Mongo behaviour (PSA) when Primary taken down

[merceskoba (Merces)]

Hi,
I have some questions regarding the best practice of mongo Primary-Secondary-Arbiter.
Right now, i use mongo PSA in my graylog production stage.

10.0.0.1 = mongo-1 (Primary)
10.0.0.2 = mongo-2 (Secondary)
10.0.0.3 = mongo-3 (Arbiter)

root@production-mongo-1:/etc# mongod --version
db version v4.0.27
git version: d47b151b55f286546e7c7c98888ae0577856ca20

Graylog 3.2.4+a407287, codename Ethereal Elk

My config in graylog server.conf:
mongodb_uri = mongodb://10.0.0.1:27017,10.0.0.2:27017/graylog?replicaSet=graylog2

Now, i have plan to add more CPU and Memory of mongo-1 (Primary). I need to shutdown the VM mongo-1.

According to this Mongo docs, i need to do mongod --shutdown to shutdown gracefully the mongo server.

However, if i shutdown the mongo-1 (Primary);

• what happen to graylog master and slave if the mongo-1 shutdown-ed ?
• is it ok for graylog master and slave when mongo-2 (Secondary) promoted to be (Primary) ?
• will mongo-2 be promoted from Secondary to Primary automatically ? or must be set manually ?
• or, mongo-3 as Arbiter will promote it automatically and the data sync automatically ?
• After, mongo-1 upgraded and started on. is mongo-1 promoted back as primary ?

I am not mongo user, sorry if my question is too dumb. Reading replica set docs, but still suffering to understand behaviour for Graylog

I want to test in staging mongo server, but sometimes i really know first from your pov. Because my test Graylog is like Production needs. Cannot taken down even a minute

If you have any steps or personal docs to maintain mongo server, i am so glad to read it. Really want to deep learning for this mongo stuff.

Thank you so much for your time and help.

[gsmith]

Hello @merceskoba

Good question.

Back to the basics, Clusters are normally for redundancy incase an issue/s arise. So I would assume if you shutdown a node in the cluster the cluster should still function. Once you bring that node back up, the master should resume.

@merceskoba

I am not mongo user, sorry if my question is too dumb. Reading replica set docs, but still suffering to understand behaviour for Graylog

As for MongoDb, I would look into the MongoDb forum here. I’m quite sure there are some user/s there that do this all the time. Unfortunately, I haven’t done this in a long time but what I can remember, I had Graylog/MongoDb on the same node and I had three nodes. So when I shut down Node-01 my load balancer ( Nginx) re-route to Graylog/MongDb Node-02. Once Node-01 was back up that became my primary again.

@merceskoba

If you have any steps or personal docs to maintain mongo server, I am so glad to read it. Really want to deep learning for this mongo stuff.

To be honest, sometimes just doing it and making a mess can help the learning process
In a lab environment create three nodes as shown above and give it a try, see how it function. Keep an eye on the logs, find out how it fails over when shutting down a node, etc…Seeing is believing.

Sorry I cant be more help, if I come across ant documentation Ill post it here.