☁Cloud-Computing

We want to optimize the parts of the datacenter hardware such that the cost of operating the datacenter as a whole would be lower, we need to think about it as a whole. Datacenter CPUs Different requirements Hardware needs high level isolation (because it will be shared among different users). Usually high workloads and moving a lot of data around. They have a spectrum of low and high end cores, so that if you have high parallelism you can use lower cores, while for resource intensive tasks, its better to have high end cores, especially for latency critical tasks. ...

HTTP is the acronym for HyperText Transfer Protocol. Caratteristiche principali (3) Comunicazioni fra client e server, e quanto sono comunicate le cose si chiude la connessione e ci sono politiche di caching molto bone (tipo con i proxy) Generico: perché è un protocollo utilizzato per caricare moltissime tipologie di risorse! Stateless, ossia non vengono mantenute informazioni su scambi vecchi, in un certo modo ne abbiamo parlato in Sicurezza delle reti quando abbiamo parlato di firewall stateless. Solitamente possiamo intendere questo protocollo come utile per scambiare risorse di cui abbiamo parlato in Uniform Resource Identifier. ...

Object Stores Characteristics of Cloud Systems Object storage design principles 🟨++ We don’t want the hierarchy that is common in Filesystems, so we need to simplify that and have these four principles: Black-box objects Flat and global key-value model (trivial model, easy to access, without the need to trasverse a file hierarchy). Flexible metadata Commodity hardware (the battery idea of Tesla until 2017). Object storage usages 🟩 Object storage are useful to store things that are usually read-intensive. Some examples are ...

CDNs are intermediary servers that replicate read intensive data to provide better performance when user requests them. A close relative of CDNs is edge computing (e.g. gaming stations) where lots of computation is done directly close to the user. Types of CDNs 🟨– Mainly three types of CDNs: Highly distributed ones. Database based ones. Ad-hoc CDNs. Advantages and disadvantages 🟩 The main reason we use CDNs is to lower the value of latency: we are in fact bringing the data closer to the user. We have much less data in length to be transmitted. Yet we have some disadvantages too: ...

Introduzione ai Redundant Array of Indipendent Disks I RAID ne abbiamo citato per la prima volta in Memoria. Come facciamo a stare su alla velocità del processore se questa va a crescere in modo esponenziale? Parallelizzazione della ricerca!. Ecco perché ci serve raid (oltre alla ridondanza quindi più sicuro). E possono anche fallire. → ammette recovery. E una altra cosa bella dei raid è che sono hot-swappable cioè li puoi sostituire anche quando stanno runnando. ...

A seconda dell’utilizzatore l’OS può essere molte cose, come solamente l’interfaccia se sei un programmatore, servizi (se sei un utente, ma gran parte dei servizi sono astratti e l’utente ne può anche essere a non-conoscenza). Ma se sei un programmatore OS ti interessa capire le componenti principali dell’OS Slide componenti OS alto livello Introduzione sui componenti (salto) Questa parte la salto perché è una descrizione molto generale di cosa si occupa L’os verso drivers, processi, filesystem I/O, quindi non è molto importante ...

The fundamental idea behind a virtual machine is to abstract the hardware of a single computer (the CPU, memory, disk drives, network interface cards, and so forth) into several different execution environments, thereby creating the illusion that each separate environment is running on its own private computer. (Silberschatz et al. 2018). Virtualization allows a single computer to host multiple virtual machines, each potentially running a completely different operating system. È virtuale nel senso che la macchina virtuale ha la stessa percezione della realtà di una macchina reale. Qualcosa che non è la realtà ma appare molto simile ad essa. ...

How can we coordinate services to actually understand what they are doing, or what the user wants them to do? How to manage networks errors? This note will mainly focus on high level communication protocols to coordinate this kind of communication. Remote Procedure Calls History and Basic Idea This has been the main idea, introduced in 1984, using the idea of stubs, see (Birrell & Nelson 1984). The system basically calls the remote procedure as if it was local on the high level, but on a lower level a network request is sent. The architecture has remained the same in these years. It hides all the complexity in the stub (marshaling, binding and sending, without caring about the sockets and communication matters). One problem is that it might be hiding the complexity too well. The programmer has surely an ease of programming, but design consideration should consider overloads generated by the network communication. ...

Reliability is the ability of a system to remain operational over time, i.e., to offer the service it was designed for. Cloud Hardware and software fails. In this note, we will try to find methods to analyze and predict when components fail, and how we can prevent this problem. Defining the vocabulary Availability $$ \text{Availability} = \frac{\text{Uptime}}{\text{Uptime} + \text{Downtime}} $$MTTF: Mean Time To Failure $$ \text{MTTF} = \frac{1}{r} $$ This definition does not include repair time, and assumes the failures are independent with each other. ...

Queueing theory is the theory behind what happens when you have lots of jobs, scarce resources, and subsequently long queues and delays. It is literally the “theory of queues”: what makes queues appear and how to make them go away. This is basically what happens in clusters, where you have a limited number of workers that need to execute a number of jobs. We need some little maths to model the stochastic process of request arrivals. ...