> For the complete documentation index, see [llms.txt](https://stephen-tsoi.gitbook.io/stephen-tsoi-docs/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://stephen-tsoi.gitbook.io/stephen-tsoi-docs/messaging-troubleshooting/a-missing-event-among-billions-how-we-found-it-on-an-event-mesh/day-7-13-use-case-2-oauth-authentication-via-mqtt-connection-failure.md).
# Day 7/13 Use Case 2 - OAuth authentication via MQTT Connection Failure
MQTT stands out for giving detailed error codes that clarify why a connection was refused, making it simple to trace the issue’s origin. Under the OAuth 2.0 framework, the event broker periodically syncs with the CIAM to obtain a public key, which is then used to authenticate the JWT for each connection attempt. There are many potential causes for connection rejections, but the most common ones include:
**Rejection due to expired JWT** Clients must update their JWT before it expires to maintain their connection with the event broker. The event broker keeps track of the token’s expiration and will automatically sever connections when that time comes. Occasionally, an application cannot update its JWT in time due to ongoing transactions, leading to missed events when the event broker disconnects the session.
**Rejection due to invalid connection** After the client utilizes the refreshed JWT to re-establish a connection with the event broker, it inadvertently continues to use the prior connection to transmit events. Consequently, the event broker identifies and rejects this invalidated connection.
*Rejection due to an excessive number of concurrent connections*\* The client establishes a distinct connection on each page without terminating the previous one. Consequently, if the cumulative number of concurrent connections surpasses the event broker’s maximum allowed concurrency for each client, the event broker will refuse the new connection.
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