Node.js Single-thread Mechanism

I learnt Node.js is single-threaded and non-blocking. Here I saw a nice explanation How, in general, does Node.js handle 10,000 concurrent requests? But the first answer says The

Solution 1:

Since you asked about my answer I guess I can help clarify.

1

For the specific case of handling multiple parallel client requests which triggers multiple parallel MongoDB queries you asked:

Are those queries handled by different threads?

On node.js since MongoDB connects via the network stack (tcp/ip) all parallel requests are handled in a single thread. The magic is a system API that allows your program to wait in parallel. Node.js uses libuv to select which API to use depending on which OS at compile time. But which API does not matter. It is enough to know that all modern OSes have APIs that allow you to wait on multiple sockets in parallel (instead of the usual waiting for a single socket in multiple threads/processes). These APIs are collectively called asynchronous I/O APIs.

On MongoDB.. I don't know much about MongoDB. Mongo may be implemented in multiple threads or it may be singlethreaded like node.js. Disk I/O are themselves handled in parallel by the OS without using threads but instead use I/O channels (eg, PCI lanes) and DMA channels. Basically both threads/processes and asynchronous I/O are generally implemented by the OS (at least on Linux and Mac) using the same underlying system: OS events. And OS events are just functions that handle interrupts. Anyway, this is straying quite far from the discussion about databases..

I know that MySQL and Postgresql are both multithreaded to handle parsing the SQL query loop (query processing in SQL are basically operations that loop through rows and filter the result - this requires both I/O and CPU which is why they're multithreaded)

If you are still curious how computers can do things (like wait for I/O) without the CPU executing a single instruction you can check out my answers to the following related questions:

Is there any other way to implement a "listening" function without an infinite while loop?

What is the mechanism that allows the scheduler to switch which threads are executing?

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Security is ensured by the language being interpreted and making sure the interpreter does not have any stack overflow or underflow bugs. For the most part this is true for all modern javascript engines. The main mechanism to inject code and execute foreign code via program input is via buffer overflow or underflow. Being able to execute foreign code then allows you to access memory. If you cannot execute foreign code being able to access memory is kind of moot.

There is a second mechanism to inject foreign code which is prevalent in some programming language cultures: code eval (I'm looking at you PHP!). Using string substitution to construct database queries in any language open you up to sql code eval attack (more commonly called sql injection) regardless of your program's memory model. Javascript itself has an eval() function. To protect against this javascript programmers simply consider eval evil. Basically protection against eval is down to good programming practices and Node.js being open source allows anyone to look at the code and report any cases where code evaluation attack is possible. Historically Node.js has been quite good in this regards - so your main guarantee about security from code eval is Node's reputation.

Solution 2:

(1) The big picture goes like this; for nodejs there are 2 types of thread: event (single) and workers (pool). So long you don't block the event loop, after nodejs placed the blocked I/O call to worker thread; nodejs goes on to service next request. The worker will place the completed I/O back to the event loop for next course of action.

In short the main thread: "Do something else when it need to wait, come back and continue when the wait is over, and it does this one at a time".

And this reactive mechanism has nothing to do with thread running in another process (ie database). The database may deploy other type of thread management scheme.

(2) The 'memory space' in your question is in the same process space. A thread belongs to a process (ie Express app A) never run in other process (ie Fastify app B) space.