Custom SerDes in Kafka - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this custom SerDes serialization?

Given the following Kafka custom serializer code snippet, what will be the output byte array when serializing the string "abc"?

Kafka

public class CustomStringSerializer implements org.apache.kafka.common.serialization.Serializer<String> {
    @Override
    public byte[] serialize(String topic, String data) {
        if (data == null) return null;
        byte[] originalBytes = data.getBytes();
        byte[] result = new byte[originalBytes.length + 1];
        result[0] = (byte) originalBytes.length;
        System.arraycopy(originalBytes, 0, result, 1, originalBytes.length);
        return result;
    }
}

A [99, 98, 97]

B [0, 97, 98, 99]

C [3, 97, 98, 99]

D [97, 98, 99]

💡 Hint

Think about how the length of the string is stored in the first byte.

The serializer prepends the length of the string as the first byte, then copies the original bytes. For "abc", length is 3, ASCII codes are 97, 98, 99.

❓ Predict Output

intermediate

What error occurs when deserializing null data?

Consider this Kafka custom deserializer snippet. What error will occur if data is null?

Kafka

public class CustomStringDeserializer implements org.apache.kafka.common.serialization.Deserializer<String> {
    @Override
    public String deserialize(String topic, byte[] data) {
        int length = data[0];
        return new String(data, 1, length);
    }
}

A ClassCastException

B ArrayIndexOutOfBoundsException

C No error, returns empty string

D NullPointerException

💡 Hint

What happens if you try to access an element of a null array?

If data is null, accessing data[0] causes a NullPointerException.

🧠 Conceptual

advanced

Which statement best describes the role of a custom SerDes in Kafka?

Choose the most accurate description of what a custom SerDes (Serializer/Deserializer) does in Kafka.

A It manages Kafka topic partitions and offsets automatically.

B It converts data between Kafka's internal byte format and application-specific objects.

C It encrypts messages to secure Kafka communication.

D It balances load between Kafka brokers.

💡 Hint

Think about how Kafka sends and receives data as bytes but applications use objects.

Custom SerDes convert between bytes and objects so Kafka can store and transmit data while applications can work with meaningful types.

🔧 Debug

advanced

Why does this custom deserializer produce incorrect strings?

Given this deserializer code, why might the output strings be incorrect or corrupted?

Kafka

public class FaultyDeserializer implements org.apache.kafka.common.serialization.Deserializer<String> {
    @Override
    public String deserialize(String topic, byte[] data) {
        if (data == null) return null;
        int length = data[0];
        return new String(data, 0, length);
    }
}

A It includes the length byte in the string, causing corruption.

B It throws NullPointerException when data is null.

C It uses the wrong character encoding by default.

D It reverses the byte order before decoding.

💡 Hint

Check the substring start index and length used in the String constructor.

The code uses new String(data, 0, length), which includes the first byte (length byte) as a character, corrupting the output.

🚀 Application

expert

How many items are in the map after this custom SerDes deserialization?

Assume a custom deserializer converts a byte array representing multiple key-value pairs into a Map<String, Integer>. The byte array encodes 3 pairs: {"a":1, "b":2, "c":3}. After deserialization, how many entries does the map contain?

Kafka

byte[] data = new byte[]{3, 97, 1, 98, 2, 99, 3}; // Custom format: first byte count, then pairs of (keyByte, valueByte)
// Deserializer logic:
// int count = data[0];
// for i in 0..count-1:
//   key = String.valueOf((char) data[2*i+1]);
//   value = data[2*i+2];
//   map.put(key, value);

A 3

B 6

C 1

D 0

💡 Hint

Count how many key-value pairs the first byte indicates and how the loop processes them.

The first byte is 3, indicating 3 pairs. The loop runs 3 times, adding 3 entries to the map.