diff --git a/app/src/main.rs b/app/src/main.rs
index fbcb2e7..7027986 100644
--- a/app/src/main.rs
+++ b/app/src/main.rs
@@ -12,6 +12,7 @@ use chgk_ledb_lib::source;
 
 use crate::questions::{Question, QuestionsConverter};
 use crate::source::ReadSourceQuestionsBatches;
+use chgk_ledb_lib::util::ErrorToString;
 
 const ZIP_FILENAME: &str = "json.zip";
 const NEW_DB_FILENAME: &str = "db.dat";
@@ -41,70 +42,6 @@ struct Cli {
     measure: bool,
 }
 
-fn zip_reader_task(tx: mpsc::Sender<Question>) {
-    let zip_file = fs::File::open(ZIP_FILENAME).unwrap();
-    let zip_reader = io::BufReader::new(zip_file);
-    let archive = zip::ZipArchive::new(zip_reader).unwrap();
-    let mut source_questions = archive.source_questions();
-
-    let questions = source_questions
-        .convert()
-        .enumerate()
-        .map(|(num, mut question)| {
-            question.num = 1 + num as u32;
-            question
-        });
-    for question in questions {
-        let res = tx.send(question);
-        if res.is_err() {
-            break;
-        }
-    }
-    println!("read done");
-}
-
-fn print_question_from<F>(get_q: F)
-where
-    F: FnOnce() -> Option<Question>,
-{
-    let q = get_q().expect("question not found");
-    println!("{:#?}", q)
-}
-
-fn read_from_zip(file_num: usize, mut num: usize) -> Option<Question> {
-    let mut rng = rand::thread_rng();
-    let zip_file = fs::File::open(ZIP_FILENAME).unwrap();
-    let zip_reader = io::BufReader::new(zip_file);
-    let archive = zip::ZipArchive::new(zip_reader).unwrap();
-
-    let mut source_questions = archive.source_questions();
-    let (filename, batch) = if file_num == 0 {
-        source_questions.choose(&mut rng).unwrap()
-    } else {
-        source_questions.nth(file_num - 1).unwrap()
-    };
-    let mut batch = batch.unwrap();
-    batch.filename = filename;
-    let questions: Vec<Question> = batch.into();
-    if num == 0 {
-        num = (1..=questions.len()).choose(&mut rng).unwrap();
-    }
-    Some(questions[num - 1].clone())
-}
-
-// measure and return time elapsed in `func` in seconds
-pub fn measure<F: FnOnce()>(func: F) -> f64 {
-    let start = Instant::now();
-    func();
-    let elapsed = start.elapsed();
-    (elapsed.as_secs() as f64) + (elapsed.subsec_nanos() as f64 / 1_000_000_000.0)
-}
-
-pub fn measure_and_print<F: FnOnce()>(func: F) {
-    let m = measure(func);
-    eprintln!("{}", m);
-}
-
 fn main() {
     let args = Cli::parse();
 
@@ -127,20 +64,75 @@ fn main() {
     action();
 }
 
-fn read_from_db(id: u32) -> Option<Question> {
-    let reader: db::Reader<Question> =
-        db::Reader::new(NEW_DB_FILENAME, 2048).expect("new db reader");
+// measure and return time elapsed in `func` in seconds
+pub fn measure<F: FnOnce()>(func: F) -> f64 {
+    let start = Instant::now();
+    func();
+    let elapsed = start.elapsed();
+    (elapsed.as_secs() as f64) + (elapsed.subsec_nanos() as f64 / 1_000_000_000.0)
+}
 
+pub fn measure_and_print<F: FnOnce()>(func: F) {
+    let m = measure(func);
+    eprintln!("{}", m);
+}
+
+fn print_question_from<F>(get_q: F)
+where
+    F: FnOnce() -> Result<Question, String>,
+{
+    let q = get_q().expect("question not found");
+    println!("{:#?}", q)
+}
+
+fn read_from_zip(file_num: usize, mut num: usize) -> Result<Question, String> {
+    let mut rng = rand::thread_rng();
+    let zip_file = fs::File::open(ZIP_FILENAME).str_err()?;
+    let zip_reader = io::BufReader::new(zip_file);
+    let archive = zip::ZipArchive::new(zip_reader).str_err()?;
+
+    let mut source_questions = archive.source_questions();
+    let (filename, batch) = if file_num == 0 {
+        source_questions
+            .choose(&mut rng)
+            .ok_or("rand choose".to_string())?
+    } else {
+        source_questions
+            .nth(file_num - 1)
+            .ok_or(format!("file nth #{file_num} => None"))?
+    };
+    let mut batch = batch.map_err(|e| format!("get batch from file #{file_num} => {e}"))?;
+    batch.filename = filename;
+    let questions: Vec<Question> = batch.into();
+    if num == 0 {
+        num = (1..=questions.len())
+            .choose(&mut rng)
+            .ok_or("rand choose".to_string())?;
+    }
+    Ok(questions[num - 1].clone())
+}
+
+fn read_from_db(id: u32) -> Result<Question, String> {
+    let reader: db::Reader<Question> = db::Reader::new(NEW_DB_FILENAME, 2048)?;
+
+    let len = reader.len();
     let mut questions = reader.into_iter();
 
-    match id {
+    let question = match id {
         0 => {
             let mut rng = rand::thread_rng();
-            questions.choose(&mut rng)
+            questions
+                .choose(&mut rng)
+                .ok_or(format!("rand choose, len = {len}"))?
         }
-        _ => questions.nth((id - 1) as usize),
-    }
+        _ => questions
+            .nth((id - 1) as usize)
+            .ok_or(format!("get nth #{id} => None"))?,
+    };
+
+    Ok(question)
 }
+
 fn write_db() {
     let (tx, rx) = mpsc::channel::<Question>();
     [
@@ -151,6 +143,29 @@ fn write_db() {
     .for_each(|handle| handle.join().expect("thread panic"));
     println!("all done");
 }
+
+fn zip_reader_task(tx: mpsc::Sender<Question>) {
+    let zip_file = fs::File::open(ZIP_FILENAME).unwrap();
+    let zip_reader = io::BufReader::new(zip_file);
+    let archive = zip::ZipArchive::new(zip_reader).unwrap();
+    let mut source_questions = archive.source_questions();
+
+    let questions = source_questions
+        .convert()
+        .enumerate()
+        .map(|(num, mut question)| {
+            question.num = 1 + num as u32;
+            question
+        });
+    for question in questions {
+        let res = tx.send(question);
+        if res.is_err() {
+            break;
+        }
+    }
+    println!("read done");
+}
+
 fn db_writer_task(rx: mpsc::Receiver<Question>) {
     let writer_opts = db::WriterOpts::default();
     let mut writer: db::Writer<Question> =
diff --git a/app_async/src/main.rs b/app_async/src/main.rs
index 6b21fb8..fc8d9b3 100644
--- a/app_async/src/main.rs
+++ b/app_async/src/main.rs
@@ -19,6 +19,7 @@ use chgk_ledb_lib::async_db;
 use chgk_ledb_lib::questions::Question;
 use chgk_ledb_lib::questions::QuestionsConverterAsyncForStream;
 use chgk_ledb_lib::source::ReadSourceQuestionsBatchesAsync;
+use chgk_ledb_lib::util::ErrorToString;
 
 const ZIP_FILENAME: &str = "json.zip";
 const NEW_DB_FILENAME: &str = "db.dat";
@@ -48,6 +49,113 @@ struct Cli {
     measure: bool,
 }
 
+#[tokio::main]
+async fn main() {
+    let args = Cli::parse();
+
+    let mut action: Box<dyn Future<Output = _>> = match &args.command {
+        Command::Write => Box::new(write_db()),
+        Command::Print { id } => {
+            let get_question = read_from_db(*id);
+            Box::new(print_question_from(get_question))
+        }
+        Command::ZipPrint { file_num, num } => {
+            let get_question = read_from_zip(*file_num, *num);
+            Box::new(print_question_from(get_question))
+        }
+    };
+
+    if args.measure {
+        action = Box::new(measure_and_print(Box::into_pin(action)));
+    }
+
+    Box::into_pin(action).await;
+}
+
+// measure and return time elapsed in `fut` in seconds
+pub async fn measure<F: Future>(fut: F) -> f64 {
+    let start = Instant::now();
+    fut.await;
+    let elapsed = start.elapsed();
+    (elapsed.as_secs() as f64) + (elapsed.subsec_nanos() as f64 / 1_000_000_000.0)
+}
+
+pub async fn measure_and_print<F: Future>(fut: F) {
+    let m = measure(fut).await;
+    eprintln!("{}", m);
+}
+
+async fn print_question_from<F>(get_q: F)
+where
+    F: Future<Output = Result<Question, String>>,
+{
+    let q = get_q.await.expect("question not found");
+    println!("{:#?}", q)
+}
+
+async fn read_from_zip(file_num: usize, mut num: usize) -> Result<Question, String> {
+    let mut rng = thread_rng();
+    let zip_file = fs::File::open(ZIP_FILENAME).await.str_err()?;
+    let mut zip_reader = io::BufReader::new(zip_file);
+    let archive = ZipFileReader::with_tokio(&mut zip_reader).await.str_err()?;
+
+    let mut source = archive.source_questions();
+    let files_count = source.len();
+    let file_index = if file_num == 0 {
+        let files = Uniform::new(0, files_count);
+        rng.sample(files)
+    } else {
+        file_num - 1
+    };
+
+    let src = source
+        .get(file_index)
+        .await
+        .map_err(|e| format!("get file {file_num} => {e}"))?;
+    let src = stream::once(async { src });
+    pin_mut!(src);
+    let converter = src.converter();
+    let questions: Vec<_> = converter.convert().collect().await;
+    if num == 0 {
+        num = (1..=questions.len()).choose(&mut rng).unwrap();
+    }
+    let mut question = questions
+        .get(num - 1)
+        .ok_or(format!("get question #{num} => None"))?
+        .clone();
+    question.num = num as u32;
+    Ok(question)
+}
+
+async fn read_from_db(id: u32) -> Result<Question, String> {
+    let reader: async_db::Reader<Question> = async_db::Reader::new(NEW_DB_FILENAME).await?;
+
+    let len = reader.len();
+
+    let index = if id == 0 {
+        let mut rng = thread_rng();
+        let questions = Uniform::new(0, len);
+        rng.sample(questions)
+    } else {
+        id as usize - 1
+    };
+
+    reader
+        .get(index)
+        .await
+        .map_err(|e| format!("get #{index} => {e}"))
+}
+
+async fn write_db() {
+    let (tx, rx) = mpsc::unbounded_channel::<Question>();
+    tokio::try_join!(
+        tokio::spawn(zip_reader_task(tx)),
+        tokio::spawn(db_writer_task(rx))
+    )
+    .expect("tokio join");
+    println!("all done");
+}
+
 async fn zip_reader_task(tx: UnboundedSender<Question>) {
     let mut file = fs::File::open(ZIP_FILENAME).await.expect("open zip");
     let archive = ZipFileReader::with_tokio(&mut file)
@@ -73,109 +181,6 @@ async fn zip_reader_task(tx: UnboundedSender<Question>) {
     println!("read done");
 }
 
-async fn print_question_from<F>(get_q: F)
-where
-    F: Future<Output = Option<Question>>,
-{
-    let q = get_q.await.expect("question not found");
-    println!("{:#?}", q)
-}
-
-async fn read_from_zip(file_num: usize, mut num: usize) -> Option<Question> {
-    let mut rng = thread_rng();
-    let zip_file = fs::File::open(ZIP_FILENAME).await.expect("open zip file");
-    let mut zip_reader = io::BufReader::new(zip_file);
-    let archive = ZipFileReader::with_tokio(&mut zip_reader)
-        .await
-        .expect("open zip file reader");
-
-    let mut source = archive.source_questions();
-    let files_count = source.len();
-    let file_index = if file_num == 0 {
-        let files = Uniform::new(0, files_count);
-        rng.sample(files)
-    } else {
-        file_num - 1
-    };
-
-    let src = source.get(file_index).await;
-    let src = stream::once(async { src.expect("get source file") });
-    pin_mut!(src);
-    let converter = src.converter();
-    let questions: Vec<_> = converter.convert().collect().await;
-    if num == 0 {
-        num = (1..=questions.len()).choose(&mut rng).unwrap();
-    }
-    let mut question = questions.get(num - 1).expect("get question").clone();
-    question.num = num as u32;
-    Some(question)
-}
-
-// measure and return time elapsed in `fut` in seconds
-pub async fn measure<F: Future>(fut: F) -> f64 {
-    let start = Instant::now();
-    fut.await;
-    let elapsed = start.elapsed();
-    (elapsed.as_secs() as f64) + (elapsed.subsec_nanos() as f64 / 1_000_000_000.0)
-}
-
-pub async fn measure_and_print<F: Future>(fut: F) {
-    let m = measure(fut).await;
-    eprintln!("{}", m);
-}
-
-#[tokio::main]
-async fn main() {
-    let args = Cli::parse();
-
-    let mut action: Box<dyn Future<Output = _>> = match &args.command {
-        Command::Write => Box::new(write_db()),
-        Command::Print { id } => {
-            let get_question = read_from_db(*id);
-            Box::new(print_question_from(get_question))
-        }
-        Command::ZipPrint { file_num, num } => {
-            let get_question = read_from_zip(*file_num, *num);
-            Box::new(print_question_from(get_question))
-        }
-    };
-
-    if args.measure {
-        action = Box::new(measure_and_print(Box::into_pin(action)));
-    }
-
-    Box::into_pin(action).await;
-}
-
-async fn read_from_db(id: u32) -> Option<Question> {
-    let reader: async_db::Reader<Question> = async_db::Reader::new(NEW_DB_FILENAME)
-        .await
-        .expect("new db reader");
-
-    let len = reader.len();
-
-    let index = if id == 0 {
-        let mut rng = thread_rng();
-        let questions = Uniform::new(0, len);
-        rng.sample(questions)
-    } else {
-        id as usize - 1
-    };
-
-    match reader.get(index).await {
-        Ok(question) => Some(question),
-        Err(_) => None,
-    }
-}
-async fn write_db() {
-    let (tx, rx) = mpsc::unbounded_channel::<Question>();
-    tokio::try_join!(
-        tokio::spawn(zip_reader_task(tx)),
-        tokio::spawn(db_writer_task(rx))
-    )
-    .expect("tokio join");
-    println!("all done");
-}
 async fn db_writer_task(rx: UnboundedReceiver<Question>) {
     let writer_opts = WriterOpts::default();
     let mut writer: async_db::Writer<Question> =