Each UDx written in C++, Java, or Python has an associated instance of ServerInterface.

Each UDx written in C++, Java, or Python has an associated instance of ServerInterface. The ServerInterface class provides a function to write to the Vertica log, and the C++ implementation also provides a function to log events in a system table.

Writing messages to the Vertica log

You can write to log files using the ServerInterface.log() function. The function acts similarly to printf(), taking a formatted string and an optional set of values and writing the string to the log file. Where the message is written depends on whether your function runs in fenced mode or unfenced mode:

  • Functions running in unfenced mode write their messages into the vertica.log file in the catalog directory.

  • Functions running in fenced mode write their messages into a log file named UDxLogs/UDxFencedProcesses.log in the catalog directory.

To help identify your function's output, Vertica adds the SQL function name bound to your UDx to the log message.

The following example logs a UDx's input values:

    virtual void processBlock(ServerInterface &srvInterface,
                              BlockReader &argReader,
                              BlockWriter &resWriter)
        try {
            // While we have inputs to process
            do {
                if (argReader.isNull(0) || argReader.isNull(1)) {
                } else {
                    const vint a = argReader.getIntRef(0);
                    const vint b = argReader.getIntRef(1);
                    srvInterface.log("got a: %d and b: %d", (int) a, (int) b);
            } while (argReader.next());
        } catch(std::exception& e) {
            // Standard exception. Quit.
            vt_report_error(0, "Exception while processing block: [%s]", e.what());
        public void processBlock(ServerInterface srvInterface,
                                 BlockReader argReader,
                                 BlockWriter resWriter)
                    throws UdfException, DestroyInvocation
            do {
                // Get the two integer arguments from the BlockReader
                long a = argReader.getLong(0);
                long b = argReader.getLong(1);

                // Log the input values
                srvInterface.log("Got values a=%d and b=%d", a, b);

                long result = a+b;
            } while (argReader.next());
    def processBlock(self, server_interface, arg_reader, res_writer):
        server_interface.log("Python UDx - Adding 2 ints!")
            first_int = block_reader.getInt(0)
            second_int = block_reader.getInt(1)
            block_writer.setInt(first_int + second_int)
            server_interface.log("Values: first_int is {} second_int is {}".format(first_int, second_int))
            if not block_reader.next():

The log() function generates entries in the log file like the following:

$ tail /home/dbadmin/py_db/v_py_db_node0001_catalog/UDxLogs/UDxFencedProcesses.log
 07:52:12.862 [Python-v_py_db_node0001-7524:0x206c-40575]  0x7f70eee2f780 PythonExecContext::processBlock
 07:52:12.862 [Python-v_py_db_node0001-7524:0x206c-40575]  0x7f70eee2f780 [UserMessage] add2ints - Python UDx - Adding 2 ints!
 07:52:12.862 [Python-v_py_db_node0001-7524:0x206c-40575]  0x7f70eee2f780 [UserMessage] add2ints - Values: first_int is 100 second_int is 100

For details on viewing the Vertica log files, see Monitoring log files.

Writing messages to the UDX_EVENTS table (C++ only)

In the C++ API, you can write messages to the UDX_EVENTS system table instead of or in addition to writing to the log. Writing to a system table allows you to collect events from all nodes in one place.

You can write to this table using the ServerInterface.logEvent() function. The function takes one argument, a map. The map is written into the __RAW__ column of the table as a Flex VMap. The following example shows how the Parquet exporter creates and logs this map.

// Log exported parquet file details to v_monitor.udx_events
std::map<std::string, std::string> details;
details["file"] = escapedPath;
details["created"] = create_timestamp_;
details["closed"] = close_timestamp_;
details["rows"] = std::to_string(num_rows_in_file);
details["row_groups"] = std::to_string(num_row_groups_in_file);
details["size_mb"] = std::to_string((double)outputStream->Tell()/(1024*1024));

You can select individual fields from the VMap as in the following example.

(2 rows)

Alternatively, you can define a view to make it easier to query fields directly, as columns. See Monitoring exports for an example.