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From Jan Lehnardt <...@apache.org>
Subject [discussion] New Blog Series: The Little Things
Date Mon, 07 Apr 2014 20:16:08 GMT
Hey all

I have an idea for a new blog series:

Category: The Little Things

Theme: Explaining some of the little features we put into CouchDB that make people’s lives
easier.

First draft of the first article: https://blogs.apache.org/roller-ui/authoring/preview/couchdb/?previewEntry=the_little_things_1_do
(pasted below for those of you who don’t have a blog account)

What do you think? :)

I’m imagining to solicit more articles from the developers on dev@ as new things arrive
in the code-base. And we can take inspirations from user@ and IRC when we explain a certain
behaviour to a user and what the reasoning behind that is.

I think this is a good way to get people to learn and talk about a number of clever things
we are doing outside of the regular channels.

Best
Jan

* * *

<p>CouchDB takes data storage extremely seriously. This usually means we work hard to
make sure that the underlying storage modules are as robust as we can make them. Sometimes
though, we go all the way to the HTTP API to secure against accidental data loss, saving users
from their mistakes, rather than dealing with hard drives and kernel caches that usually stand
in the way of safe data storage.</p>

<h2>The scenario:</h2>

<p>To delete a document in CouchDB, you issue the following HTTP request:</p>

<code><pre>DELETE /database/docid?rev=12345 HTTP/1.1</pre></code>

<p>A common way to program this looks like this:</p>

<code><pre>http.request('DELETE', db + '/' + docId + '?rev=' + docRev);</pre></code>

<p>So far so innocent. Sometimes though, users came to us and complained that their
whole database was deleted by that code.</p>

<p>Turns out the above code creates a request that deletes the whole database, if the
docId variable isn’t set correctly. The request then looks like:</p>

<code><pre>DELETE /database/?rev=12345 HTTP/1.1</pre></code>

<p>It looks like an honest mistake, once you check the CouchDB log file, but good old
CouchDB would just go ahead and delete the database, ignoring the <code>?rev=</code>
value.</p>

<p>We thought this is a good opportunity to help users not accidentally losing their
data. So since late 2009 (yes, this is an oldie, but it came up in a recent discussion and
we thought it is worth writing about :), CouchDB will not delete a database, if it sees that
a <code>?rev=</code> parameter is present and it looks like that this is just
a malformed request, as database deletions have no business requiring a <code>?rev=</code>.</p>

<p>One can make an easy argument that the code sample is fairly shoddy and we’d agree.
But we are not here to argue how our users use our database beyond complying with the API
and recommended use-cases. And if we can help them keep their data, that’s a win in our
book</p>

<p>Continuing down this thought, we thought we could do one better. You know that to
delete a document, you must pass the current rev value, like you see above. This is to ensure
that we don’t delete the document accidentally without knowing that someone else may have
added an update to it that we don’t actually want to delete. It’s CouchDB’s standard
multi version currency control (MVCC) mechanism at work.</p>

<p>Databases don’t have revisions like documents, and deleting a database is a simple
<code>HTTP DELETE /database</code> away. Databases, however, do have a sequence
id, it’s the ID you get from the changes feed, it’s an number that starts at 0 when the
database is created and increments by 1 each time a document is added, updated or deleted.
Each state of the database has a single sequence ID associated with it.</p>

<p>Similar to a rev, we could require the latest sequence ID to delete a database, as
in:</p>

<code><pre>DELETE /database?seq_id=6789</pre></code>

<p>And deny database deletes that don’t carry the latest <code>seq_id</code>.
We think this is a decent idea, but unfortunately, this would break backwards compatibility
with older versions of CouchDB and it would break a good amount of code in the field, so we
are hesitant to add this feature. In addition, sequence IDs change a little when BigCouch
finally gets merged, so we’d have to look at this again then.</p>

<p>In the meantime, we have the protection against simple coding errors and we are happy
that our users keep their hard earned data more often now.</p>

-- 



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