Do Leases Purchase Us Anything?
Consider the atomic storage problem, which is a simpler work than consensus. CAP theorem says that when in that place is a partition, yous volition require to sacrifice strong-consistency or high-availability.
Using leases, it is possible to sacrifice availability for sometime (until lease expires), in addition to reconfigure the storage organisation (often past times shrinking the quorum) to transcend on providing availability. Consistency is preserved throughout, in addition to availability is sacrificed solely during lease expiration time. This is a expert tradeoff. (I am going to convey upwards the enquiry of whether this may attention circumvent CAP at the halt of the post.)
But is this mightiness unique to leases? Is in that place a manner to explicate this inwards an alternate manner using solely failure detectors instead of leases? This is the enquiry I desire to explore here.
Many distributed systems implement leases amongst but countdown timers in addition to without NTP timestamps. This is because inwards the short-term the charge per unit of measurement of clocks at processes don't drift every bit good much.
So maybe, nosotros tin copy a lease expiration past times a node suspecting itself every bit failed. If the other nodes have got noesis of the timeout on the failure detector of this expired node, they tin facial expression that fourth dimension out, in addition to initiatory of all reconfiguration of the storage organisation after that. While a failure detector requires solely unilateral decision, this explanation requires that other nodes know almost the jump on the failure detector at the expired node. Let's meet if nosotros tin produce without that requirement.
For reconfiguration, 2 options are possible. One is decentralized reconfiguration implemented over the nodes themselves, in addition to the other is past times using a reconfiguration box (often implemented past times Paxos) every bit the arbiter.
An instance of the onetime is the chain replication. Consider that the tail node is partitioned. The master copy chain cannot consummate serving writes anymore, because the tail is partitioned. The tail tin notwithstanding serve reads though for the clients that contact it straight for to a greater extent than or less time.
Here the reconfiguration box tin reconfigure the chain to take away the partitioned tail. To explicate this amongst failure detector terminology, let's tell the reconfiguration box has its failure detector itself. The failure detector suspects the tail, in addition to passes a reconfiguration amongst a higher epoch in addition to takes the tail off. But earlier reconfiguring the chain to take away the master copy tail, the reconfiguration box should brand certain that the tail stops serving reads to client. How tin this hold upwards accomplished? The reconfiguration message volition non rank the partitioned old tail. So the tail should know almost the reconfiguration box's failure detector timeout duration. Without this knowledge, without tying the reconfiguration server's failure detector almost the tail to the tail's failure detector almost itself, nosotros wouldn't know when it is prophylactic to switch from the old configuration in addition to initiatory of all the novel configuration. (The alternative is that the tail checks amongst the reconfiguration box for each operation, in addition to then it confirms its condition inwards the configuration. Even amongst this one, due to asymmetric message delay, the reconfiguration box may require to facial expression to a greater extent than or less duration earlier reconfiguring.)
In price of trading off availability to teach availability, it seems similar leases provides to a greater extent than data than a unilateral failure detector in addition to tin purchase yous consistency inwards the presence of a partitioned node. This comes amongst the loss of to a greater extent than or less availability because reconfiguration for quorum shrinking needs to facial expression the lease fourth dimension to expire.
Leases also render an payoff for reconfiguration inwards the presence of partitions. Leases sacrifice availability to restore availability (while preserving safety) for the storage system. This functionality requires to a greater extent than than the unilateral conclusion taken past times failure detectors, but rather a bilateral data on the expiration of the timeouts.
CAP defines availability as "Each asking for read or write eventually receives a response." Even amongst leases in addition to reconfiguration, in that place volition hold upwards a asking that does non have a response. In my example, the old tail volition non answer to read asking after the expiration of the lease. But since at that indicate the old tail is non constituent of the organisation anymore, why does that count against the availability of the system? But the formulation of CAP is every bit good strict in addition to defines the organisation every bit the initial laid upwards of nodes inwards the system. That formulation prohibits whatsoever reconfiguration of the organisation fifty-fifty when in that place are no partitions.
I intend nosotros require to a greater extent than refined versions of CAP. It has a really fossil oil granularity formulation.
Using leases, it is possible to sacrifice availability for sometime (until lease expires), in addition to reconfigure the storage organisation (often past times shrinking the quorum) to transcend on providing availability. Consistency is preserved throughout, in addition to availability is sacrificed solely during lease expiration time. This is a expert tradeoff. (I am going to convey upwards the enquiry of whether this may attention circumvent CAP at the halt of the post.)
But is this mightiness unique to leases? Is in that place a manner to explicate this inwards an alternate manner using solely failure detectors instead of leases? This is the enquiry I desire to explore here.
Many distributed systems implement leases amongst but countdown timers in addition to without NTP timestamps. This is because inwards the short-term the charge per unit of measurement of clocks at processes don't drift every bit good much.
So maybe, nosotros tin copy a lease expiration past times a node suspecting itself every bit failed. If the other nodes have got noesis of the timeout on the failure detector of this expired node, they tin facial expression that fourth dimension out, in addition to initiatory of all reconfiguration of the storage organisation after that. While a failure detector requires solely unilateral decision, this explanation requires that other nodes know almost the jump on the failure detector at the expired node. Let's meet if nosotros tin produce without that requirement.
For reconfiguration, 2 options are possible. One is decentralized reconfiguration implemented over the nodes themselves, in addition to the other is past times using a reconfiguration box (often implemented past times Paxos) every bit the arbiter.
An instance of the onetime is the chain replication. Consider that the tail node is partitioned. The master copy chain cannot consummate serving writes anymore, because the tail is partitioned. The tail tin notwithstanding serve reads though for the clients that contact it straight for to a greater extent than or less time.
Here the reconfiguration box tin reconfigure the chain to take away the partitioned tail. To explicate this amongst failure detector terminology, let's tell the reconfiguration box has its failure detector itself. The failure detector suspects the tail, in addition to passes a reconfiguration amongst a higher epoch in addition to takes the tail off. But earlier reconfiguring the chain to take away the master copy tail, the reconfiguration box should brand certain that the tail stops serving reads to client. How tin this hold upwards accomplished? The reconfiguration message volition non rank the partitioned old tail. So the tail should know almost the reconfiguration box's failure detector timeout duration. Without this knowledge, without tying the reconfiguration server's failure detector almost the tail to the tail's failure detector almost itself, nosotros wouldn't know when it is prophylactic to switch from the old configuration in addition to initiatory of all the novel configuration. (The alternative is that the tail checks amongst the reconfiguration box for each operation, in addition to then it confirms its condition inwards the configuration. Even amongst this one, due to asymmetric message delay, the reconfiguration box may require to facial expression to a greater extent than or less duration earlier reconfiguring.)
Leases purchase us fourth dimension in addition to optionality
Using leases, a node does non have got to banking concern agree its condition inwards a configuration for each operation. Provided that the lease holds, the nodes condition inwards the configuration is unchanged. Using leases on the acceptors, a Paxos leader tin serve reads locally, without checking amongst a quorum. And using leases, a tail inwards chain replication tin serve reads without checking if it is notwithstanding the tail. This translates to efficiency because checking your condition inwards the configuration is non done for each operation, but rather batched in addition to done in 1 lawsuit for each lease renewal.In price of trading off availability to teach availability, it seems similar leases provides to a greater extent than data than a unilateral failure detector in addition to tin purchase yous consistency inwards the presence of a partitioned node. This comes amongst the loss of to a greater extent than or less availability because reconfiguration for quorum shrinking needs to facial expression the lease fourth dimension to expire.
Leases also render an payoff for reconfiguration inwards the presence of partitions. Leases sacrifice availability to restore availability (while preserving safety) for the storage system. This functionality requires to a greater extent than than the unilateral conclusion taken past times failure detectors, but rather a bilateral data on the expiration of the timeouts.
MAD questions
1. Did nosotros circumvent the CAP result?CAP defines availability as "Each asking for read or write eventually receives a response." Even amongst leases in addition to reconfiguration, in that place volition hold upwards a asking that does non have a response. In my example, the old tail volition non answer to read asking after the expiration of the lease. But since at that indicate the old tail is non constituent of the organisation anymore, why does that count against the availability of the system? But the formulation of CAP is every bit good strict in addition to defines the organisation every bit the initial laid upwards of nodes inwards the system. That formulation prohibits whatsoever reconfiguration of the organisation fifty-fifty when in that place are no partitions.
I intend nosotros require to a greater extent than refined versions of CAP. It has a really fossil oil granularity formulation.
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