Paper Summary: Coordination Avoidance Inward Database Systems
Serializing transactions is sufficient for correctness, but it is non necessary for all operations of all applications. The downside of serialization is that it kills scalability in addition to is overkill inward many cases.
This newspaper (which volition seem inward VLDB'15) has the next insight: Given noesis of application transactions in addition to correctness criteria (i.e., invariants), it is possible to avoid this over-coordination of serializability in addition to execute approximately transactions without coordination acre all the same preserving those correctness criteria (invariants).
In item the authors suggest the concept of "invariant confluence" to relax the role of serialization for approximately operations of a coordination-requiring application. By operating on application-level invariants over database states (e.g., integrity constraints), the invariant confluence analysis provides a necessary in addition to sufficient status for safe, coordination-free execution. When programmers specify application invariants, this analysis allows databases to coordinate entirely when concurrency may violate those application invariants.
So how produce they larn the application invariants? "Many production databases today already back upwardly invariants inward the shape of primary key, uniqueness, unusual key, in addition to row-level cheque constraints. We analyze this in addition to exhibit that many are invariant-confluent, including forms of unusual fundamental constraints unique value generation, in addition to cheque constraints, acre others, similar primary fundamental constraints are, inward general, not."
They claim that many mutual integrity constraints flora inward SQL in addition to standardized benchmarks are invariant confluent, allowing order-of-magnitude performance gains over coordinated execution. To substantiate this claim, they apply invariant confluence analysis to a database image in addition to exhibit 25-fold improvement over prior TPC-C New-Order performance on a 200 server cluster. They observe that 10 out of 12 of TPC-C's invariants are invariant-confluent, nether the workload transaction.
They model transactions to operate over independent logical snapshots of database state. Transaction writes are applied at 1 or to a greater extent than snapshots initially when the transaction commits in addition to are so integrated into other snapshots asynchronously via a merge operator that incorporates those changes into the snapshot's state. "Merge" is merely the laid spousal human relationship of versions, in addition to is used to capture the procedure of reconciling divergent states.
In effect, this model states that each transaction tin modify its replica solid set down without modifying whatever other concurrently executing transactions' replica state. Replicas thence render transactions amongst partial snapshot views of global state. They define local validity/consistency equally a security property, but global replica consistency is non defined equally a security property. Instead it is defined equally a liveness belongings nether the refer "convergence".
(Formal Definition of invariant-confluent:)
A laid of transactions T is I-confluent amongst honour to invariant I if, for all I-T-reachable states Di, Dj amongst a mutual ancestor state, Di spousal human relationship Dj is I-valid.
Table iii summarizes the 12 invariants flora inward TPC-C benchmark equally good equally their I-confluence analysis results equally determined past times Table 2. They assort the invariants into iii wide categories: materialized sentiment maintenance, unusual fundamental constraint maintenance, in addition to unique ID assignment.
Figure v shows the concurrency/throughput improvement made possible past times applying the invariant-confluence analysis to the TPC-C workload.
The "Scalable commutatitivity rule" paper, which was 1 of the best papers inward SOSP'13, is a closely related work. In gild to relax serializability in addition to boost concurrency, that operate prescribes exploiting the commutativity of operations. Another related operate that exploits commutativity to relax serializability is the "Making Geo-Replicated Systems Fast equally Possible, Consistent when Necessary" paper. The invariant confluence analysis concept is to a greater extent than full general (but in all probability harder to apply) than the commutativity dominion approach, because acre commutativity is sufficient for correctness it is non ever necessary.
In our previous work, the slow-fast newspaper (2010), nosotros had likewise used the concept of "invariant-relaxed serializability" inward distributed systems domain, peculiarly inward application to wireless sensor/actor network concurrency control. (Maybe somewhat of a misnomer nosotros called a tedious activity equally 1 which tin endure executed inward a concurrent/uncoordinated/nonatomic manner, in addition to a fast activity equally 1 which needs to endure executed inward an atomic/coordinated/no-conflicts manner.)
I suspect our slow-fast approach used a less aggressive optimization than invariant-confluence: Slow-fast did non require/inspect computer program invariants explicitly, it entirely required access to the computer program actions (i.e., transactions). The slow-fast approach inferred that the invariant holds when computer program actions (transactions) execute atomically. (Invariant-confluence may potentially role fifty-fifty a weaker invariant than what slow-fast used.)
Our slow-fast analysis inspects the computer program actions, which are precondition guarded assignment statements, in addition to determined for which actions the atomicity tin endure relaxed so that they tin execute inward an uncoordinated manner. Our finding was that if the precondition of a computer program activity is "locally-stable" (i.e., this precondition predicate cannot larn falsified past times execution of other computer program actions), so it is rubber to execute this computer program activity inward an uncoordinated/nonatomic manner. (This cheque in all probability implies "mergeability" of the state.) Our analysis likewise prescribed ways to intermission a coordination requiring activity into 2 smaller actions to larn inward coordination free.
The "coordination avoidance inward databases" newspaper applies the "invariant-relaxed serializability" thought inward a to a greater extent than restricted in addition to to a greater extent than useful domain, database transactions, in addition to demonstrates the thought inward a rattling practical way.
This newspaper (which volition seem inward VLDB'15) has the next insight: Given noesis of application transactions in addition to correctness criteria (i.e., invariants), it is possible to avoid this over-coordination of serializability in addition to execute approximately transactions without coordination acre all the same preserving those correctness criteria (invariants).
In item the authors suggest the concept of "invariant confluence" to relax the role of serialization for approximately operations of a coordination-requiring application. By operating on application-level invariants over database states (e.g., integrity constraints), the invariant confluence analysis provides a necessary in addition to sufficient status for safe, coordination-free execution. When programmers specify application invariants, this analysis allows databases to coordinate entirely when concurrency may violate those application invariants.
So how produce they larn the application invariants? "Many production databases today already back upwardly invariants inward the shape of primary key, uniqueness, unusual key, in addition to row-level cheque constraints. We analyze this in addition to exhibit that many are invariant-confluent, including forms of unusual fundamental constraints unique value generation, in addition to cheque constraints, acre others, similar primary fundamental constraints are, inward general, not."
They claim that many mutual integrity constraints flora inward SQL in addition to standardized benchmarks are invariant confluent, allowing order-of-magnitude performance gains over coordinated execution. To substantiate this claim, they apply invariant confluence analysis to a database image in addition to exhibit 25-fold improvement over prior TPC-C New-Order performance on a 200 server cluster. They observe that 10 out of 12 of TPC-C's invariants are invariant-confluent, nether the workload transaction.
The invariant-confluence model
Invariant-confluence captures a uncomplicated (informal) rule: coordination tin endure avoided if in addition to entirely if all local commit decisions are globally valid. (In other words, the commit decisions are composable.)They model transactions to operate over independent logical snapshots of database state. Transaction writes are applied at 1 or to a greater extent than snapshots initially when the transaction commits in addition to are so integrated into other snapshots asynchronously via a merge operator that incorporates those changes into the snapshot's state. "Merge" is merely the laid spousal human relationship of versions, in addition to is used to capture the procedure of reconciling divergent states.
In effect, this model states that each transaction tin modify its replica solid set down without modifying whatever other concurrently executing transactions' replica state. Replicas thence render transactions amongst partial snapshot views of global state. They define local validity/consistency equally a security property, but global replica consistency is non defined equally a security property. Instead it is defined equally a liveness belongings nether the refer "convergence".
(Formal Definition of invariant-confluent:)
A laid of transactions T is I-confluent amongst honour to invariant I if, for all I-T-reachable states Di, Dj amongst a mutual ancestor state, Di spousal human relationship Dj is I-valid.
Applying the invariant-confluence concept
As the Definition implies, I-confluence holds for specific combinations of invariants in addition to transactions. Removing a user from the database is I-confluent amongst honour to the invariant that the user IDs are unique. However, 2 transactions that take 2 dissimilar users from the database are non I-confluent amongst honour to the invariant that at that spot exists at to the lowest degree 1 user inward the database at all times. As approximately other example, uniqueness is non I-confluent for inserts of unique values. However, reads in addition to deletions are both I-confluent nether uniqueness invariants: reading in addition to removing items cannot innovate duplicates.Table iii summarizes the 12 invariants flora inward TPC-C benchmark equally good equally their I-confluence analysis results equally determined past times Table 2. They assort the invariants into iii wide categories: materialized sentiment maintenance, unusual fundamental constraint maintenance, in addition to unique ID assignment.
Figure v shows the concurrency/throughput improvement made possible past times applying the invariant-confluence analysis to the TPC-C workload.
Related work
This newspaper is an extension of the "CALM" approach that uses monotonicity in addition to convergence concepts to relax the coordination needs of applications.The "Scalable commutatitivity rule" paper, which was 1 of the best papers inward SOSP'13, is a closely related work. In gild to relax serializability in addition to boost concurrency, that operate prescribes exploiting the commutativity of operations. Another related operate that exploits commutativity to relax serializability is the "Making Geo-Replicated Systems Fast equally Possible, Consistent when Necessary" paper. The invariant confluence analysis concept is to a greater extent than full general (but in all probability harder to apply) than the commutativity dominion approach, because acre commutativity is sufficient for correctness it is non ever necessary.
In our previous work, the slow-fast newspaper (2010), nosotros had likewise used the concept of "invariant-relaxed serializability" inward distributed systems domain, peculiarly inward application to wireless sensor/actor network concurrency control. (Maybe somewhat of a misnomer nosotros called a tedious activity equally 1 which tin endure executed inward a concurrent/uncoordinated/nonatomic manner, in addition to a fast activity equally 1 which needs to endure executed inward an atomic/coordinated/no-conflicts manner.)
I suspect our slow-fast approach used a less aggressive optimization than invariant-confluence: Slow-fast did non require/inspect computer program invariants explicitly, it entirely required access to the computer program actions (i.e., transactions). The slow-fast approach inferred that the invariant holds when computer program actions (transactions) execute atomically. (Invariant-confluence may potentially role fifty-fifty a weaker invariant than what slow-fast used.)
Our slow-fast analysis inspects the computer program actions, which are precondition guarded assignment statements, in addition to determined for which actions the atomicity tin endure relaxed so that they tin execute inward an uncoordinated manner. Our finding was that if the precondition of a computer program activity is "locally-stable" (i.e., this precondition predicate cannot larn falsified past times execution of other computer program actions), so it is rubber to execute this computer program activity inward an uncoordinated/nonatomic manner. (This cheque in all probability implies "mergeability" of the state.) Our analysis likewise prescribed ways to intermission a coordination requiring activity into 2 smaller actions to larn inward coordination free.
The "coordination avoidance inward databases" newspaper applies the "invariant-relaxed serializability" thought inward a to a greater extent than restricted in addition to to a greater extent than useful domain, database transactions, in addition to demonstrates the thought inward a rattling practical way.
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