There are a number of quarky little MongoDB commands and queries hidden in the docs, two of them are $size and $type.

Does it really matter? $size that is …

The $size operator matches any array with the specified number of elements.

The basic use would be something like …

> db.ideas.find( { votes : { $size: 2 } } );

That would get you all the documents in the ideas collection that have exactly 2 items in the votes array (if say, you kept an array of votes.)

This however is of somewhat dubous use as:

You cannot use $size to find a range of sizes.

Meaning you can’t query for ideas that have 2 or more votes, or between 2 and 25 votes.

Are you her $type?

Another interesting query modifier is $type …

The $type operator matches values based on their BSON type.

This might be useful to seek out documents that have a property that is an int and not a string; the types correspond numeric IDs (see the list below.)

To get documents with a property which has a value that is of the type of int the query might look something like this …

> db.mycollection.find( { foo : { $type : 16 } } );

Again, the practical usefulness of this might be rather thin, but who knows … it might come in handy someday!

MongoDB BSON Types

Does this senario sound familiar?

The good news: your new super amazing mashup is becoming super popular, its getting featured on tech blogs and podcasts w00t!

You’ve used MongoDB for your database backed and so things have been scaling pretty well, and your analytics you have been tracking are really adding up, like millions and millions of documents.

The bad news is, well pretty much the same as the good — you’re starting to fill up your MongoDB Collection pretty quick and you want to break out your  analytics by month, and since the it’s analytic data it’s pretty much read only so you can move it around … but how?

Now, of course you could do some fancy sharding or something like that, but lets keep things simply why don’t we?

Copying Particular Documents Between Collections

So, say you want to take all the documents (records) that were created in May and move them to a stats_2012_05 collection?

Turns out this is pretty simple with MongoDB, much like a SELECT INTO statement in SQL you can make a copy of the May documents and insert them into your new collection, and then remove them from your source collection.

To do this, we need to remember that the shell of MongoDB uses javascript, so instead of a long query like one would use in SQL we will instead use the power of javascript and write a small function.

Grab Just the Docs You Want …

First off we’ll gather the documents we want and store them in a javascript variable, switch databases (or don’t if you just want to move between collections) then loop over the documents you saved in your varible and insert them into your new collection.

> use source_database;
> var docs = db.source_collection.find({ accessed: {
     '$gte': new Date(2012, 4, 1), '$lt': new Date(2012, 5, 1)
} }); 
> use new_database;
switched to db new_database
> docs.forEach(function(doc) { db.new_collection.insert(doc) });

Let’s Breakdown What We Did There …

First, we got all the documents that have an accessed date in May, i.e. where the date is greater than or equal ($gte) to 5/1/2012 and less than ($lt) 6/1/2012 and loaded all the documents into a varible called docs.

Javascript has super weird 0 based months, so May = 4 not 5, I know … weird.

Then we switched databases, and looped over each document in our docs variable, loaded it into a variable called doc and inserted it into our new collection. And you’re done!

Optionally, you can remove() the documents form your source collection when you are done.

Other Uses

You could use this for all sorts of purposes of course, pretty much any query you can think of to help you break out your data into separate collections for lots of useful reasons.

I’m happy to (sorta) announce I (Justin) will be giving a talk at Mongo Seattle 2011 on Dec 1st in downtown Seattle.

According my contacts at 10Gen the registration is going gang-busters and there are going to be some really great sessions to attend including one from our buddy Damon Cortesi at Simply Measured as well as a bunch of 10Gen engineers (the people that make MongoDB) plus MongoLab, Geek.net, VMWare and WordSquared (a super fun game!)

So, what are you waiting for?

Go ahead a register here  … and if you send me a DM or @Reply on Twitter (@learnmongo) I might just give you a pretty decent discount code!

A while back we wrote a post explaining how to compact MongoDB data files, that example shows how to use some server side javascript and a cron job to automatically compact the data files on a schedule … however this isn’t always ideal as it will compact all the collections in a database at one time.

If you have a very large Collection you didn’t want to run the operation on …or a number of Collections in your database you didn’t wish to compact you were a bit out of luck.

Also, using repairDatabase() requires the disk space for the current database plus the repaired copy (i.e. double the disk space.)

Enter MongoDB 2.0 … in 2.0+ you can now use the simple compact command to target the compact to a single collection.

Running compact has three major performance benefits:

• Compacts collection (less disk space.)
• Defragments a collection (data pages are aligned better.)
• Rebuilds and compacts the collection’s indexes (less RAM needed, and better perf.)

If  you have a lot of read/write/delete operations going on in your Collection this could possibly have a fairly noticeable performance impact.

There are two ways to run the compact command:

> db.yourCollection.runCommand("compact");

> db.runCommand({ compact : 'yourCollection' });

Now, that said there are two big downsides to using compact …

• The compact command blocks operations on the collection until it’s done compacting (so it’s best to run this off hours during scheduled maintenance.)
• It’s typically slower than repairDatabase in its actual operational time.

Those aside, as part of routine maintenance compact is a really helpful new feature and might justify an upgrade to 2.0 all on it’s own!

Since MongoDB will allow you to store more than just string and int values but also things like arrays … from time to time you might need to know how many items are in an array in your document.

For example, say you have a hosting business and to make a good profit you need to sell at least three upgrades to each hosting client.

To make sure you market to the correct people you want to find all the clients with two upgrades (and get them to sign up for just one more.)

Your general document structure looks something like this:

{
 customer_name : "Sam Taylor",
 email : "sam@widgetxyz.com",
 upgrades: ["ssl", "diskspace 3", "RAM 5"]
}

{
 customer_name : "Kyle Lopez",
 email : "klopez@buymybikes.com",
 upgrades: ["diskspace 3", "RAM 5"]
}

To get all the clients that have two upgrades we’ll use the $size operator:

> db.clients.find( { upgrades : { $size: 2 } } );

Now you’ll get back all the documents that have two upgrades in the “upgrades” array (or in this example, the document for “Kyle Lopez”.)

What about a range?

Unfortunately the $size operator doesn’t support ranges (like getting all documents with 2 to 4 array values.) To do that you’d need to create your own separate field to keep track of the count and query on that field.

Inspired by this Stackoverflow question I thought it would be worth while to post this quick tip …

If you ever need to copy (or “clone”) a MongoDB databse between two MongoDB servers its as easy as running the copyDatabase() command.

First, change into your admin db …

Then, run copyDatabase() supplying the needed information, the “from_db” (the database to copy) the “to_db” and the host name of the database you are copying …

> use admin;
> db.copyDatabase(…);

You’ll need to run the copyDatabase() command on the sever you want to “copy” the database to, aka the “destantion” server …

> db.copyDatabase(from_db, to_db, from_host);

If you are running MongoDB with accounts/password via –auth you’ll need to add that info on there too …

> db.copyDatabase(from_db, to_db, from_host, username, password);

A simple example for a database named “weyoun” on a host called “delta.quad.com” would be like so …

> db.copyDatabase('weyoun', 'weyoun', 'delta.quad.com');

There ya go, you should be good to go copying your MongoDB database!

Dear Readers,

Sorry it’s been so quiet lately … thing is LearnMongo is moving!

Well not moving sites or anything … but I’m moving … to the sunny North West (at least that’s what my new boss keeps telling me?) …

Seattle Bound (wait that sounds wrong)

Once all the moving stuff is taken care of LearnMongo will be back to regular posts (staring in March.)

I’m sure I’ll have plenty of coffee to fuel some awesome Mongoness.

Thanks for your support so far, and I’m looking forward to lots more Mongo in 2011.

~ Justin

MongoLA 2011

Here are the slides from my talk at MongoLA last week, hopefully the audio will be up soon (I will post it here.)

There is a nice write up of the event here… thanks to everyone that came!

Beyond Logging

Using MongoDB to Power a Private Social Network

(Oh, and log millions of e-mails too.)

StackOverflow Anwser Series

This is a new “concept” post based on StackOverflow Questions/Answers.

Feel free to comment below if this format is helpful & informative!

Overview

This Answer covers the following MongoDB topics:

• Data Files
• Compacting Data Files
• The db.repairDatabase(); command
• Capped Collections
• Server-side Javascript

Question

StackOverflow Link

Is there a way to auto compact MongoDB Data Files?

The mongodb documentation says that:

To compact this space, run db.repairDatabase() from the mongo shell (note this operation will block and is slow).

I’m wondering how to make the mongodb free deleted disk space automatically?

Answer

StackOverflow Link

In general if you don’t need to shrink your datafiles you shouldn’t shrink them at all. This is because “growing” your datafiles on disk is a fairly expensive operation and the more space that MongoDB can allocate in datafiles the less fragmentation you will have.

Also doing any sort of “shrink” will likely be a rather expensive operation and will likely lock your database while it’s running!

So, you should try avoid shrinking and provide as much disk-space as possible for the database.

However if you must shrink the database you should keep two things in mind.

• MongoDB grows it’s data files by doubling so the datafiles may be 64MB, then 128MB, etc up to 2GB (at which point it stops doubling to keep files until 2GB.)
• As with most any database … to do operations like shrinking you’ll need to schedule a separate job to do so, there is no “autoshrink” in MongoDB. In fact of the major noSQL databases (hate that name) only Riak will autoshrink. So, you’ll need to create a job using your OS’s scheduler to run a shrink. You could use an bash script, or have a job run a php script, etc.

Server-side Javascript

You can use server-side Javascript to do the shrink and run that JS via mongo’s shell on a regular bases via a job (like cron or the windows scheduling service) …

Assuming a collection called foo you would save the javascript below into a file called bar.js and run …

$ mongo foo bar.js

The javascript file would look something like …

bar.js

// Get a the current collection size.
var storage = db.foo.storageSize();
var total = db.foo.totalSize();

print('Storage Size: ' + tojson(storage));

print('TotalSize: ' + tojson(total));

print('-----------------------');
print('Running db.repairDatabase()');
print('-----------------------');

// Run repair
db.repairDatabase()

// Get new collection sizes.
var storage_a = db.foo.storageSize();
var total_a = db.foo.totalSize();

print('Storage Size: ' + tojson(storage_a));
print('TotalSize: ' + tojson(total_a));

This will run and return something like …

MongoDB shell version: 1.6.4
connecting to: foo
Storage Size: 51351
TotalSize: 79152
-----------------------
Running db.repairDatabase()
-----------------------
Storage Size: 40960
TotalSize: 65153

Run this on a schedule (during non-peak hours) and you are good to go.

Capped Collections

However there is one other option, capped collections.

Capped collections are fixed sized collections that have a very high performance auto-FIFO age-out feature (age out is based on insertion order). They are a bit like the “RRD” concept if you are familiar with that.

In addition, capped collections automatically, with high performance, maintain insertion order for the objects in the collection; this is very powerful for certain use cases such as logging.

Basically you can limit the size of (or number of documents in ) a collection to say .. 20GB and once that limit is reached MongoDB will start to throw out the oldest records and replace them with newer entries as they come in.

This is a great way to keep a large amount of data, discarding the older data as time goes by and keeping the same amount of disk-space used.

By default a MongoDB install does not use a username/password combination to access the database.

No Password, But Why?

This is down to the design philosophy of MongoDB which is to push much of the “logic” to the application level and keep the database doing what databases do best! Hence,  given the way that MongoDB is normally used it’s generally not necessary.

In a SQL environment you might have multiple users with multiple groups and schemas to lock down different levels of the database, tables, views, stored procedures, etc. …

The idea of this is of course to 1) protect data from unauthorized modification or deletion as well as 2)  limiting the tug ‘o war between users by clearly defining their rights.

So, you can give the accounting department different privileges then the marketing department or given root access to the developers (of course.)

Conversely MongoDB’s design philosophy would pass these (generally) over to the application itself to handle.

I Don’t Care, I Still Want Passwords!

No worries, if the idea of no users/passwords keeps you up at nights you can still have them!

MongoDB (currently) supports users on the database level with both “read/write” and “read only” options.

You can see all the users in a database in the system.users collection …

> use mydatabase
> db.system.users.find()

Creating MongoDB Users

To adding a new user is fairly straight forward …

Read/Write User

$ ./mongo
> use mydatabase
> db.addUser("admin", "Sup3rG00dP@azzword")

This creates a read/write user for the database mydatabase (you can choose any username you wish.)

This user will be “good for” this database alone.

Read Only User

> db.addUser("web", "prettyGoodPass", true)

The “true” parameter there makes the user read only (great for parts of the application code that you want to make sure never accidentally preform a write operation.)

Using A User: Authenticate

To “login” as a user you’ll need to authenticate, simply use …

> db.auth("admin", "Sup3rG00dP@azzword")

You can also authenticate via the command line using the mongo parameters below …

  -u [ --username ] arg username for authentication
  -p [ --password ] arg password for authentication

Changing Passwords

To change the password simply run the addUser command again with a new password.

> db.addUser("web", "wayGooderPass", true)

Delete a User

To remove a user you need to remove the corresponding document for that user in the system.users collection.

> db.system.users.remove({"user" : "web"});

Set MongoDB to Force Authentication

To force MongoDB to use authentication you’ll need to add the –auth parameter to MongoDB at startup (so you would need to restart.)

If you are using an unauthorized user you’ll get an error something like …

error: { “$err” : “unauthorized for db [mydatabase] lock type: -1 ” }

If you have a read only user and attempt a write option you’ll simply get back …

unauthorized

Lastly, if you try to use a user that doesn’t have read/write privlages on the admin database …

So, if you try to do something like list the avaible database on the server ( > show dbs ) you’ll get error too and you’ll need to make sure you switch to an account with read/write privlages in admin.