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Appending to Files

This tutorial shows how to add data to existing files on an OPC UA server without overwriting them.

When to Use Append Mode​

Use WriteAppend mode when you need to:

  • Add log entries to existing log files
  • Continuously record sensor data
  • Build cumulative reports
  • Preserve existing file content while adding new data

Quick Start​

Basic Append Flow​

[ Inject ] → [ Function ] → [ OPC UA File Operation ] → [ Debug ]

Function Node:

msg.payload = "New log entry: " + new Date().toISOString() + "\n";
return msg;

OPC UA File Operation Configuration:

  • Mode: WriteAppend
  • NodeId: ns=1;s=LogFile
  • Encoding: utf8

Each time you trigger the inject, a new line is added to the end of the file.

Common Use Cases​

Logging Events​

// Function node
const timestamp = new Date().toISOString();
const level = msg.level || "INFO";
msg.payload = `[${timestamp}] ${level}: ${msg.payload}\n`;
return msg;

Result in file:

[2025-11-24T10:00:00.000Z] INFO: System started
[2025-11-24T10:15:23.000Z] WARN: High memory usage
[2025-11-24T10:30:45.000Z] ERROR: Connection failed

Recording Sensor Data​

// Function node
const data = {
    timestamp: new Date().toISOString(),
    temperature: msg.payload.temp,
    humidity: msg.payload.humidity
};
msg.payload = JSON.stringify(data) + "\n";
return msg;

Result in file (JSONL format):

{"timestamp":"2025-11-24T10:00:00.000Z","temperature":22.5,"humidity":55}
{"timestamp":"2025-11-24T10:01:00.000Z","temperature":22.7,"humidity":54}
{"timestamp":"2025-11-24T10:02:00.000Z","temperature":22.6,"humidity":55}

Appending to CSV Files​

// Function node - assumes CSV file already has headers
const row = `${msg.id},${msg.name},${msg.value}`;
msg.payload = row + "\n";
return msg;

Result in file:

ID,Name,Value
1,Sensor1,42.5
2,Sensor2,38.2
3,Sensor3,45.7

Important Notes​

  • Always add newline characters (\n) at the end of your payload to keep entries on separate lines
  • File must exist - create it first with Write mode if needed
  • No size limit - the node handles large files automatically
  • Thread-safe - multiple appends are handled sequentially by the server

Append vs Write​

ModeEffectUse Case
WriteReplaces entire fileReports, configs, complete data exports
WriteAppendAdds to end of fileLogs, continuous monitoring, incremental data

Error Handling​

// In a Catch node
if (msg.error && msg.error.includes("BadNodeIdUnknown")) {
    // File doesn't exist - create it first
    msg.payload = "File Header\n" + msg.payload;
    msg.mode = "Write";  // Switch to Write mode
    return msg;
}

See Also​