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And if you happen to like Perl, the module Net::CouchDb will make your life really a lot easier. Great fun to play with.

So to get some practical fun with CouchDb, I incorporated it as a back end type in my tamper-resistant HammerServer. By definition, there is more network overhead when storing data in CouchDb, since all is sent over HTTP. So when compared to a flat file, or a PostgreSQL database, CouchDb is slower. However, CouchDb's strenghts should become apparent when the datastore is spread over many nodes which balance-out inserting and retrieving records (which I didn't try out).

So if you like, have a look at HammerServer 1.02.

One of the fun things I found was the following. The HammerServer relies on sequential ID's to store data. Each record at a given ID is signed using its own data, but also using the signature of the previous ID (which is ID-1). This is done to form a "chain" of secure data, where deletions and insertions are detected. Therefore, the correct working of the HammerServer relies on atomic increments of some ID counter. Concurrent requests to insert new data may never wind up using the same ID!

In RDBMS terms this is easily accomplished as something like:

select nextval(sequence)

select sequence.nextval from dual

A too simplistic application-oriented attempt might be:

• Initially, create a document in the database, named e.g. lastid, with a data element nr initialized to the value 0.
• When getting the next value, the process might be:
  • Fetch the document lastid from the database,
  • Get its field nr,
  • Increment it,
  • Write it back to the database.

• Fetch the document lastid from the database,
• Get its field nr,
• Increment it,
• Attempt to write it back to the database,
• If there is a conflict error, then redo all - starting at the top.
• Otherwise we're done, the last value of nr is the atomically incremented ID.

# Connect to the CouchDb server
my $connection = Net::CouchDb->new('http://hostname:5984')
  or die("Can't connect to server\n");
# Connect to the database
my $cdb = $connection->db('database')
  or die("Can't connect to the database\n");

# The next ID to determine:
my $nextid;

# Fetch the "lastid" document, increment the field "nr". If an update
# conflict occurs, then redo until we're out of time. Oh, and die if
# some other weird error occurs.
my $start_t = time();
my $try = 1;
while (1) {
    # Fetch the counter document, increment number.
    my $doc = $cdb->get('lastid');
    $doc->nr += 1;

    # Write it back to the database.
    print("Try $try: incrementing to ", $doc->nr, "\n");
    my $result = $cdb->post($doc)
      or die("Failed to re-contact the CouchDB server\n");
    if ($result->{ok}) {
	# Data committed. We're done.
	$nextid = $doc->id;
	last;
    } elsif ($result->{error} eq 'conflict') {
	# Conflict error. Die miserably if this has been going on
	# for 60 seconds or more.
	die("Failed to increment lastid counter after 60 sec\n")
	  if (time() - $start_t > 60);
	# The 1 min hasn't elapsed yet, let's retry.
	print("Concurrency error, retrying\n");
	next;
    } else {
	# Some other error.
	die("CouchDB error: ", $res->{error}, ' ',
	    $res->{description}, "\n");
    }
}

# All done.
print("ID succesfully and atomically incremented to $nextid\n");