2. Request-Reply and Java UDP

07/02/23

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Introduction

• Network protocol - Set of rules on how to communicate. If everyone follows the same rules then effective communication is achieved
• Distributed System Challenges - Common challenges consist of
  • Heterogeneity - coping with system component variability
  • Failure Handing - Coping with partial failure
  • Concurrency - Correctness and performance with concurrency
  • Openness - To extension or reproduction
  • Security - Protection form threats
  • Transparency - Selective abstraction
  • Quality of Service - Performance & other non-functional characteristics

Heterogeneity

• All the parts of a distributed system has to work together, even if they are different
• Will mean using common protocols, interfaces and encoding
• E.g. Java Data IO Streams

Failure Handling

• Can fail in complicated ways, (different parts can fail at different times)
• Coping with failure typically requires some level of redundancy, and specific means detecting and responding to partial failures
• E,g, simple TCP server

Concurrency

• The different processes in a distributed system normally run concurrently
• This may requires additional checks and communication to ensure global consistency
• Use alternative approaches with weaker consistency
• E.g. The example TCP server is multi-threaded so that it can serve multiple independent clients concurrently

Request-reply Protocols

Typical pattern for client-server interaction. Request and response are messages carried by selected inter process communication mechanism (TCP, UDP)

• Within the client, the server needs to be identified by a remote reference; IP, port, optional additional process-internal identifiers.
• Server may support several different operations
• Client and server must agree on how arguments are encoded
• Client normally blocks until the response is received. Protocols can be reply-less(one-way) or asynchronous (reply is read by the client and later operated on)

UDP Request - Only know if its been successful if you receive a response. Don't know if the response or reply got lost etc. Can set a timer to resend out request incase it got lost.

Implementation

• Message Identifiers - Each request is given a unique ID. Useful to help match responses to requests at the client
• Failure Model - Can suffer from communication omission faults and process omission faults.
• Timeouts - Client typically uses a timeout to detect a missing reply. Client will re-transmit the request if time exceeds limit.
• Duplicate request messages - Can be detected in the server from the duplicate message identifier. If response has already been sent then the response will need to be sent again.
• History - With idempotent operations performing the operation twice makes no difference. Yet sometimes the server should not execute a duplicate request. Results get held in history.

Idempotent/Idempotence - Making multiple identical requests that have the same effect as making a single request.

TCP vs UDP

TCP	UDP
Establish connection first	No connection overhead
Request/reply = stream of bytes	Request/reply = 1 message
No size limit	Few KB
Reliably delivered in order	May be lost or out of order
HTTP, SMTP	DNS, DHCP

Java UDP Programming

• DatagramPacket - Representing a UDP datagram
• DatagramSocket - Representing a UDP socket

UDP Considerations

• Message Size - Hard limit of 65508 bytes, but often 8k or less. Larger sizes are broken up into fragments
• Blocking - Receive normally blocks until a message is received. Socket timeout can be specified
• Receive from any - Normally a socket will accept messages from any sender
• Failure Model - Best effort, checksum used to check for corruption, out of order packets etc
• Uses of UDP - Small messages and very simple interactions (DNS). Also supports Multicast communicaiton
  • Avoids overheads of TCP

Java Data Encoding

TCP - Sequence of bytes UDP - Array of bytes

Marshalling - Process of converting program data to network form

Unmarshalling - The converse

JSON - Cross-language UTF-8 text-based encoding including maps and arrays XML - Cross-language text-based encoding with HTML-like markup