Safe appendRow
onFormSubmit waits for lock, appendRow to Log, increments Script Properties ticket counter, then releases lock in finally.
What is LockService in Google Sheets Apps Script?
LockService gives your script a mutex so only one execution holds a lock at a time. In busy spreadsheets, two triggers firing together can both read the same last row and write duplicates. LockService.waitLock and releaseLock serialize critical sections like appendRow, counter increments, or quota-sensitive API calls.
When to use it
Use LockService when parallel runs are possible: dual form submits, overlapping time triggers, onEdit plus manual menu running together, or any shared counter and log append pattern.
When to skip it
Skip locks for read-only work or single-threaded manual runs. Over-locking everything slows the script and can cause wait timeouts if one run hangs.
How it works
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const lock = LockService.getScriptLock() for project-wide coordination.
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lock.waitLock(30000) waits up to 30 seconds to acquire the lock.
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Wrap the critical section in try/finally and call lock.releaseLock() in finally.
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{:"Keep locked sections short"=>"read, compute, write, then release."}
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Document lock and user lock exist for narrower scopes when appropriate.
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Log when waitLock times out so you know contention is too high.
Examples in Google Sheets
Shared invoice number
Two sales triggers read max invoice id, increment, and write back. Lock prevents duplicate invoice numbers.
API rate guard
Only one execution at a time calls a strict external API, avoiding parallel UrlFetchApp bursts.
Build this without starting from a blank cell
Use a Better Sheets tool for LockService, then watch a walkthrough when you want the full pattern.
Better Sheets resources
Common mistakes
- Forgetting releaseLock in finally, blocking all future runs until timeout.
- Holding a lock during long UrlFetchApp or MailApp calls, stalling every other trigger.
- Using getUserLock when all triggers share one script account and need Script lock.
- waitLock with zero timeout, failing instantly under normal contention.
- Nested locks across functions without a clear order, risking deadlocks in complex projects.
Frequently asked questions
- What happens if waitLock times out?
- It throws an exception. Catch it, log, and decide whether to retry or alert an admin.
- Do I always need LockService for appendRow?
- Low-traffic sheets often work without it. High concurrency form or webhook intake benefits strongly.
- What is the difference between script and document lock?
- Script lock coordinates across the whole Apps Script project. Document lock scopes to one spreadsheet file.
- Can simple onEdit use locks?
- Yes, if the handler does restricted work in an installable trigger. Keep the locked section minimal.
- Does lock work across spreadsheet copies?
- No. Each file and script project has its own lock namespace.
- How long can a lock be held?
- Until releaseLock or the execution ends. Long runs block others for the whole wait window.
- Can two different functions share one lock?
- Yes. getScriptLock returns the same mutex for the project when you use the same lock type.
- Is LockService related to Sheet protected ranges?
- No. Protected ranges are UI permissions. LockService is runtime concurrency control in Apps Script.
Related Tutorials
Watch how LockService works
Lock Your Sheets with OnlySheets
Create an Internal Google Sheets Add-on
Related glossary terms
Apps Script triggers
Triggers tell Apps Script when to run a function: on edit, on open, on a schedule, or when a form is submitted. Simple triggers use reserved names like onEdit. Installable triggers are created in the Apps Script Triggers page and can do more, including running as a specific user or on a timer.
Read guide →Google Sheets automation
Automation means work happens without repeating the same clicks every day: imports update, emails send, rows move, and dashboards refresh. In Sheets, automation usually stacks built-in features, Apps Script, and sometimes the Sheets API or add-ons. Start with the lightest tool that still solves the job.
Read guide →Google Sheets workflows
A workflow is the path work takes through your sheet: intake, review, approval, done. Good workflows use consistent columns, statuses everyone understands, and just enough automation to move tasks forward without hiding steps from the team.
Read guide →MailApp
MailApp is an Apps Script service that sends email from your Google account without opening Gmail. You call MailApp.sendEmail with a recipient, subject, and body, often using cell values as the content. It is the fastest way to notify someone when a row is added or a deadline hits in a tracker.
Read guide →Done reading about LockService?
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