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Consistency-Based Service Level Agreements
for Cloud Storage
Douglas B. Terry, Vijayan Prabhakaran, Ramakrishna Kotla, Mahesh Balakrishnan, Marcos K. Aguilera, Hussam Abu-Libdeh
Microsoft Research
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“A foolish consistency is the hobgoblin of little minds”
-- Ralph Waldo Emerson (1841)
“… and of large clouds”-- Douglas Brian Terry (2013)
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Today’s Cloud Storage Providers
• Replicate data widely• Offer choice of strong or eventual
consistencye.g. Amazon DynamoDB, Yahoo PNUTS, Google App Engine, Oracle NoSQL, Cassandra, … Microsoft Windows Azure
• Tradeoff consistency, availability and performance
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Problem
• Developers must choose consistency • No single choice is best for all clients and situations
Client
ConsistencyU.S.
(secondary)England (primary)
India (secondary)
China (client only)
strong 147.5 1.2 435.5 307.23
eventual 1.1 1.0 1.1 160.2
Shopping cart: Want read in
under 300 ms.
roundtrip times in milliseconds
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Pileus key features
•Replicated, partitioned key-value store
•Choice of consistency
•Consistency-based service level agreements (SLAs)
a cap cloud
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Pileus System Model
primary core
secondary nodes
Get(key, SLA)
Put(key, value)
Get(key, SLA)
syncreplication
lazyreplication
Get(key, SLA)
API
BeginSession (SLA)BeginTx (SLA)
Put (key, value)Get (key, SLA) returns value, consistency
EndTx ()EndSession ()
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Read Consistency Guarantees
Strong Consistency Return value of latest Put.
Causal Consistency Return value of latest causally preceding Put.
Bounded Staleness (t) Return value that is stale by at most t seconds.
Read My Writes Return value of latest Put in client session or a later value.
Monotonic Reads Return same or later value as earlier Get in client session.
Eventual Consistency Return value of any Put.
[COPS 2011]
[TACT 2002]
[Bayou 1994]
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Read LatenciesClient/Consistency U.S.
(secondary)England
(primary)India
(secondary)China
(client only)
strong 147.5 1.2 435.5 307.3
causal 146.3 1.0 431.6 306.4
bounded(30) 75.1 1.0 234.6 241.9
read-my-writes 13.0 1.1 18.4 166.8
monotonic 1.1 1.0 1.1 160.2
eventual 1.1 1.0 1.1 160.2
roundtrip times in milliseconds
consistency affects latency
client location affects latency
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Consistency-based SLA
• Applications declare desired consistency/latency
strong 300 ms.consistency latency
read my writes 300 ms.
1.
2.
1.0utility
0.5
eventual 300 ms.3. 0.1
Shopping Cart:
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SLA Enforcement: Client Monitoring
Node Primary? RTTs High Timestamp
A yes 210
B no 166
C no 203
For each tablet:
from configuration
service
measured on Gets, Puts, and pings
returned from Gets, Puts, and
pings
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SLA Enforcement: Node Selection
1. For each subSLA and node, a. compute Platency
b. compute Pconsistency
c. compute Platency x Pconsistency x utility2. Select node with maximum expected utility3. Send Get operation to node4. Measure RTT and update records5. Return data and delivered consistency to caller
On Get (key, SLA):
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Experimental SetupSystem configuration:
Primary: EnglandSecondaries: U.S., IndiaClients: U.S., England, India, China
Benchmark:• YCSB with 50/50 Gets/Puts• 500-op sessions
Node selection schemes:• Primary = get from primary• Random = get from random node• Closest = get from closest node• Pileus = get from node with highest
expected utility
Measurement: • Average utility for Get operations
U.S.England
China
India
149
287436
161
308
181
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Experiment #1: SLA
Simplified shopping cart SLA:
consistency latency
read my writes 300 ms.1.
2.
utility
1.0
eventual 300 ms. 0.5
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Experiment #1: Delivered UtilityAv
erag
e uti
lity
per G
et
Client datacenter
(secondary) (secondary)(primary) (client only)
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Experiment #1: Delivered UtilityAv
erag
e uti
lity
per G
et
Client datacenter
(secondary) (secondary)(primary) (client only)
Primary selection works well when close to the primary,
but poorly when distant
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Experiment #1: Delivered UtilityAv
erag
e uti
lity
per G
et
Client datacenter
(secondary) (secondary)(primary) (client only)
Random selection rarely works well
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Experiment #1: Delivered UtilityAv
erag
e uti
lity
per G
et
Client datacenter
(secondary) (secondary)(primary) (client only)
100% Gets from England;
100% meet top subSLA
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Aver
age
utilit
ype
r Get
Experiment #1: Delivered Utility
Client datacenter
(secondary) (secondary)(primary) (client only)
91% from U.S.;9% from England;100% meets top
subSLA
14.5 ms. avg. latency
vs. 148 ms. for primary
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Experiment #1: Delivered UtilityAv
erag
e uti
lity
per G
et
Client datacenter
(secondary) (secondary)(primary) (client only)
99.6% from U.S.;0.4% from India; 96% meets top
subSLA
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(secondary) (secondary)(primary) (client only)
Client datacenter
Experiment #1: Delivered Utility
Pileus always delivers the most utility!
Aver
age
utilit
ype
r Get
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Experiment #1: Delivered UtilityAv
erag
e uti
lity
per G
et
Client datacenter
(secondary) (secondary)(primary) (client only)
9% fail to meet read-my-write
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Experiment #2: SLA
Password checking SLA:
consistency latency
strong 150 ms.1.
2.
utility
1.0
eventual 150 ms. 0.5
3. strong 1000 ms. 0.25
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Experiment #2: Delivered UtilityAv
erag
e uti
lity
per G
et
Client datacenter
(secondary) (secondary)(primary) (client only)
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Conclusions: Main Contributions
Our Pileus system• provides a broad choice of consistency guarantees and
range of delivered read latency• allows declarative specification of desired consistency
and latency through consistency-based SLAs• selects nodes to maximize expected utility while
adapting to varying conditions