Transcript slides15

ICS 214B: Transaction Processing and
Distributed Data Management
Lecture 15: Data Replication with Network Partitions
& Transaction Processing Systems
Professor Chen Li
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Network partitions
• Groups of nodes may be isolated or
nodes may be slow in responding
P
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Net
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Network Partitions
• No replicated data
– Quorum-based 3PC
 abort, commit quorums
• Replicated data
– at most one operational group
 coterie
 propagation of updates
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. a
Quorums
. c
b .
. d
C1 = {{a,b,c}, {a,b,d}, {a,c,d}, {b,c,d}}
A1 = {{a,b}, {a,c}, {a,d}, {b,c}, {b,d}, {c,d}}
Important property: XC1  YA1 XYØ
YA1  XC1 XYØ
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• Some quorums can be implemented with
vote assignments
1
. a
To commit  3
To abort  2
. c
b .
1
. d
1
1
• Votes to commit + votes to abort > total votes
• Why?
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Not all quorums can be implemented via
votes
C2 = {{a,b}, {c,d}}
A2 = {{a,c}, {a,d}, {b,c}, {b,d}}
b
a
d
c
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Proof
C2 = {{a,b}, {c,d}}
A2 = {{a,c}, {a,d}, {b,c}, {b,d}}
Suppose it can be implemented via votes
Let each site symbol represent its vote
Let x be the commit threshold
Let y be the abort threshold
Commit:
Abort:
a + b >= x
a + c >= y
y>c+d
x>b+d
c + d >= x
a + d >= y
y>a+b
x>b+c
b + c >= y
x>a+d
b + d >= y
x>a+c
2y > 2(a+b+c+d) >= 2x
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4x > 4(a+b+c+d) >= 4y
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Another Proof
C2 = {{a,b}, {c,d}}
A2 = {{a,c}, {a,d}, {b,c}, {b,d}}
Commit:
a + b >= x
c + d >= x
2 * ()
Abort:
a + c >= y
a + d >= y
b + c >= y
b + d >= y
+ 1 * ()
4(a+b+c+d) >= 4x + 4y
a+b+c+d >= x + y  contradiction!
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Partitions and data replication
Options:
(1) All copies required for updates
(2) Group may update, but at most one (at a time)
(3) Any group may update
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Updates by at most one group
. a
. c
b .
Coterie
. d
C1 = {{a,b,c}, {a,b,d}, {a,c,d}, {b,c,d}}
C2 = {{a,b}, {a,c}, {a,d}, {b,c,d}}
X1= {{a,b}, {c,d}} not valid
Important property:
SC  GC, SGØ
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Reading replicated data . a
. c
b .
. d
C1 = {{a,b,c}, {a,b,d}, {a,c,d}, {b,c,d}}
R1 = {{a,b}, {a,c}, {a,d},{b,c}, {b,d}, {c,d}}
C2 = R2 = {{a,b}, {a,c}, {a,d}, {b,c,d}}
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Reading replicated data - Votes
C1 = {{a,b,c}, {a,b,d}, {a,c,d}, {b,c,d}}
R1 = {{a,b}, {a,c}, {a,d},{b,c}, {b,d}, {c,d}}
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to write get 3 votes (Vw)
to read get 2 votes (VR)
. a
1
. c
b .
1
. d
1
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Reading replicated data - Votes
C2 = R2 = {{a,b}, {a,c}, {a,d}, {b,c,d}}
2
to write get 3 votes (Vw)
to read get 3 votes (VR)
. a
1
. c
b .
1
. d
1
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A Problem
Example: a 3-node system, 1 vote each node,
replicated data
Now:
T1
.a
T1 is committed at a,b.
But after T1 writes data
at b before it writes
data at a, the network
gets repartitioned.
T1 . b
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.c
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Later:
T1
.a
T2 reads at c
(not seeing T1);
.b
.c
then writes and commits
at a, c
Then c doesn’t see T1!
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Solution
• Each node keeps list of committed transactions
• Whenever a new quorum is formed
– Compare sequence numbers to see which node has
the latest updates
– All node apply latest updates
– Then resume processing
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Network Partitions: Summary
• No replicated data
– abort, commit quorums
• Replicated data
– at most one operational group
 coterie
 propagation of updates
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Next:
Transaction Processing Systems
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Basic concepts
• Business Transactions
– Real-world interaction
• On-line Transactions
– execution of program that performs some
function(s) of business transaction
– steps:
 get inputs
 do work (access DB’s)
 produce response
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Example applications
• ATMs
– E.g., 1 trans per minute per ATM
 60 ATMs  1 trans/sec
• Stock exchange
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servers
clients
Transaction Processing System (TPS)
Presentation
manager
Presentation
manager
workflow control
transaction programs
resource managers
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TP Monitor
• The “operating system” for the TPS
• Functions:
– manage processes that run trans. programs
– load balancing
– commit distributed transactions
– route requests from clients to servers
– etc.
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TP monitor architecture
Presentation
server
requests
Network
manager
queues
Workflow
controller
Transaction
server
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….
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net
Transaction
server
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Two-tier
Three-tier
Client
presentation
database
server
workflow
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transaction
servers
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TP monitor = “glue” and “veneer”
TP application programs
TP monitor API veneer
request
routing
user
interface
services
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transactional
communications
operating
system
services
communication
services
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2 phase
commit
...
database
system
services
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TP monitor functions:
presentation
communication clients-servers
workflow management
commit coordination
process management
security
queue management
...
•
•
•
•
•
•
•
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Presentation services
•
•
•
•
•
user interaction
constructing requests
authentication
communication
logging
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User interaction
= Go Airlines =
agent:
Fred Smith
task:
do it
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check-in
reservation
query
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Flight no:
=Check In =
Date
today
Passenger:
Search for passenger
Announcements:
Storm heading our way!!
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Old-fashioned approach for client
Display
menu box
of screen
Wait for
user input
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Code
that does
everything
---------------------------------
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send msg
to server
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Client using TP monitor services:
...
DISPLAY_FORM (CHECK_IN);
...
some logic [no date validation?]
Check in schema:
Flight: ___
Date: ___
Do_check_in schema:
Flight: ___
Date: ___
...
SEND_REQ (DO_CHECK_IN);
...
some logic (what next?)...
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Data validation in forms
• Best done using cached copy of valid values
• OK for static values
e.g.: product codes, flight numbers
• Not OK for dynamic values
e.g.: seats left in plane; account balance
do this validation at server (can access DB)
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Authentication: 2 levels
user
client
server
Do I know/trust
user?
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Do I know/trust
client?
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Process Management
server
db
client
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REQ
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?
code that
does the
work
more
code
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OS Solution
server
Code for all REQ’s
client1
db
...
process
1
db
...
...
Code for all REQ’s
clientX
process
X
 Problem: too many processes…..
does not scale!
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Old fashioned TP solution
server
client1
db
single process:
...
- code for all REQ’s
db
- threading implementation
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todo
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waiting
...
clientX
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 Problem:
– complexity
– need all application code
in one process
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Server processes
server
db
client1
...
...
clientX
workflow/
routing
code for req#1
code for req#2
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db
db
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client1
routing
Can be generalized:
db
code for req#1
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routing
...
...
clientX
code for req#2
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db
db
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Routing
REQ
?
• Consider factors such as load balancing,
failures…
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NEXT: Inter-process communication
router
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do work
server
process
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do work
another
process
(eg DBMS)
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Transactional RPC
• Transaction info (e.g., TRANS ID) is passed
with calls
• Execution of server code becomes part of
transaction
... ... ... ...
client:
start-trans
server
T35
( T35 )
commit
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work
...
call
lock data
for T35
DB
involve all resources for T35
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Example:
credit_card
...
pay_cc
work
333
...
...
T88
...
... ... ... ... ...
T88
start-trans
T88
lock
client:
pay_cc
commit
debit_bank
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T88
...
work
commit changes to
credit_card and accounts
accounts
777
... ... ...
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lock
T88
...
debit_bank
Fault tolerance in RPC
client:
server:
____
____
____
____
____
____
call
timeout: ??
work
NO REPLY
Timeout: retry?
give up?
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Motivation for queues
(a) server unavailable
?
client
server
(b) server loses request
req
client
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server
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Motivation for queues
(c) client down
req
client
server
?
ans
(d) queues also useful for
- load balancing
- request priorities
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client
enqueue
req
req
req
req
server
dequeue
queue
other
clients
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other
servers
(load balancing)
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client
Trans#1:
start_trans:
get inputs
construct req
enqueue (Q1)
commit
Trans#3:
start_trans:
dequeue (Q2)
decode reply
process output
commit
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server
Q1:
request queue
Q2:
reply queue
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Trans#2:
start_trans:
dequeue (Q1);
process req;
enqueue (Q2);
commit
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TP monitors and the web
web
http
browser
web server
daemon
TP monitor
client
TP monitor protocol
TP
system
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Summary of TPS
• Architecture
– Presentation services
– Routing and workflow control
– Process management
• Communication services
– RPC
– Queues
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