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Message from discussion MapReduce on SECONDARY in a ReplicaSet
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Stephen Steneker  
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 More options Sep 30 2012, 9:34 pm
From: Stephen Steneker <stephen.stene...@10gen.com>
Date: Sun, 30 Sep 2012 18:34:10 -0700 (PDT)
Local: Sun, Sep 30 2012 9:34 pm
Subject: Re: MapReduce on SECONDARY in a ReplicaSet
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> Running 2.2.0 3 box replicaset with one primary.

> PRIMARY is write-intensive and loads data all day long from various
> sources.

> The 2 SECONDARY server are supposed to support the read-only end of the
> application.  However, the "reads" involve lots of map-reduces that produce
> large results we store in temporary collections for sorting, etc.

> All was working OK in dev when we had one machine.  Now, trying to go to
> production we just learned that we can't write the map-reduce results to a
> temporary collection on the SECONDARY.  (per
> http://www.mongodb.org/display/DOCS/MapReduce#MapReduce-Outputoptions).
>  When we try (C# driver), we get Command 'mapreduce' failed: not master
> (response: { "errmsg" : "not master", "ok" : 0.0 }

Hi Mark,

If you want to run Map/Reduce on a secondary you will need to use SlaveOK
and the "inline" output option, which does not require write access.  Note
that inline results are limited to the maximum document size (currently
16Mb).  To save those results to a collection you would still have to
commit via the primary, from your C# code.

If you are using the C# 1.6 driver or newer, you can use the additional
Read Preferences supported by MongoDB 2.2:
 http://docs.mongodb.org/manual/applications/replication/#read-prefere...

Depending on the sort of processing you are doing, you may be able to use a
query filter to create a more efficient incremental Map/Reduce:
 http://www.mongodb.org/display/DOCS/MapReduce#MapReduce-IncrementalMa...

Another option for read-only queries in MongoDB 2.2 is the new Aggregation
Framework:
 http://docs.mongodb.org/manual/applications/aggregation/

Cheers,
Stephen


 
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