I'm creating a 3-nodes Replica set in MongoDB. When I launch the rs.initiate() query, it creates one node as a Secondary, while the other two remains in the "STARTUP" state.
In this situation, without any Primary, it is impossible to run any other query into the mongo shell and as a consequence I can't even reconfigure the replica set by deleting and adding later the other nodes.
I want to specify that I created this cluster cloning all the config files from another Replica Set that I created some months ago, and it works perfectly.
There is someone who faced this issue and know how to fix it?
Thanks a lot!
Related
I am using MongoDB with Loopback in my application with a loopback connector to the MongoDB. My application was working fine but now it throws an error
not master and slaveOk=false.
try running rs.slaveOk() in a mongoDB shell
You are attempting to connect to secondary replica whilst previously your app (connection) was set to connect likely to the primary, hence the error. If you use rs.secondaryOk() (slaveOk is deprecated now) you will possibly solve the connection problem but it might not be what you want.
To make sure you are doing the right thing, think if you want to connect to the secondary replica instead of primary. Usually, it's not what you want.
If you have permissions to amend the replica configuration
I suggest to connect using MongoDB Compass and execute rs.status() first to see the existing state and configuration for the cluster. Then, verify which replica is primary.
If necessary, adjust priorities in the replicaset configuration to assign primary status to the right replica. The highest priority number sets the replica as primary. This article shows how to do it right.
If you aren't able to change the replica configuration
Try a few things:
make sure your hostname points to the primary replica
if it is a local environment issue - make sure you added your local replica hostnames to the /etc/hosts pointing to 127.0.0.1
experiment with directConnection=true
experiment with multiple replica hosts and ?replicaSet=<name> - read this article (switch tabs to replica)
The best bet is that your database configuration has changed and your connection string no longer reflects it correctly. Usually, slight adjustments in the connection string are needed or just checking to what instance you want to connect.
MongoDB is always showing me this error message when I insert any data in my collection.
I am trying to configure ElasticSearch with mongodb, when I realized my Replica. I try to add something, but no results.
Mongo Shell always shows me the same message:
WritResult({"writeError":{"code":undefined,"errmsg":"not master"}})
This happens when you do not have a 3 node Replica Set, and you start the replica in Master-Slave configuration and after that your master goes down or the secondary goes down. Since there is not third node or Arbiter to elect a new primary, the primary steps down as master and in pure read only mode. The only way to bring the replica set up is to create a new server with the same repl-set name and add the current stepped down master as secondary to it.
Here is a newbie trying to play around Mongodb. I am trying to demonstrate scaling in my class, meaning, I need to show that I have 2 instances of mongoDB up and running and I need to replicate them, set one as master and the other as secondary.
Can any of you suggest me a simple way to demonstrate that if primary/master fails the slave/secondary comes up as the master?
Please keep it as simple as possible as I am teaching to a fairly beginners of MongoDB
MongoDB replica sets are not master/slave. In order to achieve automatic failover you need to have a majority of nodes in the replica set able to elect a new primary. The minimum number of nodes in your replica set should be 3, which can either be 3 data-bearing nodes or 2 data-bearing nodes and an arbiter, which is a node that votes in elections.
A demo using replication alone is more about failover and redundancy than scaling (better demo'd with sharding).
If you want a very simple (and non-production) way to stand up a replica set or sharded cluster in a development environment, I would suggest using the mlaunch script which is part of mtools.
For example, to create a 3-node replica set with an arbiter:
mlaunch --replicaset --nodes 2 --arbiter
To create a sharded cluster with 3 shards backed by a replica set (plus mongos and config server):
mlaunch --replicaset --sharded 3
As mentioned in the other comments here, the free MMS Monitoring service is a good way to visualise activity in your MongoDB deployment, and you can use db.shutdownServer() to shutdown specific nodes to see the outcome.
The easiest way would be to set up the MongoDB monitoring service. Stop the MongoD process on one and watch the other take over. But, use replica sets rather than master/secondary as they are the recommended approach.
Actually, it is pretty easy
Set up a replica set with 2 "normal" mongods and an arbiter
Connect to both of the normal mongods using mongo
Show the output of rs.status(). (Note the selffield)
Shut down the current primary
Show the output of rs.status() again and again, until the former secondary is elected primary
Another option would be to write a simple java app which utilizes the driver, put it in an infinite loop which writes one entry every second and puts out the number of objects in the database. Catch exceptions and write out that a problem occurred. Start the sharded cluster, then start your application. Shut down the primary while the program is running. during the elections, there may be exceptions be thrown. As soon as the former secondary is elected primary, the document count should start to rise again.
We are using MongoDB in production environment and now, due to some issues of current servers, I'm going to change the server and start a new MongoDB instance.
We have a replica set and a single mongod instance (two different MongoDB networks for different purposes). Now, first I should migrate the single mongod instance and then the whole replica set to the new server.
What I want to know is, how can I migrate both instances with no down-time? I don't want to shutdown the server or stop write operations.
Thanks in advance.
So first of all you should never run mongodb as a single instance for production. At a minimum you should have 1 primary, 1 secondary and 1 arbiter.
Second, even with a replica set you will always have a bit of write downtime when you switch primaries, as writes are not possible during the election process. From the docs:
IMPORTANT Elections are essential for independent operation of a
replica set; however, elections take time to complete. While an
election is in process, the replica set has no primary and cannot
accept writes. MongoDB avoids elections unless necessary.
Elections are going to occur when for example you bring down the primary to move it to a new server or virtual instance, or upgrade the database version (like going from 2.4 to 2.6).
You can keep downtime to a minimum with an existing replica set by setting the appropriate options to allow queries to run against secondaries. Again from the docs:
Maintaining availability during a failover. Use primaryPreferred if
you want an application to read from the primary under normal
circumstances, but to allow stale reads from secondaries in an
emergency. This provides a “read-only mode” for your application
during a failover.
This takes care of reads at least. Writes are best dealt with by having your application retry failed writes, or queue them up.
Regarding your standalone the documented procedures for converting to a replica set are well tested and can be completed very quickly with minimal downtime:
http://docs.mongodb.org/manual/tutorial/convert-standalone-to-replica-set/
You cannot have no downtime (a new mongod will run on new IP so you need to at least connect to it). But you can minimize downtime by making geographically distributed replica set.
Please Read
http://docs.mongodb.org/manual/tutorial/deploy-geographically-distributed-replica-set/
Use the given process but please note:
Do not set priority 0 of instances at New Location so that they become primary when old ones at Old Location step down.
You still need to restart mongod in replica set mode at Old Location.
You need 3 instances including an arbiter at New Location, if you want it to be
replica set.
When complete data is in sync with instances at New Location, step down instances at Old Location (one by one). Now everything will go to New Location but the problem is that it is directed through a distant mongod.
So stop mongod at Old Location and start a new one at new Location. Connect your applications to New Location Mongod.
Note: I have not done the same so far. I had planned it once but then I got the problem and it was not of hosting provider. Practically you may get some issues.
Replica Set is the feature provided by the Mongodb database to achieve high availability and automatic failover.
It is kinda traditional master-slave configuration but have capability of automatic failover.
It is basically group/cluster of the mongod instances which communicates, replicates to each other to provide high availability and to do automatic failover
Basically, in replica sets there are minimum 2 and maximum of 12 mongod instances can exist
In replica set following types of server exist. out of all, one server is always primary.
http://blog.ajduke.in/2013/05/31/setup-mongodb-replica-set-in-4-steps/
John answer is right, btw in your case you have no way to avoid downtime, you can just try to make it shorter as possible.
You can prepare the new replica set and save its configuration.
Same for the single mongod instance, prepare a js file with specific configuration (ie: stuff going on the admin database).
disable client connections on production servers.
copy the datafiles from the old servers to the new ones (http://docs.mongodb.org/manual/core/backups/#backup-with-file-copies)
apply your previous saved replica set config and configuration.
done
you can use diffent ways as add an hidden secondary member on the replica set if you have a lot of data, so you can wait it's is up-to-date before stopping the production server. Basically for the replica set you have many ways to handle a migration, with the single instance instead you don't have such features.
My test system (due to lack of resources) has a dual mongodb replicaset. There is no arbiter.
During some system changes one of the servers got put out of action and will not be coming back. This server happened to host the primary mongo node. This left the only other member of the set as a secondary.
I know I should have had at least three nodes for the cluster (our prod setup does).
Is there a way I can make the primary that is now offline to step down? I haven't been able to change any of the rs.conf() settings because the only working node is secondary. starting an arbiter doesn't seem to work because I cannot add it to the replset as the primary is down.
Has anyone encountered this before and managed to resolve it?
To recap:
SERVER A (PRIMARY) - OFFLINE
SERVER B (SECONDARY) - ONLINE
A + B = REPLSET
Any help would be greatly appreciated.
The mongodb website has documentation for what to do (in an emergency only) when you need to reconfigure a replica set when members are down. This sounds like the situation you are in.
Basically, if you're on version >= 2.0, and it's an emergency, you can add force: true to the replica set configuration command.