MongoDB Replica Sets are an awesome included feature that gives you High Availability with automatic failover via a consensus election. Having been a pager-carrying DBA in my previous life, I can tell you that if failover involves a human (usually me) being woken up in the middle of the night to promote a secondary to be the new master, I do not consider that "high" availability at all! Working in the middle of the night is when I make all the errors that catch up with me weeks or months later, so I'd rather the system "do the right thing", and replica sets do just that.
Of course you also get extra copies of your data (for Disaster Recovery or just for compliance reasons) and other benefits, like ability to do maintenance without scheduling downtime for the database, but what if you don't need any of that?
What if you just want to have an extra copy of your data, but you don't want to have high availability?
I occasionally want to have that myself, when I need to test replication of data but don't want to deal with elections, arbiters or for whatever other reason want to pretend that my "primary" is basically a stand-alone node, it just happens to be replicating to another node (that might even be running on the same laptop).
In MongoDB the replica set architecture is set up to prevent two primaries from existing at the same time (this allows the data to stay consistent and avoids needing to conflict-resolve in cases of network partition) - consensus elections where a node must receive >50% of the votes to assume a PRIMARY role guarantees that two nodes cannot be elected at the same time. But that also means that if you have an even number of nodes (like two) you must get both votes to become or stay a primary.
In order to avoid losing the primary whenever I happen to take down my secondary node, I adjust the replica set configuration to make sure that only one node gets to participate in elections. Having only one vote means that majority is one, and my intended primary can "unanimously" elect itself every time - as long as it's up!
Here is the configuration:
Of course you also get extra copies of your data (for Disaster Recovery or just for compliance reasons) and other benefits, like ability to do maintenance without scheduling downtime for the database, but what if you don't need any of that?
What if you just want to have an extra copy of your data, but you don't want to have high availability?
I occasionally want to have that myself, when I need to test replication of data but don't want to deal with elections, arbiters or for whatever other reason want to pretend that my "primary" is basically a stand-alone node, it just happens to be replicating to another node (that might even be running on the same laptop).
In MongoDB the replica set architecture is set up to prevent two primaries from existing at the same time (this allows the data to stay consistent and avoids needing to conflict-resolve in cases of network partition) - consensus elections where a node must receive >50% of the votes to assume a PRIMARY role guarantees that two nodes cannot be elected at the same time. But that also means that if you have an even number of nodes (like two) you must get both votes to become or stay a primary.
In order to avoid losing the primary whenever I happen to take down my secondary node, I adjust the replica set configuration to make sure that only one node gets to participate in elections. Having only one vote means that majority is one, and my intended primary can "unanimously" elect itself every time - as long as it's up!
Here is the configuration:
The "priority" field says whether a node can be elected as a primary - zero means not eligible. Other numbers would influence the nodes relative standing against other equally eligible nodes during an election; "hidden" means the node's presence won't be revealed to clients/drivers that are connecting to the replica set as a whole; "votes" expresses whether the node gets a vote in the election - note that whether a node can vote and whether it can become a primary are two separate settings!
This means that if I lose the primary node, the secondary will remain as a secondary since it cannot elect itself - and I won't have high availability, and that's exactly what I was trying to achieve here.
This means that if I lose the primary node, the secondary will remain as a secondary since it cannot elect itself - and I won't have high availability, and that's exactly what I was trying to achieve here.
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