MongoDB does not remove documents from a collection with a configured TTL Index - mongodb

Question

I try to get started with TTL indices in mongo, and wanted to get a simple demo setup running, but I just cannot get it to work and I am not sure what I do wrong.
Mongod version:
$ mongod --version
db version v4.4.5
Build Info: {
"version": "4.4.5",
"gitVersion": "ff5cb77101b052fa02da43b8538093486cf9b3f7",
"openSSLVersion": "OpenSSL 1.1.1m 14 Dec 2021",
"modules": [],
"allocator": "tcmalloc",
"environment": {
"distmod": "debian10",
"distarch": "x86_64",
"target_arch": "x86_64"
}
}
I start with a completely fresh, default configured mongod instance:
$ mkdir /tmp/db && mongod --dbpath /tmp/db
Then I run the following commands
use test
db.ttldemo.insertOne({
created_at: Date(Date.now() + 60_000)
})
db.ttldemo.createIndex({created_at: 1}, {expireAfterSeconds: 1})
db.ttldemo.find()
I would expect the document to vanish after some time, but even after running find() after waiting for a few minutes, the document is still present.
Is there anything I am missing here?
db.adminCommand({getParameter:1, ttlMonitorSleepSecs: 1}) yields 60 to me

Answer 1

I tried in mongo 4.2.17, your insertion of documents yields a document like:
{
"_id" : ObjectId("61f17ea34844b5f0505d80ea"),
"created_at" : "Wed Jan 26 2022 19:02:27 GMT+0200 (GTB Standard Time)"
}
when the
db.getCollection('ttldemo').insertOne({created_at: new Date()})
yields a document like:
{
"_id" : ObjectId("61f17f404844b5f0505d80ec"),
"created_at" : ISODate("2022-01-26T17:05:04.023Z")
}
Emphasis here on the created_at field, where this on the first case is a string, in the second case is an ISODate.
as from the documentation , this index works on dates only:
To create a TTL index, use the createIndex() method on a field whose value is either a date or an array that contains date values, and specify the expireAfterSeconds option with the desired TTL value in seconds.
The solution here is to construct the date as follows:
db.getCollection('ttldemo').insertOne({created_at: new Date(new Date().getTime()+60000)})
this will create a new ISODate based on the current date + 60 seconds.