How to write CouchDB view to get currently active servers given start timestamp and end timestamp of each server? - mapreduce

Question

I have set of documents which has the server name, with the start timestamp and end timestamp of that server. eg.
[
{
serverName: "Houston",
startTimestamp: "2018/03/07 17:52:13 +000",
endTimestamp: "2018/03/07 18:50:10 +000"
},
{
serverName: "Canberra",
startTimestamp: "2018/03/07 18:48:09 +000",
endTimestamp: "2018/03/07 20:10:00 +000"
},
{
serverName: "Melbourne",
startTimestamp: "2018/03/08 01:43:13 +000",
endTimestamp: "2018/03/08 12:09:10 +000"
}
]
With this data, given a Timestamp I need to get the list of active servers at that point of time.
For example. for TS="2018/03/07 18:50:00 +000" from the above data the list of active servers are ["Huston", "Canberra"]
Is it possible to achieve this using only CouchDB views. If so how to go about it?
Note: Initially I tried the following approach. In the map function I emit two documents
1 with key=doc.startTimestsamp and value={"station_add": doc.station}
1 with key=doc.startEndtsamp and value={"station_rem": doc.station}
My intention was to iterate through these in the reduce function adding stations present in "station_add" and removing stations in "stations_rem". But I found that CouchDB does not mention anything about the ordering of values in the reduce function.

Answer 1

If you can live with fixed periods and don't mind the extra disk space that might be needed for the view results, you can create a view of active servers per hour, for example.
Iterate over the periods between start and end and emit the time that each server was online during this period:
function(doc) {
var start = new Date(doc.startTimestamp).getTime()
var end = new Date(doc.endTimestamp).getTime()
var msPerPeriod = 60*60*1000
var msOfflineInFirstPeriod = start % msPerPeriod
var firstPeriod = start - msOfflineInFirstPeriod
var msOnlineInLastPeriod = end % msPerPeriod
var lastPeriod = end - msOnlineInLastPeriod
if (firstPeriod === lastPeriod) {
// The server was only online within one period.
emit([new Date(firstPeriod), doc.serverName], [1, msOnlineInLastPeriod - msOfflineInFirstPeriod])
} else {
// The server was online over multiple periods.
emit([new Date(firstPeriod), doc.serverName], [1,msPerPeriod - msOfflineInFirstPeriod])
for (var period = firstPeriod + msPerPeriod; period < lastPeriod; period += msPerPeriod) {
emit([new Date(period), doc.serverName], [1, msPerPeriod])
}
emit([new Date(lastPeriod), doc.serverName], [1,msOnlineInLastPeriod])
}
}
If you want the total without the server names, just add a reduce function with the built-in shortcut _sum. You'll get the number of servers online during the period as the first number and the milliseconds that the servers were online in that period as the second number.
You can play with the view if you emit the year, month and day as the first keys. Then you can use the group_level at query time to get a finer or more coarse overview.
Bear in mind that this view might get large on disk, as each row has to be stored, and also the intermediate results for each group level are stored. So you shouldn't set the period duration too small – emitting a row for each second would take a lot of disk space, for example.