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Do not call HttpClient inside an open write transaction
Contributions welcome — open a PR to refine or extend this article.
Description
The first database write opens an AL write transaction that the runtime holds until the execution completes or Commit() runs — see understand-implicit-transaction-boundary.md. HttpClient blocks the session until the remote call returns. Any locks taken by earlier Insert/Modify/Delete therefore stay held for the HTTP wall-clock time, and interactive users see a spinner. This is not generic "don't block": it is the AL transaction model plus lock lifetime around outbound I/O.
Best Practice
Defer the HTTP call to a separate session. When the external operation must correspond to a committed database change, insert an outbox work item in the same transaction as that change and process committed outbox rows with a recurring job queue entry. The change and work item then commit or roll back together, and the worker performs HTTP before deleting the item so it holds no write lock during the call. The separate retry-safety requirement is covered by job-queue-external-effects-must-be-idempotent.md.
A directly created scheduled task is suitable only when its work is independent of the caller's commit. An immediately ready task can run concurrently with the caller, so it must not assume that the caller's writes are already committed. Do not use Commit() as a general remedy: it irrevocably commits all prior writes in the current transaction, so any subsequent failure cannot roll them back. Commit() is appropriate only at top-level entry points where partial persistence is intentional and understood.
See sample: httpclient-inside-write-transaction-holds-locks.good.al.
Anti Pattern
Modify/Insert followed by HttpClient in the same procedure with no Commit between them. Detection signal: any HttpClient use after a write on the same execution path, especially in posting, page actions, or subscribers.
See sample: httpclient-inside-write-transaction-holds-locks.bad.al.