Consistency-Based Service Marker Agreements For Cloud Storage
This paper is from Microsoft Research in addition to appeared inwards SOSP'13. This newspaper is to a greater extent than of a seat in addition to vision paper. The newspaper introduces a consistency-based SLA concept, which tin furnish a win-win organisation for cloud providers in addition to developers.
(Actually at that topographic point is simply about other option: You tin purpose a strongly consistent multiversion information shop similar BigTable or Spanner, in addition to relax it past times reading slightly stale information in addition to acquire flexibility. I volition revisit this choice inwards the discussion.)
| Rank | Consistency | Latency | Utility |
| 1. | potent | 150 ms | 1.0 |
| 2. | eventual | 150 ms | 0.5 |
| 3. | potent | 500 ms | 0.25 |
Enter the consistency-based SLA concept. The SLA acts equally an interface betwixt the application developer in addition to the inners of the cloud. The developer provides a wishlist for their acquire (i.e., read) operations from the key-value shop equally above. Here the developer says "I want a reply inwards nether 150 ms in addition to prefer strongly consistent information but volition bring whatever data; if no information tin hold upward obtained speedily thus I am willing to expect upward to 500ms for up-to-date data." The cloud-provider backend is structured such that it keeps rails of which of these reads is viable currently for that place in addition to it satisfies the highest ranked 1 it tin inwards gild to plough over the best utility to the developer.
Using such an SLA makes skillful delineate of piece of work concern sense. With this SLA the developers seat their coin where their oral fissure is. They concord to pay to a greater extent than for amend utility provided to them. The cloud-providers tin purpose the SLAs to prioritize the read requests: they tin plough over to a greater extent than priority to consistency requiring higher paying (higher utility) customers.
To illustrate, Figure iii shows simply about read latencies at a given dot from given locations. The developer does non have got access to all per portion or per client latencies similar this, but inwards the SLA she tin set down her ranked preferences for latency in addition to consistency of the reads she thinks would brand most feel for her application, in addition to through this interface she has access to dynamic tuning of performance of her application.
Some storage nodes are designated equally master copy nodes, which grip the master copy data, piece others are secondary nodes. All Puts (i.e., writes) inwards Pileus are performed in addition to strictly ordered at a master copy site. Secondary nodes eventually have from the master copy amount all the updated objects along amongst their update timestamps. Since all Put operations are assigned increasing update timestamps from the master copy site in addition to the asyncronous replication protocol transfers updated objects inwards timestamp order, at whatever dot inwards time, each secondary node has received a prefix of the overall sequence of Put operations.
When selecting the node to which a Get functioning should hold upward sent, the desired consistency guarantee, along amongst the previous object versions that have got been read or written inwards the electrical current session in addition to the cardinal beingness read, determines the minimum acceptable read timestamp. The minimum acceptable read timestamp indicates how far a secondary node tin lag behind the master copy in addition to soundless furnish an respond to the given Get functioning amongst the desired consistency. This is beingness decided past times the client library of Pileus.
This architecture forces all the writes to hold upward performed on a unmarried master copy amount limits the occupation space, in addition to simplifies things for ensuring consistency for the reads inwards the consistency-spectrum. But this too limits the performance on reads (except for eventual-consistency reads). Moreover, amongst this setup you lot don't acquire to specify latency bounds for writes.
Evaluation results present that consistency-based SLAs tin indeed improve application-specific levels of service (i.e., utility).
A: I am soundless confused close this. It sounds similar this tin hold upward applicable to a large course of written report of applications, but sometimes I revert to thinking possibly non that big.
For latency-favoring (eventual-consistency happy) applications at that topographic point are existing solutions: DynamoDB, in addition to several key-value stores. And the target applications are those that tolerate relaxed consistency but, nevertheless, exercise goodness from improved consistency. It may seem that these are already served to simply about extent past times the eventual-consistent key-value stores. They are simply best effort. You don't know what you lot get, but fresher to a greater extent than consistent information improves service the same equally inwards Pileus. Pileus gives you lot tuned performance, but possibly you lot could have got gotten that performance past times probabilistic way also. (Peter Bailis has a real overnice operate on probabilistically bounded staleness, which is too a related approach here.)
For consistency-favoring applications, at that topographic point are existing solutions similar Bigtable, Spanner. And you lot tin soundless exercise a quick muddy read from Spanner, past times giving a slightly past times read timestamp. This industrial plant because Spanner is a multiversion key-value store. But I gauge you lot soundless require to acquire by when you lot would want to revert to the quick muddy reads.
Q: How does Pileus alter the application code?
A: Yes nosotros larn from API when nosotros acquire dorsum a consistent read in addition to when not, but reacting on the type of reads may Pb to polluting my plan amongst a lot of branches in addition to checks. Maybe programming languages people may have got an respond to that. I guess, this way is soundless amend than monitoring for latencies in addition to implement these tuning inwards your application.
Problem
For performing reads from key-value stores, currently you lot have got 2 options. You tin exercise strongly-consistent reads past times increasing the size of your read replica quorum, but this volition increase latency of the responses, in addition to you lot don't acquire the flexibility to revert to a quick muddy (eventually-consistent) read if a strong-consistent read would bring a long fourth dimension to respond. Or you lot become amongst best endeavour reads (which are eventually-consistent) from the cardinal value shop because you lot insist on low-latency answers.(Actually at that topographic point is simply about other option: You tin purpose a strongly consistent multiversion information shop similar BigTable or Spanner, in addition to relax it past times reading slightly stale information in addition to acquire flexibility. I volition revisit this choice inwards the discussion.)
| Rank | Consistency | Latency | Utility |
| 1. | potent | 150 ms | 1.0 |
| 2. | eventual | 150 ms | 0.5 |
| 3. | potent | 500 ms | 0.25 |
Enter the consistency-based SLA concept. The SLA acts equally an interface betwixt the application developer in addition to the inners of the cloud. The developer provides a wishlist for their acquire (i.e., read) operations from the key-value shop equally above. Here the developer says "I want a reply inwards nether 150 ms in addition to prefer strongly consistent information but volition bring whatever data; if no information tin hold upward obtained speedily thus I am willing to expect upward to 500ms for up-to-date data." The cloud-provider backend is structured such that it keeps rails of which of these reads is viable currently for that place in addition to it satisfies the highest ranked 1 it tin inwards gild to plough over the best utility to the developer.
Using such an SLA makes skillful delineate of piece of work concern sense. With this SLA the developers seat their coin where their oral fissure is. They concord to pay to a greater extent than for amend utility provided to them. The cloud-providers tin purpose the SLAs to prioritize the read requests: they tin plough over to a greater extent than priority to consistency requiring higher paying (higher utility) customers.
To illustrate, Figure iii shows simply about read latencies at a given dot from given locations. The developer does non have got access to all per portion or per client latencies similar this, but inwards the SLA she tin set down her ranked preferences for latency in addition to consistency of the reads she thinks would brand most feel for her application, in addition to through this interface she has access to dynamic tuning of performance of her application.
Pileus Architecture:
To showcase the SLA, the authors developed a replicated key-value shop called Pileus. Pileus is a type of cloud formation, it is a cap cloud. (Get it? A "CAP" cloud.) Pileus dynamically selects which servers to access inwards gild to deliver the best service given the electrical current configuration in addition to scheme conditions.Some storage nodes are designated equally master copy nodes, which grip the master copy data, piece others are secondary nodes. All Puts (i.e., writes) inwards Pileus are performed in addition to strictly ordered at a master copy site. Secondary nodes eventually have from the master copy amount all the updated objects along amongst their update timestamps. Since all Put operations are assigned increasing update timestamps from the master copy site in addition to the asyncronous replication protocol transfers updated objects inwards timestamp order, at whatever dot inwards time, each secondary node has received a prefix of the overall sequence of Put operations.
When selecting the node to which a Get functioning should hold upward sent, the desired consistency guarantee, along amongst the previous object versions that have got been read or written inwards the electrical current session in addition to the cardinal beingness read, determines the minimum acceptable read timestamp. The minimum acceptable read timestamp indicates how far a secondary node tin lag behind the master copy in addition to soundless furnish an respond to the given Get functioning amongst the desired consistency. This is beingness decided past times the client library of Pileus.
This architecture forces all the writes to hold upward performed on a unmarried master copy amount limits the occupation space, in addition to simplifies things for ensuring consistency for the reads inwards the consistency-spectrum. But this too limits the performance on reads (except for eventual-consistency reads). Moreover, amongst this setup you lot don't acquire to specify latency bounds for writes.
Evaluation results present that consistency-based SLAs tin indeed improve application-specific levels of service (i.e., utility).
Discussion
Q: How rich is the course of written report of applications that exercise goodness from this SLA?A: I am soundless confused close this. It sounds similar this tin hold upward applicable to a large course of written report of applications, but sometimes I revert to thinking possibly non that big.
For latency-favoring (eventual-consistency happy) applications at that topographic point are existing solutions: DynamoDB, in addition to several key-value stores. And the target applications are those that tolerate relaxed consistency but, nevertheless, exercise goodness from improved consistency. It may seem that these are already served to simply about extent past times the eventual-consistent key-value stores. They are simply best effort. You don't know what you lot get, but fresher to a greater extent than consistent information improves service the same equally inwards Pileus. Pileus gives you lot tuned performance, but possibly you lot could have got gotten that performance past times probabilistic way also. (Peter Bailis has a real overnice operate on probabilistically bounded staleness, which is too a related approach here.)
For consistency-favoring applications, at that topographic point are existing solutions similar Bigtable, Spanner. And you lot tin soundless exercise a quick muddy read from Spanner, past times giving a slightly past times read timestamp. This industrial plant because Spanner is a multiversion key-value store. But I gauge you lot soundless require to acquire by when you lot would want to revert to the quick muddy reads.
Q: How does Pileus alter the application code?
A: Yes nosotros larn from API when nosotros acquire dorsum a consistent read in addition to when not, but reacting on the type of reads may Pb to polluting my plan amongst a lot of branches in addition to checks. Maybe programming languages people may have got an respond to that. I guess, this way is soundless amend than monitoring for latencies in addition to implement these tuning inwards your application.
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