influxdata/influxdb · critical · WalBufferErrorState

another process has written to the WAL ahead of this one

Error message

another process has written to the WAL ahead of this one

What it means

WalBufferErrorState::WalAlreadyWrittenTo is a buffered-write error set when the WAL discovers that a file it was about to write already exists on the object store at the expected sequence position — i.e. another writer has advanced the WAL ahead of this one. The buffer flips into the Error state, stops accepting new writes, and pending write callers receive 'another process has written to the WAL ahead of this one'. The library deliberately halts rather than corrupting the sequence.

Solutions

1. Check for a second process using the same --node-id against the same object store and stop one of them.
2. Restart the affected node so it either picks up the new WAL position or fails cleanly; new writes will then flow to the healthy writer.
3. If the conflict is stale data from an older build, remove or archive the conflicting WAL file(s) at the conflicting path after confirming no live writer owns them.
4. Enable write verification (conditional puts) on your object store so duplicate writes are detected before this state is reached.

Example fix

# before: two nodes, same node id
influxdb3 serve --node-id node_a ...   # process 1
influxdb3 serve --node-id node_a ...   # process 2 -> WalAlreadyWrittenTo
# after: unique node ids per writer
influxdb3 serve --node-id node_a ...
influxdb3 serve --node-id node_b ...

# before starting a writer, verify no other process holds the node id
# check running processes for the same --node-id and the same object-store prefix

fn is_wal_ahead(err_msg: &str) -> bool { err_msg.contains("another process has written to the WAL ahead") }

match wal.write_buffer(buffer, gen1).await {
    Err(e) if e.to_string().contains("another process has written to the WAL ahead") => {
        // terminal: stop this writer; an operator must resolve the duplicate --node-id
    }
    other => { /* handle normally */ }
}

When it happens

Trigger: During flush/rotation, the object-store put fails with Error::AlreadyExists for a WAL file path this writer did not write (object_store.rs:366-383); the flush buffer then latches WalAlreadyWrittenTo and returns this message to all waiting and subsequent writes.

Common situations: Two influxdb3 processes started with the same --node-id writing to the same object store; write-verification disabled in a store that drops object metadata (so AlreadyExists is not detected reliably); leftover WAL files from an older build at the next expected sequence number.

Understand the failure class

Background: "already exists" / EEXIST / FileAlreadyExistsException: what the 'file already exists' error means and how to fix it — this error's family across 37 libraries.

Related errors

• object store error
• bitcode error
• Cannot parse object store config
• crc32 checksum mismatch
• deserialize error

AI-assisted analysis of influxdata/influxdb@06200ef96b (2026-09-19). Data as JSON: /api/errors/54d82baaf8876e2e. Report an issue: GitHub.

Appendix: source

Thrown at influxdb3_wal/src/object_store.rs:852

        self.state = state;
    }

    fn is_accepting_writes(&self) -> bool {
        matches!(self.state, WalBufferState::AcceptingWrites)
    }
}

#[derive(Debug, Default, Copy, Clone)]
enum WalBufferState {
    #[default]
    AcceptingWrites,
    ShuttingDown,
    Error(WalBufferErrorState),
}

#[derive(Debug, thiserror::Error, Copy, Clone)]
enum WalBufferErrorState {
    #[error("another process has written to the WAL ahead of this one")]
    WalAlreadyWrittenTo,
}

// Writes should only fail if the underlying WAL throws an error. They are validated before they
// are buffered. The WAL should continuously retry the write until it succeeds. But if a timeout
// passes, we can use this to pass the object store error back to the client.
#[derive(Debug, Clone)]
pub enum WriteResult {
    Success(()),
    Error(String),
}

impl WalBuffer {
    fn write_ops_unconfirmed(&mut self, ops: Vec<WalOp>) -> crate::Result<(), crate::Error> {
        if !self.is_accepting_writes() {
            return Err(crate::Error::Shutdown);
        }
        if self.op_count >= self.op_limit {

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