> For the complete documentation index, see [llms.txt](https://ebs-projects-knowledgebase.gitbook.io/ebs-projects-knowledgebase/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://ebs-projects-knowledgebase.gitbook.io/ebs-projects-knowledgebase/double-opt-in-solution/psm-doi-middleware.md).

# PSM DOI Middleware

The **DOI PSM** is a mission-critical middleware solution designed to handle high-throughput transactional requests (primarily DOI/Push notifications). It serves as a robust buffer between client applications and sensitive downstream infrastructure (like OTA/HSM servers), ensuring reliability through **asynchronous processing**, **rate limiting**, and **failure recovery**.

The PSM is a critical middleware solution designed to bridge the gap between modern, high-velocity web applications and legacy telecommunications infrastructure.

Its primary mandate is to facilitate secure **Over-The-Air (OTA)** updates to subscriber SIM cards. By abstracting the complexities of cryptographic hardware (HSM) and binary SMS delivery (SMPP), the PSM provides a clean, RESTful interface for upstream clients while ensuring strict compliance, auditability, and reliability in downstream delivery.