fermata-couchdb

NOTE: the currently-published fermata@0.10.6 already bundles this plugin, this is split out for an upcoming version!

This is a CouchDB plugin for Fermata. It primarily offers a watchChanges helper for monitoring a _changes feed, as well as a few other convenience helpers where it makes sense.

Setup

For classic in-browser usage, copy the main script file for both Fermata and this plugin alongside your static files. Then:

<script src="/your/static/files/fermata.js"></script>
<script src="/your/static/files/fermata-couchdb.js"></script>
<script>
var db = fermata.couchdb()('my-database');    // will try to auto-detect server

// … see example usage below …
</script>

Under node.js/browserify you will again need to npm install fermata fermata-couchdb. Then in your code, you will need to register this plugin manually:

var fermata = require('fermata'),
    couchdb = require('fermata-couchdb);
fermata.registerPlugin('couchdb', couchdb);   // you could pick a different name if you'd like

var db = fermata.couchdb()('my-database');
// …

Example usage

TBD: improve, and actually test, this…

var autoDatabase = fermata.couchdb()('my-database'),    // will try to auto-detect server
    corsDatabase = fermata.couchdb("https://other-server.example.com")('your-database');

// implement a lousy sort of "replicator", copying changed docs from the other server into our own…
fermata.plugins.couchb.watchChanges(corsDatabase({since:'now', include_docs:true}), function (arr) {
    arr.forEach(function (change) {
      autoDatabase.put(change.doc, function (e,d) {
          if (e) console.error(e, d);
          else console.log("New rev is:", d.rev);
      });
    });
});

Plugin API

The following documentation assumes this plugin has been registered with the name 'couchdb' (as by default the browser, or if you follow the node.js example above).

• fermata.couchdb([serverUrl]) — Returns a "URL proxy" object for the server's base URL. This gives you access to CouchDB's complete API via Fermata's usual API, following its philosophy of direct REST access. Pass a string with the serverUrl or omit to autodect. (TBD: The "autodection" currently just uses "http://localhost:5984" if no serverUrl is provided, but this could get a bit smarter in the future, issue #1)
• fermata.plugins.couchdb.watchChanges(databaseUrl, lastSeq, callback[, interval]) — Starts monitoring changes, returning a watcher object whose API is documented below, along with this method's parameters. This helper encapsulates best-practice polling behaviors depending on the monitoring mode (determined via interval). For the continuous and long-polling cases (see below), this includes exponential backoff on error.
• fermata.plugins.couchdb.warn ≈ console.warn.bind(console); — The changes watcher may emit some warnings to the console when it is available. You can override this method if you wish to handle these in some custom way. Your function should accept multiple arguments of any type, as console.warn does, but doesn't need to support any string substition (format specifiers).

Changes watcher API

Parameters to the .watchChanges(databaseUrl, lastSeq, callback[, interval]) method introduced above are as follows:

• required parameter: databaseUrl — must be a Fermata URL object, using pre-set query parameters to customize many of the monitoring options, e.g. db({include_docs:true, timeout:5e3}).
• required parameter: lastSeq — Changes will be monitored starting from (and not including) this sequence, which may be 'now' if you aren't trying to catch up a set of existing data [TBD: why isn't this handled via databaseUrl? issue #3].
• required parameter: callback — whenever a set of changes is available, this function will be called with an /array/ of changes as its first argument. In the event of an error, it will be called with an empty array and an error object (from Fermata) as the second argument. This differs from normal node.js callback practice, so think of this more like an event listener if you prefer. Having the parameters in this order encourages you to ignore errors, as the watcher's built-in handling should be sufficient in many cases.
• optional parameter: interval parameter controls how changes are monitored. If omitted, it will long-poll using any timeout query parameter of databaseURL. If set to a number, the watcher will issue simple polls with interval milliseconds between the previous response and the next request. Under node.js you can also pass 'continuous' to keep a persistent/streaming connection open.

The .watchChanges(…) method will return an watcher object with the following methods:

• watcher.cancel() — Stops (or pauses) monitoring changes. Any pending request will be aborted and no more callbacks will be sent; however, if you keep a reference to the watcher, its current status is preserved.
• watcher.pause() — Alias for .cancel().
• watcher.restart() — Resumes monitoring of changes. Use this to "unpause" a cancelled watcher.
• watcher.getStatus() — Returns an object with information about the monitoring status. Currently the only proprerty provided is update_seq, which is the last processed sequence.

Alternate Cloudant / BigCouch (/ CouchDB 2.0?) extended plugin

Cloudant's "BigCouch" version of CouchDB does not actually enforce read quorums! This means that if a quorum write request yields a 202 response, there is no way to ever know whether the write ended up conflicting or not. So my recommendation is to treat such writes as failed from the upstream code's perspective.

To facilitate this, this plugin offers a variant which, in the browser, is registered under the "bigcouch" name. To register under node.js you can follow this example, changing the variable names and/or registered name if you'd like:

var fermata = require('fermata'),
    plugin = require('fermata-couchdb');
fermata.registerPlugin('bigcouch', plugin.bigcouch);

var db = fermata.bigcouch()('my-database');
// …

This alternate plugin is used exactly as the main plugin. The only difference is that a response with a 202 status code will cause an error callback rather than a successful callback.

License (MIT)

Copyright © 2011–2015 Nathan Vander Wilt.

Released under the terms of the MIT License:

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.