#mongo admin

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MongoDB: The Optime

Mongo records all operations that modify the data in a replica set in the oplog. Each entry in the oplog has a special timestamp called the optime.

The optime is a couple of integers: a first value which is a timestamp with precision to the second, and a 32 bit incremental value for operations "in the same second". So, basically, every time the first value increases, the second one restarts from zero.

Secondary members of the set will receive the optime from the primary with the statement they synchronize to, and store it unchanged. So, on secondaries the last optime states which operation has just been received from the primary and is now executing locally.

In the Replica Set Admin UI you can get the optime for the last entry in all the members of a replica set, and that gives a simple way to see if some servers are lagging behind the primary: it shows both how much time and how many operations the secondary is behind.

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Mongo Admin Tools

To get information on the state of a running mongod process, some external tools are available.

The only one bundled with MongoDB is the HTTP console, which provides a simple web interface listing information of interest to administrators. It can be accessed at the port with numeric value 1000 more than the configured mongod port: that is, for the mongod listening on port 27017, the console can be accessed at http://localhost:28018 in a web browser. Some of these information can be accessed by launching mongotop or running the db.currentOp() command in the shell.

By launching mongod with the additional --rest parameter, two more services are available on the same port. One is the Simple REST interface, which only supports data retrieval: no create, update or delete but it allows to issue any kind of query using URLs and returns the result as strict JSON. It can be found at http://localhost:28018/dbName/ where dbName is the name of the database to query.

Finally, there's the Replica Set Admin UI. For some reason, you also need to have ran your server with:

mongod --rest someOtheStuffHere

It can be found at http://localhost:28017/_replSet or by just following the link from the root page. It has additional information on the status of the replica sets, if any set is available. Some of these infp can be accessed with the db.printReplicationInfo() shell command.