concurrency

Topics…

• What to keep in mind when developing server
• When a function is:
  • thread safe?
  • synchronized?
• When multiple requests are hit on the server, how is it handled
  • connection pool size
• What are the database considerations while developing the server?
  • @Transaction??

Books and References

• https://docs.oracle.com/javase/tutorial/essential/concurrency/index.html
• https://github.com/preslavmihaylov/booknotes/tree/master/java/java-concurrency-in-practice/chapter-01
• Java Concurrency in practice by Joshua Bloch
• The art of multiprocessor programming by Maurice Herlihy
• https://www.youtube.com/playlist?list=PLBBK9G6O8MPDEeSVG17Pwya2xlUNbZn30 (Code with Ease by Varsha)
• Day to Day concurrency issues:
  • https://github.com/code-review-checklists/java-concurrency
  • https://stackoverflow.com/questions/461896/what-is-the-most-frequent-concurrency-issue-youve-encountered-in-java
• Practice problems
  • https://codesignal.com/learn/courses/introduction-to-java-concurrency
  • https://github.com/hemelnj/edu/tree/master/Java%20Multithreading%20for%20Senior%20Engineering%20Interviews%20-%20Learn%20Interactively

Concurrency Interview Questions

• Design Bounded Blocking Queue
• Multithreaded Web Crawler
• Print Zero Even Odd
• Fizz Buzz Multithreaded
• The Dining Philosophers

Concurrency vs Parallel

• In Computer Science world they are different
• Concurrency is when the execution of multiple tasks is interleaved, instead of each task being executed sequentially one after another.
• Parallelism is when these tasks are actually being executed in parallel. For parallelism to happen, we need more than one cpu core. Hence Parallelism is limited by hardware.
• In scenario 1 (concurrency) green task is completed partially in two different steps in different time. Concurrent tasks look like they are executing in parallel.
• In scenario 2 (parallel), green task is completely executing in another processing unit and completely parallel to other tasks.
• https://stackoverflow.com/questions/4844637/what-is-the-difference-between-concurrency-parallelism-and-asynchronous-methods
• https://stackoverflow.com/questions/1050222/what-is-the-difference-between-concurrency-and-parallelism
• Tips:
  • It’s not worth it to parallelize small tasks
  • Do not parallelize infinite data source
  • parallelism only benefits if we have multiple cores available

Sync vs Async

• An asynchronous method call is normally used for a process that needs to do work away from the current application and we don’t want to wait and block our application awaiting the response.
• Nodejs is asynchronous in nature and JS is single threaded language

Multithread vs Multicore vs Hyperthreading

• Thread is a software concept.
• A core can run multiple threads concurrently which will appear to be running in parallel but in reality it is switching between multiple threads very fast.
• Multicore is hardware concept. It refers to a processor that has more than one logical CPU core, and that can physically execute multiple instructions at the same time.
• Hyperthreading is a hardware concept according to which it means that the core can run two parallel threads, as if there were two cores.
• Hyperthreading increases the number of logical cores available
• https://stackoverflow.com/questions/11835046/multithreading-and-multicore-differences

Concurrency Models

• https://jenkov.com/tutorials/java-concurrency/concurrency-models.html
• Parallel Workers
• Assembly Line
• Functional Parallelism