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storage: add min proposal timestamp tracker
Implement the minimum proposal timestamp tracker outlined in the [follower reads RFC]. The implementation will likely need follow-up work to reduce lock contention. The intended usage will place a call to Track and to the returned closure into the write path. Touches cockroachdb#16593. Release note: None [follower reads RFC]: cockroachdb#26362
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| // Copyright 2018 The Cockroach Authors. | ||
| // | ||
| // Licensed under the Apache License, Version 2.0 (the "License"); | ||
| // you may not use this file except in compliance with the License. | ||
| // You may obtain a copy of the License at | ||
| // | ||
| // http://www.apache.org/licenses/LICENSE-2.0 | ||
| // | ||
| // Unless required by applicable law or agreed to in writing, software | ||
| // distributed under the License is distributed on an "AS IS" BASIS, | ||
| // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or | ||
| // implied. See the License for the specific language governing | ||
| // permissions and limitations under the License. | ||
|
|
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| // Package minprop exports a main data structure, Tracker, which checkpoints | ||
| // closed timestamps and associated Raft Lease Applied indexes positions for | ||
| // which (under additional conditions) it is legal to serve follower reads. It | ||
| // does so by maintaining a 'next' timestamp above which new command evaluations | ||
| // are forced, and by tracking when all in-flight evaluations below this | ||
| // timestamp have completed (at which point a call to the Close method succeeds: | ||
| // 'next' becomes closed, and a new 'next' is initialized with a future | ||
| // timestamp). | ||
| // | ||
| // In-flight command evaluations are tracked via the Track method which acquires | ||
| // a reference with the tracker, returns a minimum timestamp to be used for the | ||
| // proposal evaluation, and provides a closure that releases the reference with | ||
| // a lease applied index used for the proposal. | ||
| package minprop |
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| // Copyright 2018 The Cockroach Authors. | ||
| // | ||
| // Licensed under the Apache License, Version 2.0 (the "License"); | ||
| // you may not use this file except in compliance with the License. | ||
| // You may obtain a copy of the License at | ||
| // | ||
| // http://www.apache.org/licenses/LICENSE-2.0 | ||
| // | ||
| // Unless required by applicable law or agreed to in writing, software | ||
| // distributed under the License is distributed on an "AS IS" BASIS, | ||
| // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or | ||
| // implied. See the License for the specific language governing | ||
| // permissions and limitations under the License. | ||
|
|
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| package minprop | ||
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|
||
| import ( | ||
| "context" | ||
| "fmt" | ||
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| "github.com/cockroachdb/cockroach/pkg/util/hlc" | ||
| ) | ||
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| func Example() { | ||
| ctx := context.TODO() | ||
|
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| tracker := NewTracker() | ||
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| fmt.Println("The newly initialized tracker has a zero closed timestamp:") | ||
| fmt.Println(tracker) | ||
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| fmt.Println("A first command arrives on range 12 (though the range isn't known yet to the Tracker).") | ||
| ts, done1 := tracker.Track(ctx) | ||
| fmt.Println("All commands initially start out on the right. The command has its timestamp forwarded to", ts, ".") | ||
| fmt.Println(tracker) | ||
|
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| fmt.Println("Two more commands arrive, on r1 and r12.") | ||
| _, done2 := tracker.Track(ctx) | ||
| _, done3 := tracker.Track(ctx) | ||
| fmt.Println(tracker) | ||
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| fmt.Println("The command on r1 finishes evaluating at Lease Applied Index 10 and lets the Tracker know.") | ||
| done2(ctx, 1, 10) | ||
| fmt.Println(tracker) | ||
|
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| fmt.Println("The command on r12 also finishes quickly, at LAI 77.") | ||
| done3(ctx, 12, 77) | ||
| fmt.Println(tracker) | ||
|
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| fmt.Println("The system closes out a timestamp (registering 1000 as the next timestamp to close out).") | ||
| closed1, mlai1 := tracker.Close(hlc.Timestamp{WallTime: 1E9}) | ||
| fmt.Println("No problem: nothing is tracked on the left side; returns:", closed1, "and", mlai1) | ||
| fmt.Println("Note how the items on the right have moved to the left, as they are relevant for the") | ||
| fmt.Println("next call to Close.") | ||
| fmt.Println(tracker) | ||
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| fmt.Println("Nothing happens for a while until the system tries to close out the next timestamp.") | ||
| fmt.Println("However, the very first proposal is still tracked and blocks progress.") | ||
| closed2, mlai2 := tracker.Close(hlc.Timestamp{WallTime: 2E9}) | ||
| fmt.Println("The call returns a no-op in the form", closed2, mlai2, ".") | ||
| fmt.Println(tracker) | ||
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| ts4, done4 := tracker.Track(ctx) | ||
| fmt.Println("A new command gets tracked on r12 (and is forwarded to", ts4, "(if necessary).") | ||
| fmt.Println("It terminates quickly, leaving an MLAI entry of 78 behind.") | ||
| done4(ctx, 12, 78) | ||
| fmt.Println(tracker) | ||
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| fmt.Println("Finally! The slow evaluation finishes and the command gets proposed at index 79.") | ||
| fmt.Println("Note that the right now tracks a smaller value of 78. Consumers have to keep the") | ||
| fmt.Println("maximum they've seen.") | ||
| done1(ctx, 12, 79) | ||
| fmt.Println(tracker) | ||
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| closed3, mlai3 := tracker.Close(hlc.Timestamp{WallTime: 3E9}) | ||
| fmt.Println("The next call to Close() is successful and returns:", closed3, "and", mlai3) | ||
| fmt.Println(tracker) | ||
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| // Output: | ||
| // The newly initialized tracker has a zero closed timestamp: | ||
| // | ||
| // closed next | ||
| // | left | right | ||
| // | 0 # 0 | ||
| // v v | ||
| // ---------------------------------------------------------> time | ||
| // | ||
| // closed = 0.000000000,0 | ||
| // next = 0.000000000,1 | ||
| // | ||
| // A first command arrives on range 12 (though the range isn't known yet to the Tracker). | ||
| // All commands initially start out on the right. The command has its timestamp forwarded to 0.000000000,2 . | ||
| // | ||
| // closed next | ||
| // | left | right | ||
| // | 0 # 1 | ||
| // v v | ||
| // ---------------------------------------------------------> time | ||
| // | ||
| // closed = 0.000000000,0 | ||
| // next = 0.000000000,1 | ||
| // | ||
| // Two more commands arrive, on r1 and r12. | ||
| // | ||
| // closed next | ||
| // | left | right | ||
| // | 0 # 3 | ||
| // v v | ||
| // ---------------------------------------------------------> time | ||
| // | ||
| // closed = 0.000000000,0 | ||
| // next = 0.000000000,1 | ||
| // | ||
| // The command on r1 finishes evaluating at Lease Applied Index 10 and lets the Tracker know. | ||
| // | ||
| // closed next | ||
| // | left | right | ||
| // | 0 # 2 | ||
| // | @ 10 (r1) | ||
| // v v | ||
| // ---------------------------------------------------------> time | ||
| // | ||
| // closed = 0.000000000,0 | ||
| // next = 0.000000000,1 | ||
| // | ||
| // The command on r12 also finishes quickly, at LAI 77. | ||
| // | ||
| // closed next | ||
| // | left | right | ||
| // | 0 # 1 | ||
| // | @ 10 (r1) | ||
| // | @ 77 (r12) | ||
| // v v | ||
| // ---------------------------------------------------------> time | ||
| // | ||
| // closed = 0.000000000,0 | ||
| // next = 0.000000000,1 | ||
| // | ||
| // The system closes out a timestamp (registering 1000 as the next timestamp to close out). | ||
| // No problem: nothing is tracked on the left side; returns: 0.000000000,1 and map[] | ||
| // Note how the items on the right have moved to the left, as they are relevant for the | ||
| // next call to Close. | ||
| // | ||
| // closed next | ||
| // | left | right | ||
| // | 1 # 0 | ||
| // | 10 @ (r1) | ||
| // | 77 @ (r12) | ||
| // v v | ||
| // ---------------------------------------------------------> time | ||
| // | ||
| // closed = 0.000000000,1 | ||
| // next = 1.000000000,0 | ||
| // | ||
| // Nothing happens for a while until the system tries to close out the next timestamp. | ||
| // However, the very first proposal is still tracked and blocks progress. | ||
| // The call returns a no-op in the form 0.000000000,1 map[] . | ||
| // | ||
| // closed next | ||
| // | left | right | ||
| // | 1 # 0 | ||
| // | 10 @ (r1) | ||
| // | 77 @ (r12) | ||
| // v v | ||
| // ---------------------------------------------------------> time | ||
| // | ||
| // closed = 0.000000000,1 | ||
| // next = 1.000000000,0 | ||
| // | ||
| // A new command gets tracked on r12 (and is forwarded to 1.000000000,1 (if necessary). | ||
| // It terminates quickly, leaving an MLAI entry of 78 behind. | ||
| // | ||
| // closed next | ||
| // | left | right | ||
| // | 1 # 0 | ||
| // | 10 @ (r1) | ||
| // | 77 @ (r12) | ||
| // | @ 78 (r12) | ||
| // v v | ||
| // ---------------------------------------------------------> time | ||
| // | ||
| // closed = 0.000000000,1 | ||
| // next = 1.000000000,0 | ||
| // | ||
| // Finally! The slow evaluation finishes and the command gets proposed at index 79. | ||
| // Note that the right now tracks a smaller value of 78. Consumers have to keep the | ||
| // maximum they've seen. | ||
| // | ||
| // closed next | ||
| // | left | right | ||
| // | 0 # 0 | ||
| // | 10 @ (r1) | ||
| // | @ 78 (r12) | ||
| // | 79 @ (r12) | ||
| // v v | ||
| // ---------------------------------------------------------> time | ||
| // | ||
| // closed = 0.000000000,1 | ||
| // next = 1.000000000,0 | ||
| // | ||
| // The next call to Close() is successful and returns: 1.000000000,0 and map[1:10 12:79] | ||
| // | ||
| // closed next | ||
| // | left | right | ||
| // | 0 # 0 | ||
| // | 78 @ (r12) | ||
| // v v | ||
| // ---------------------------------------------------------> time | ||
| // | ||
| // closed = 1.000000000,0 | ||
| // next = 3.000000000,0 | ||
| } |
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