A: To quote from the Why TouchDB? document: “TouchDB is a lightweight CouchDB -compatible database engine suitable for embedding into mobile apps. Think of it this way: If CouchDB is MySQL, then TouchDB is SQLite.” For more details, read that page.
A: The reference Objective-C implementation runs on iOS 5+ and Mac OS X 10.7+. There is a Java port for Android devices.
The Objective-C implementation has been partially adapted to run in the GNUstep environment, which makes it portable to Linux, BSD, Solaris, etc. and even Microsoft Windows; at this time (May 2012) it’s not fully functional yet.
A: Sure, if you understand that it’s currently (May 2012) pre-beta. In general it is pretty solid, but there are some rough edges involving replication, especially using authentication or SSL.
A: There’s a Grocery Sync app for iOS that implements a simple checklist that can sync with a server.
A: TouchDB itself is released under the Apache License 2.0 (the same as CouchDB and Couchbase Mobile.)
Some libraries that TouchDB uses (FMDB, MYUtilities, and CocoaHTTPServer) have MIT or BSD licenses. We may seek relicensing or rewrite those parts of the code, to get everything under a single (Apache) license.
A: Yes; you can see a list on the “TouchDB In The Wild” page.
A: Sure — the Google group is the best place. (You can access it on the web or subscribe to it as a mailing list.)
A: Right now (May 2012) it compiles to about 250k bytes of optimized ARM7 code. And the source code is about 10,000 lines of Objective-C (plus another ~2500 lines from some existing utility libraries.)
By comparison, Couchbase Mobile 2.0 is about 3.5MB of ARM7 code.
A: About 100ms to initialize the library and open a small database. That’s a cold launch; if the app has been launched and quit recently, leaving stuff in cache, it’s more like 60ms. This is on an iPad 2; older devices will be a bit slower.
By comparison, Couchbase Mobile 2.0 takes about 5 seconds to initialize on the same hardware.
A: I don’t know yet; I haven’t done any real benchmarks or stress-testing, nor have I put much work into optimization. With small databases, everything is pretty much instant. The most complex thing I’ve run is pulling a database of about 3,000 revisions from either a local server or from IrisCouch; this seems to go about the same speed as CouchDB.
I’m sure that it will not scale as well as CouchDB to large data sets. But I also think it’s unlikely that mobile apps are going to need multi-gigabyte databases or millions of documents.
A: There aren’t any hard limits in TouchDB itself, nor to my knowledge in SQLite. The most likely practical limit is the available disk/flash storage on the device, and of course app responsiveness as query times increase (see above).
I’ve heard that Android has a 2GB file size limit; but this should be less of a problem for TouchDB than CouchDB, because the database file doesn’t grow as fast (it doesn’t need explicit compaction) and because it doesn’t store attachments inside the database file.
A: In the ways that matter, yes. The REST API is compatible, although you talk to the engine in-process rather than over a socket. Some of the more server-centric features of CouchDB, like user accounts, aren’t supported.
A: Yes, its replication protocol is entirely compatible. That’s a very important goal. Apps using TouchDB can sync with servers running Apache CouchDB or BigCouch, and with hosting services like Cloudant and IrisCouch.
A: Couchbase Server 2.0 uses CouchDB technology but has been heavily customized to optimize performance. As a side effect it no longer replicates with CouchDB, or therefore with TouchDB. This is obviously an issue! We are working on a product called [Syncpoint](https://github.com/couchbaselabs/Syncpoint-API) that will act as a server-side bridge between Couchbase Server and mobile clients.
A: Yes, the fundamental data model is also compatible. That’s pretty much required for replication compatibility, anyway.
A: Yes. Revision trees are implemented, and preserved across replication.
A: Pretty much. TouchDB has its own native API, but there’s a REST adapter above it that understands standard CouchDB requests. The iOS implementation registers a custom URL protocol handler with the CFNetwork framework, which means that when the app uses the Cocoa NSURLConnection API to access “touchdb:” URLs, it talks to the TouchDB library in-process instead of to the network. There are no actual sockets involved at all. This makes it very easy to use existing CouchDB libraries (such as CouchCocoa) with little or no change.
In the long run, since everyone (to my knowledge) is using CouchCocoa rather than direct HTTP calls, I’d like to re-implement CouchCocoa to talk directly to the TouchDB API and avoid the overhead of going through the HTTP stack. But that’ll be primarily an optimization, that shouldn’t affect developers’ code.
A: There’s an experimental HTTP server extension called TouchDBListener. It’s mostly there to enable TouchDB-to-TouchDB (P2P) replication as well as making testing easier. It’s not meant to be the primary way for an app to access TouchDB.
A: TouchDB doesn’t currently support every detail of the CouchDB REST API. We’ll add support for things that turn out to be important to mobile apps, and will probably ignore things that aren’t (like /restart, /_config and /users.)
Other things that aren’t compatible: The database file format. Configuration options (no .ini files.) Functions in interpreted languages (JS or Erlang). List and show functions.
A: I did, but it doesn’t seem possible to optimize CouchDB’s size and startup time enough for mobile apps. We were able to get some significant improvements in Couchbase Mobile 2.0, but beyond that, further shrinking and speeding-up possibilities seemed pretty minor (on the order of 5%) when what we wanted were orders of magnitude. Most of this was intrinsic in the fact that it’s implemented in an interpreted language (Erlang) whose runtime library needs to be bundled in.
A: The design is cross-platform. After considering various interpreted languages and cross-platform libraries, I decided they had too much overhead for mobile apps, or would make the implementation too awkward. So instead the goal is to have separate implementations for each platform, but share aspects like the SQL schema, class architecture and algorithms.
A: True, from that standpoint it might have been better to start with a Java implementation for Android, since more people read Java fluently than read Objective-C. In the end it came down to the fact that I (Jens) like Objective-C better than Java, and have experience developing for iOS but not Android.
A: Yes, my colleague Marty Schoch has been working on a port of my implementation. It’s also on GitHub.
I have heard interest in an ANSI C implementation (possibly using the Apache Portable Runtime or GLib) but to my knowledge no one is working on one yet. It’s not unlikely that Couchbase will develop one, since there may be customer interest.
A: No. I am honestly not very familiar with the CouchDB implementation. I don’t think it would have made sense to refer directly to the Erlang code anyway, as the way code is structured in that language (functional, highly parallelized, non-OOP) is very different from imperative OOP languages like Objective-C and Java.
And in any case, a lot of the code is of necessity very different because it’s using SQL for storage, rather than a custom append-only B-tree implementation.
A: No, it lets SQLite take care of file storage and indexing. TouchDB database files are regular SQLite databases. There’s documentation on the schema, if you’re curious.
A: Largely because SQLite is already available as a shared library on every platform we’re interested in; this keeps our code size down and simplifies the build process.
Additionally, both Berkeley and Kyoto have GPL-like licenses that are less friendly to commercial developers (especially iOS developers) and incompatible with the Apache license of TouchDB itself.
A: At this point (May 2012) the existing Couchbase Mobile 2.0 is more mature, and has been used in several shipping apps. However, we see TouchDB as the way forward, and aren’t putting any development effort into the old implementation beyond maintenance.
If you have a project you want to ship in the very near future, it’s best to be conservative and use Couchbase Mobile. Beyond that, TouchDB is clearly better.
A: Because it’s a lot smaller, starts up a lot more quickly, and is easily embeddable into an app. Those are important factors for mobile app developers (and some desktop app developers too.) If you’re working on server-side software they probably don’t matter to you, or at least don’t outweigh the drawbacks.
A: Because CouchDB is much more mature, scales better, interoperates easily with other server processes using a REST API, and runs on more platforms.
A: Basically for the same reasons that are already outlined in the Couchbase Mobile marketing materials. The primary one is world-class, highly-flexible data sync capabilities that go way beyond what you can get from iCloud. Another factor is that the CouchCocoa API is (we think) simpler and easier to use than CoreData’s.
A: As with the previous comparison to CoreData: the big reason is sync. If your users want to work with their data on multiple devices or platforms (including the Web), or have it transparently backed up, the replication capabilities in TouchDB and CouchDB will make it very easy compared to the pain of implementing sync yourself, or trying to duct-tape your custom SQLite database to the iCloud APIs.