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72 lines
4.6 KiB
Markdown
72 lines
4.6 KiB
Markdown
# Usage
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TSDB can be - and is - used by other applications such as [Cortex](https://cortexmetrics.io/) and [Thanos](https://thanos.io/).
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This directory contains documentation for any developers who wish to work on or with TSDB.
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For a full example of instantiating a database, adding and querying data, see the [tsdb example in the docs](https://pkg.go.dev/github.com/prometheus/prometheus/tsdb).
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`tsdb/db_test.go` also demonstrates various specific usages of the TSDB library.
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## Instantiating a database
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Callers should use [`tsdb.Open`](https://pkg.go.dev/github.com/prometheus/prometheus/tsdb#Open) to open a TSDB
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(the directory may be new or pre-existing).
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This returns a [`*tsdb.DB`](https://pkg.go.dev/github.com/prometheus/prometheus/tsdb#DB) which is the actual database.
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A `DB` has the following main components:
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* Compactor: a [leveled compactor](https://pkg.go.dev/github.com/prometheus/prometheus/tsdb#LeveledCompactor). Note: it is currently the only compactor implementation. It runs automatically.
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* [`Head`](https://pkg.go.dev/github.com/prometheus/prometheus/tsdb#DB.Head)
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* [Blocks (persistent blocks)](https://pkg.go.dev/github.com/prometheus/prometheus/tsdb#DB.Blocks)
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The `Head` is responsible for a lot. Here are its main components:
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* [WAL](https://pkg.go.dev/github.com/prometheus/prometheus/tsdb/wal#WAL) (Write Ahead Log).
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* [`stripeSeries`](https://github.com/prometheus/prometheus/blob/411021ada9ab41095923b8d2df9365b632fd40c3/tsdb/head.go#L1292):
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this holds all the active series by linking to [`memSeries`](https://github.com/prometheus/prometheus/blob/411021ada9ab41095923b8d2df9365b632fd40c3/tsdb/head.go#L1462)
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by an ID (aka "ref") and by labels hash.
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* Postings list (reverse index): For any label-value pair, holds all the corresponding series refs. Used for queries.
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* Tombstones.
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## Adding data
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Use [`db.Appender()`](https://pkg.go.dev/github.com/prometheus/prometheus/tsdb#DB.Appender) to obtain an "appender".
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The [golang docs](https://pkg.go.dev/github.com/prometheus/prometheus/storage#Appender) speak mostly for themselves.
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Remember:
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* Use `Commit()` to add the samples to the DB and update the WAL.
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* Create a new appender each time you commit.
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* Appenders are not concurrency safe, but scrapes run concurrently and as such, leverage multiple appenders concurrently.
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This reduces contention, although Commit() contend the same critical section (writing to the WAL is serialized), and may
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inflate append tail latency if multiple appenders try to commit at the same time.
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Append may reject data due to these conditions:
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1) `timestamp < minValidTime` where `minValidTime` is the highest of:
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* the maxTime of the last block (i.e. the last truncation time of Head) - updated via [`Head.Truncate()`](https://pkg.go.dev/github.com/prometheus/prometheus/tsdb#Head.Truncate) and [`DB.compactHead()`](https://github.com/prometheus/prometheus/blob/411021ada9ab41095923b8d2df9365b632fd40c3/tsdb/db.go#L968)
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* `tsdb.min-block-duration/2` older than the max time in the Head block. Note that while technically `storage.tsdb.min-block-duration` is configurable, it's a hidden option and changing it is discouraged. So We can assume this value to be 2h.
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Breaching this condition results in "out of bounds" errors.
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The first condition assures the block that will be generated doesn't overlap with the previous one (which simplifies querying)
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The second condition assures the sample won't go into the so called "compaction window", that is the section of the data that might be in process of being saved into a persistent block on disk. (because that logic runs concurrently with ingestion without a lock)
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2) The labels don't validate. (if the set is empty or contains duplicate label names)
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3) If the sample, for the respective series (based on all the labels) is out of order or has a different value for the last (highest) timestamp seen. (results in `storage.ErrOutOfOrderSample` and `storage.ErrDuplicateSampleForTimestamp` respectively)
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`Commit()` may also refuse data that is out of order with respect to samples that were added via a different appender.
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## Querying data
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Use [`db.Querier()`](https://pkg.go.dev/github.com/prometheus/prometheus/tsdb#DB.Querier) to obtain a "querier".
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The [golang docs](https://pkg.go.dev/github.com/prometheus/prometheus/storage#Querier) speak mostly for themselves.
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Remember:
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* A querier can only see data that was committed when it was created. This limits the lifetime of a querier.
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* A querier should be closed when you're done with it.
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* Use mint/maxt to avoid loading unneeded data.
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## Example code
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Find the example code for ingesting samples and querying them in [`tsdb/example_test.go`](../example_test.go)
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