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2Cand o aplicatie web creste in termeni de utilizatori, date si functionalitati, scalabilitatea devine o prioritate.
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4## Scalabilitate verticala vs orizontala
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6**Scalabilitate verticala (Scale Up):** cresterea resurselor (CPU, RAM, stocare) unui singur server.
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8**Scalabilitate orizontala (Scale Out):** adaugarea mai multor servere/noduri care lucreaza impreuna.
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10```mermaid
11flowchart LR
12    A[Users] --> B[Load Balancer]
13    B --> S1[Server 1]
14    B --> S2[Server 2]
15    B --> S3[Server 3]
16```
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18## Caching: Accelerarea raspunsurilor
19~
20```mermaid
21flowchart TD
22    U[User] --> CDN[CDN]
23    CDN --> App[Application]
24    App --> DB[Database]
25```
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27## Load Balancing: Distribuirea traficului
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29```mermaid
30flowchart TD
31    U[User] --> LB[Load Balancer]
32    LB --> S1[Server 1]
33    LB --> S2[Server 2]
34    LB --> S3[Server 3]
35```
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37## Scalarea bazei de date: Replicare si Sharding
38~
39```mermaid
40flowchart TD
41    App[Application] --> DB1[Shard 1]
42    App --> DB2[Shard 2]
43    App --> DB3[Shard 3]
44```
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46## Microservicii si arhitecturi distribuite
47~
48```mermaid
49flowchart TD
50    U[User] --> API[API Gateway]
51    API --> MS1[Microservice 1]
52    API --> MS2[Microservice 2]
53    API --> MS3[Microservice 3]
54    MS1 --> DB1[(DB 1)]
55    MS2 --> DB2[(DB 2)]
56    MS3 --> DB3[(DB 3)]
57```
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59## Asincronie si cozi de lucru
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61```mermaid
62flowchart TD
63    App[Application] -- send task --> Queue[Queue]
64    Queue --> Worker[Worker]
65    Worker --> DB[Database]
66```
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68## Modele avansate de scalabilitate
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70- **Circuit Breaker:** previne defectiunile in cascada intre servicii.
71- **Bulkhead:** izoleaza resursele intre componente.
72- **Retry and Backoff:** reincearca automat cererile esuate cu intervale crescatoare.
73- **Rate Limiting:** limiteaza numarul de cereri acceptate.
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75```mermaid
76flowchart TD
77    Client --> API[API Gateway]
78    API --> CB[Circuit Breaker]
79    CB --> Svc[Service]
80    Svc --> DB[Database]
81    API --> RL[Rate Limiter]
82    RL --> CB
83```
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85## Stiva tehnologica completa
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87```mermaid
88flowchart TD
89    CDN[CDN] --> LB[Load Balancer]
90    LB --> API[API Gateway]
91    API --> MS1[Microservice 1]
92    API --> MS2[Microservice 2]
93    MS1 --> Redis[Redis Cache]
94    MS1 --> DB1[(Relational DB)]
95    MS2 --> MQ[Message Queue]
96    MQ --> Worker[Worker]
97    Worker --> DB2[(NoSQL DB)]
98```
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100## Fluxul complet al unei cereri
101~
102```mermaid
103flowchart LR
104    U[User] --> CDN[CDN]
105    CDN --> LB[Load Balancer]
106    LB --> API[API Gateway]
107    API --> MS[Microservices]
108    MS --> MQ[Message Queue]
109    MS --> Redis[Cache]
110    MS --> DB[Database]
111    MQ --> Worker[Worker]
112    Worker --> DB
113```
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115## Concluzie
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117Scalarea unei aplicatii web necesita o viziune holistica: arhitectura, instrumente, automatizare, monitorizare si cultura DevOps. Studierea modelelor avansate, adoptarea bunelor practici si invatarea din greselile companiilor mari este cheia pentru construirea sistemelor reziliente gata sa creasca.
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