Performance issue when a user changed username

Description:

When a user changes username in chat, all messages in the collection are updated. This places a heavy load on the database if there are many documents in the collection.

rocketchat_message collection size: ~88 GB, 57 million documents

CPU usage

Steps to reproduce:

For reproduce needs a lot of documents in the collection rocketchat_message and a user should have a lot of messages

1. Go to profile
2. change username

Expected behavior:

Chat works

Actual behavior:

Chat is not available

Server Setup Information:

• Version of Rocket.Chat Server: 3.6.3
• Operating System: Debian
• Deployment Method: Docker
• Number of Running Instances: 4
• DB Replicaset Oplog: enabled
• MongoDB Version: 4.0

Relevant logs:

Logs from DB

update rocketchat.rocketchat_message command: { q: { u._id: "kPzqtR7ahm5z5Wgtx" }, u: { $set: { u.username: "Kerry", _updatedAt: new Date(1606316642299) } }, multi: true, upsert: false } planSummary: IXSCAN { u._id: 1 } keysExamined:52516 docsExamined:52516 nMatched:52516 nModified:52516 keysInserted:105032 keysDeleted:105032 numYields:7797 locks:{ Global: { acquireCount: { r: 7798, w: 7798 } }, Database: { acquireCount: { w: 7798 }, acquireWaitCount: { w: 4 }, timeAcquiringMicros: { w: 105158 } }, Collection: { acquireCount: { w: 7798 } } } storage:{ data: { bytesRead: 3349435742, timeReadingMicros: 64408232 } } 97265ms

Start by tracing the username-change path that updates the rocketchat_message collection and review the reported MongoDB update, including its index scan and multi-document write. Done means changing a username no longer makes chat unavailable or creates the reported heavy database load, but the issue does not name files or tests to run.