CSV logging server

server.js

/**
 * Usage: node server.js foo.csv
 *
 * Test with: curl -d "{\"Foo\":1,\"Bar\":\"baz\"}" -X POST http://localhost:8000/
 *
 */
const http = require('http');
const fs = require('fs');
const fsPromises = require('fs/promises');

const PORT = 8000;
const MAX_REQUEST_SIZE = 1e6;

const outFilename = process.argv[2];

if ( ! outFilename ) {
	console.error( `Usage: ${process.argv[0]} ${process.argv[1]} OUTPUT_FILENAME`);
	process.exit();
}

const escapeField = ( val ) => {
	if ( typeof val === 'string' && val.includes( ',' ) ) {
		return '"' + val.replace( '"', '""' ) + '"';
	}
	return val;
}

// Use the stream API to read headers so we don't read in giant files just
// to get the first line
const getHeaders = async ( filename ) => {
	return new Promise( ( resolve, reject ) => {
		let headerLine = '';
		const stream = fs.createReadStream( filename, { encoding: 'utf8', start: 0 } );
		stream.on( 'error', ( err ) => {
			reject( err );
		} );

		function onData( chunk ) {
			headerLine += chunk;
			if ( headerLine.includes('\n') ) {
				const headerCSV = headerLine.split('\n')[0];
				const headers = headerCSV.split(',').slice(1);
				resolve( headers );
				stream.removeListener( 'data', onData );
				stream.destroy();
			}
		}

		stream.on( 'data', onData );
	} );
}

( async () => {
	let headersWritten = false;
	let fields = [];

	try {
		fields = await getHeaders( outFilename );
		console.log( 'Using existing fields: ' + fields.join(', ') + '\n');
		headersWritten = true;
	} catch ( err ) {
		console.error( err );
		console.log( 'Creating new file ' + outFilename + '\n');
		headersWritten = false;
	}

	let filehandle = await fsPromises.open( outFilename, 'a' );
	process.on('SIGINT', async () => {
		try {
			await filehandle.close();
		} catch ( err ) {
			console.error( err );
		}
		process.exit();
	});

	// kill -HUP to rotate CSV files
	process.on('SIGHUP', async () => {
		await filehandle.close();
		filehandle = await fsPromises.open( outFilename, 'a' );
		await filehandle.write( ['Date', ...fields ].join(',') + '\n' );
	});

	const server = http.createServer(async ( req, res ) => {
		if ( req.method === 'POST' ) {
			let body = '';
			req.on( 'data', ( data ) => {
				body += data;
				if ( body.length > MAX_REQUEST_SIZE ) {
					req.destroy();
				}
			 } );

			req.on( 'end', async () => {
				try {
					const postData = JSON.parse( body );
					if ( ! headersWritten ) {
						fields = Object.keys( postData );
						await filehandle.write( ['Date', ...fields ].join(',') + '\n' );
						console.log( 'Using fields: ' + fields.join(', ') + '\n');
						headersWritten = true;
					}
					const row = fields.map( field => escapeField( postData[field] ) );
					await filehandle.write( [(new Date()).toISOString(), ...row].join(',') + '\n' );
					res.writeHead( 200 );
					res.end( http.STATUS_CODES[200] + '\n' );
				} catch ( err ) {
					console.error( body );
					console.error( err );
					res.writeHead( 500 );
					res.end( err + '\n' );
				}
			} );
		}
	});

	server.listen( PORT );
})();