What does "supportsCommittedReads" truly mean ?

Hakan

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Dec 19, 2015, 6:07:38 AM12/19/15

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Hi,

I use the latest version of MongoDB (that is, 3.2), After examining the server stats of wiredTiger usingdb.serverStatus(), I have realised that there is a new field called supportsCommittedReads in the "storage" object:

 "storageEngine" : {
                "name" : "wiredTiger",
                "supportsCommittedReads" : true
 },

What should I understand if supportsCommittedReads is true? Plus I did not see any documentation about it on https://docs.mongodb.org/manual/reference/server-status/

Andy Schwerin

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Dec 19, 2015, 4:26:59 PM12/19/15

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This indicates that the storage engine supports the "majority" read concern level. The documentation of the new readConcern feature is at the following URL:

https://docs.mongodb.org/manual/reference/readConcern

The name should really be "supportsMajorityReadConcern", but we must have missed it when we finally named that feature. The documentation on serverStatus is still to be updated.

Hakan

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Dec 20, 2015, 3:09:03 AM12/20/15

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Hi Andy, thanks for the quick reply, I found a flag that you pointed out on https://docs.mongodb.org/manual/reference/program/mongod/#cmdoption--enableMajorityReadConcern It says that you must enable it before starting a mongod. However, this is active by default in wiredTiger?

Dwight Merriman

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Dec 20, 2015, 11:28:27 AM12/20/15

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the docs say the default is "local" (not majority) and i believe that is correct.

Asya Kamsky

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Dec 23, 2015, 12:01:33 PM12/23/15

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I think you're focused too much on the meaning of "supportsCommittedReads": true.

Yes, Wired Tiger supports committed reads. No, majority committed read concern is not enabled by default, hence there is an option you can use to enable it.

Asya

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Eric Sedor

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Oct 23, 2016, 2:00:56 PM10/23/16

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To guide on setting this option, how does a cluster behave differently when enableMajorityReadConcern is enabled, especially with respect to changes in behavior that impact performance?