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docs: Added native histogram WAL record documentation.
Signed-off-by: bwplotka <bwplotka@gmail.com>
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# WAL Disk Format
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This document describes the official Prometheus WAL format.
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The write ahead log operates in segments that are numbered and sequential,
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e.g. `000000`, `000001`, `000002`, etc., and are limited to 128MB by default.
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A segment is written to in pages of 32KB. Only the last page of the most recent segment
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and are limited to 128MB by default.
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## Segment filename
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The sequence number is captured in the segment filename,
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e.g. `000000`, `000001`, `000002`, etc. The first unsigned integer represents
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the sequence number of the segment, typically encoded with six digits.
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## Segment encoding
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This section describes the segment encoding.
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A segment encodes an array of records. It does not contain any header. A segment
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is written to pages of 32KB. Only the last page of the most recent segment
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may be partial. A WAL record is an opaque byte slice that gets split up into sub-records
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should it exceed the remaining space of the current page. Records are never split across
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segment boundaries. If a single record exceeds the default segment size, a segment with
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a larger size will be created.
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The encoding of pages is largely borrowed from [LevelDB's/RocksDB's write ahead log.](https://github.com/facebook/rocksdb/wiki/Write-Ahead-Log-File-Format)
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Notable deviations are that the record fragment is encoded as:
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### Records encoding
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Each record fragment is encoded as:
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```
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┌───────────┬──────────┬────────────┬──────────────┐
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@ -17,7 +34,8 @@ Notable deviations are that the record fragment is encoded as:
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└───────────┴──────────┴────────────┴──────────────┘
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```
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The initial type byte is made up of three components: a 3-bit reserved field, a 1-bit zstd compression flag, a 1-bit snappy compression flag, and a 3-bit type flag.
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The initial type byte is made up of three components: a 3-bit reserved field,
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a 1-bit zstd compression flag, a 1-bit snappy compression flag, and a 3-bit type flag.
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```
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┌─────────────────┬──────────────────┬────────────────────┬──────────────────┐
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@ -25,7 +43,7 @@ The initial type byte is made up of three components: a 3-bit reserved field, a
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└─────────────────┴──────────────────┴────────────────────┴──────────────────┘
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```
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The lowest 3 bits within this flag represent the record type as follows:
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The lowest 3 bits within the type flag represent the record type as follows:
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* `0`: rest of page will be empty
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* `1`: a full record encoded in a single fragment
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@ -33,11 +51,16 @@ The lowest 3 bits within this flag represent the record type as follows:
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* `3`: middle fragment of a record
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* `4`: final fragment of a record
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## Record encoding
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After the type byte, 2-byte length and then 4-byte checksum of the following data are encoded.
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The records written to the write ahead log are encoded as follows:
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All float values are represented using the [IEEE 754 format](https://en.wikipedia.org/wiki/IEEE_754).
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### Series records
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### Record types
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In the following sections, all the known record types are described. New types,
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can be added in the future.
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#### Series records
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Series records encode the labels that identifies a series and its unique ID.
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@ -58,7 +81,7 @@ Series records encode the labels that identifies a series and its unique ID.
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└────────────────────────────────────────────┘
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```
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### Sample records
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#### Sample records
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Sample records encode samples as a list of triples `(series_id, timestamp, value)`.
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Series reference and timestamp are encoded as deltas w.r.t the first sample.
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@ -79,7 +102,7 @@ The first sample record begins at the second row.
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└──────────────────────────────────────────────────────────────────┘
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```
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### Tombstone records
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#### Tombstone records
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Tombstone records encode tombstones as a list of triples `(series_id, min_time, max_time)`
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and specify an interval for which samples of a series got deleted.
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@ -95,7 +118,7 @@ and specify an interval for which samples of a series got deleted.
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└─────────────────────────────────────────────────────┘
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```
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### Exemplar records
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#### Exemplar records
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Exemplar records encode exemplars as a list of triples `(series_id, timestamp, value)`
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plus the length of the labels list, and all the labels.
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@ -127,7 +150,7 @@ See: https://github.com/OpenObservability/OpenMetrics/blob/main/specification/Op
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└──────────────────────────────────────────────────────────────────┘
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```
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### Metadata records
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#### Metadata records
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Metadata records encode the metadata updates associated with a series.
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@ -156,3 +179,90 @@ Metadata records encode the metadata updates associated with a series.
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└────────────────────────────────────────────┘
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```
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#### Histogram records
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Histogram records encode the integer and float native histogram samples.
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A record with the integer native histograms with the exponential bucketing:
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```
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┌───────────────────────────────────────────────────────────────────────┐
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│ type = 7 <1b> │
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├───────────────────────────────────────────────────────────────────────┤
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│ ┌────────────────────┬───────────────────────────┐ │
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│ │ id <8b> │ timestamp <8b> │ │
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│ └────────────────────┴───────────────────────────┘ │
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│ ┌────────────────────┬──────────────────────────────────────────────┐ │
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│ │ id_delta <uvarint> │ timestamp_delta <uvarint> │ │
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│ ├────────────────────┴────┬─────────────────────────────────────────┤ │
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│ │ counter_reset_hint <1b> │ schema <varint> │ │
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│ ├─────────────────────────┴────┬────────────────────────────────────┤ │
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│ │ zero_threshold (float) <8b> │ zero_count <uvarint> │ │
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│ ├─────────────────┬────────────┴────────────────────────────────────┤ │
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│ │ count <uvarint> │ sum (float) <8b> │ │
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│ ├─────────────────┴─────────────────────────────────────────────────┤ │
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│ │ positive_spans_num <uvarint> │ │
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│ ├─────────────────────────────────┬─────────────────────────────────┤ │
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│ │ positive_span_offset_1 <varint> │ positive_span_len_1 <uvarint32> │ │
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│ ├─────────────────────────────────┴─────────────────────────────────┤ │
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│ │ . . . │ │
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│ ├───────────────────────────────────────────────────────────────────┤ │
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│ │ negative_spans_num <uvarint> │ │
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│ ├───────────────────────────────┬───────────────────────────────────┤ │
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│ │ negative_span_offset <varint> │ negative_span_len <uvarint32> │ │
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│ ├───────────────────────────────┴───────────────────────────────────┤ │
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│ │ . . . │ │
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│ ├───────────────────────────────────────────────────────────────────┤ │
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│ │ positive_bkts_num <uvarint> │ │
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│ ├─────────────────────────┬───────┬─────────────────────────────────┤ │
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│ │ positive_bkt_1 <varint> │ . . . │ positive_bkt_n <varint> │ │
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│ ├─────────────────────────┴───────┴─────────────────────────────────┤ │
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│ │ negative_bkts_num <uvarint> │ │
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│ ├─────────────────────────┬───────┬─────────────────────────────────┤ │
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│ │ negative_bkt_1 <varint> │ . . . │ negative_bkt_n <varint> │ │
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│ └─────────────────────────┴───────┴─────────────────────────────────┘ │
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│ . . . │
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└───────────────────────────────────────────────────────────────────────┘
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```
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A records with the Float histograms:
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```
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┌───────────────────────────────────────────────────────────────────────┐
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│ type = 8 <1b> │
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├───────────────────────────────────────────────────────────────────────┤
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│ ┌────────────────────┬───────────────────────────┐ │
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│ │ id <8b> │ timestamp <8b> │ │
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│ └────────────────────┴───────────────────────────┘ │
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│ ┌────────────────────┬──────────────────────────────────────────────┐ │
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│ │ id_delta <uvarint> │ timestamp_delta <uvarint> │ │
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│ ├────────────────────┴────┬─────────────────────────────────────────┤ │
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│ │ counter_reset_hint <1b> │ schema <varint> │ │
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│ ├─────────────────────────┴────┬────────────────────────────────────┤ │
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│ │ zero_threshold (float) <8b> │ zero_count (float) <8b> │ │
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│ ├────────────────────┬─────────┴────────────────────────────────────┤ │
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│ │ count (float) <8b> │ sum (float) <8b> │ │
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│ ├────────────────────┴──────────────────────────────────────────────┤ │
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│ │ positive_spans_num <uvarint> │ │
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│ ├─────────────────────────────────┬─────────────────────────────────┤ │
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│ │ positive_span_offset_1 <varint> │ positive_span_len_1 <uvarint32> │ │
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│ ├─────────────────────────────────┴─────────────────────────────────┤ │
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│ │ . . . │ │
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│ ├───────────────────────────────────────────────────────────────────┤ │
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│ │ negative_spans_num <uvarint> │ │
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│ ├───────────────────────────────┬───────────────────────────────────┤ │
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│ │ negative_span_offset <varint> │ negative_span_len <uvarint32> │ │
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│ ├───────────────────────────────┴───────────────────────────────────┤ │
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│ │ . . . │ │
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│ ├───────────────────────────────────────────────────────────────────┤ │
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│ │ positive_bkts_num <uvarint> │ │
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│ ├─────────────────────────────┬───────┬─────────────────────────────┤ │
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│ │ positive_bkt_1 (float) <8b> │ . . . │ positive_bkt_n (float) <8b> │ │
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│ ├─────────────────────────────┴───────┴─────────────────────────────┤ │
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│ │ negative_bkts_num <uvarint> │ │
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│ ├─────────────────────────────┬───────┬─────────────────────────────┤ │
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│ │ negative_bkt_1 (float) <8b> │ . . . │ negative_bkt_n (float) <8b> │ │
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│ └─────────────────────────────┴───────┴─────────────────────────────┘ │
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│ . . . │
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└───────────────────────────────────────────────────────────────────────┘
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```
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