MongoDB 不尊重上限集合的最大值

Question

Here's how I'm creating the collection:

def connect(self):
    self.client = MongoClient(self.ip, self.port)
    try:
        capped = self.max_size != None
        print("capped", capped)
        print("max", self.max_size)
        self.coll = self.client[self.db_name].create_collection(self.coll_name, capped=capped, max=self.max_size)
    except:
        self.coll = self.client[self.db_name][self.coll_name]
    self.coll.create_index([('expireAt', 1)], expireAfterSeconds=0)

I set self.max_size to 10 .

Then, I tried this:

for i in range(20):
    cache.put(i, {'id': i})

I expected to see exactly 10 documents in the collection, but in reality all the 20 documents are in the collection.

Why?

Update: I ran the following in mongo shell:

db.getCollection('test').isCapped()

And I got

false

Answer 1

You need both size and max parameters to create the capped collection:

conn.test.create_collection('test', capped=True, size=10000, max=5)

for i in range(10):
    conn.test.test.insert_one({'_id': i})

In the mongo shell:

> db.test.find()
{ "_id" : 5 }
{ "_id" : 6 }
{ "_id" : 7 }
{ "_id" : 8 }
{ "_id" : 9 }

> db.test.isCapped()
true

If you try to create it without the size parameter, it will output an error:

pymongo.errors.OperationFailure: the 'size' field is required when 'capped' is true

However your code have a try...except parameter. So it hit this error, then proceeded to the except clause, so you didn't see that an error was produced and the capped collection was not created.