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Service Primitives for Internet Scale Applications
Amr Awadallah, Armando Fox, Ben Ling
Computer Systems LabStanford University
2 Service Primitives for Interactive Internet-Scale Applications. Amr Awadallah, Armando Fox, Ben Ling. ROC, Sep 2002.
Interactive Internet-Scale Application?
Millions of users. Global LB
Local LB
PresentationServers + $
LB
Application Servers + $
Fail over
State Replica
Local LB
PresentationServers + $
PresentationServers + $
Application Servers + $
Application Servers + $
Data Center
State
PS + $
LB
AS + $
Fail over
Local LB
State
PS + $
LB
AS + $
Fail over
3 Service Primitives for Interactive Internet-Scale Applications. Amr Awadallah, Armando Fox, Ben Ling. ROC, Sep 2002.
Motivation
A general framework to describe IIA’s and characterize the functional properties that can be traded away to improve the following operational metrics: Throughput (how many user requests/sec?)
Interactivity (latency, how fast user requests finish?)
Availability (% of time user perceives service as up), including fast recovery to improve availability
TCO (Total Cost of Ownership)
In particular, enumerate architectural primitives that expose partial degradation of functional properties and illustrate how they can be built with “commodity” HW.
4 Service Primitives for Interactive Internet-Scale Applications. Amr Awadallah, Armando Fox, Ben Ling. ROC, Sep 2002.
Recall ACID
Atomicity: For a transaction involving two or more discrete pieces of information, either all pieces changed are committed or none.
Consistency: A transaction creates a new valid state obeying all user integrity constraints.
Isolation: Changes from non-committed transactions remains hidden from all other concurrent transactions (Serializable, Repeatable-R, Commited-R, Uncommit-R)
Durability: Committed data survives beyond system restarts and storage failures.
5 Service Primitives for Interactive Internet-Scale Applications. Amr Awadallah, Armando Fox, Ben Ling. ROC, Sep 2002.
ACID is too much for Internet scale
Yahoo UDB: tens of thousands of reads/sec, up to 10k writes/sec
Geoplexing used for both disaster recovery and scalability, but eager replication (strong consistency) across replicas scales poorly If total DB size grows with # nodes, deadlock rate
increases at the same rate as number of nodes
If DB size grows sublinearly, deadlock rate increases as cube of number of nodes
Even if we could use transactional DB’s and eager replication, cost would be too high
6 Service Primitives for Interactive Internet-Scale Applications. Amr Awadallah, Armando Fox, Ben Ling. ROC, Sep 2002.
The New Properties
Durability (State): Hard, Soft, Stateless
Consistency: Strong, Eventual, Weak, NonC
Completeness: Full, Incomp-R, Lossy-W
Visibility: User, Entity, World
7 Service Primitives for Interactive Internet-Scale Applications. Amr Awadallah, Armando Fox, Ben Ling. ROC, Sep 2002.
Durability (Hard, Soft, Stateless)
Hard: This is permanent state in the original sense of the D in ACID.
Soft: This is temporary storage in the RAM sense, i.e. if power fails then data is lost. This is cheaper and acceptable if user can rebuild state quickly.
Stateless: No need to store state on behalf of the user.
8 Service Primitives for Interactive Internet-Scale Applications. Amr Awadallah, Armando Fox, Ben Ling. ROC, Sep 2002.
Consistency (Strong, Eventual, Weak)
Eventual: after a write, there is some time t after which all reads see the new value. (eg caching)
Strong: in addition, before time t, no reads see the new value (single-copy ACID consistency)
Weak: This is weak consistency in the TACT sense - captures ordering inaccuracies, or persistent staleness.
9 Service Primitives for Interactive Internet-Scale Applications. Amr Awadallah, Armando Fox, Ben Ling. ROC, Sep 2002.
Completeness (Full, Incomp, Lossy)
Complete: all updates either succeed, or fail synchronously. All queries return 100% accurate data.
Incomplete Queries: This is aggregated lossy reads over partitioned state, or state sampling. The best example here is Inktomi’s distributed search where its ok that some partitions not return results under load.
Lossy Updates: This means that its ok for some commited writes to not make it. Example: Lossy Counters and online polls.
10 Service Primitives for Interactive Internet-Scale Applications. Amr Awadallah, Armando Fox, Ben Ling. ROC, Sep 2002.
Visibility (World, Entity, User)
World: The state and changes to it are visible to all the world, e.g. listing a product on eBay.
Entity: State is only visible to a group of users, or within a specific subset of the data (e.g. eBay Jewlery)
User: The state and changes to it are only visible to the user interacting with it, e.g. the MyYahoo user profile. This could be simpler to implement using ReadMyWrites techniques.
11 Service Primitives for Interactive Internet-Scale Applications. Amr Awadallah, Armando Fox, Ben Ling. ROC, Sep 2002.
Architectural Primitives
Primitives Trades Gains
Caching, Replication Eventual Consistency
Interactiveness, Availability, Throughput
Partitioning Entity Visibility Interactiveness, Graceful Degradation
Lossy/Sampled Aggregation
Weak Consistency
Interactiveness, Graceful Degradation
12 Service Primitives for Interactive Internet-Scale Applications. Amr Awadallah, Armando Fox, Ben Ling. ROC, Sep 2002.
Examples of Primitives
LossyUpdate(key,newVal)
LossyAccumulator(key, updateOp) - for commutative ops
LossyAggregate(searchKeys) - lossy search of an index
13 Service Primitives for Interactive Internet-Scale Applications. Amr Awadallah, Armando Fox, Ben Ling. ROC, Sep 2002.
LossyUpdate implementation
LossyUpdate Steve Gribble’s DHT: atomic ops, single-copy consistency; during
failure recovery, reads are slower and writes are refused
If update occurs while updated partition is recovering => fail
Otherwise, update is persistent
When is this useful?
LossyAccumulator (for hit counter, online poll, etc) Every period T, in-memory sub-accumulators from worker nodes
are swept to persistent copy
At the same time, current value of master accumulator is read by each worker node, to serve reads locally
Worker nodes don’t backup in-memory copy => fast restart
Can bound loss rate of accumulator and inconsistency in read
14 Service Primitives for Interactive Internet-Scale Applications. Amr Awadallah, Armando Fox, Ben Ling. ROC, Sep 2002.
What is given up
What is given up Strict consistency of read copies of accumulator
Precision of accumulator value (lost updates)
What is gained: fast recovery for each node, continuous operation despite transient per-node failures
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