Middleware

• Middleware is software that sits between application programs and the operating system
• Databases, Runtimes, etc

Resources

• Limited CPU time, disk access bandwidth, memory bandwidth
• Multiple applications using resources simultaneously

Concurrency

• Make efficient use of resources
• When we aren’t actively using the CPU because we are waiting on I/O, another program can use it

Thread

• Fundamental unit of concurrency
• Sequence of programmed actions
• Each program includes at least one thread

Process

• Container for one or more threads
• Protects threads from interactions from unrelated threads running on the same computer
• Threads in different processes do not share memory

Thread APIs

• Create
• Kill
• Switch

Scheduling

• OS will have many threads that can be run
• Limited CPUs are available
• OS must select thread to run

Dependencies

• Sometimes threads are not fully independent
• One thread may generate data that another consumes
• These threads need to interact via synchronization

What if thread A is waiting for thread B and thread B is waiting for thread A?

Deadlock

• Cyclic waiting
• Must be addressed to prevent freezes
• Difficult to solve generally

Transaction

• Unit of computation with no externally visible internal state
• Computation succeeds or fails as a unit
• Useful concurrency primitive

Accidental Interaction

• Threads share memory, so they may interact unintentionally
• This can be mitigated on some systems using virtual memory