When I say "extensibility," I specifically refer to replacing backend components or adding new components to certain type categories. I do recognize that things like the power of the address book API is important for extensions, but I don't consider that "extensibility" (perhaps "API usability" is a better name?). I am also going to focus exclusively on mailnews components here.
This blog post is a set of my opinions on the state of extensibility in Thunderbird and SeaMonkey. With one exception, I am not recommending what should or shouldn't be worked on: I am just giving my biased beliefs on these topics. I also want to mention that I focus primarily on the backend, so I have a very distorted view of what can be done in extensions and how easily it can be done.
The first metric I consider is possibility, the ability of one to adequately and easily make an extension in the given field in Thunderbird 3. Since it is theoretically possible to replace nearly any component given enough effort, a key component is the ease with which one can do it. High numbers correspond to requiring a little bit of JS and a bit of UI to go along with it; low numbers generally indicate that thousands of lines of C++ reimplementing large swathes of platform code is needed.
The second metric I consider is difficulty, the amount of effort it would take to make this facet of extensibility rather close to perfect. For this metric, having a lower number is better: a very low number indicates that it would only take a few hours to produce a patch (if necessary) to fix it, while a very high number indicates that several people would have to work for a few months to produce a patch, not including the review cycles.
The final metric I consider is desirability, how useful it would be to improve this facet. Again, higher numbers are better.
A final note about numbers: they're just qualitative indicators of relative value. Don't read too much into actual values, as I more or less came up with these numbers and then tweaked them up or down as I wrote more of this blog. A low value for possibility in particular doesn't mean it's very difficult to do right now, it just means it's not as easy as some other things.Facets of extensibility:
- Folder pane views
- Alternative database views
- Message visualization
- Protocol-level extensions
- Search backends
- Filter backends
- Specifying new address book types
- Message storage
- Metadata storage
- Specifying new account types
Here, when I talk about UI, I specifically mean the ability to add, remove, replace, or rearrange elements of the UI, including toolbars, menus, and keyboard shortcuts, among others. To my knowledge, there aren't many problems with being able to modify the UI modulo the usability of XPCOM, Mozilla's toolkit, or other components; any problems that come up could be worked out mostly by adding an id attribute or perhaps reshuffling a portion of the dialog. Then again, as I mostly stick to the backend, I could be dead wrong about this.
Folder pane views:Possibility: 9
Right now my 18 accounts (19 if you include the "Smart Folders" account) are arrayed in a specific order with trees of various folders hanging off of them. Perhaps I don't want to see them like that. After all, there is rather little correlation between an account and the contents of folders (in my case at least). None of the other layouts are particularly useful for my needs as well. Why not make a layout of folders that suits my needs better? For that matter, why limit it to looking at folders at all?
In a supreme effort, Joey Minta replaced the folder pane implementation with an extensible JS version (with regressions tracked down and fixed by myriads of others). This is perhaps the only major part where SeaMonkey and Thunderbird diverge greatly: SeaMonkey has yet to port the patch, for good reason (the original produced some 33 regressions, so it's a rather risky and complex bug to port). Asides from that, there's probably not much that would have to be done to get easy-to-implement extensions working here.
Alternative database views:Possibility: 4
This concept loosely ties into the creation of new account types, but I think many of the more exotic account types could use other ways of selecting messages than a tree of threads. Even email lists could sometimes use better visualization; here is a graphical thread view discussed by Andrew Sutherland that struck my eyes a year ago.
If you don't want to set your sights so high, making essentially a new filtered view is a matter of mere C++ extension combined with a touch of UI; replacing the <tree> XUL tag would probably be a lot more involved. Making the latter part easier would involve removing or shifting around a fair amount of logic around.
Message display:Possibility: 5
Not all messages are best viewed as static rich text. For example, feed summaries in many cases don't include the full article, so it's sometimes better to think of it as the URI that the summary is associated with. I also imagine that there are some wacky message visualizations for some domain-specific messages that would be nice to play with. Though I say that, I'm at a loss to think of any of them as being particularly useful right now.
It's not terribly hard to change message display right now, as one can replace the preview pane or full window with another URI on message load—this is what RSS does right now. At the same time, though, a lot of information in libmime is rather locked away, and more primitive forms of extension (replacing consideration of parts in specific circumstances) would require changing mostly-hardcoded libmime code. Which would probably require a C++ infection first.
Protocol-level extensions:Possibility: 1
There are 38 official IMAP extensions; the IMAP client code implements far fewer. In addition, there is also the possibility that servers may implement their own custom XAWESOMEIMAPCOMMAND which might be ideal to implement in an extension. Of all of the facets I describe here, this is the most difficult to do in an extension right now: you'd have to rewrite the protocol objects themselves. Making this possible to do is also no cakewalk, as this is inherently connection-level, but the server/service interfaces tend to abstract away connection issues. As for desirability, I mostly consider this a "neat concept," but probably not worth spending time to sit down and actually do. There is one example which recently came up, though: the IMAP ANNOTATE command is something that seems perfect to implement in an extension.
A subcategory of this which is probably more useful is the ability to merely add in new authentication schemes. All of IMAP, NNTP, POP, and SMTP have a mechanism for generic SASL authentication schemes; HTTP authentication (for RSS and possibly others) also has a similar generic authentication measure (I don't actually know how well HTTP-authed RSS feeds work, but I'd be surprised if there were a major problem).
Coding an importer into an extension isn't too hard: I've done it before. The hardest issue I had was the fact that import is actually multithreaded (unlike most actions, which merely run on event queues). My understanding is that this implies that you cannot write it in JS, only C++.
That said, though, I'm not sure that it's something to be concerned with. Import is not something that I think a lot of people will be doing time and time again (if they were, you've pretty much entered synchronization land). Instead, import is something that matters more for people redistributing mailnews code, such as Linux distributions. The interesting cases, then, would mostly be other system applications, which generally implies that you're writing stuff in native code anyways (although JSCtypes can access those native APIs from JS, as long as they are written in C and not C++ (so no qt) and you don't need to expand dozens of macros to get a simple method (so no gtk)).
By synchronization, I mean specifically the ability to synchronize data—be it preferences, mail, contacts, calendar, or anything else people come up with—between Thunderbird and any other accepting client, which includes other instances of Thunderbird or SeaMonkey, online web services, PDAs, and smartphones, among others. You can already do it now, as evidenced by two of the more popular extensions doing this. Indeed, until recently, comm-central had Palm synchronization code in its tree. The problem is that some APIs could stand to cater more to this use case; I would even go so far as to say that a centralized framework might be useful.
Search backends:Possibility: 6
I've never precisely tabulated how much mail I get in a day, but I reliably receive around 100 messages a day from someone called "email@example.com", another 50 or so from various other mailing lists, on top of innumerable RSS feed messages (I subscribe to 60 or so feeds) and NNTP messages. Compared to some other people, that list seems small. In short, many users may be suffering from acute information overload, and how does one remedy that? With comprehensive searching and filtering.
It seems to me, therefore, that search extensibility is vital. For example, for triage purposes, I might want to search for all bugzilla threads whose bugs are currently fixed (irrespective of the bug's status when I received the email). This area has really improved recently in Thunderbird 3, but I don't think it's quite there yet. For example, searching on the current status of a bug in bugzilla is not really feasible under the gloda search engine, but it may be possible under the conventional search. The high value for the difficulty is due to the fact that I think to put this where it truly ought to be, one needs to do some massive database overhauls.
Filter backends:Possibility: 5
My rationale for the desirability here is similar to searching, but it's slightly lower since I think that filtering is slightly less useful than searching (then again, my modus operandi is typically a trigger-happy finger on the Delete key). Again, filters have had massive improvement in the course of Thunderbird 3 development, but I don't think they're quite where they should be.
What makes this more annoying than search in terms of ease of use and hacking is the fact that filters live in a neverland where the headers have not yet been added to the database, so some potentially useful operations (notably, thread lookup) won't work. Changing this would require one to audit carefully the entire new message process and handle the database manipulation in precise ways. Nevertheless, I think this stands out as one of the more likely things to be improved, since the APIs seem to be slowly moving this way as a result of the other new message bugs.
Creating new address book types:Possibility: 6
This was an idea that I was once a very big proponent of; in recent months, though, I've tempered my enthusiasm for this. Unlike accounts, I don't think there's a lot of reason to prefer a new address book type over synchronization. There is also the possibility of faking a new address book type by selecting an address book to be automatically and zealously synchronized.
At this point, it was rather possible to create a new type—provided you write it in C++, you do some internal sleights of hand (it's a MAPI address book! I swear! Or maybe "none," which (oddly enough) also seems to work…), and you remember to make sure it's an RDF resource. Actually, getting a very rudimentary, read-only SQL address book implementation to work only took a few hours. The remaining internal work, to my knowledge, is merely to finish eradicating RDF, overhaul nsDirPrefs, and stop assuming the presence of nsIAddrDatabase in so many places. I'm not including the related address book API overhauls in the difficulty metric, although it seems the hardest part there is getting people to review the changes.
Message storage:Possibility: 2
Message storage is the ability to specify other ways to store messages. Predominantly this is is the storage for local folders and offline storage, although theoretically one can consider the remote storage of new account types to fall under this depending on how the API is done. Simply put, right now it is really difficult to do this in an extension: for all of mailnews's heavy usage of XPCOM, this is one area where the using classes create instances via new.
This is probably the most difficult of the facets to make extensible, even more so than the ability to make new account types. Why? First you have to create a new API, and then make everyone who does offline storage or local folders use it. The next thing is to consider the optimizations made under the hood to speed up local copy/moves and work out how to optimize them for more general APIs. Finally, you have to set up the UI. Flame wars of what to make the default may be involved as well.
With all that said, I don't think it's particularly desirable. For all of its foibles, I'm not particularly convinced that the proposed replacements for mbox are appreciably better. With that said, a well-designed API for this feature would also make creating new account types easier, as it would likely cause simplification of the nsIMsgFolder interface.
Metadata storage:Possibility: 2
This facet of extensibility is very similar to what I call message storage, except this is concerned with the ability to replace the database storage format itself. Creating an extension to do this now is at least as hard as creating one that replaces the message storage, I think. On the other hand, the core work needed to make it easier to do seems to mostly involve creating a few new interfaces and a large automated rewrite (depending on how much you want to depart from the current mork interface). Switching between databases would probably be largely lossy, though.
Of all the ways to extend Thunderbird, I think this is one of the least interesting. Seperate from message storage (which tends to imply a need to rework metadata as well), there is little value to being able to create new storage backends for metadata. That is not to say that I think this area couldn't use an entirely new format, I just think that the work needed to make it more pluggable is less desirable. I put it in here mostly to bring up for discussion of an approach mentioned in bug 11050.
Creating new account types:Possibility: 3
Last, but not least, is what I would consider to be the holy grail of extensibility, to be able to create new account types. While I have several reasons for considering this so important, the best reason in my mind is that this, more than any other facet of extensibility, would be able to best showcase the ingenuity of the world at large.
Assigning possibility and difficulty numbers here is difficult, as something like this has not really been fully attempted in recent years. Making a server in the ilk of POP, movemail, or RSS (one whose mail is downloaded and stored locally) is probably rather possible and not too difficult; making one in the format of IMAP or NNTP appears to only have been attempted once, and in that case, the author just ended up writing an IMAP server to bridge the two ends. I myself have tried more than once to create a working prototype; my furthest effort got to the downloading mail stage before I lost the will to continue.
As far as I know, there is nothing absolutely stopping somebody from just implementing this, at least in C++. That said, truly exotic account types (instant messaging or social networking are the common examples here) would probably require large contortions to work right, and the frontend is littered with manual type checks for account types that would probably have to be overriden in a more generic format. One large sector of possible account types is hampered by a difficulty in turning HTML text into a DOM tree for easier manipulation (i.e., screenscraping). I am therefore assigning rather pessimistic values for possibility and difficulty, since implementation currently requires brute force to attempt and the difficulty of making this easier to use requires a prototype implementation to fully ascertain.
On top of that, though, there is the issue that many account types would also like to be able to send stuff via the compose window. That part is probably the single hardest part, as compose cannot fathom the existence of anything other than SMTP or NNTP. One would therefore have to create new APIs for extensions, ramrod support for these into compose, and probably fix up nsIMsgIdentity to not be so mail-centric. Right now, the easiest way to do it is probably to make an SMTP server that acts as a middleman. I doubt a good way to do this can be finished before 3.next, though.
With that said, of all the facets I mention, the ability to create new account types is certainly the one I will put the most effort into personally. I have a goal of producing a usable extension in this regard, implemented entirely in JS, by the first beta, and this is my highest-priority 3.next goal.