@Override
    public CompletableFuture<AppendResult> append(AppendContext context, RecordBatch recordBatch) {
        if (sealed) {
            return FutureUtil.failedFuture(new IllegalStateException("stream segment " + this + " is sealed"));
        }
        return stream.append(context, recordBatch).thenApply(AppendResultWrapper::new);
    }
the real start offset of this segment in the stream.
core/src/main/scala/kafka/log/streamaspect/DefaultElasticStreamSlice.java