We had a problem with an application accessing our MongoDB cluster, and the log files didn’t provide much useful information. I could see the client connect and disconnect … but nothing in between. I discovered that the default logging level is very low. Good for disk utilization and I/O, but not great for troubleshooting.

db.runCommand({getParameter: 1, logLevel: 1}) # Get the current logging level
db.setLogLevel(3) # Fairly robust logging
db.setLogLevel(5) # don't try this is prod huge stream of text super logging
db.setLogLevel(0) # and set logging back to a low level once you are done troubleshooting

You can also increase the log level for individual components of MongoDB to minimize logging I/O:

db.setLogLevel(2, "accessControl" )

On one server create a key file. Copy this key file to all other servers that will participate in the replica set

mkdir -p /opt/mongodb/keys/
openssl rand -base64 756 > /opt/mongodb/keys/$(date '+%Y-%m-%d').key
chmod 400 /opt/mongodb/keys/$(date '+%Y-%m-%d').key
chown -R mongodb:mongodb /opt/mongodb/keys/$(date '+%Y-%m-%d').key

On each server, edit /etc/mongo.conf and add the keyfile to the security section and define a replica set

security:
 authorization: enabled
 keyFile:  /etc/mongodb/keys/mongo-key
#replication:
replication:
  replSetName: "myReplicaSet"

Restart MongoDB on each node.

On one server, use mongosh to enter the MongDB shell.

rs.initiate(
{
_id: "myReplicaSet",
members: [
{ _id: 0, host: "mongohost1.example.net" },
{ _id: 1, host: "mongohost2.example.net" },
{ _id: 2, host: "mongohost3.example.net" }
]
})

Use rs.status() to view the status of the replica set. If it is stuck in STARTING … check connectivity. If the port is open, I ran into a snag with some replacement servers. They’ve got temporary hostnames. But you cannot add a host on itself — it ignores that you typed mongohost1.example.net … and it takes it’s hostname value. And then sends that value to the other servers in the replica set. If you cannot change the hostname to match what you want, there is a process to change the hostname in a replicaset.

We are upgrading servers from really old MongoDB (4.2.15) to much new MongoDB (6.something). I am used to getting into the MongoDB shell using:

mongoserver:~ # mongo -u $STRMONGOUSER -p $STRMONGOPASS
MongoDB shell version v4.2.15
connecting to: mongodb://127.0.0.1:27017/?compressors=disabled&gssapiServiceName=mongodb
Implicit session: session { "id" : UUID("5658a72f-fea0-4316-aa97-4b0c0ffab7ff") }
MongoDB server version: 4.2.15

Except the new server says there’s no such file. And there isn’t. A bit of research later, I learn that the shell is now called mongosh … which is a more reasonable name. It works the same way: mongosh -u $STRMONGOUSER -p $STRMONGOPASS gets me there, and all of the commands I know work.

The documentation on Mongo’s website tells you to use mongodump with a username, password, destination, and which database you want to back up. Except I wanted to back up and restore everything. Users, multiple databases, I don’t really know what else is in there hence I want everything instead of enumerating the things I want.

Turns out you can just omit the database name and it dumps everything

mongodump --uri="mongodb://<host URL/IP>:<Port>" -u $STRMONGODBUSER -p $STRMONGODBPASS

And restore with

mongorestore --uri="mongodb://<host URL/IP>:<Port>"

Since it’s a blank slate with no authentication or users defined yet.

We inherited a system that uses MongoDB, and I managed to get the sandbox online without actually learning anything about Mongo. The other environments, though, have data people care about set up in a replicated cluster of database servers. That seems like the sort of thing that’s going to require knowing more than “it’s a NoSQL database of some sort”.

It is a NoSQL database — documents are organized into ‘collections’ within the database. You can have multiple databases hosted on a server, too. A document is a group of key/value pairs with dynamic schema (i.e. you can just make up keys as you go).

There are GUI clients and a command-line shell … of course I’m going with the shell 🙂 There is a db function for basic CRUD operations using db.nameOfCollection then the operation type:

db.collectionName.insert({"key1": "string1", "key2" : false, "key3": 12345})

db.collectionName.find({key3 : {$gt : 10000} })

db.collectionName.update({key1 : "string1"}, {$set: {key3: 100}})

db.collectionName.remove({key1: "string1"});

CRUD operations can also be performed with NodeJS code — create a file with the script you want to run, then run “node myfile.js”

Create a document in a collection

var objMongoClient = require('mongodb').MongoClient;
var strMongoDBURI = "mongodb://mongodb.example.com:27017/";
  
objMongoClient.connect(strMongoDBURI, function(err, db) {
  if (err) throw err;
    var dbo = db.db("dbNameToSelect");
    var objRecord = { key1: "String Value1", key2: false };
    dbo.collection("collectionName").insertOne(objRecord, function(err, res) {
         if (err) throw err;
         console.log("document inserted");
         db.close();
    });
}); 

Read a document in a collection

var objMongoClient = require('mongodb').MongoClient;
var strMongoDBURI = "mongodb://mongodb.example.com:27017/";

objMongoClient.connect(strMongoDBURI, function(err, db) {
  if (err) throw err;
    var dbo = db.db("dbNameToSelect");
    var objQuery = { key1: "String Value 1" };
    dbo.collection("collectionName").find(objQuery).toArray(function(err, result) {
     if (err) throw err;
     console.log(result);
     db.close();
  });
}); 

Update a document in a collection

var objMongoClient = require('mongodb').MongoClient;
var strMongoDBURI = "mongodb://mongodb.example.com:27017/";

objMongoClient.connect(strMongoDBURI, function(err, db) {
if (err) throw err;
  var dbo = db.db("dbNameToSelect");
  var objRecord= { key1: "String Value 1" };
  dbo.collection("collectionName").deleteOne(objRecord, function(err, obj) {
    if (err) throw err;
    console.log("Record deleted");
    db.close();
});
}); 

Delete a document in a collection

var objMongoClient = require('mongodb').MongoClient;
var strMongoDBURI = "mongodb://mongodb.example.com:27017/";

objMongoClient.connect(strMongoDBURI, function(err, db) {
if (err) throw err;
  var dbo = db.db("dbNameToSelect");
  var objQuery = { key1: "String Value 1" };
  var objNewValues = { $set: {key3: 12345, key4: "Another string value" } };
  dbo.collection("collectionName").updateOne(objQuery, objNewValues , function(err, res) {
    if (err) throw err;
    console.log("Record updated");
    db.close();
   });
});