Distribute reads across replica sets

I’m trying to understand the behavior of reads in a mongodb replica set. In particular I have an environment with high rate of reads, low rate of writes, and relatively small data set.

I read this document:

In particular:

    primary Default mode. All operations read from the current replica set primary.
    primaryPreferred In most situations, operations read from the primary but if it is unavailable, operations read from secondary members.
    secondary All operations read from the secondary members of the replica set.
    secondaryPreferred In most situations, operations read from secondary members but if no secondary members are available, operations read from the primary.
    nearest Operations read from the nearest member of the replica set, irrespective of the member’s type.

So my understanding is that reads by default go to the primary. There are read preferences that allow reading from secondary ( secondary , and secondaryPreferred ). In these cases stale data may be served.

It seems to me that it would be preferable to distribute the reads across both primary and secondary machines, so that I can make best use off all 3 machines. But I don’t really see this as an option. The following statement in particular perplexes me:

    If read operations account for a large percentage of your application’s traffic, distributing reads to secondary members can improve read throughput. However, in most cases sharding provides better support for larger scale operations, as clusters can distribute read and write operations across a group of machines.

However, in the case of a relatively small data set, sharding simply doesn’t make sense. Can someone shed some light on the right configuration?

TL;DR: Use nearest.

Indeed, sharding your cluster would definitely solve the problem as it would force you to split your data set into pieces (shards) and your reads and writes operations would be distributed evenly by the mongos servers - granted that you chose a good shard key.
But, as you found out, it doesn’t really makes sense for a relatively little data set and it would be more costly.

Our documentation doesn’t really reveals all the magic behind the “nearest” option, but there is actually a round-robin algorithm implemented behind it.
In our specifications, you can read more about it - especially about the options that you can set to tune the round-robin algorithm.

To distribute reads across all members evenly regardless of RTT, users should use mode ‘nearest’ without tag_sets or maxStalenessSeconds and set localThresholdMS very high so that all servers fall within the latency window.

Here is more documentation about the ping times.

Especially this part:

Once the driver or mongos has found a list of candidate members based on mode and tag sets, determine the “nearest” member as the one with the quickest response to a periodic ping command. (The driver already knows ping times to all members, see “assumptions” above.) Choose a member randomly from those at most 15ms “farther” than the nearest member. The 15ms cutoff is overridable with “secondaryAcceptableLatencyMS”.

Also, the more RAM you have, the less documents will need to be retrieved from disk. If your working set is large, you should considered adding some RAM to reduce the IOPS and overall latency.

I hope this helps :slight_smile: !

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Thank you, yes, this helps a lot, I was running into dead-ends everywhere before you replied.

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@MaBeuLux88_xxx there is inconsistency in terms used in https://github.com/mongodb/specifications/blob/master/source/driver-read-preferences.rst#nearest , https://github.com/mongodb/specifications/blob/master/source/server-selection/server-selection.rst#nearest and https://docs.mongodb.com/manual/core/read-preference-mechanics/#default-threshold like secondaryAcceptableLatencyMS, localThreshold and the correct one of localThresholdMS.

Server Selection in Next Generation MongoDB Drivers | MongoDB Blog clarified that
The ‘localThresholdMS’ variable used to be called secondaryAcceptableLatencyMS, but was renamed for more consistency with mongos (which already had localThreshold as a configuration option) and because it no longer applies only to secondaries.

we should consider updating it.

I have escalated this internally. Thanks :+1:

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