How do I write binary data to an output file?

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You can write the data to a byte array in your Groovy Script Evaluator, and assign that to a record field. For example:

// Loop through the records in the batch
for (record in records) {
  try {
    // Write dummy data to a byte array, based on an incoming 'count' field
    int count = record.value['count'].toInteger()
    byte[] bytes = new byte[count]
    for (int i = 0; i < count; i++) {
      bytes[i] = (byte)(i & 0x000000ff)
    }

    // Assign the byte array to a record field
    record.value['bytes'] = bytes

    // Write the record to the processor output
    output.write(record)
  } catch (e) {
    // Write a record to the error pipeline
    log.error(e.toString(), e)
    error.write(record, e.toString())
  }
}

Now in the Local FS destination, select Binary as the Data Format, and choose the relevant field:

We can verify that the file is written to the configured location:

$ hexdump -C sdc-24e75fba-bd00-42fd-80c3-1f591e200ca6_26187b53-6efa-4765-b790-757149baf75f | head -n 10
00000000  00 01 02 03 04 05 06 07  08 09 0a 0b 0c 0d 0e 0f  |................|
00000010  10 11 12 13 14 15 16 17  18 19 1a 1b 1c 1d 1e 1f  |................|
00000020  20 21 22 23 24 25 26 27  28 29 2a 2b 2c 2d 2e 2f  | !"#$%&'()*+,-./|
00000030  30 31 32 33 34 35 36 37  38 39 3a 3b 3c 3d 3e 3f  |0123456789:;<=>?|
00000040  40 41 42 43 44 45 46 47  48 49 4a 4b 4c 4d 4e 4f  |@ABCDEFGHIJKLMNO|
00000050  50 51 52 53 54 55 56 57  58 59 5a 5b 5c 5d 5e 5f  |PQRSTUVWXYZ[\]^_|
00000060  60 61 62 63 64 65 66 67  68 69 6a 6b 6c 6d 6e 6f  |`abcdefghijklmno|
00000070  70 71 72 73 74 75 76 77  78 79 7a 7b 7c 7d 7e 7f  |pqrstuvwxyz{|}~.|
00000080  80 81 82 83 84 85 86 87  88 89 8a 8b 8c 8d 8e 8f  |................|
00000090  90 91 92 93 94 95 96 97  98 99 9a 9b 9c 9d 9e 9f  |................|