如何在MongoDB 3.6.3中启用ReadConcernMajority？解决pymongo watch报错

Hey there! I’ve dealt with this exact error before when working with MongoDB 3.6 and pymongo’s watch() method. Let’s walk through how to fix it step by step.

The ReadConcernMajorityNotEnabled error pops up because either your MongoDB deployment doesn’t support read concern "majority" (like a single-node instance), or the feature is explicitly disabled in your configuration. Change streams (used by watch()) rely on read concern "majority" to ensure you’re seeing consistent, committed data.

If you’re running a replica set (the recommended setup for production and change streams), follow these steps:

a. Update Your MongoDB Configuration File #

Locate your mongod.conf (Linux: /etc/mongod.conf; Windows: C:\Program Files\MongoDB\Server\3.6\bin\mongod.cfg) and edit the replication block to include the enableMajorityReadConcern setting:

replication:
  replSetName: "yourReplicaSetName"  # You should already have this for a replica set
  enableMajorityReadConcern: true    # Add this line if missing, or set to true

b. Restart the MongoDB Service #

Save the config file, then restart the mongod process to apply changes:

• Linux: sudo systemctl restart mongod
• Windows: Open Command Prompt as admin and run net stop MongoDB && net start MongoDB

c. Verify the Configuration #

Connect to your MongoDB instance via the mongo shell, then run this command to confirm the setting is active:

db.adminCommand({ getParameter: 1, enableMajorityReadConcern: 1 })

You should see output like: { "enableMajorityReadConcern" : true, "ok" : 1 }

If you’re running a single-node MongoDB instance, read concern "majority" isn’t supported out of the box (since there’s no "majority" of nodes to confirm writes). To use watch(), you’ll need to convert it to a single-node replica set:

a. Stop the Current mongod Instance #

First, shut down your running MongoDB service:

• Linux: sudo systemctl stop mongod
• Windows: net stop MongoDB

b. Restart mongod with Replica Set Enabled #

Either update your config file to add replSetName: "rs0" under the replication block, or run mongod directly with the replica set flag:

mongod --replSet "rs0" --dbpath /path/to/your/database/folder --port 27017

Adjust the --dbpath and --port values to match your setup.

c. Initialize the Replica Set #

Connect to the mongo shell, then run this command to initialize the single-node replica set:

rs.initiate()

Once this completes, MongoDB will enable read concern "majority" by default, and your watch() method should work.

If you’re using a sharded cluster, you need to enable enableMajorityReadConcern on all shard replica sets and the config server replica set. Follow the same steps as the replica set section for each of these components.

While the main fix is on the MongoDB side, you can explicitly set read concern "majority" in your pymongo code to ensure it’s using the correct setting:

from pymongo import MongoClient, ReadConcern

client = MongoClient("mongodb://your-host:27017/")
collection = client["your-database"]["your-collection"]
cursor = collection.watch(read_concern=ReadConcern("majority"))

内容的提问来源于stack exchange，提问作者Alberto