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    <p>Here is what I do. This is borderline CouchDB abuse however I have had much success.</p> <p>Usually, <code>reduce</code> will compute a sum, or a count, or something like that. However, think of reduce as an elimination tournament. Many values go in. Only one comes out. A reduction! Repeat over and over and you have the ultimate winner (a re-reduction). In this case, the log with the latest timestamp is the winner.</p> <p>Of course, welterweights can't fight heavyweights. There have to be leagues and weight classes. It only makes sense for <em>certain</em> documents to do battle with <em>certain other</em> similar documents. That is exactly what the reduce <strong>group</strong> parameter will do. It will ensure that only evenly-matched gladiators enter the steel cage in our bloodsport. (Coffee is kicking in.)</p> <p>First, emit all logs keyed by device. The <code>value</code> emitted is simply a copy of the document.</p> <pre><code>function(doc) { emit(doc.name, doc); } </code></pre> <p>Next, write a reduce function to return the latest timestamp of all given values. If you see a fight between two gladiators from different leagues (two logs from different systems), stop the fight! Something went wrong (somebody queried without the correct <code>group</code> value).</p> <pre><code>function(keys, vals, re) { var challenger, winner = null; for(var a = 0; a &lt; vals.length; a++) { challenger = vals[a]; if(!winner) { // The title is unchallenged. This value is the winner. winner = challenger; } else { // Fight! if(winner.name !== challenger.name) { // Stop the fight! He's gonna kill him! return null; // With a grouping query, this will never happen. } else if(winner.timestamp &gt; challenger.timestamp) { // The champ wins! (Nothing to do.) } else { // The challenger wins! winner = challenger; } } } // Today's champion lives to fight another day. return winner; } </code></pre> <p>(Note, the timestamp comparison is probably wrong. You will have to convert to a <code>Date</code> probably.)</p> <p>Now, when you query a view with <code>?group=true</code>, then CouchDB will only reduce (find the winner between) values with the same <code>key</code>, which is your machine name.</p> <p>(You can also emit an array as a key, which gives a bit more flexibility. You could <code>emit([doc.name, doc.timestamp], doc)</code> instead. So you can see all logs by system with a query like <code>?reduce=false&amp;startkey=["NAS", null]&amp;endkey=["NAS", {}]</code> or you could see latest logs by system with <code>?group_level=1</code>.</p> <p>Finally, the "stop the fight" stuff is optional. You could simply always return the document with the latest timestamp. However, I prefer to keep it there because in similar situations, I want to see if I am map-reducing incorrectly, and a null reduce output is my big clue.</p>
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    1. COThank you. That works great. And what a nice cinematographic answer :-) P.S. I'll give it an upvote as soon as i can ;-)
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