bcquality/microsoft/knowledge/performance/oncompanyopen-subscribers-must-not-do-io.md
Jesper Schulz-Wedde 42ff075793 Promote knowledge for Microsoft review skills
Move canonical knowledge for Microsoft-owned review domains into the Microsoft layer and document the skill/knowledge co-location policy.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Copilot-Session: 2a6ea875-d38e-4f30-aadb-0d606f9be231
2026-09-03 14:59:44 +02:00

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---
bc-version: [all]
domain: performance
keywords: [oncompanyopen, onafterlogin, session-start, httpclient, subscriber, login]
technologies: [al]
countries: [w1]
application-area: [all]
---
# Session-open subscribers must not do I/O
> Contributions welcome — open a PR to refine or extend this article.
## Description
`OnCompanyOpen`, `OnCompanyOpenCompleted`, and `System Initialization`.OnAfterLogin run while the session is being created. The platform waits until every subscriber returns before the UI, an API call, or a background session can proceed. `HttpClient` or a heavy `FindSet` here delays **every** session type, not just the user who "opened the company". Agents still put warmup sync, license checks, and HTTP probes on these events because they look like an application startup hook.
## Best Practice
Keep company-open subscribers to cheap in-memory work: set a flag, enqueue a job-queue entry, or `TaskScheduler.CreateTask`. Perform HTTP and large SQL after the session is running, in that background work.
See sample: `oncompanyopen-subscribers-must-not-do-io.good.al`.
## Anti Pattern
An `OnAfterLogin` / `OnCompanyOpenCompleted` subscriber that calls `HttpClient` or scans a ledger. Detection signal: `HttpClient`, `FindSet`, or `CalcFields` inside a subscriber bound to those events.
See sample: `oncompanyopen-subscribers-must-not-do-io.bad.al`.