How to print a sentence when users on the server run mongo shell?

For DBAs to limit some dangerous functionalities like dropping a database or it can be a helpful message. Or a greeting message. Or even printing the default database that he is already connected to.

By default mongoDB looks for a file named .mongorc.js in your home directory. So create a file ~/.mongorc.js and put a content like below inside of it:

print("Hello! Welcome to Alireza company :)");
 print("Your database is set to: " + db);

That's it! Save the file and run mongo in your terminal, the output should be similar to the following:

$ mongo
 MongoDB shell version v3.6.2
 connecting to: mongodb://127.0.0.1:27017
 MongoDB server version: 3.6.2
 Your database is set to: test
 Hello! Welcome to Alireza company :)
 >

In order to disable dangerous functionalities:

var no = function() { print("oops! You are not allowed to drop anything!!");};

 // Prevent dropping databases
 db.dropDatabase = DB.prototype.dropDatabase = no;

 // Prevent dropping collections
 DBCollection.prototype.drop =no;

 // Prevent dropping indexes
 DBCollection.prototype.dropIndex = no;

Sample output:

> db.dropDatabase('bi')
 oops! You are not allowed to drop anything!!.

NOTE: You can disable loading your .mongorc.js by using the --norc option when starting the shell.

#mongodb #mongo #shell #mongorc

#Google on #valentine.

In order to connect to MongoDB replica set in Python you can give all server node addersses to MongoClient. Addresses passed to MongoClient() are called the seeds. As long as at least one of the seeds is online, MongoClient discovers all the members in the replica set, and determines which is the current primary and which are secondaries or arbiters.


Sample usages:

>>> MongoClient('localhost', replicaset='foo')
     MongoClient(host=['localhost:27017'], replicaset='foo', ...)
 >>> MongoClient('localhost:27018', replicaset='foo')
     MongoClient(['localhost:27018'], replicaset='foo', ...)
 >>> MongoClient('localhost', 27019, replicaset='foo')
     MongoClient(['localhost:27019'], replicaset='foo', ...)
 >>> MongoClient('mongodb://localhost:27017,localhost:27018/?replicaSet=foo')
     MongoClient(['localhost:27017', 'localhost:27018'], replicaset='foo', ...)

Read full details here:

- http://api.mongodb.com/python/current/examples/high_availability.html#connecting-to-a-replica-set


#database #mongodb #mongo #replica_set #replication #pymongo #arbiter #master #primary #slave

Secondary Reads

By default an instance of MongoClient sends queries to the primary member of the replica set. To use secondaries for queries we have to change the read preference:

>>> client = MongoClient(
 ...     'localhost:27017',
 ...     replicaSet='foo',
 ...     readPreference='secondaryPreferred')
 >>> client.read_preference
 SecondaryPreferred(tag_sets=None)

Now all queries will be sent to the secondary members of the set. If there are no secondary members the primary will be used as a fallback. If you have queries you would prefer to never send to the primary you can specify that using the secondary read preference.

#mongodb #replica_set #replication #secondary #slave #pymongo

If you are building a docker image using docker build and you want to force building the package use --no-cache:

docker build --no-cache

#docker #build #no_cache #cache #force

Today we had hundreds of connections to MongoDB that compared to previous days was so high and one of our modules initiating the connections every time a new request was initiated. To address the issue, I used a singleton design pattern to prevent creating many connections. Although pymongo has its own connection pool, we reduced the connections by 40 to 60!

Singleton design pattern makes sure to only create a new object from your class only ONCE:

class Singleton(object):
     _instance = None

     def __new__(class_, *args, **kwargs):
         if not isinstance(class_._instance, class_):
             class_._instance = object.__new__(class_, *args, **kwargs)

         return class_._instance

__new__ will be called automatically when a new instance of the class is initiated. Here we check whether _instance is set or not, in case it is set we will return the old instance and will not create a new object.

Now you just need to send Singleton as parent class to your classes:

class MongoConnnection(Singleton):
     def __init__(self):
         pass

     def get_connection(self):
         pass

Create as many object as you want from MongoConnnection. And now when you print the initiated class you will see one memory address not different ones.

#mongodb #mongo #singleton #design_pattern #__new__

How to determine fragmentation in MongoDB?

use mydb
 var s = db.my_collection.stats()
 var frag = s.storageSize / (s.size + s.totalIndexSize)

NOTE: A frag value larger than 1 indicates some level of fragmentation.


#mongodb #mongo #stats #fragmentation

http://learnmongodbthehardway.com/schema/introduction/

#mongodb #mongo #tutorial

In order to check the filesystem of the partition and where that partition maps to that address use:

mount -l

It will show for example that you have a partition that mounted by NFS or that a partition is ext4 and so on:

/dev/mapper/vg-var on /var type ext4 (rw,relatime,stripe=384,data=ordered)

#linux #mount #ext4 #nfs #filesystem #partition

For optimal performance in terms of the storage layer, use disks backed by RAID-10. RAID-5 and RAID-6 do not typically provide sufficient performance to support a MongoDB deployment.

Avoid RAID-0 with MongoDB deployments. While RAID-0 provides good write performance, it also provides limited availability and can lead to reduced performance on read operations, particularly when using Amazon’s EBS volumes.


#mongodb #mongo #disk #raid #SSD

How to detect unused indexes on MongoDB collections?

db.your_collection.aggregate([{$indexStats:{}}]).pretty()

ops displays operations per second on a specific index. If ops is very low compared to other indexes you can drop the index.

#mongodb #mongo #index #unused_indexes

#mongodb #backlog #road_map

The good news is MongoDB will support transactions from version 4.0 like any other DBMS systems like MySQL, Oracle,...

Scaling MongoDB - Avoiding Common Pitfalls

#mongodb #video #tutorial

Looks like they need to defrag the port.

OnePHP a super simple and super lightweight PHP framework that works in just a few lines:

// index.php
 require_once('src/OnePHP/one_framework.php');
 $app = new \OnePHP\App();

 $app->get('/:name',function( $name ) use ( $app ){//Action
   echo "Hello $name";
 });
 $app->listen();

To install using composer:

composer create-project julces/oneframework

Why OnePHP:

1- Restful Routes

2- Easy and clean (GET, POST, PUT, DELETE...) Requests management

3- Restful Response with HTTP Status Code and custom Headers

4- PHP native Views

5- No dependencies, add extra libraries only when you need it.

#php #framework #onePHP

If you have a dedicated server only for MongoDB database I would I highly recommend to set block size as mongo suggested for their engines.


For the WiredTiger storage engine:

Set the readahead setting to 0 regardless of storage media type (spinning, SSD, etc.).

Setting a higher readahead benefits sequential I/O operations. However, since MongoDB disk access patterns are generally random, setting a higher readahead provides limited benefit or performance degradation. As such, for most workloads, a readahead of 0 provides optimal MongoDB performance.

In general, set the readahead setting to 0 unless testing shows a measurable, repeatable, and reliable benefit in a higher readahead value. MongoDB Professional Support can provide advice and guidance on non-zero readahead configurations.


For the MMAPv1 storage engine:

Ensure that readahead settings for the block devices that store the database files are appropriate. For random access use patterns, set low readahead values. A readahead of 32 (16 kB) often works well.

For a standard block device, you can run sudo blockdev --report to get the readahead settings and sudo blockdev --setra <value> <device> to change the readahead settings. Refer to your specific operating system manual for more information.


#mongodb #mongo #ra #readahead #mmapv1 #wiredtiger

In python in order to swap variables you can use unpack feature:

>>> a, b = 1, 2
 >>> a, b = b, a
 >>> a, b
 (2, 1)

This is a nice way to swap variables without using a third variable.

#python #swap #swap_variables

Extended unpacking (`Python 3 Only`):

>>> a, *b, c = [1, 2, 3, 4, 5]
 >>> a
 1
 >>> b
 [2, 3, 4]
 >>> c
 5

This way first and last list element will be unpacked into a and c and other values will be inside of b variable.

#python #python3 #unpack #extended_unpack