Server Selector Example

Users can exert fine-grained control over the server selection algorithm by setting the server_selector option on the MongoClient to an appropriate callable. This example shows how to use this functionality to prefer servers running on localhost.


Use of custom server selector functions is a power user feature. Misusing custom server selectors can have unintended consequences such as degraded read/write performance.

Example: Selecting Servers Running on localhost

To start, we need to write the server selector function that will be used. The server selector function should accept a list of ServerDescription objects and return a list of server descriptions that are suitable for the read or write operation. A server selector must not create or modify ServerDescription objects, and must return the selected instances unchanged.

In this example, we write a server selector that prioritizes servers running on localhost. This can be desirable when using a sharded cluster with multiple mongos, as locally run queries are likely to see lower latency and higher throughput. Please note, however, that it is highly dependent on the application if preferring localhost is beneficial or not.

In addition to comparing the hostname with localhost, our server selector function accounts for the edge case when no servers are running on localhost. In this case, we allow the default server selection logic to prevail by passing through the received server description list unchanged. Failure to do this would render the client unable to communicate with MongoDB in the event that no servers were running on localhost.

The described server selection logic is implemented in the following server selector function:

>>> def server_selector(server_descriptions):
...     servers = [
...         server for server in server_descriptions
...         if server.address[0] == 'localhost'
...     ]
...     if not servers:
...         return server_descriptions
...     return servers

Finally, we can create a MongoClient instance with this server selector.

>>> client = MongoClient(server_selector=server_selector)

Server Selection Process

This section dives deeper into the server selection process for reads and writes. In the case of a write, the driver performs the following operations (in order) during the selection process:

  1. Select all writeable servers from the list of known hosts. For a replica set this is the primary, while for a sharded cluster this is all the known mongoses.
  2. Apply the user-defined server selector function. Note that the custom server selector is not called if there are no servers left from the previous filtering stage.
  3. Apply the localThresholdMS setting to the list of remaining hosts. This whittles the host list down to only contain servers whose latency is at most localThresholdMS milliseconds higher than the lowest observed latency.
  4. Select a server at random from the remaining host list. The desired operation is then performed against the selected server.

In the case of reads the process is identical except for the first step. Here, instead of selecting all writeable servers, we select all servers matching the user’s ReadPreference from the list of known hosts. As an example, for a 3-member replica set with a Secondary read preference, we would select all available secondaries.