Pymongo: Advantage of using MongoReplicaSetClient?

Question

It seems that MongoClient and MongoReplicaSetClient can connect to mongo replica sets. In fact, their documentation pages are almost identical - the same parameters, the same methods, etc. - except that for the last constructor I need to specify replicaSet.

In both cases, we can indicate a reading preference. In both cases, we must handle the AutoReconnect exception if indentation occurs.

So my questions are:

• Why can one use one of them, since it is possible to perform the same operations with both?

• Both can do secondary reads, right? The documentation says that the advantage of ReplicaSetClient is that we can do secondary reads, but clearly , they are supported in both.

• the documentation says that ReplicaSetClient has the function "replica set health monitoring". What exactly does this mean? Are there any new methods that I can call that will tell me about the health of the replacement, which I otherwise cannot do with MongoClient?

• In theory, MongoReplicaSetClient will connect to all replset members, not just one. This is not true: you can skip or omit any of the servers in the connection string, and MongoClient and MongoReplicaSetClient can still connect. Did I miss something?

Answer 1

It was a confusing API choice, which we regret in PyMongo 2.x. We will combine all client classes in MongoClient in PyMongo 3, in April 2015:

http://emptysqua.re/blog/good-idea-at-the-time-pymongo-mongoreplicasetclient/

Topics:

• Use MongoReplicaSetClient when you plan to connect to the entire replica set. MongoClient connects to only one .
• For performing primary or secondary readings, one MongoReplicaSetClient can be used, as well as more complex decision making with reading preferences, see my related blog post . MongoClient will connect to one member of the replica set (primary) and always read from it, unless you make a direct connection to the secondary using MongoClient, in which case it will always read from this secondary.
• MongoReplicaSetClient monitors the specified health with a background thread that periodically checks all members. The client keeps track of whether members are present, tracks their ping time and notices when a member is added. This will reduce the number of exceptions that you see on the elephant network or when the replica set configuration changes, and this allows the client to correctly implement the read settings.
• MongoReplicaSetClient really connects to all members, while MongoClient only connects to one . MongoReplicaSetClient tries to connect to each member specified in the connection string; as soon as he connects to one, he asks the member for a list of all the other members. From now on, it ignores your connection string and uses the list that it received from the member to which it connected.