function write(key, value, prevEtag) {
  var etag = GUID.NewGuid();
  var record = {
    prevEtag: prevEtag,
    value: value,
    etag: etag
  };
  log(`${DateTime.now()}: writing(${key}, ${record})`);
  db.updateWhen(key, record, x => x.etag == prevEtag);
  log(`${DateTime.now()}: written(${key}, ${record})`);
}

function read(key) {
  log(`${DateTime.now()}: reading(${key})`);
  var record = db.read(key);
  log(`${DateTime.now()}: read(${key}, ${record})`);
}