Created: Oct, 2015
Last modified: Sep, 2016
A list of MongoDB glossaries can be found here. But at its minimum, there are three terms: database, collection (like the “table” in a RBDMS) and document (the unit of a database, like the row in a RBDMS). For example:
A MongoDB server can have multiple databases, one database can have multiple collections, and a collection can have multiple documents. The the shell commands below to have a grasp of what it looks like.
MongodDB is schema-less (a feature of NoSQL), meaning that the documents in a collection don’t have to have the same structure.
Another feature of MongoDB is that a database or collection can be created on the fly, but its existence can only come into being when the first document under it is created.
Install MongoDB on Ubuntu: doc from mongodb.org
MongoDB failed to start on Ubuntu, “errno:98 Address already in use for socket: 0.0.0.0:27017 “: restart mongod on Ubuntu to release the occupied port:
sudo service mongod restart
mongod --smallfiles --dbpath /path/to/database
mongo
show dbs
use datadump
Note that if it is a new database “datadump”, it will not be created until the first record is inserted below.
At this point, the shell variable “db” will be referenced to “datadump” (also see below).
show collections
db.mycollection1.insert({"title": "user1", "email": "[email protected]"})
this will create a collection named “mycollection1” if it hasn’t be created, and add a document to this collection. From this point, this collection can be referenced as “db.mycollection1” in the shell.
newrec = [{"title": "user2", "email": "[email protected]"}, {"title": "user3", "email": "[email protected]"}]
db.mycollection1.insert(newrec)
db.mycollection1.find()
db.mycollection1.find().forEach(printjson)
db.mycollection1.remove({})
A query is passed to the remove() function to match the target queries. See query operator for details.
Note that even all documents in a collection are removed, the collection still exists, and show up in the results of show collections.
db.mycollection1.drop()
This will decommission a collection from a database, making it disappear in the results of show collections.
db.dropDatabase()
Press Ctrl+”d” to quit the console.
A list of commonly used MongoDB shell commands is here.
MongoDB has a Python driver called PyMongo. First initiate a Virtualenv container:
virtualenv pymongo_env
. ./pymongo_env/bin/activate
pip install --upgrade pip
And install dependencies and pymongo
sudo apt-get install python-dev libffi-dev libssl-dev
pip install requests[security]
pip install pymongo
Otherwise, there will be “InsecurePlatformWarning” after installing pymongo (see this post).
Refer to PyMongo documentation and tutorials on the usage.
Note that PyMongo is a blocking MongoDB driver, which may be fine for basic use. If non-blocking CRUD are needed, check out Motor.
MongoDB listens only to localhost by default. One basic scenario is that I need to access a database running on Server A and listening 27017 from another Server B. I find the SSH tunneling method in this post a simple solution that is also secure enough. It can be further supplemented by autossh to keep the SSH tunnel alive. The following assumes the MongoDB on Server A uses default settings.
First install autossh
sudo apt-get install autossh
Now generate SSH public-private key pairs on Server B
ssh-keygen
and concatenate the public key on Server B to “~/.ssh/authorized_keys” on Server A. I leave the key passphrase empty for convenience.
Create an autossh tunnel on Server B:
autossh -M 0 -fN -L 4321:localhost:27017 UserA@ServerA
The command above will create a tunnel between the port 4321 of localhost on Server B to the port 27017 of localhost on Server A. Autossh has many powerful options that need further reading. If the public key of Server B is not at “~/.ssh/id_rsa.pub”, specify the correct path with -i path_to_pub_key_server_b option.
Now, one can access the database on Server A from Server B with port 4321 of localhost.