4.1 Docker the program

notes/4_under_the_hood.md

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+# 4. Under the Hood
+
+## Docker the program
+
+### What Kernels Do
+
+* software core of computer
+* Run directly on hardware, had many jobs
+  * Respond to messages from the hardware
+  * Start and schedule programs
+  * Control and organize storage
+  * Pass messages between programs
+  * Allocate resources, memory, CPU, network, etc
+  * **Create containers by Docker configuring the kernel**
+
+### What Docker Does
+
+* Program written in Go
+* Manages kernel features
+  * Docker uses "cgroups" to contain processes, so each container (with its process) is isolated
+  * Docker uses "namespaces" (feature of Linux Kernel) to contain networks
+  * USes "copy-on-write" filesystems to build images
+* Above features has been used by industry for years before Docker.
+
+### What Docker Really Does
+
+* Makes scripting distributed systems "easy"
+
+### The Docker Control Socket
+
+* Docker is two programs: a **client** & a **server**
+  * They communicate via a socket
+* The server receives commands over a socket (either over a network or through a "file")
+  * When client & server are on same computer, a special "file" `socket` is created for communication
+* The client can even run inside a docker container itself
+
+location of docker socket file on linux: `/var/run/docker.sock`
+
+We can control the docker server using docker client inside a container: `docker run -it --rm -v /var/run/docker.sock:/var/run/docker.sock docker sh`
+
+##