A Few Useful Commmands for MongoDB

About four months ago I was handed an already existing MongoDB infrastructure and told to take care of it. It’s been a long road, and I’ve had to train up quite quickly on this system.

While I’m certainly no expert at this point, it is what I’ve been spending the bulk of my time doing. Now that I’ve got som actual administration time under my belt I thought I’d relay a few commands and tricks that I wished I’d known from the outset.

db.setProfilingLevel

db.setProfilingLevel(level, slowms)

This turns on the performance monitoring in MongoDB. The level portion sets the level of the profiling (logging) you want.

0 = Off
1 = Only slow operations
2 = All operations

The slowms is optional, but sets the threshold for what is considered a “slow” operation. It is 100ms by default and generally you can leave it there.

Keep in mind that if you are using the cloud-based MMS solution from MongoDB, this will send information to them. If your business falls under HIPAA or some similar security requirements, you may not want to use this.

Note: You can find out if profiling is enabled by using db.getProfilingStatus()

show profile

show profile

This, when used in conjunction with the previous command will show the last 5 slow operations. Very useful if you think there might be an errant query or index problem that is slowing down your DB.

db.serverCmdLineOpts()

db.serverCmdLineOpts() will give you output similar to this.

db.serverCmdLineOpts()

I was handed an already existing infrastructure and had to do a little bit of walking backwards through it to put my own configuration documentation together.

This is a command that tells you what command line options were being used when this particular mongo instance was started. It includes everything entered in the command line as well as anything used in the config file.

Collection Schema

var findschema = db.Collection.findOne();
for (var key in findschema) {print (key) ; }

While MongoDB is a schemaless* database, it does help to know what is being stored where. To build a layout of the database, you can use the above two lines to report all the keys in a Collection. Make sure you swap out the Collection portion to the collection you want to find the schema for.

logRotate

db.runCommand( { logRotate : 1})

If you’re running MongoDB on Linux, then you have a few more choices via syslog, but if you’re like me and using it in Windows, then this is really your only option to rotate the logs.

That is, it stops writing to one log file and starts writing to another, appending the previous log file with a time stamp. This rotates all logs as far as I can tell, including audit logs if you’re using those. I personally run this command once a month just to keep the files manageable.

More to Come

This is just a quick starter set that I had specifically noted as “things I wished I knew when I started,” and I’ve got quite a few more. In the future I’ll delve into some of my simpler metrics and stat commands that I use in conjunction with MMS.

-Charlton Julius

* It’s not really schema-less as MongoDB does use a schema, but it doesn’t enforce that schema. Objects can be added and removed as needed.