Resume and Checkpoint Restart

Every long transfer is a bet that nothing will interrupt it - and the longer it runs, the worse the odds. When the bet is lost, the difference between resume and restart is the difference between a two-minute recovery and repeating the whole night. Resume is an old capability that most protocols support and most automation forgets to use.

This series makes it deliberate. It covers why transfers get interrupted and how resume actually works in FTP, SFTP, HTTP, and rsync. It covers what to check in clients and servers before relying on it. Then it covers checkpointing at the job level so multi-file runs restart where they stopped. It covers verifying a resumed file properly and choosing a resume strategy per flow. It pairs with our partial-file safety series, which covers the consumer's side of half-written files.

Articles in This Series

• Why Transfers Get Interrupted (and Why Starting Over Hurts)
  This article catalogs interruptions: idle timeouts, network blips, reboots, address-table expiry, and the far end's maintenance. It covers the cost of restarting at scale and the resume-or-restart decision.
• How Resume Works in FTP, SFTP, HTTP, and rsync
  This article covers restart markers and append in FTP, and offset reads and writes in SFTP. It covers byte ranges in HTTP, and partial and append modes in rsync. It explains what each supports, what it assumes, and where it breaks.
• Resume Support Compared: Clients, Servers, and Settings
  What to check on each side, enabling resume deliberately, the modified-source danger of resuming onto a file that changed, and testing resume before trusting it.
• Checkpointing in Automated Jobs
  Job-level state that survives a crash: what was done, restartable steps, per-file ledgers, and the restart-where-you-stopped pattern for multi-file runs.
• Verifying a Resumed Transfer
  Why a resumed file needs a hash, why size alone lies, detecting stale partial files, and the verification step that belongs in every resume-capable job.
• A Resume Strategy for Your Flows
  This article covers per-flow decisions from file sizes, link quality, and protocol, and the settings checklist for both ends. It covers killing a transfer on purpose to test resume, and documenting what you chose.

Explore More Topics

This series is part of the Sysax file transfer topic library. The library covers the protocols, security practices, automation techniques, and operational skills behind reliable file transfer. The library pairs well with the practical tools we build. Sysax Multi Server is a secure FTP, FTPS, SFTP, and HTTPS server for Windows. Sysax FTP Automation schedules and scripts secure transfers so the routine ones run themselves.