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Web Engineering/Foundations of the Web.md

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+---
+type: practical
+---
+The web $\neq$ The internet
+
+![As portrayed in CN](Pasted%20image%2020241121115442.png)
+As portrayed in CN
+
+
+## URIs
+A **URI (Uniform Resource Identifier)** identifies a resource, either by its name or location, and includes both URLs and URNs. A **URL (Uniform Resource Locator)** is a type of URI that specifies the exact location of a resource on the web, including its access method (e.g., HTTP, FTP).
+
+Bad naming...
+
+* URLs and URNs are **URI Schemes**
+* URLs  - `transport://user:password@host:port/path[?search][#fragmentid]`
+* URNs - Is a logical address of a resource
+
+
+## HTTP
+- Port 80 usually
+- TCP
+### MIME
+**MIME (Multipurpose Internet Mail Extensions)** is a standard that extends email and web protocols to support text in character sets other than ASCII, as well as attachments like images, audio, video, and other file types.
+
+**HTTP does not have these built-in!**
+
+- No state retained between request/response pairs
+- Connections **can** persist
+
+
+![Server_client](server_client.canvas)
+
+
+| Term          | Explanation                                     |
+| ------------- | ----------------------------------------------- |
+| Origin Server | Server where resources reside                   |
+| Proxy         | Program that can act on behalf of origin server |
+| Gateway       | Intermediary for some other server              |
+| Tunnel        | Relay between two connections (blindly)         |
+
+---
+### Content negotiation
+- Content can be available in multiple variants
+- Representation is to be served based on the **content negotiation mechanism**
+
+
+| Type of content negotiation | Description                                                          |
+| --------------------------- | -------------------------------------------------------------------- |
+| Server-driven               | Client includes headers and server tries to find stuff based on them |
+| Client-driven               | Server responds with a list of available types and client picks      |
+
+---
+
+### Messages & methods
+
+Cool diagram expressing how protocols are being nested:
+![](Pasted%20image%2020241121120647.png)
+
+#### Request message (BNF)
+
+```js
+
+HTTP-message = request-line | status-line message-header* CRLF message-body? message-header = general-header | entity-header | request-header | response-header (general | entity | request | response)-header = field-name ":" field-content CRLF
+```
+
+
+#### **General Headers**
+- `Cache-Control`: Caching directives.
+- `Connection`: Manage persistent connections.
+- `Transfer-Encoding`: Encoding (e.g., `chunked`, `gzip`).
+- `Via`: Tracks intermediaries and routing.
+
+#### **Entity Headers**
+- `Content-Encoding`: Compression (e.g., `gzip`).
+- `Content-MD5`: Body integrity check.
+- `Expires`: Response expiry date.
+- `Last-Modified`: Last entity update timestamp.
+
+
+
+### Request methods
+- GET, DELETE, POST, PUT, etc.
+- They NEED to be used accordingly, although you are not forced to do it at all
+- Idempotent[^1] requests are side-effects free
+
+![](Pasted%20image%2020241121121317.png)
+
+
+[^1]: Describing an action which, when performed multiple times, has no further effect on its subject after the first time it is performed.