mongodb single DB replication

I've a working MongoDB "replica set" made up by 3 servers. It is storing two DBs, I wonder if is it possible to replicate only one of the DBs without running more than one mongoDB instance(one per DB).

Here is a sketch of the "problem"

      Server1   Server2   Server3
DB1      X         X         X
DB2      X         X

X 开发者_如何学Gostands for Server where DBs have to be replicated in.

thank

I don't believe it is possible.

Unlike sharding, where you specify down to the collection level what gets sharded, with replica sets you're defining that a given MongoDB instance is part of a replica set. As only one node in a replica set can be the master at any given time, based on the scenario you are talking about, then there would be a problem if e.g. Server1 went down and Server3 was promoted to master - as DB2 would then not be able to be written to.

I had a simliar problem and found a quite easy solution in javascript to be executed in a mongo-shell.

Sourcecode available here: http://www.suenkel.de/blog/2012/02/mongodb-replicate-one-database-or-collection/

With opening a tailable cursor on the oplog of the master server each operation could be applied to another server (of course you can filter by the namespace of the collections or even the databases...)

According to current MongoDB ReplicaSet architecture, you can't use a single Replica Set with some members having parts of the databases or collections.

However, if you have the requirement of replicating a single database or collection in real-time in another location, I ended up with following workaround:

1. Use directoryPerDB to separate the desired database files (Create a new replica with this option enabled if you don't have this already)
2. Copy the directory of desired database to the new location.
3. Deploy a new ReplicaSet with this single database.
4. Write a simple script and use Change Streams to perform the replication for you.

As I said, you will end up with another Replica Set dedicated for this database, but replication is done in real-time and both Replica Sets has the data in a consistent way (You have to perform your write operations on first ReplicaSet, though).