ELI5: v8 Isolates and Contexts

This is a very good question. Let's start first with understanding a Unix process: A place that a particular chunk of code is being executed by the systems processing units, either their CPU or GPU or more. A process is where the Operating System makes space for:

• Priority - Knowing how important the process is to the operating system, so that as the processor has idle cycles it can apply them in the most valuable order
• Memory Allocation - Gaining access to memory so the code can store variables, with some affordances for marking the memory as available the data in the variables isn't relevant anymore.
• Memory Isolation - Confidence that other bits of code cannot access the data you store in your codes variables means malicious actors can't grab the users password from your memory.

This is, of course, an over-simplification. Process prioritization, memory allocation, and memory isolation are all very deeply complex and nuanced topics that fly right over my head most of the time.

The other piece of Unix infrastructure that's worth wrapping ones head around to understand v8 Isolates and Contexts is the notion of a thread.

Threads, like processes, take responsibility for memory isolation and allocation and priority. However, they run within a parent process. The outer operating system isn't expected to know about or have to think about them. It only has to think about the outer process.

This makes starting and terminating threads potentially cheaper computationally; and provides in-application affordances for managing the work being done in those threads potentially independent of the underlying operating system. You don't have to worry about whether the Unix kernel manages memory in this way and Windows kernel manages it in another way, or Android having a different process prioritization schema than iOS. Hooray! This saves cognitive load on developers at the cost of an orchestration layer with some, but hopefully minimal, performance impact.

So what's happening in v8 with Isolates and Contexts?

The role of v8 in the Node/JavaScript ecosystem is approximately that of the "operating system." In fact, you may hear of v8 as being called a JavaScript "Virtual Machine." It's not, but it's pretty darn close.

An Isolate is a mechanism for v8 to isolate and allocate memory to the code that is running. Code running within an isolate cannot read data from code running within another isolate.

A Context is very very similar. However, it appears that multiple chunks of running code may share a context.

Essentially, Isolates are closer to the Processes in Unix, whereas contexts are closer to Threads.

Hopefully this is close enough to ELI 5 :).