That it doesn’t work in Ambari, at least to the point of writing thru to the config file (whether or not it actually works with ES), is quite mystifying.  Silly question, but, are you pressing the “Save” button after changing the field?  Are you perhaps not logged into Ambari with an ambari user id that has privs to change this config?  Did you use non-default user configuration with Ambari, and if so are you running with an admin-priv ambari account?

From: Laurens Vets <[EMAIL PROTECTED]>
Date: Tuesday, September 12, 2017 at 8:32 PM
Cc: Michael Miklavcic <[EMAIL PROTECTED]>, Matt Foley <[EMAIL PROTECTED]>
Subject: Re: Elasticsearch masters_also_are_datanodes doesn't work.

See inline.
Now, Laurens, two question for you:   First, please confirm: Are you setting the value of masters_also_are_datanodes via the masters_also_are_datanodes field in the Ambari configuration GUI?  Or directly editing files?
I'm using Ambari to edit the configuration. However, I have to manually edit the file locally as the Ambari configuration doesn't work.
Second, do you set it at the initial install time, or afterward?  I’m pretty sure it needs to be set at initial install time, before you ever launch the Elasticsearch nodes.  I’m dubious about whether an Elasticsearch master can learn to be a datanode if it first wakes up and configures itself to not be one.  But I’m not an Elasticsearch expert, so if someone else knows different, please say so.
In this case, I forgot to set it initially, so I tried to change it afterwards to no avail.
Hope this helps more than obfuscates.

From: Michael Miklavcic <[EMAIL PROTECTED]>
Date: Tuesday, September 12, 2017 at 4:54 PM
Cc: Matt Foley <[EMAIL PROTECTED]>
Subject: Re: Elasticsearch masters_also_are_datanodes doesn't work.
At the very least, the value provided by default seems to have changed to a "1" instead of "true" without the tooltip having been updated to match.
Error! Filename not specified.
On Tue, Sep 12, 2017 at 4:00 PM, Michael Miklavcic <[EMAIL PROTECTED]<mailto:[EMAIL PROTECTED]>> wrote:
I think this is our default setup for full dev. It's only a 1-node VM, so I'm pretty sure that it would not work otherwise. I'm spinning up full dev now and will look into it also.
On Tue, Sep 12, 2017 at 3:04 PM, Laurens Vets <[EMAIL PROTECTED]<mailto:[EMAIL PROTECTED]>> wrote:

I'll also spin up a full-dev environment and see what happens there.
On 2017-09-10 21:50, Matt Foley wrote:
Laurens, please open a jira.  Altho it may seem obvious, please
include full repro.
This may be a showstopper, as it presumably (?) prevents ES from
working on a single-node deployment?

On 9/10/17, 4:01 PM, "Laurens Vets" <[EMAIL PROTECTED]<mailto:[EMAIL PROTECTED]>> wrote:

    Another issue I noticed. Setting "masters_also_are_datanodes" in Ambari
    to "true" does not work.
    The settings in /etc/elasticsearch/elasticsearch.yml will always be
    false when restarting elasticsearch...
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