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61 lines
2.5 KiB
Go
61 lines
2.5 KiB
Go
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// Copyright 2019, OpenTelemetry Authors
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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/*
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Package metric implements the OpenTelemetry `Meter` API. The SDK
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supports configurable metrics export behavior through a
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`export.MetricBatcher` API. Most metrics behavior is controlled
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by the `MetricBatcher`, including:
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1. Selecting the concrete type of aggregation to use
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2. Receiving exported data during SDK.Collect()
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The call to SDK.Collect() initiates collection. The SDK calls the
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`MetricBatcher` for each current record, asking the aggregator to
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export itself. Aggregators, found in `./aggregators`, are responsible
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for receiving updates and exporting their current state.
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The SDK.Collect() API should be called by an exporter. During the
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call to Collect(), the exporter receives calls in a single-threaded
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context. No locking is required because the SDK.Collect() call
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prevents concurrency.
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The SDK uses lock-free algorithms to maintain its internal state.
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There are three central data structures at work:
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1. A sync.Map maps unique (InstrumentID, LabelSet) to records
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2. A "primary" atomic list of records
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3. A "reclaim" atomic list of records
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Collection is oriented around epochs. The SDK internally has a
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notion of the "current" epoch, which is incremented each time
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Collect() is called. Records contain two atomic counter values,
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the epoch in which it was last modified and the epoch in which it
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was last collected. Records may be garbage collected when the
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epoch in which they were last updated is less than the epoch in
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which they were last collected.
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Collect() performs a record-by-record scan of all active records
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and exports their current state, before incrementing the current
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epoch. Collection events happen at a point in time during
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`Collect()`, but all records are not collected in the same instant.
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The purpose of the two lists: the primary list is appended-to when
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new handles are created and atomically cleared during collect. The
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reclaim list is used as a second chance, in case there is a race
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between looking up a record and record deletion.
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*/
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package metric
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