Using recent client libraries (pymongo 3.4, mongodb (nodejs) 2.2.27), I am having trouble connecting to my mongodb servers with replication.
The replicaset configuration contains either the internal ips of the servers or the hostnames. I'm getting the following error:
pymongo.errors.ServerSelectionTimeoutError: mongodbdriver20151129-arbiter-1:27017: [Errno 8] nodename nor servname provided, or not known,mongodbdriver20151129-instance-1:27017: [Errno 8] nodename nor servname provided, or not known,mongodbdriver20151129-instance-2:27017: [Errno 8] nodename nor servname provided, or not known
or
pymongo.errors.ServerSelectionTimeoutError: 10.0.0.5:27017: timed out,10.0.0.6:27017: timed out,10.0.0.4:27017: timed out
I am currently working around it by changing the replicaset config to contain the external ips for the servers but I guess that would slow down the inter-server communication. How can I connect to my servers from an external location with the original rsconf?
[update] Note: I am trying to connect to the external ip of the servers and this worked fine when using pymongo 2.8 or mongodb (js) 2.1.4
[update] Follow this chat for more details/examples
Later versions of all officially supported MongoDB drivers (including the node driver) follows the Server Discovery and Monitoring spec (SDAM), which mandates that all drivers to monitor all nodes in a replica set (see Monitoring).
The reason for this monitoring is to be able to discover the status of the whole replica set at all times, and reconnect to a new primary should the current primary goes offline for any reason. See What's the point of periodic monitoring
To be able to monitor all nodes in a replica set, the driver must have access to each of the replica set member. Since your replica set is defined using internal IPs inaccessible by the driver, the driver cannot connect to them. This is the reason for the error you're seeing.
There are a couple of ways to solve this issue:
Use IP addresses or hostnames for the replica set configuration that are accessible by the driver (recommended).
Connect to one of the nodes without specifying a replica set, essentially treating the node as a standalone (not recommended).
If the older driver can connect without complaint, then either the driver is very outdated or doesn't follow the proper SDAM spec and should not be used, since its behaviour cannot be guaranteed. MongoDB publishes the SDAM spec and mandates all drivers to follow it for a good reason.
Related
When connecting to mongo cluster do we need replicaSet option in connection URI like below
mongodb://db1.example.net:27017,db2.example.net:2500/?replicaSet=test
What happens if replicaSet option is not used but all the nodes are given in connection URI like below
mongodb://db1.example.net:27017,db2.example.net:2500/
What is the advantage of giving and not giving replicaSet in the connection URI for the above 2 cases.
It's always recommended to include the replicaSet in the MongoDB connection String URI Format as a best practice. Enabling this will help to explore much more options for better application connectivity.
Advantages of including replicaSet:
Once enabled the client/driver will be aware of the all other members in the replica set.
If fail-over occurs client/driver automatically connects to next available member with zero downtime.
Using readConcern we can scale the reads better with other replica members.
replicaSet=myRepl&readConcernLevel=majority
To acknowledge all the write operation we can use write concern along with the URI
replicaSet=myRepl&w=majority&wtimeoutMS=5000
We can enable the connection timeout to maintain a better connectivity.
replicaSet=test&connectTimeoutMS=200000
Securing the application to use only TLS/SSL encrypted connections.
replicaSet=myRepl&ssl=true
For better secured application support and connectivity always use the replicaSet on connection String URI.
You should specify also the replicaSet.
If you don't specify the replicaSet then you still connect to the PRIMARY node. However, in case the PRIMARY becomes unavailable then your connection is lost. If you specify the replicaSet then the client will automatically re-connect to the new primary member.
You an play around and make test with these commands:
db.hello().primary returns the current primary member
db.hostInfo().system.hostname returns the member where you are currently connected to
if you have an endpoint like: mongodb+srv://username:password#mycluster.mik05g.mongodb.net
run the nslookup CLI
nslookup -type=TXT mycluster.mik05g.mongodb.net
Server: 127.0.0.53
Address: 127.0.0.53#53
Non-authoritative answer:
mycluster.mik05g.mongodb.net text = "authSource=admin&replicaSet=MyCluster-shard-0"
So the replicaSet name here is: MyCluster-shard-0
While creating new connection on Mongo Compass the UI try to discover the entire replica set topology and connect to the primary/secondary IP.
Is there a way to create a DIRECT connection to an HOST/IP+Port just like the clients api and disable the topology discovery step?
From pymongo documentation:
directConnection (optional): if True, forces this client to
connect directly to the specified MongoDB host as a standalone. If false, the client connects to the entire replica set of which the given MongoDB host(s) is a part
If you don't want to connect to a replicaset, use a mongodb connection string (as opposed to mongodb+srv), use a host IP / port, and drop the &replicaSet= parameter.
A bug ticket was opened to Mongo on Jira
https://jira.mongodb.org/browse/COMPASS-4534
The fix should be in version 1.42.2
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.
I have a sharded, replicated mongodb cluster. I'm in the process of re-IPing the shards to be on a different subnet. Just started by re-IPing one secondary mongod. It now has the new IP and I flushed the DNS. However in Cloud Manager, that server shows as DOWN now.
What can I do to make MongoCloud see that server again? I know MongoCloud communicates with the shards via the mongo automation service that is installed on them, but I don't see any configuration in there about the IP address, etc.
Found the answer:
there was a hardcoded hostfile always forcing the mongod server to the old IP address. Once the host file entries were removed, everything was OK. The host file (in Windows) is found in C:\Windows\system32\drivers\etc\hosts on the 2 other replica set members (I have a sharded env. w/ 1 primary and 2 secondaries).
The host file contains a line per entry, host ip
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