderContext implements the view; it is responsible for rendering a sheet into DOM objects.
• TableControl is the main controller, accepting mouse and keyboard events. As it receives view events such as scrolling and resizing, it updates its associated RenderContext object. As it receives update events that affects the sheet's content, it schedules new commands to the sheet's command queue.
• SpreadSheetControl is the top-level UI with toolbars, status bars, dialog boxes and color pickers.
• SpreadSheetViewer is an alternate top-level UI that provides a read-only interactive view.

We adopted a minimal class-based object system with simple composition/delegation, and make no use of inheritance or object prototypes. All symbols are placed under the SocialCalc.* namespace to avoid naming conflicts.

Each update on the sheet goes through the ScheduleSheetCommands method, which takes a command string representing the edit. (Some common commands are show in Table 19.2.) The application embedding SocialCalc may define extra commands on their own, by adding named callbacks into the SocialCalc.SheetCommandInfo.CmdExtensionCallbacks object, and use the startcmdextension command to invoke them.

    set     sheet defaultcolor blue
    set     A width 100
    set     A1 value n 42
    set     A2 text t Hello
    set     A3 formula A1*2
    set     A4 empty
    set     A5 bgcolor green
    merge   A1:B2
    unmerge A1

    erase   A2
    cut     A3
    paste   A4
    copy    A5
    sort    A1:B9 A up B down
    name    define Foo A1:A5
    name    desc   Foo Used in formulas like SUM(Foo)
    name    delete Foo
    startcmdextension UserDefined args

19.3. Command Run-loop

To improve responsiveness, SocialCalc performs all recalculation and DOM updates in the background, so the user can keep making changes to several cells while the engine catches up on earlier changes in the command queue.

Figure 19.7: SocialCalc Command Run-loop

When a command is running, the TableEditor object sets its busy flag to true; subsequent commands are then pushed into the deferredCommands queue, ensuring a sequential order of execution. As the event loop diagram in Figure 19.7 shows, the Sheet object keeps sending StatusCallback events to notify the user of the current state of command execution, through each of the four steps:

• ExecuteCommand: Sends cmdstart upon start, and cmdend when the command finishes execution. If the command changed a cell's value indirectly, enter the Recalc step. Otherwise, if the command changed the visual appearance of one or more on-screen cells, enter the Render step. If neither of the above applies (for example with the copy command), skip to the PositionCalculations step.
• Recalc (asneeded): Sends calcstart upon start, calcorder every 100ms when checking the dependency chain of cells, calccheckdone when the check finishes, and calcfinished when all affected cells received their re-calculated values. This step is always followed by the Render step.
• Render (as needed): Sends schedrender upon start, and renderdone when the <table> element is updated with formatted cells. This step is always followed by PositionCalculations.
• PositionCalculations: Sends schedposcalc upon start, and doneposcalc after updating the scrollbars, the current editable cell cursor, and other visual components of the TableEditor.

Because all commands are saved as they are executed, we naturally get an audit log of all operations. The Sheet.CreateAuditString method provides a newline-delimited string as the audit trail, with each command in a single line.

ExecuteSheetCommand also creates an undo command for each command it executes. For example, if the cell A1 contains "Foo" and the user executes set A1 text Bar, then an undo-command set A1 text Foo is pushed to the undo stack. If the user clicks Undo, then the undo-command is executed to restore A1 to its original value.

19.4. Table Editor

Now let's look at the TableEditor layer. It calculates the on-screen coordinates of its RenderContext, and manages horizontal/vertical scroll bars through two TableControl instances.

Figure 19.8: TableControl Instances Manage Scroll Bars

The view layer, handled by the RenderContext class, also differs from WikiCalc's design. Instead of mapping each cell to a <td> element, we now simply create a fixed-size <table> that fits the browser's visible area, and pre-populate it with <td> elements.

As the user scrolls the spreadsheet through our custom-drawn scroll bars, we dynamically update the innerHTML of the pre-drawn <td> elements. This means we don't need to create or destroy any <tr> or <td> elements in many common cases, which greatly speeds up response time.

Because RenderContext only renders the visible region, the size of Sheet object can be arbitrarily large without affecting its performance.

TableEditor also contains a CellHandles object, which implements the radial fill/move/slide menu attached to the bottom-right corner to the current editable cell, known as the ECell, shown in Figure 19.9.

Figure 19.9: Current Editable Cell, Known as the ECell

The input box is managed by two classes: InputBox and InputEcho. The former manages the above-the-grid edit row, while the latter shows an updated-as-you-type preview layer, overlaying the ECell's content (Figure 19.10).

Figure 19.10: The Input Box is Managed by Two Classes

Usually, the SocialCalc engine only needs to communicate to the server when opening a spreadsheet for edit, and when saving it back to server. For this purpose, the Sheet.ParseSheetSave method parses a save format string into a Sheet object, and the Sheet.CreateSheetSave method serializes a Sheet object back into the save format.

Formulas may refer to values from any remote spreadsheet with a URL. The recalc command re-fetches the externally referenced spreadsheets, parses them again with Sheet.ParseSheetSave, and stores them in a cache so the user can refer to other cells in the same remote spreadsheets without re-fetching its content.

19.5. Save Format

The save format is in standard MIME multipart/mixed format, consisting of four text/plain; charset=UTF-8 parts, each part containing newline-delimited text with colon-delimited data fields. The parts are:

• The meta part lists the types of the other parts.
• The sheet part lists each cell's format and content, each column's width (if not default), the sheet's default format, followed by a list of fonts, colors and borders used in the sheet.
• The optional edit part saves the TableEditor's edit state, including ECell's last position, as well as the fixed sizes of row/column panes.
• The optional audit part contains the history of commands executed in the previous editing session.

For example, Figure 19.11 shows a spreadsheet with three cells, with 1874 in A1 as the ECell, the formula 2^2*43 in A2, and the formula SUM(Foo) in A3 rendered in bold, referring to the named range Foo over A1:A2.

Figure 19.11: A Spreadsheet with Three Cells

The serialized save format for the spreadsheet looks like this:

    socialcalc:version:1.0
    MIME-Version: 1.0
    Content-Type: multipart/mixed; boundary=SocialCalcSpreadsheetControlSave
    --SocialCalcSpreadsheetControlSave
    Content-type: text/plain; charset=UTF-8

    # SocialCalc Spreadsheet Control Save
    version:1.0
    part:sheet
    part:edit
    part:audit
    --SocialCalcSpreadsheetControlSave
    Content-type: text/plain; charset=UTF-8

    version:1.5
    cell:A1:v:1874
    cell:A2:vtf:n:172:2^2*43
    cell:A3:vtf:n:2046:SUM(Foo):f:1
    sheet:c:1:r:3
    font:1:normal bold * *
    name:FOO::A1\cA2
    --SocialCalcSpreadsheetControlSave
    Content-type: text/plain; charset=UTF-8

    version:1.0
    rowpane:0:1:14
    colpane:0:1:16
    ecell:A1
    --SocialCalcSpreadsheetControlSave
    Content-type: text/plain; charset=UTF-8

    set A1 value n 1874
    set A2 formula 2^2*43
    name define Foo A1:A2
    set A3 formula SUM(Foo)
    --SocialCalcSpreadsheetControlSave--

This format is designed to be human-readable, as well as being relatively easy to generate programmatically. This makes it possible for Drupal's Sheetnode plugin to use PHP to convert between this format and other popular spreadsheet formats, such as Excel (.xls) and OpenDocument (.ods).

Now that we have a good idea about how the pieces in SocialCalc fit together, let's look at two real-world examples of extending SocialCalc.

19.6. Rich-text Editing

The first example we'll look at is enhancing SocialCalc's text cells with wiki markup to display its rich-text rendering right in the table editor (Figure 19.12).

Figure 19.12: Rich Text Rendering in the Table Editor

We added this feature to SocialCalc right after its 1.0 release, to address the popular request of inserting images, links and text markups using a unified syntax. Since Socialtext already has an open-source wiki platform, it was natural to re-use the syntax for SocialCalc as well.

To implement this, we need a custom renderer for the textvalueformat of text-wiki, and to change the default format for text cells to use it.

What is this textvalueformat, you ask? Read on.

19.6.1. Types and Formats

In SocialCalc, each cell has a datatype and a valuetype. Data cells with text or numbers correspond to text/numeric value types, and formula cells with datatype="f" may generate either numeric or text values.

Recall that on the Render step, the Sheet object generates HTML from each of its cells. It does so by inspecting each cell's valuetype: If it begins with t, then the cell's textvalueformat attribute determines how generation is done. If it begins with n, then the nontextvalueformat attribute is used instead.

However, if the cell's textvalueformat or nontextvalueformat attribute is not defined explicitly, then a default format is looked up from its valuetype, as shown in Figure 19.13.

Figure 19.13: Value Types

Support for the text-wiki value format is coded in SocialCalc.format_text_for_display:

if (SocialCalc.Callbacks.expand_wiki && /^text-wiki/.test(valueformat)) {
    // do general wiki markup
    displayvalue = SocialCalc.Callbacks.expand_wiki(
        displayvalue, sheetobj, linkstyle, valueformat
    );
}

Instead of inlining the wiki-to-HTML expander in format_text_for_display, we will define a new hook in SocialCalc.Callbacks. This is the recommended style throughout the SocialCalc codebase; it improves modularity by making it possible to plug in different ways of expanding wikitext, as well as keeping compatibility with embedders that do not desire this feature.

19.6.2. Rendering Wikitext

Next, we'll make use of Wikiwyg¹, a Javascript library offering two-way conversions between wikitext and HTML.

We define the expand_wiki function by taking the cell's text, running it through Wikiwyg's wikitext parser and its HTML emitter:

var parser = new Document.Parser.Wikitext();
var emitter = new Document.Emitter.HTML();
SocialCalc.Callbacks.expand_wiki = function(val) {
    // Convert val from Wikitext to HTML
    return parser.parse(val, emitter);
}

The final step involves scheduling the set sheet defaulttextvalueformat text-wiki command right after the spreadsheet initializes:

// We assume there's a <div id="tableeditor"/> in the DOM already
var spreadsheet = new SocialCalc.SpreadsheetControl();
spreadsheet.InitializeSpreadsheetControl("tableeditor", 0, 0, 0);
spreadsheet.ExecuteCommand('set sheet defaulttextvalueformat text-wiki');

Taken together, the Render step now works as shown in Figure 19.14.

Figure 19.14: Render Step

That's all! The enhanced SocialCalc now supports a rich set of wiki markup syntax:

*bold* _italic_ `monospace` 
> indented text
* unordered list
# ordered list
"Hyperlink with label"<http://softwaregarden.com/>
{image: http://www.socialtext.com/static/logo.png}

Try entering *bold* _italic_ `monospace` in A1, and you'll see it rendered as rich text (Figure 19.15).

Figure 19.15: Wikywyg Example

19.7. Real-time Collaboration

The next example we'll explore is multi-user, real-time editing on a shared spreadsheet. This may seem complicated at first, but thanks to SocialCalc's modular design all it takes is for each on-line user to broadcast their commands to other participants.

To distinguish between locally-issued commands and remote commands, we add an isRemote parameter to the ScheduleSheetCommands method:

SocialCalc.ScheduleSheetCommands = function(sheet, cmdstr, saveundo, isRemote) {
   if (SocialCalc.Callbacks.broadcast && !isRemote) {
       SocialCalc.Callbacks.broadcast('execute', {
           cmdstr: cmdstr, saveundo: saveundo
       });
   }
   // …original ScheduleSheetCommands code here…
}

Now all we need to do is to define a suitable SocialCalc.Callbacks.broadcast callback function. Once it's in place, the same commands will be executed on all users connected to the same spreadsheet.

When this feature was first implemented for OLPC (One Laptop Per Child²) by SEETA's Sugar Labs³ in 2009, the broadcast function was built with XPCOM calls into D-Bus/Telepathy, the standard transport for OLPC/Sugar networks (see Figure 19.16).

Figure 19.16: OLPC Implementation

That worked reasonably well, enabling XO instances in the same Sugar network to collaborate on a common SocialCalc spreadsheet. However, it is both specific to the Mozilla/XPCOM browser platform, as well as to the D-Bus/Telepathy messaging platform.

19.7.1. Cross-browser Transport

To make this work across browsers and operating systems, we use the Web::Hippie⁴ framework, a high-level abstraction of JSON-over-WebSocket with convenient jQuery bindings, with MXHR (Multipart XML HTTP Request⁵) as the fallback transport mechanism if WebSocket is not available.

For browsers with Adobe Flash plugin installed but without native WebSocket support, we use the web_socket.js⁶ project's Flash emulation of WebSocket, which is often faster and more reliable than MXHR. The operation flow is shown in Figure 19.17.

Figure 19.17: Cross-Browser Flow

The client-side SocialCalc.Callbacks.broadcast function is defined as:

var hpipe = new Hippie.Pipe();

SocialCalc.Callbacks.broadcast = function(type, data) {
    hpipe.send({ type: type, data: data });
};

$(hpipe).bind("message.execute", function (e, d) {
    var sheet = SocialCalc.CurrentSpreadsheetControlObject.context.sheetobj;
    sheet.ScheduleSheetCommands(
        d.data.cmdstr, d.data.saveundo, true // isRemote = true
    );
    break;
});

Although this works quite well, there are still two remaining issues to resolve.

19.7.2. Conflict Resolution

The first one is a race-condition in the order of commands executed: If users A and B simultaneously perform an operation affecting the same cells, then receive and execute commands broadcast from the other user, they will end up in different states, as shown in Figure 19.18.

Figure 19.18: Race Condition Conflict

We can resolve this with SocialCalc's built-in undo/redo mechanism, as shown in Figure 19.19.

Figure 19.19: Race Condition Conflict Resolution

The process used to resolve the conflict is as follows. When a client broadcasts a command, it adds the command to a Pending queue. When a client receives a command, it checks the remote command against the Pending queue.

If the Pending queue is empty, then the command is simply executed as a remote action. If the remote command matches a command in the Pending queue, then the local command is removed from the queue.

Otherwise, the client checks if there are any queued commands that conflict with the received command. If there are conflicting commands, the client first Undoes those commands and marks them for later Redo. After undoing the conflicting commands (if any), the remote command is executed as usual.

When a marked-for-redo command is received from the server, the client will execute it again, then remove it from the queue.

19.7.3. Remote Cursors

Even with race conditions resolved, it is still suboptimal to accidentally overwrite the cell another user is currently editing. A simple improvement is for each client to broadcast its cursor position to other users, so everyone can see which cells are being worked on.

To implement this idea, we add another broadcast handler to the MoveECellCallback event:

editor.MoveECellCallback.broadcast = function(e) {
    hpipe.send({
        type: 'ecell',
        data: e.ecell.coord
    });
};

$(hpipe).bind("message.ecell", function (e, d) {
    var cr = SocialCalc.coordToCr(d.data);
    var cell = SocialCalc.GetEditorCellElement(editor, cr.row, cr.col);
    // …decorate cell with styles specific to the remote user(s) on it…
});

To mark cell focus in spreadsheets, it's common to use colored borders. However, a cell may already define its own border property, and since border is mono-colored, it can only represent one cursor on the same cell.

Therefore, on browsers with support for CSS3, we use the box-shadow property to represent multiple peer cursors in the same cell:

/* Two cursors on the same cell */
box-shadow: inset 0 0 0 4px red, inset 0 0 0 2px green;

Figure 19.20 shows how the screen would look with four people editing on the same spreadsheet.

Figure 19.20: Four Users Editing One Spreadsheet

19.8. Lessons Learned

We delivered SocialCalc 1.0 on October 19th, 2009, the 30th anniversary of the initial release of VisiCalc. The experience of collaborating with my colleagues at Socialtext under Dan Bricklin's guidance was very valuable to me, and I'd like to share some lessons I learned during that time.