microsoft/FASTER · error · FasterException

Cannot commit in read-only mode

Error message

Cannot commit in read-only mode

What it means

FasterLog.CommitAsync (or Commit) was called on a FasterLog opened in read-only mode. A read-only log is typically attached for tailing/recovery on a replica and has no commit capability, so the library refuses the operation up front via this guard in FasterLog.cs:2704.

Solutions

1. Check FasterLog.ReadOnlyMode (or how the instance was constructed) before calling any commit API and skip/branch the commit.
2. Open the log in writable mode on the node that owns the log (do not pass read-only/recovery-only settings).
3. On replicas, rely on log replication/checkpoint streaming from the primary instead of local commits.
4. Wrap shared append helpers so commit calls are conditional on writability.

Example fix

// before
await log.CommitAsync();

// after
if (!log.ReadOnlyMode)
    await log.CommitAsync();

if (log.ReadOnlyMode) throw new InvalidOperationException("This node cannot commit; log is read-only");

try { await log.CommitAsync(); } catch (FasterException ex) when (ex.Message == "Cannot commit in read-only mode") { /* route write to primary */ }

When it happens

Trigger: Calling CommitAsync/Commit (including the fast-forward overload) on a FasterLog created with FastCommitMode or opened as a read-only subscriber/recover-only log where readOnlyMode is true.

Common situations: Opening a log on a backup node or with LogCommitManager pointing at an immutable snapshot, then reusing shared append code that unconditionally commits; connecting to a replicated log as a follower and calling the same enqueue-and-commit path as the primary.

Understand the failure class

Background: "Invalid state transition" errors: "status must be X, actually Y", "already rejected/charging/uninstalled", "cannot ... while running" — what they mean when a library rejects your call — this error's family across 31 libraries.

Related errors

• Already recovered until address
• Cannot use named iterators with read-only FasterLog
• Cannot use scanUncommitted with read-only FasterLog
• This method can only be used with a read-only FasterLog…
• requested commit num is not available

AI-assisted analysis of microsoft/FASTER@321d872eab (2026-09-15). Data as JSON: /api/errors/04e5e354b2728e9f. Report an issue: GitHub.

Appendix: source

Thrown at cs/src/core/FasterLog/FasterLog.cs:2704

                length = GetLength(ptr);

                // forego checksum verification since record may not be read in full by AsyncGetHeaderOnlyFromDiskCallback()
            }

            record.Return();
            return length;
        }


        private bool CommitInternal(out long commitTail, out long actualCommitNum, bool fastForwardAllowed, byte[] cookie, long proposedCommitNum, Action callback)
        {
            if (cannedException != null)
                throw cannedException;

            commitTail = actualCommitNum = 0;

            if (readOnlyMode)
                throw new FasterException("Cannot commit in read-only mode");

            if (fastForwardAllowed && (cookie != null || proposedCommitNum != -1 || callback != null))
                throw new FasterException(
                    "Fast forwarding a commit is only allowed when no cookie, commit num, or callback is specified");

            var info = new FasterLogRecoveryInfo
            {
                FastForwardAllowed = fastForwardAllowed,
                Cookie = cookie,
                Callback = callback,
            };
            info.SnapshotIterators(PersistedIterators);
            var commitRequired = ShouldCommmitMetadata(ref info) || (commitCoveredAddress < TailAddress);
            // Only apply commit policy if not a strong commit
            if (fastForwardAllowed && !commitPolicy.AdmitCommit(TailAddress, commitRequired))
                return false;

            // This critical section serializes commit record creation / commit content generation and ensures that the

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