Spanner: Google's Globally-Distributed Database
The Spanner newspaper past times Google (appeared inwards OSDI'12) is cryptic too difficult to understand. When I get-go read it, I visit I understood the top dog idea, too that the practise goodness of TrueTime was to enable lock-free read-only transactions inwards Spanner. Then, I slow realized things didn't check; it was possible to accomplish lock-free read-only transactions without TrueTime every bit well. I did some other read, too visit for some time, too had a amend agreement of how TrueTime benefits Spanner, too how to improve its shortcomings.
I volition get-go furnish a summary of the Spanner operate (borrowing sentences too figures from the Spanner paper), too hence beak close what TrueTime is genuinely proficient for.
Spanner evolved from a Bigtable-like versioned key-value shop into a temporal multi-version database. Data is stored inwards semi-relational tables, too Spanner provides a SQL-based query linguistic communication too supports general-purpose long-lived transactions (e.g, for study generation —on the gild of minutes). The Spanner squad believes it is amend to convey application programmers bargain amongst functioning problems due to overuse of transactions every bit bottlenecks arise, rather than ever coding roughly the lack of transactions.
TT.now() returns a TTinterval that is guaranteed to incorporate the absolute fourth dimension during which TT.now() was invoked. In other words, TrueTime guarantees that for an invocation tt = TT.now(), tt.earliest < tabs(enow) < tt.latest, where enow is the invocation final result too tabs(enow) is the absolute fourth dimension of final result enow. The instantaneous mistake saltation is denoted every bit ε, which is one-half of the interval’s width.
Google keeps incertitude modest (bounded past times roughly 6ms) past times using multiple modern clock references (GPS too atomic clocks). TrueTime is implemented past times a laid of fourth dimension original machines per datacenter too a fourth dimension slave daemon per machine. The bulk of masters convey GPS receivers amongst dedicated antennas. The remaining masters (Armageddon masters) are equipped amongst atomic clocks.
Every daemon polls a diversity of masters to trim back vulnerability to errors from whatever 1 master. Between synchronizations, a daemon advertises a slow increasing fourth dimension uncertainty. ε is derived from conservatively applied worst-case local clock drift. The daemon's poll interval is currently thirty seconds, too the electrical current applied drift charge per unit of measurement is laid at 200 μ sec/second, which together draw organisation human relationship for the saltation on incertitude at 6ms.
The TrueTime API direct exposes clock uncertainty, too the guarantees on Spanner's timestamps depend on the bounds that the implementation provides. If the incertitude is large, Spanner slows downwardly to hold off out that uncertainty.
A zone has 1 zonemaster too 100 to 1000s of spanservers. Zonemaster assigns information to spanservers; spanserver serves information to clients. Location proxies help clients to locate the spanservers assigned to serve their data. The universe original displays condition information close all the zones for interactive debugging. The placement driver handles automated movement of information across zones on the timescale of minutes.
Each spanserver is responsible for 100 to grand tablets. A tablet implements a pocketbook of the next mappings: (key:string, timestamp:int64) → string. To back upwards replication, each spanserver implements a unmarried Paxos acre machine on top of each tablet.
At every replica that is a leader, each spanserver implements: a lock tabular array (mapping ranges of keys to lock states) to implement concurrency control, too a transaction director to back upwards distributed transactions. If a transaction involves only 1 Paxos grouping (as is the illustration for most transactions), it tin bypass the transaction manager, since the lock tabular array too Paxos together furnish transactionality.
If a transaction involves to a greater extent than than 1 Paxos group, those groups' leaders coordinate to perform 2-phase commit. One of the player groups is chosen every bit the coordinator: the player leader of that grouping volition endure referred to every bit the coordinator leader, too the slaves of that grouping every bit coordinator slaves.
Read-write transactions exercise 2-phase locking too 2-phase commit. First, the customer issues reads to the leader replica of the appropriate group, which acquires read locks too hence reads the most recent data. When a customer has completed all reads too buffered all writes, it starts 2-phase commit. Read-write transactions tin endure assigned commit timestamps past times the coordinator leader at whatever fourth dimension when all locks convey been acquired, but before whatever locks convey been released. For a given transaction, Spanner assigns it the timestamp that the coordinating leader assigns to the Paxos write that represents the transaction commit. To hold off out the incertitude inwards TrueTime, at that topographic point is a Commit Wait: The coordinator leader ensures that clients cannot come across whatever information committed past times Ti until TT.after(si) is true.
Serving reads at a timestamp. Every replica tracks a value called prophylactic fourth dimension tsafe which is the maximum timestamp at which a replica is up-to-date. A replica tin satisfy a read at a timestamp t, if t ≤ tsafe. We define tsafe = min(tPaxos, tTM), where each Paxos acre machine has a prophylactic fourth dimension tPaxos and each transaction director has a prophylactic fourth dimension tTM.
tPaxos is the timestamp of the highest-applied Paxos write. Because timestamps growth monotonically too writes are applied inwards order, writes volition no longer plough over at or below tPaxos with honour to Paxos.
tTM is ∞ at a replica if at that topographic point are nix prepared (but prophylactic non committed) transactions—that is, transactions inwards betwixt the 2 phases of 2-phase commit. Otherwise, for every player grouping g, over all transactions Ti prepared at g, tTM= mini (spreparei,g)-1. In other words, tTM denotes the asking timestamp of the earliest prepared but non committed transaction.
Assigning timestamps to read-only transactions. The uncomplicated assignment of sread = TT.now().latest to a read-only transaction preserves external consistency. However, such a timestamp may require the execution of the information reads at sread to block if tsafe has non advanced sufficiently. To trim back the chances of blocking, Spanner should assign the oldest timestamp that preserves external consistency. (External consistency constraint dictates that y'all cannot exercise an older version of variable to read, too y'all cannot assign a timestamp before than pending read-write transaction on whatever of the variables involved inwards the read-only transaction). Spanner implements a simpler selection when multiple Paxos groups are involved. The customer avoids a negotiation round, too only has its reads execute at sread= TT.now().latest, which may hold off for prophylactic fourth dimension to advance.
tPaxos is also advanced past times heartbeats to help tsafe advance at the replicas. (This does non require high precision clock synchronization, too NTP easily suffices for this.)
After re-reading the newspaper only amongst this query inwards mind, I was left to a greater extent than puzzled. In my get-go read of the paper, I visit TrueTime enabled lock-free reads inwards Spanner. After the 2nd reading, I realized that lock-free reads could endure implemented without TrueTime, only past times using version numbers, because read-only transactions were also serialized past times coordinating leaders too Paxos groups. TrueTime wasn't speeding upwards read-only transactions either. Even using TrueTime, read-only transaction yet needs to hold off to hear/learn the commit information from whatever variable/tablet-overlapping pending/prepared read-write transaction.
Maybe TrueTime benefited past times making Spanner implementation simple, but Section 4.2.4 lists several unavoidable implementation hardships fifty-fifty amongst TrueTime. It looks similar using version numbers wouldn't endure significantly to a greater extent than complicated. Also, for schema alter too paxos leader replacement (which the newspaper claims TrueTime simplified a lot), the NTP synchronization (several tens of ms accuracy) easily suffices. We could convey easily avoided the to a greater extent than precise TrueTime implementation amongst GPS too atomic clocks for these.
But tin nosotros notice whatever clever solutions to circumvent that work without resorting to the high-precision clocks inwards TrueTime?
My unopen friend too frequent-collaborator Sandeep Kulkarni at Michigan State University had proposed HybridClocks in 2001. HybridClocks also exposed clock incertitude ε inwards physical clocks, but it also used logical clocks inwards add-on to the physical clocks to capture the finer-grain causality relationships that falls into the ε grayness area.
Using HybridClocks it tin endure possible to salvage the atomic clock requirement inwards Spanner too exercise NTP instead. Even amongst ε at 100 msecs, nosotros tin yet runway finer-grain dependencies betwixt variables using HybridClocks. The proficient tidings is that the size of the HybridClocks tin endure kept really succinct too tin endure recorded inwards Spanner database every bit timestamp to enable snapshot reads easily.
HybridClocks visit may also help speed upwards Spanner's high-precision clock-based implementation. In Spanner ε determines the charge per unit of measurement of read-write transactions on a tablet. This is because, the coordinating leaders delay read-write transactions to commit amongst at to the lowest degree ε fourth dimension apart inwards gild to ensure that at that topographic point is no incertitude inwards past times snapshot reads. It is possible to avoid this wasteful waiting past times adding some logical clocks information to TrueTime every bit prescribed inwards HybridClocks.
Sandeep too I are currently exploring these ideas.
Beyond TrueTime: Using AugmentedTime for Improving Google Spanner
Hybrid Logical Clocks
I volition get-go furnish a summary of the Spanner operate (borrowing sentences too figures from the Spanner paper), too hence beak close what TrueTime is genuinely proficient for.
In a nutshell
Spanner is Google's scalable, multi-version, globally-distributed, too synchronously-replicated database. Spanner supports non-blocking reads inwards the past, lock-free read-only transactions, too atomic schema changes. In gild to back upwards externally-consistent distributed transactions at global scale, it uses a novel TrueTime API that exposes clock uncertainty.From NoSQL to NewSQL!
The ask to back upwards semi-relational tables too synchronous replication inwards Spanner has been motivated past times the popularity of Megastore. Many applications at Google (e.g., Gmail, Picasa, Calendar, Android Market, too AppEngine) chose to exercise Megastore because of its semi-relational information model too synchronous replication, despite its pitiable write throughput.Spanner evolved from a Bigtable-like versioned key-value shop into a temporal multi-version database. Data is stored inwards semi-relational tables, too Spanner provides a SQL-based query linguistic communication too supports general-purpose long-lived transactions (e.g, for study generation —on the gild of minutes). The Spanner squad believes it is amend to convey application programmers bargain amongst functioning problems due to overuse of transactions every bit bottlenecks arise, rather than ever coding roughly the lack of transactions.
Spanner distributed database
Data is versioned, too each version is automatically timestamped amongst its commit fourth dimension past times the TrueTime API. Spanner provides externally consistent reads too writes, too globally-consistent reads across the database at a timestamp. External consistency (or equivalently, linearizability) is defined every bit follows: if a transaction T1 commits before some other transaction T2 starts, hence T1's commit timestamp is smaller than T2's. Using TrueTime Spanner is able to assign globally-meaningful commit timestamps to transactions, which reverberate the serialization order.2 TrueTime API
Google keeps incertitude modest (bounded past times roughly 6ms) past times using multiple modern clock references (GPS too atomic clocks). TrueTime is implemented past times a laid of fourth dimension original machines per datacenter too a fourth dimension slave daemon per machine. The bulk of masters convey GPS receivers amongst dedicated antennas. The remaining masters (Armageddon masters) are equipped amongst atomic clocks.
Every daemon polls a diversity of masters to trim back vulnerability to errors from whatever 1 master. Between synchronizations, a daemon advertises a slow increasing fourth dimension uncertainty. ε is derived from conservatively applied worst-case local clock drift. The daemon's poll interval is currently thirty seconds, too the electrical current applied drift charge per unit of measurement is laid at 200 μ sec/second, which together draw organisation human relationship for the saltation on incertitude at 6ms.
The TrueTime API direct exposes clock uncertainty, too the guarantees on Spanner's timestamps depend on the bounds that the implementation provides. If the incertitude is large, Spanner slows downwardly to hold off out that uncertainty.
three Spanner Implementation
At every replica that is a leader, each spanserver implements: a lock tabular array (mapping ranges of keys to lock states) to implement concurrency control, too a transaction director to back upwards distributed transactions. If a transaction involves only 1 Paxos grouping (as is the illustration for most transactions), it tin bypass the transaction manager, since the lock tabular array too Paxos together furnish transactionality.
If a transaction involves to a greater extent than than 1 Paxos group, those groups' leaders coordinate to perform 2-phase commit. One of the player groups is chosen every bit the coordinator: the player leader of that grouping volition endure referred to every bit the coordinator leader, too the slaves of that grouping every bit coordinator slaves.
iv Concurrency control
The Spanner implementation supports read-write transactions, read-only transactions, too snapshot reads. Standalone writes are implemented every bit read-write transactions; non-snapshot standalone reads are implemented every bit read-only transactions. A snapshot read is a read inwards the past times that executes without locking.4.1 Read-Write transactions
4.2 Read-only transactions
Reads inwards a read-only transaction execute at a system-chosen timestamp without locking, hence that incoming writes are non blocked. A read-only transaction executes inwards 2 phases: assign a timestamp sread, too hence execute the transaction's reads every bit snapshot reads at sread. The snapshot reads tin execute at whatever replicas that are sufficiently up-to-date.Serving reads at a timestamp. Every replica tracks a value called prophylactic fourth dimension tsafe which is the maximum timestamp at which a replica is up-to-date. A replica tin satisfy a read at a timestamp t, if t ≤ tsafe. We define tsafe = min(tPaxos, tTM), where each Paxos acre machine has a prophylactic fourth dimension tPaxos and each transaction director has a prophylactic fourth dimension tTM.
tPaxos is the timestamp of the highest-applied Paxos write. Because timestamps growth monotonically too writes are applied inwards order, writes volition no longer plough over at or below tPaxos with honour to Paxos.
tTM is ∞ at a replica if at that topographic point are nix prepared (but prophylactic non committed) transactions—that is, transactions inwards betwixt the 2 phases of 2-phase commit. Otherwise, for every player grouping g, over all transactions Ti prepared at g, tTM= mini (spreparei,g)-1. In other words, tTM denotes the asking timestamp of the earliest prepared but non committed transaction.
Assigning timestamps to read-only transactions. The uncomplicated assignment of sread = TT.now().latest to a read-only transaction preserves external consistency. However, such a timestamp may require the execution of the information reads at sread to block if tsafe has non advanced sufficiently. To trim back the chances of blocking, Spanner should assign the oldest timestamp that preserves external consistency. (External consistency constraint dictates that y'all cannot exercise an older version of variable to read, too y'all cannot assign a timestamp before than pending read-write transaction on whatever of the variables involved inwards the read-only transaction). Spanner implements a simpler selection when multiple Paxos groups are involved. The customer avoids a negotiation round, too only has its reads execute at sread= TT.now().latest, which may hold off for prophylactic fourth dimension to advance.
4.3 Refinements
tTM as defined inwards a higher house has a weakness, inwards that a unmarried prepared transaction prevents tsafe from advancing. Such simulated conflicts tin endure removed past times augmenting tTM with a fine-grained mapping from cardinal ranges to prepared-transaction timestamps. When a read arrives, it only needs to endure checked against the fine-grained prophylactic fourth dimension for cardinal ranges amongst which the read conflicts.tPaxos is also advanced past times heartbeats to help tsafe advance at the replicas. (This does non require high precision clock synchronization, too NTP easily suffices for this.)
v TrueTime, what is it proficient for?
The newspaper does non direct hash out what TrueTime buys for Spanner. It says this inwards the conclusions: "One aspect of our pattern stands out: the linchpin of Spanner's characteristic laid is TrueTime. We convey shown that reifying clock incertitude inwards the fourth dimension API makes it possible to ready distributed systems amongst much stronger fourth dimension semantics." What does this hateful exactly? What TrueTime buys Spanner is left unclear inwards the paper.After re-reading the newspaper only amongst this query inwards mind, I was left to a greater extent than puzzled. In my get-go read of the paper, I visit TrueTime enabled lock-free reads inwards Spanner. After the 2nd reading, I realized that lock-free reads could endure implemented without TrueTime, only past times using version numbers, because read-only transactions were also serialized past times coordinating leaders too Paxos groups. TrueTime wasn't speeding upwards read-only transactions either. Even using TrueTime, read-only transaction yet needs to hold off to hear/learn the commit information from whatever variable/tablet-overlapping pending/prepared read-write transaction.
Maybe TrueTime benefited past times making Spanner implementation simple, but Section 4.2.4 lists several unavoidable implementation hardships fifty-fifty amongst TrueTime. It looks similar using version numbers wouldn't endure significantly to a greater extent than complicated. Also, for schema alter too paxos leader replacement (which the newspaper claims TrueTime simplified a lot), the NTP synchronization (several tens of ms accuracy) easily suffices. We could convey easily avoided the to a greater extent than precise TrueTime implementation amongst GPS too atomic clocks for these.
TrueTime too consistent-cuts inwards the past
After I was almost convinced that TrueTime was non proficient for anything, I realized this: TrueTime benefits snapshot reads (reads inwards the past) the most! By only giving a fourth dimension inwards the past, the snapshot read tin acquire a consistent cutting read of all the variables requested at that given time. This is non an like shooting fish in a barrel feat to accomplish inwards a distributed organisation without using TrueTime too high-precision synchronized clocks, every bit it would require capturing too recording causality relationships across many unlike versions of the variables involved hence that a consistent cutting tin endure identified for all the variables requested inwards the snapshot read. That would surely endure highly prohibitive to shop inwards the multiversion database too really difficult to query every bit well. TrueTime provides a convenient too succinct agency of encoding too accessing past times consistent-cuts of the Spanner multiversion database.But tin nosotros notice whatever clever solutions to circumvent that work without resorting to the high-precision clocks inwards TrueTime?
My unopen friend too frequent-collaborator Sandeep Kulkarni at Michigan State University had proposed HybridClocks in 2001. HybridClocks also exposed clock incertitude ε inwards physical clocks, but it also used logical clocks inwards add-on to the physical clocks to capture the finer-grain causality relationships that falls into the ε grayness area.
Using HybridClocks it tin endure possible to salvage the atomic clock requirement inwards Spanner too exercise NTP instead. Even amongst ε at 100 msecs, nosotros tin yet runway finer-grain dependencies betwixt variables using HybridClocks. The proficient tidings is that the size of the HybridClocks tin endure kept really succinct too tin endure recorded inwards Spanner database every bit timestamp to enable snapshot reads easily.
HybridClocks visit may also help speed upwards Spanner's high-precision clock-based implementation. In Spanner ε determines the charge per unit of measurement of read-write transactions on a tablet. This is because, the coordinating leaders delay read-write transactions to commit amongst at to the lowest degree ε fourth dimension apart inwards gild to ensure that at that topographic point is no incertitude inwards past times snapshot reads. It is possible to avoid this wasteful waiting past times adding some logical clocks information to TrueTime every bit prescribed inwards HybridClocks.
Sandeep too I are currently exploring these ideas.
Updates
Our 2 followup operate edifice on this topic are:Beyond TrueTime: Using AugmentedTime for Improving Google Spanner
Hybrid Logical Clocks
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