Copy MongoDB collections from remote to local instance

This is revisiting the blog I wrote a while back, which showed using mongodump and mongorestore to copy a MongoDB database from one machine (a Unifi CloudKey) to another. This time instead of a manual lift and shift, I wanted a simple way to automate the update of the target with changes made on the source.

The source is as before, Unifi’s CloudKey, which runs MongoDB to store its data about the network - devices, access points, events, and so on.

Why?

So that I can get this interesting data from somewhere that I cannot mess with into a place from which I can easily stream it into Kafka.

The constraints

• Unifi uses MongoDB v2.4.10 which is pretty darn old

• I must not break the source - it runs my home network and my kids will be mad if it stops working just so that I can hack around on it. I want the least-impact option, and I definitely don’t want to start reconfiguring MongoDB with replicas and stuff like that

• My target MongoDB is running in Docker

The solution

It’s as hacky as they come. Instead of mongodump to a file which I then copy to the target and repopulate with mongorestore, I use Linux pipes and remote command execution with SSH to stream the dump from the source directly into the target.

First bring up a MongoDB container:

docker run --rm \
           --publish 27017 \
           --name mongodb \
           mongo:4.2.2

Then copy across the data:

ssh robin@unifi \
    mongodump --port 27117 --db=ace --collection=device --out=- | \
docker exec mongodb \
    mongorestore --dir=- --db=ace --collection=device

In detail:

• ssh robin@unifi connects remotely to the Unifi cloudkey server (using existing SSH keys for password-less authentication)

• \ is the line continuation character just to make all of this easier to read

• mongodump is called and the important point here is that I specify --out=- which instead of dumping to file dumps to stdout

• | the magic pipe! This routes the stdout from mongodump into…

• docker exec runs the following command on the MongoDB container. Because I specify the --interactive argument it passes the stdout of the previous step as stdin into…

• mongorestore which reads from stdin because I’ve specified --dir=-

The rest of the parameters are just specifying the database and collection that I want to copy. When this runs you can see the documents get created:

[…]
2019-12-17T22:10:51.609+0000 restoring ace.device from -
connected to: 127.0.0.1:27117
2019-12-17T22:10:51.807+0000 no indexes to restore
2019-12-17T22:10:51.807+0000 finished restoring ace.device (5 documents, 0 failures)
2019-12-17T22:10:51.807+0000 5 document(s) restored successfully. 0 document(s) failed to restore.

If you re-run it you just get duplicate key violations, which is to be expected, and means that in theory I can just rerun this process on a timed loop and pick up any new documents whilst ignoring existing ones. Not very efficient, but fairly effective :)

2019-12-17T22:12:18.253+0000 continuing through error: E11000 duplicate key error collection: ace.device index: _id_ dup key: { _id: ObjectId('583854cde4b001431e4e4982') }
2019-12-17T22:12:18.253+0000 continuing through error: E11000 duplicate key error collection: ace.device index: _id_ dup key: { _id: ObjectId('5d77b7a8cf2b2b01c42811b5') }
2019-12-17T22:12:18.253+0000 continuing through error: E11000 duplicate key error collection: ace.device index: _id_ dup key: { _id: ObjectId('58385328e4b001431e4e497a') }
2019-12-17T22:12:18.254+0000 continuing through error: E11000 duplicate key error collection: ace.device index: _id_ dup key: { _id: ObjectId('58b3fb48e4b0b79e50242621') }
2019-12-17T22:12:18.254+0000 continuing through error: E11000 duplicate key error collection: ace.device index: _id_ dup key: { _id: ObjectId('58b406f1e4b0e334d74c46e4') }
2019-12-17T22:12:18.254+0000 no indexes to restore
2019-12-17T22:12:18.254+0000 finished restoring ace.device (0 documents, 5 failures)
2019-12-17T22:12:18.254+0000 0 document(s) restored successfully. 5 document(s) failed to restore.

Iterating across collections

Because we’re using stdout/stdin we have to specify the database and collection. To loop over them you can just use a for loop in bash:

for c in collection_foo collection_bar
do
    ssh robin@unifi \
        mongodump --port 27117 --db=ace --collection=$c --out=- | \
    docker exec --interactive mongodb \
        mongorestore --dir=- --db=ace --collection=$c
done

This will run the same import/export for collection_foo and collection_bar.