System design handwritten notes guidance

System Design Basics ①
1)
Try to break the problem into simpler
modules ( Top down approach)
2) T
alk about the trade -
offs
( No solution is perfect)
calculate the impact on system based on
all the constraints and the end test cases .
←
Focus on interviewer 's
heproblem
] intentions .
T
Ask
Abstract
]
questions
( constraints &
Functionality
finding
Requirements) bottlenecks
idudas

System Design Basics ccontd ) ②
D Architectural pieces / resources available
2) How these resources work together
3) Utilization & Tradeoffs
-
consistent Hashing
-
CAP Theorem ✓
-
Load balancing ✓
-
queues
f- caching -
-
Replication
-
59L vs No -
SQL
I
-
Indexes ✓
-
Proxies
1 -
Data Partitioning
✓

Load Balancing
③
( Distributed system)
Types of distribution -
f
Random
Round -
robin
<
Random ( weights for memory
&
CPU
cycles)
To utilize full scalability &
redundancy ,
add 3 LB
D User ¥ web server
2) Web server ¥
App server 1 Cache Server
( Internal platform)
3) Internal platform DB .
#W
Client LB
er
DB
T ,, LB

Smart clients
Takes a pool of service hosts & balances load.
→ detects hosts that are not responsive
→
recovered hosts
→
addition of new hosts
Load balancing functionality to DB (cache. Service
*
Attractive solution for developers
( small scale systems)
As system grows
→ LBS ( standalone servers
)
Hardware load Balancers :
Expensive but high performance.
e.
g . Citrix Netscaler
Not trivial to configure.
Large companies tend to avoid this config .
or use it as 1st point or contact to their
system to serve user requests
&
Intra network uses smart clients / hybrid
solution → ( Next page) for
load balancing traffic .

Software Load Balancers
'
No pain of creation of smart client
No cost of purchasing dedicated hardware
[ hybrid approach
HA Proxy OSS Load balancer
-4
1)
Running on client machine
'
2
( locally bound port)
e -
g
.
local host : 9000
I
F-
managed by HA Proxy
( with efficient management
of requests on the port )
2) Running on intermediate server : Proxies
running beth
HA Proxy
[
Manages health checks
dirt server side components
removal & addition of machines
balances requests alc pools .

Wortdof
Databases
=
S9Lvs.NoS÷
iaa
1) structured D Unstructured .
2) Predefined schema 2) distributed
3) Data in rows & columns 3)
dynamic schema
Row One Entity Into
column Separate data points
mysar
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Oracle
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DB
Postgres
MariaDB

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Reasonstouses.cl#BJ
1) You need to ensure ACID compliance :
ACID compliance
Reduces anomalies
Protects integrity of the database .
for
many E -
commerce & financial app
"
→
ACID compliant DB
is the first choice .
2) Your data is structured &
unchanging .
If your business is not experiencing
rapid growth or sudden changes
→ No requirements of more servers
→ data is consistent
then there's no reason to use system design
to support variety of data &
high traffic .

Reasonstouse.NO#IB
When all other components of system are fast
→
querying &
searching for data bottleneck .
NoSQL prevent data from being bottleneck .
Big data large success for NoSQL.
1) To store large volumes of data C little Ino structure)
No limit on type of data.
Document DB Stores all data in one place
( No need of type of data)
2) Using cloud & storage to the fullest .
Excellent cost saving solution .
( Easy spread of data
across multiple servers to scale up
)
OR commodity hlw on site ( affordable , smaller )
No headache or additional Stw
& NoSQL DBS like Cassander designed to scale
across multiple data centers out of the box.
3) Useful for rapid 1 agile development.
If you're making quick iterations on schema
SQL will slow you down .

Achieved by
CAP-heore€
tenyC All nodes see same data
updating several nodes .
at same time)
before allowing
g)
reads
E
"
"
"
"
e.
%%%,
Not
g
pareieiontdera£
Availability
[cassandra . Couch DB]
Hr Hr
Every request gets System continues to work
response ( success ( failure) despite message loss ( partial
Achieved by replicating Failure .
data across different servers ( can sustain any amount
of network failure without
- resulting in failure of entire
Data is sufficiently replicated network )
across combination of nodes /
networks to keep the system up .
.in:6#eo:::i:::::ia::i:n:::ans'

We cannot build a datastore which is :
D
continually available
2)
sequentially consistent
3)
partition failure tolerant .
Because ,
To be consistent all nodes should see the same
set of updates in the same order
But if network suffers partition,
update in one partition might not make it to
other partitions
↳ client reads data from out-of-date partition
After having read from up-to-date partition .
Solutions stop serving requests from out -
of -
date
partition.
↳ service is no longer
100% available .

Redundancy&ReplicationJ
Duplication of critical data & services
↳
increasing reliability of system .
For critical services & data ensure that multiple
copies 1 versions are running simultaneously on different
servers 1 databases .
Secure against single node failures .
Provides backups if needed in crisis .
Deter Dow
Primary server secondary server
& Data
2
- - a
Ig Replication 2
Active data Mirrored data
# Service
Redundancy : shared -
nothing architecture.
Every node independent. No central service managing state .
]
More resilient ← New servers
←
Helps in
[ to failures addition without scalability
Nosing special conditions

Caching
Load balancing Scales horizontally
caching :
Locality of reference principle
I Used in almost every layer of
computing .
I Application Server cache :
Placing a cache directly on a
request layer node.
↳ Local storage of response
Requests
in€.÷.
miss
response data
# Caches on one Request layer made
T
catedy
✓
Memory (very fast) Node 's local disk
( faster than going to network storage)
# #
Bottleneck :
If LB distributes requests randomly
↳ same request different nodes
our:c!m£ More
d
by
D Global caches
2) Distributed caches

Distributed
Gimme
-
Divided
using consistent
hashing function
③ ⑤gµeautts.-€#
# #
Easy to increase cache space by adding more hordes
##
Disadvantage :
Resolving a missing node
staring multiple copies of I can be handled
by
data on
different hordes
likes making it more complicated .
# # Even
if node disappears
request can pull data from Origin.

flabellate
#
Single cache space for all the hacks .
↳
Adding a cache source I file store ( faster than original store
)
#
Difficult to manage if no
af clients I request increases -
effective if
Y
fixed dataset that needs to be cached
2)
special Hlw fast Ho .
# Forms of global cache :
GEE { %fbtae.mn?hdEtahhYm
Database
database
contains hot data at
Global
cache
%gz { Database
App
"
logic understands the eviction strategy that spoils
better than cache .

CDN : content Distribution network
4- Cache store for sites that saves
large amount
of static media .
if not
available
Request CDN -
Baek- End
if a
- server
available( L
local ( static Media)
storage
Lf the site isn't large enough to have its own CDN
4µA
transition
Some static media using separate subdomain
- ( static .
yaursuuice.com
using lightweight Nginse serves
↳ entrance DNS from your sauce
to a CDN later

Cache Invalidation
# Cached data needs to be coherent with the database
Lf data in DB modified invalidate the cached data .
# 3 schemes :
=
I Write -
through cache :
cache
Data is written
same time in
Data
DB hath cache & DB .
+
Complete data consistency C cache = DB)
+
Fault tolerance in case of failure Club data loss
)
-
high latency in writes 2 write operations
2)
Write around cache
Cache
Data
DB
+ No cache flooding foe writes
-
read request for newly written data miss
higher latency
d

B) Write back cache :
cache DB
Data
after some
£ interval as under
client
some sepciifird conditions
data is written to DB
from cache
+ law latency &
high throughput bae write -
intensive app
"
-
Data loss TT ( only one
copy
in caches
# Cache Eviction Policies
D FIFO
2) LIFO Ae FILO
3) LRV
4) MRU
5) LF U
Random Replacement

S harding 11 Data Partitioning
# Data
Partitioning :
splitting up DD I table across multiple
machines
manageability .
performance , availability & LB
**
After a certain scale paint ,
it is cheaper and more feasible
to scale
horizontally by adding money instead of
vertical
scaling by adding beefiness
# Methods of Partitioning :
1)
Horizontal Partitioning
:
Different rows into diff.
tables
Range based shading
e.
g .
staring locations by zip different
Table 1 :
Lips with L 100000
ranges in
Table 2 :
Lips with 7 Loo ooo
different tables
and so on
**
come if the value of the range not chosen
carefully
leads to unbalanced servers
e. g . Table I can have more data than table 2 .

Vertical Partitioning
# Feature wise distribution of data
↳ in
different servers .
e.
g.
Instagram - DB sauce 1 :
user info
DB sauce 2 : followers
DB server 3 :
photos
* A
straightforward to implement
* A
lane impact on app .
- -
if app
→ additional growth
need to partition feature specific DB across various sources
( e -
g. it would not be possible for a
single sewer to handle
all metadata queries for Lo billion photos by 140 mill.
users
Directory based partitioning
A
loosely coupled approach to work around issues
mentioned in above two partitioning .
** Create lookup service current partitioning scheme
& abstracts it
away from the DB access code.
Mapping l tuple key → D8 sauce)
Easy to add DD towers or
change partitioning scheme .

Partitioning Criteria
D
key or Hash based partitioning :
Kay atte-
af Hash
function →
Partition
the data number
#
Effectively fines the total number
of sauces 1partitions
So
if we add new source I partition T
o
change in hash function
downtime because
of
d
redistribution
↳
Solution :
consistent Hashing
2) List Partitioning : Each partition is
assigned a list of
values .
Nuo → hookup
record for
→
stare the record
key ( partition based on the key)

3) Round Robin
Partitioning :
uniform data distribution
with '
n .
partitions
the
'
is tuple is assigned to partition
(i mad n)
4
Composite Partitioning :
combination af above
partitioning schemes
flashing t List consistent Hashing
Hr
Hash reduces the
key space to a
size that can be listed .
# Common Problems
of Shouting :
Iharded DB :
Entree constraints on the diff .
operations
Hr
operations -
across multiple tables or
multiple rains in the same table 7
no
longer running
in
single severe.

" Jains A Denoumalizatiom :
Jains on tables on
single sauce straightforward.
* not feasible to
perform joins on shrouded tables
↳ Less efficient C data needs to be compiled from
multiple servers)
# Workaround Denarmalip the DB
so that the queries that
previously read.
jains can be
performed
from a
single table .
( coins Perils of denavmalizatiom
↳
data inconsistency
2)
Referential integrity
:
Foreign keys om shrouded D8
↳
difficult
*
Mast of the RDBMS does not support foreign keys on
stranded DB .
#
If app
"
-
demands referential integrity om shrouded DB
↳
enforce it in app
"
code C SOL
jobs to
clean up dangling references)

3)
Rebalancing :
Reasons to change sharking scheme :
a) horn -
uniform distribution C data wise )
b) non -
uniform laced balancing C request wise)
Workaround: Y add new DB
2) rebalance
↳
change in partitioning scheme
↳ data monument
{ ↳
downtime
We can use
directory -
based
partitioning
↳
highly complex
↳
single paint of failure
(
lookup service 1 table)

Indexes
Well known because -
of databases .
Improves speed of retrieval
-
Increased storage auerhead
-
Shauna writes
↳ write the data
↳
Update the index
Can be created
using one or more columns
*
Rapid random lookups
&
efficient access of ordered records .
# Data structure
column → Painter to whale raw
→ Create
different views of the -
same data .
↳
very good for filtering /
sorting of large data sets .
↳ no need to create additional copies.
# Used foe datasets ( TB in size) & small
payload ( KB)
I
spied over several
physical devices → We need some
way to find the correct
physical location i. e. Indexes

useful under high load situations
Peonies
-
if we have limitcdcaehing
↳ batches several requests into one
client
Backend
>
>
MY > sauce
>
Peony
client
>
Source
^
a
filters requests
-
log requests
transform
-
add / remain headers
encryption / decryption
frequently compression
used resources request co -
ordination
( request traffic optimization
T
we can also use
←
Collapse same data access
spatial locality request into one.
↳
collapsing requests collapsed forwarding
for data that is
spatially
Ipsf
minimize reads from -
origin .

Queues
Effectively manages requests in
large
-
scale distributed system
→
In small systems
→ writes are
fast .
→
In complex systems
→
high incoming load
4- individual writes take more time
*
To achieve high performance & availability
↳
system needs to be
asynchronous
↳
Queue
#
Synchronous behaviour →
degrades performance
d
ye
Load balancing
difficult for fair &
balanced distribution
server
C
, T,
-
T2
r
C,
-13
T ,
gag { ,
T2
↳
Queue
Cu

# Queues :
asynchronous communication protocol
↳ client sends task
↳
gets ACK
from queue lecccipt)
I
serves as
reference
for the results in
future
[ client continues its work .
# Limit on the
sispafeeguest
& number of requests in
queue
# Queue :
Provides fault -
tolerance
[
↳
protection from service
outage /
failure
highly robust
[
↳
retry failed service request
Enforces Quality of Service
guarantee
L Does NOT expose clients to outages)
# Queues :
distributed communication
↳
Open source implementations
↳ Rabbitma ,
Zoeoma ,
Active MQ ,
BeanstalkD .

Consistent Hashing
# Distributed Hash Table
index =
hash -
function C
key)
# Suppose we're
designing distributed
caching system
with n cache servers
↳ hash .
function (
key % n )
Drawbacks :
1) NOT
horizontally scalable
↳ addition of new server results in
↳
need to
change all
existing mapping.
( downtime of system)
2) NOT load balanced
l because -
af non -
uniform distribution of data )
1-
Some caches : hat & saturated
Other caches :
idk &
empty
How to tackle about problems ?
Consistent flashing

What is consistent Hashing ?
→
Very useful strategy for distributed caching & outs .
→
minimizes reorganization in
scaling up / dawn.
→
only kin keys needs to be remapped.
k total number of keys
n number of servers
How it works ?
Typical hash function suppose outputs in [ 0 .
2567
In consistent hashing ,
imagine all
of these integers are placed on a
ring .
255 0
254
,
• •
of
253
,
•
2
±
"
& we have 3 servers : A ,
B & C .

1)
Given a list
of servers ,
hash them to integers in the
range.
255 0
C A
B
2)
Map key to a serum :
a) Hash it to
single integer
b) Mane CLK wise until
you find Laura
c)
map key to that server .
255 0
hL key -
1)
of A
"
h (
key - 2)
'
B

Adding a new server
'
.
will result in
morning the
-
key -2
'
to 'D
255 0
hL key -
1)
C A
"
a
D
h (
key - 2)
'
B
Removing server IA
'
,
will result in
morning the
-
key-1
'
to II
255 O
h ( key -
1)
①
a
D
h (
key - 2)
'
B •

Consider real world scenario
data →
randomly distributed
↳ unbalanced cactus .
How to handle this issue ?
Virtual Replicas
Instead -
of mapping each node to a
single paint
we
map it to multiply paints .
↳ ( more number of replicas
↳ more
equal distribution
↳
good load
balancing)
255 0
D C
•
h ( key -
1)
A
a-
C
B
B
C
AD
D
h [ Key -
2)
^
A
B

Long -
Palling vs tikbsoekrts us Serves -
Sent Events
'
↳
Client -
Senior Communication Protocols
# HTTP Protocol :
request
NJ prepare Remorse
client
>
Serum
<
.
Response
# AJAX Patting :
Clients repeatedly palls servers for data
similar to HTTP protocol
↳
requests sent to screen at regular intervals (0.5sec
)
Drawbacks :

Client keeps asking the source now data
↳ Lot
of
uspomsgy.au
'
empty
'
↳ HTTP Overhead.
.
Request .
)
4
Response
Request.
>
Client a
response
sooner

.
Request >
<
Response

# HTTP Long Patting :
•
Hanging GET '
Sauce does NOT send empty response .
Pushes response to clients only when new data is available
☐
Client makes HTTP
Request 4- waits for the response .
2) Server delays response until update is available
or until time-out occurs.
3) When update → server sends full response.
4) Client sends now
long-
poll request
a)
immediately after receiving response
d)
after a pause to allow acceptable latency period
5) Each request has timeout.
Client needs to reconnect periodically due to timeouts
LP Request .
7
<
My
full Response
LP
Request.
>
Client <
.
full Response
Showa
LP Request >
a
full Response

Wet Sockets
→
duplex communication channel over
single TCP connection .
→ Provides
'
persistent communication
'
( client a serum can send data at anytime]
→
bidirectional communication in always open channel .
pep
socket
Handshake
Request
Handshake Success
Response
<
-
:
is i.
Client <
'
.
Source
.
Communication
}
channel
→ Lower overheads
→
Real time data transfer

Server -
sent Events ( SSE)
client establishes persistent & long-
term connection with sauna
server uses this connection to send data to client
* *
If client wants to send data to server
↳
Requires another technology / protocol .
data request
using regular HTTP
v
'
ing
=
i.
Client < .
Source
•
Always -
open unidirectional
<
communication
< .
channel
-
responses whenever now data available
→
best when we need real -
time data from soever to client
OR server is
generating data in a loop &
will be sending multiple events to the client .

System design handwritten notes guidance

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