OS: Application Interaction

# Application Interaction

1. [Why Applications Interact](./slides/why.md)
1. [Definitions and Terms](./slides/defs.md)
1. [App Classification](./slides/classification.md)
1. [Interaction Channels](./slides/channels.md)
1. [Roles of the Operating System](./slides/os-roles.md)
1. [Programming Interface](./slides/api.md)

---

## Firefox I/O

```console
$ lsof -p $(pidof firefox)
[...]
firefox 32100 student   64u     IPv4           38654330       0t0      TCP so:44750->lb-140-82-112-26-iad.github.com:https (ESTABLISHED)
firefox 32100 student   65u     IPv4           38659353       0t0      TCP so:37528->239.237.117.34.bc.googleusercontent.com:https (ESTABLISHED)
[...]
firefox 32100 student  176u     unix 0x0000000000000000       0t0 38640969 type=SEQPACKET
firefox 32100 student  178u     unix 0x0000000000000000       0t0 38650545 type=SEQPACKET
firefox 32100 student  179u     unix 0x0000000000000000       0t0 38648379 type=STREAM
firefox 32100 student  181r     FIFO               0,13       0t0 38640924 pipe
firefox 32100 student  185u  a_inode               0,14         0    11438 [eventfd]
firefox 32100 student  186uw     REG              259,5    131072 17040026 /home/student/.mozilla/firefox/inonjd9q.default-release/protections.sqlite
firefox 32100 student  188u     unix 0x0000000000000000       0t0 38635209 type=STREAM
[...]
firefox 32100 student  200r      REG                0,1      5196 14538955 /memfd:mozilla-ipc (deleted)
[...]
```

----

### Reminder: I/O: Perspective of the Process

- Communication / Interaction "outside" the process
- "Inside" the process: working with memory, CPU instructions
- "Outside" the process: I/O channels
  - File-like: data comes / goes from / to outside the system
  - Socket-like: data comes / goes from / to another process

---

## Why Do Applications Interact?

- What is the alternative?
  - An application does everything.
  - Lack of modularity
  - Security / Reliability issues
  - Issues of maintainability
- Modularity
- Integration with existing components
  - Software reuse, but without building / linking

---

## Definitions and Terms

---

- What does it mean that applications interact?
- Do the following programs interact with other applications?

---

### Exhibit 1

```c
#include <stdio.h>

int main(void)
{
	printf("Hello, world\n");
	return 0;
}
```

----

### Exhibit 1

- The program simply prints a message to the standard output.
- The terminal runs the program, so we do have some app interaction.
- Also, it can interact with other programs by doing: `./hello_world | grep Hello`
- Interaction is static.
- However, this is not a voluntary interaction on the part of the program.
- The program was designed as a standalone one.

----

### Exhibit 1 - Creating Interaction

- `chapters/app-interact/overview/guides/comm-channels/reader.c`
- `chapters/app-interact/overview/guides/comm-channels/writer.c`
- `chapters/app-interact/overview/guides/comm-channels/send_receive_pipe.c`

---

### Exhibit 2

```c
// writer.c
int main(void)
{
	FILE *fp = fopen("myf.txt", "r");
	fprintf(fp, "Hello");
	fclose(fp);
	return 0;
}
```

```c
// reader.c
int main(void)
{
	char a[20];
	FILE *fp = fopen("myf.txt", "r");
	fscanf(fp, "%s", a);
	printf("%s\n", a);
	fclose(fp);
	return 0;
}
```

----

### Exhibit 2

- The 2 programs indeed interact.
- But is there any difference from the previous example?
- There is no protocol of interaction.
- If one of the applications did not exist, the other one could still be a valid, meaningful program.

----

### Exhibit 2

- What if we start the reader before the writer?

- Synchronization is required.

----

### App Interaction

- Communication (data transfer)

- Synchronization (notification)

---

### Exhibit 3

```c
int main(void)
{
	pid_t pid = fork();
	if (pid == 0) {
		sleep(2);
		exit(EXIT_SUCCESS);
	}

waitpid(pid, &status, 0);

return 0;
}
```

----

### Exhibit 3

- Synchronization, but no communication
- The programs interact in a structured manner.
- One process waits for the other to finish.
- Interaction is dynamic - e.g. a process actively waits for the other.
- The 2 processes have been designed to work together.

---

### Exhibit 4 - WhatsApp Application

![App1](overview/media/whatsApp.svg)

----

### Exhibit 4 - WhatsApp Application

- There is interaction between the main client application and other applications: photos, location, YouTube etc.
- There is also interaction with the web server when sending messages.

---

### App Interaction

- Applications that run at the same time and that communicate with each other during their lifetimes.
- This communication involves:
  - passing messages (**communication**)
  - sending / waiting for notifications (**synchronization**)

----

### Requirements for Interaction

- Address / Identity of processes
  - Process ID
  - Hostname, port
- Communication channel
  - Buffer to store data
  - "Endpoints" to send / receive data from buffer
- Address / Identity of channel
  - Path in filesystem
  - Hostname, port, path
  - ID

----

### Metrics and Goals for Interaction

- Speed: fast transfer
- Latency: time to actual delivery
- Reliability
- Intra-system interaction
  - Security: access control
- Inter-system interaction
  - Security: confidential connection
  - Scalability: multiple connections

---

## App Classification

---

### App Components

- Threads and processes
- Processes may be colocated on the same system
- Processes may be distributed on multiple systems

----

### App Components

![App Components](overview/media/app-components.svg)

---

### App Classification

- Single-process single-threaded
- Multi-threaded
- Multi-process
- Multi-system
- These apps use OS primitives to communicate between components (threads, processes)

----

### Single-process Single-threaded

- No actual software component interaction
- Simple applications that don't rely on complex features
- `ldd <exec> | grep pthread`

- `/bin/bash`, `/bin/dd`

----

### Single-process Multi-threaded Homogeneous

- Multiple threads doing the same work
- Threading models: boss-workers, worker threads, thread pools
- Generally little interaction: join at the end

- multi-threaded web servers
- `libx264` library for `ffmpeg`
- Firefox web browser: browser tabs

----

### Single-process Multi-threaded Heterogeneous

- Multiple threads, each doing different work

- Firefox web browser: browser management processes

----

### Multi-process Homogeneous

- Multiple processes doing the same work
- Process pool
- Generally no interaction: pre-fork, get job, serve

- Multi-process web servers (Apache2 mpm-prefork)
- Google Chrome: a process per Tab

----

### Multi-process Heterogeneous

- Multiple processes doing different items
- IPC mechanisms (pipes, message queues, sockets) to interface between processes

- Postfix
- GitLab

----

### Postfix

![Postfix](overview/media/postfix-architecture.svg) ([source](https://commons.wikimedia.org/wiki/File:Postfix_architecture.svg))

----

### GitLab

<img src="overview/media/gitlab-architecture.png" width="400" /> ([source](https://docs.gitlab.com/ee/development/architecture.html))

----

### Multi-system

- Processes run on different systems
- Used in distributed systems, data centers, computing clusters
- Typically a combination of homogeneous and heterogeneous systems and processes

- Kubernetes, OpenStack, Netflix backend servers

----

### Kubernetes

![Kubernetes Architecture](overview/media/kubernetes-architecture.png) ([source](https://medium.com/devops-mojo/kubernetes-architecture-overview-introduction-to-k8s-architecture-and-understanding-k8s-cluster-components-90e11eb34ccd))

---

## Interaction Channels

---

App components interact via interaction channels.

----

### Interaction Types

- Sending / waiting for notifications (**synchronization**)
- Passing messages (**communication**)

----

### Reminder: App Components

- Threads and processes
- Processes may be colocated on the same system
- Processes may be distributed on multiple systems

----

### Interaction Use-Cases

- Inter-thread interaction (the same process)
- Inter-process colocated
- Inter-process distributed

---

### Synchronization

- One component waits, the other notifies
- A synchronization object is required
  - The object typically has a binary value: set or unset
- Notification may happen without a wait
  - Interruption of normal execution flow

----

### Synchronization

![Synchronization](overview/media/synchronization-generated.gif)

----

### Interruption

![Interruption](overview/media/interruption-generated.gif)

---

### Communication

- One component sends / writes data, the other receives / reads it
- A communication buffer is required
- Accessing the buffer
  - Directly, via memory address: shared memory
  - Indirectly, via endpoints (file descriptors)
    - Required for distributed communication

----

### Shared Memory

- Threads share the virtual address space of a process
  - Different threads use the same virtual address
- Processes use virtual memory to share a physical memory region
  - Different processes use different virtual addresses mapped to the same physical address
- Communication is not synchronized
  - Receiver doesn't know when data has been sent

----

### Shared Memory

![Shared Memory](overview/media/shared-memory.svg)

----

### Communication Channel

- Underlying buffer is not directly exposed
- Access via endpoints (file descriptors)
- The only mean for distributed communication
  - For distributed communication, there are multiple buffers: one on each host, buffers in middleboxes
- Synchronization is implicit
  - Reader is blocked (waits) until data is available
  - Writer is blocked (waits) until room is available
- Protocols are used to make communication reliable

----

### Communication Channel

![Communication Channel](overview/media/communication-channel.svg)

---

## Roles of the Operating System

---

### Reminder: Requirements

---

## OS Roles

- Operating system mediates app interaction
- Operating system provides primitives / mechanisms for app interaction
- Roles
  - registry: identifying app endpoints
  - post office: reliably passing messages between applications
  - police: ensuring access control and security
  - doorbell: app notification of incoming messages

----

## OS - The Lawmaker

- The OS is like a lawmaker the dictates how the interaction takes place
- Allows or denies the sending of data / notifications
- Ensures correct delivery of data / notification

----

## OS - The Lawmaker

![Multiple Apps Interacting through the OS](overview/media/multiple-apps-interaction.svg)

---

## Programming Interface

---

### Summary of Channel Types

- Synchronization
- Interruption
- Shared memory
- Communication channels

---

### Synchronization

- Synchronization object
  - To be created / allocated before use
- mutex / spinlock / file locking: `lock()` / `unlock()`
- semaphore: `up()` / `down()`
- monitor / condition variable: `wait()` / `notify()`
- signals (Unix / Linux): `kill()` / `sigwait()`
- Generally used for inter-thread synchronization

----

### Inter-Process Synchronization

- Named semaphores
  - Entry in filesystem
- File locking
  - Uses actual file

----

### Synchronization Demos

In `chapters/app-interact/overview/guides/lock/`

In `chapters/app-interact/overview/guides/sync/`

---

### Interruption

- Asynchronous to normal execution flow
- Generally used for inter-process interaction on the same system
- Associate function handler to run when interruption is received
- signals (Unix / Linux): `sigaction()`, `kill()`, `sigqueue()`, `sigsuspend()`
- exceptions (Windows): `AddVectoredExceptionHandler()`, `RaiseException()`

---

### Interruption Demos

In `chapters/app-interact/overview/guides/interrupt/`

---

### Shared Memory

- Implicit for inter-thread communication
- Explicit API for processes
  - Create shared memory mapping
  - Filesystem entry as backing store
- No API for using it
  - Just use memory operations

----

### Shared Memory Demos

In `chapters/app-interact/overview/guides/shared-mem/`

---

### Communication Channels

- Discussed in I/O chapter, socket-like
- Created before-hand
- `read()` / `write()`, `send()` / `recv()`
- (anonymous) pipes: related processes
- named pipes (filesystem entry): any colocated processes
- UNIX / local sockets (filesystem entry): any colocated processes
- network sockets: colocated or distributed processes

----

### Communication Channels

In `/chapters/app-interact/overview/guides/comm-channels/`

----

### Case study: Our Own Server

- Server computes Fibonacci, sends it back
- single-process server
- multi-threaded server: uses a thread per-connection
- multi-process server: use a process per-connection
- thread-pool server: uses pre-created threads to serve
- process-pool server: use pre-created processes to serve

----

## Case study: Our Own Server

In `/chapters/app-interact/overview/guides/fibonacci-server/`