Response Tokenisation

Tokenisation is very important. When working with a stream of data, as provided by TCP and UDP, you most certainly want to

  1. Wait for a complete response before processing

  2. Want to process one response at a time

Whilst this might seem to occur naturally most of the time, network contention, network errors and high data rates will trip you up. CBus is one system where I’ve often see back to back messages returned in a single IO read.

ACA Engine ships with two tokenisers to help you break up the incoming data. The default buffered tokeniser and the more advanced abstract tokeniser.

Default Tokeniser

Usage:

class Example::Driver
# Device driver helper
tokenize delimiter: "\x0D"
end

Options:

Option

Description

delimiter

sequence to detect the end of message. Supports strings and regular expressions

indicator

sequence to detect the start of a message

msg_length

can be used with an indicator if messages are always a fixed length

size_limit

prevents buffering from using all your memory if the end of a message is never detected

min_length

can help prevent false positives

encoding

defaults to ASCII-8BIT to avoid invalid characters when dealing with binary data

Example:

tokenize indicator: "\x02", delimiter: "\x03"

and data: "yu\x03\x02hello\x03\x02world\x03\x02how" Would have the following result:

  • yu\x03 would be discarded

  • hello would be returned

  • world would be returned

  • \x02how would be buffered

Abstract Tokeniser

The primary use case for this tokeniser is variable length messages, where length can be determined by the message contents. (commonly a length field in the header)

Usage:

class Samsung::Displays::MdSeries
tokenize indicator: "\xAA", callback: :check_length
# Called by the Abstract Tokenizer
def check_length(byte_str)
# Check for minimum length
return false if byte_str.bytesize <= 3
response = str_to_array(byte_str)
# data length byte + (header + checksum) == message length
len = response[2] + 4
if response.length >= len
# return the length of this message (any excess will be buffered)
return len
else
# false if the complete message hasn't arrived yet
return false
end
end
end

Options:

Option

Description

callback

callable code, proc, lambda, method etc that will return an integer or false

indicator

sequence to detect the start of a message (string or regex)

size_limit

prevents buffering from using all your memory if the end of a message is never detected

encoding

defaults to ASCII-8BIT to avoid invalid characters when dealing with binary data

Further Reading

For a detailed overview of what these tokenisers are capable of, it is worth looking at their tests.