I have some questions about the mongo replica
mongo replica
If I make 1 primary and 2 secondary MongoDB for replication. So I have 3 endpoints to 3 different DB and my apps can only write on primary DB. what if suddenly my primary shutdown and secondary DB take over the primary. Then how to automatically change the endpoint in my apps? should I use mongos (mongo routes)? but it needs sharding if I remember correctly.
Thank you.
All nodes in a replica set work together to have identical data. Secondary nodes may lag behind the primary, but you don't get "3 different DB". There is only one database of which copies exist on each node.
All MongoDB drivers know to monitor replica set members and discover which is the primary one automatically. You need to configure some drivers to do so by providing the replica set name, others do it automatically by default when they connect to a replica set node. Look up "connecting to replica set" in your driver documentation.
In a proper connection string you will provide all three RS members, e.g.
mongodb://mongodb0.example.com:27017,mongodb1.example.com:27017,mongodb2.example.com:27017/?replicaSet=myRepl
The client will detect the PRIMARY and will use it. I guess, most drivers will re-connect automatically if the PRIMARY node changes.
Most drivers will detect the PRIMARY automatically if you provide the ReplicaSet name, i.e.
mongodb://mongodb0.example.com:27017/?replicaSet=myRepl
would connect to the PRIMARY even if it is not mongodb0.example.com. However, if mongodb0.example.com does not run, then you don't connect at all. So, it is beneficial to provide all ReplicaSet members in the connection string.
See Connection String URI Format
mongos is needed only to connect to a Sharded Cluster.
Related
I have 3 questions regarding a replica set on mongodb on windows:
I currently have a standalone running with data on it, if I create a replica set (adding 2 secondries) will I have a downtime or I can create the replica set and adding 2 secondries while the standalone (now the primary) still running?
Will the 2 secondries copy all the data from the primary? Also data that was written to standalone before it became part of replica set?
Once there is an election a secondry become a primary but then it means the primary is on differnt IP + Port, this means I also need to change my write to the new primary by myself or mongodb doing it by himself? or need to use virtual ip?
Have a look at Convert a Standalone to a Replica Set.
You need to change the configuration file and restart the MongoDB, so you have a downtime of your MongoDB.
Yes, whenever you add a new member to a replica set then MonogDB performs a Initial Sync to the new secondary
You would need to change your connection, see Connect to a MongoDB Replica Set. The connection string contains all replica set members and the client will connect (by default) to the primary.
Actually you don't have to put all replica set members in your connection string, the client will discover them automatically. However, if you put only one member and by chance this member is down then you have no connection.
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.
How do you setup mongodb replication in production environments? I started using cloud formation with this template but it crashes half way. I want to setup mongo so that it has one primary and two replications.
I haven't found a good tutorial for how to setup Mongo replication.
Some other questions I have are:
How does the failover work, if I have three Ec2 instances each with mongo and the primary fails. Another instance becomes the primary but how does my client PyMongo and Scala Mongo know the IP address of the new primary.
Lets say the primary goes down for 1 hour and there are 2,000 writes. When it goes back up, how does the primary gets updated. Do I need a script for this?
I am trying to do this with flask PyMongo
I ended up testing this on my local machine here is what I found.
Failover is done by the client, in the Mongo URI you specify all your replications and when PyMongo connects to it. He checks to see which one is the primary and writes to that one.
When the database goes back up they all sync to match the same records in the all the databases.
Readthedocs has step by step manual on setting up MongoDB cluster on different platforms, including AWS EC2:
https://mongodb-documentation.readthedocs.io/en/latest/ecosystem/tutorial/install-mongodb-on-amazon-ec2.html#deploy-a-multi-node-replica-set
To provide your clients with working mongo instance you can employ several different strategies. For example:
Set up Route53 failover. Route53 will monitor health instance of primary node, and change DNS record to point to secondary in case of failure.
Use service discovery. Consul, etc, ZooKeeper and doozerd are worth exploring.
In case of failing and then coming back a mongodb node will receive latest data from other nodes — that's just what replica set does.
When opening a connection to multiple mongoDB servers with ReactiveMongo, we can pass it a list of hosts to connect to.
However I don't find any way to provide the replicaSet name (as we can do in standard MongoDB URIs). Even the URI parser of ReactiveMongo ignores it.
Is it a problem? Will the driver it work correctly without knowledge of the replicaSet name?
The list you provide is called "seed list". When your driver connects to one of these servers, be it a primary or secondary, the driver will be informed that it connected to the replica set and which server is the current primary (if there is one) amongst other details.
Your driver even should be informed when a new primary elected and it should automatically connect to the newly elected primary.
As per ReactiveMongo docs which you should read throughly:
[...]ReactiveMongo provides support for Replica Sets. That means the following:
the driver will detect if it is connected to a Replica Set;
it will probe for the other nodes in the set and connect to them;
it will detect when the primary has changed and guess which is the new one;
[...]
I am new to mongodb replica set.
According to Replic Set Ref this should be connection string in my application to connect to mongodb
mongodb://db1.example.net,db2.example.net,db3.example.net:2500/?replicaSet=test
Suppose this is production replica set (i.e. I cannot change application code or stop all the mongo servers) And, I want to add another mongo db instance db4.example.net in test replica set. How will I do that?
How my application will know about the new db4.example.net
If you are looking for real-world scenario:
In situation when any of existing server is down due to hardware failure etc, it is natural to add another db server to the replica set to preserve the redundancy. But, how to do that.
The list of replica set hosts in your connection string is a "seed list", and does not have to include all of the members of your replica set.
The MongoDB client driver used by your application will iterate through the seed list until it can successfully connect to a host, and use that host to request the current replica set configuration which will list all current members of the replica set. Per the documentation, it is recommended to include at least two hosts in the connect string so that your driver can still connect in the event the first host happens to be down.
Any changes in replica set configuration (i.e. adding/removing members or election of a new primary) are automatically discovered by your client driver so you should not have to make any changes in the application configuration to add a new member to your replica set.
A change in replica set configuration may trigger an election for a new primary, so your application code should expect to handle transient errors for a few seconds during reconfiguration.
Some helpful mongo shell commands:
rs.conf() - display the current replication configuration
db.isMaster().primary - display the current primary
You should notice a version number in the configuration document returned by rs.conf(). This version is incremented on every configuration change so drivers and replica set nodes can check if they have a stale version of the config.
How my application will know about the new db4.example.net
Just rs.add("db4.example.net") and your application should discover this host automatically.
In your scenario, if you are replacing an entirely dead host you would likely also want to rs.remove() the original host (after adding the replacement) to maintain the voting majority for your replica set.
Alternatively, rather than adding a host with a new name you could replace the dead host with a new server using the same hostname as previously configured. For example, if db3.example.net died, you could replace it with a new db3.example.net and follow the steps to Resync a replica set member.
A way to provide abstraction to your database is to set up a sharded cluster. In that case, the access point between your application and the database are the mongodb routers. What happens behind them is outside of the visibility of the application. You can add shards, remove shards, turn shards into replica-sets and change those replica-sets all you want. The application keeps talking with the routers, and the routers know which servers they need to forward them. You can change the cluster configuration at runtime by connecting to the routers with the mongo shell.
When you have questions about how to set up and administrate MongoDB clusters, please ask on http://dba.stackexchange.com.
But note that in the scenario you described, that wouldn't even be necessary. When one of your database servers has a hardware failure and your system administrators want to replace it without application downtime, they can just assign the same IP and hostname to the new server so the application doesn't even notice that it's a replacement.
When you want to know details about how to do this, you will find help on http://serverfault.com