Add documentation on how to write a RecordBuilder

docs/5.x/writing-a-record-builder.md

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+---
+category: Develop
+---
+
+<div markdown="1" class="alert alert-warning">
+**This API is unstable.**
+
+The RecordBuilder API will eventually be public and the only way to define archiving logic, but for now the API is unstable
+and subject to change. Please be aware it could potentially change between minor version releases.
+</div>
+
+# Writing a RecordBuilder
+
+RecordBuilders encapsulate the smallest units of aggregation logic required to generate records for a plugin.
+
+They define two methods: `aggregate()` which builds the actual `DataTable` & numeric records to insert into archive tables,
+and `getRecordMetadata()` which returns information about what records the `RecordBuilder` builds.
+
+`aggregate()` will generally aggregate data from log tables to create records, but it does not have to. An example of a use case
+without aggregation would be importing analytics data from another service.
+
+`getRecordMetadata()` is used when aggregating records for non-day periods. In this case, Matomo will find the record values
+for the subperiods of the non-day period and aggregate them together.
+
+If your plugin needs to insert data into the archive tables during archiving, then you'll want to create your own `RecordBuilder` classes.
+This guide describes how to do that.
+
+## How to create one
+
+### Step one: identify the list of records and log aggregation queries you want to bundle together
+
+Log aggregation queries are expensive (especially with segmentation) and Matomo wants to be able to run as few of them
+as possible at a time. A `RecordBuilder` is meant to encapsulate the smallest amount of archiving logic possible, allowing Matomo
+to run just what it needs to.
+
+Many times this will either be running a single log aggregation query to generate a single `DataTable` or running a single
+log aggregation query to generate multiple numeric metrics. Sometimes it will mean running multiple log aggregation queries
+to generate a single `DataTable` or running multiple log aggregation queries to generate multiple `DataTable`s and multiple metrics.
+
+It is up to you as a developer to find the balance between efficiency (executing the fewest log aggregation queries overall)
+and modularity (having `RecordBuilder`s that individually do as little as possible).
+
+Once you've identified the `RecordBuilder`s you'll need, create empty classes for them in a `RecordBuilders` subfolder of your plugin. For example,
+`/path/to/matomo/plugins/MyPlugin/RecordBuilders/MyRecordBuilder`.
+
+**A note about Parameterized RecordBuilders**
+
+`RecordBuilder`s that can be created without specifying constructor arguments (as in, are default constructable)
+are found and created automatically by Matomo. But it is also possible to create `RecordBuilder`s that require
+parameters. These `RecordBuilder`s are added via the `Archiver.addRecordBuilders` event.
+
+The ability to create parameterized `RecordBuilder`s may not be necessary in most cases, but if your plugin
+manages entities and provides reports about those entities, it can be used to avoid having to run a query for
+every entity in the database within a single `RecordBuilder`.
+
+Examples of plugins that use this feature are the Custom Reports premium feature and the A/B Testing premium feature.
+Each of these plugins use a `RecordBuilder` that takes an ID. For Custom Reports this is the ID of the specific custom
+report and for A/B Testing this is the ID of the experiment.
+
+### Step two: implement `getRecordMetadata()`
+
+Once you know what queries the `RecordBuilder`s you are going to create will execute, you can start coding.
+The first thing to do is implement the `getRecordMetadata()` method.
+
+All this method does is return a list of `Record` entries describing the records the builder will create:
+
+```
+use Piwik\ArchiveProcessor\Record;
+
+public function getRecordMetadata(ArchiveProcessor $archiveProcessor): array
+{
+    return [
+        Record::make(Record::TYPE_BLOB, 'MyPlugin_myRecord'),
+        Record::make(Record::TYPE_NUMERIC, 'MyPlugin_myMetric'),
+        ...
+    ];
+}
+```
+
+The above is a typical example of how this method would be implemented, but it doesn't have to just be a hard-coded array.
+You can use the `ArchiveProcessor` to get the current site/period/segment or fetch system settings or measurable
+settings and vary the result based on that information. The only requirement is that every `Record` returned matches
+what can be returned by the `aggregate()` method, which we'll look at next.
+
+### Step three: implement `aggregate()`
+
+The next step is to implement your actual log aggregation logic in the `aggregate()` method. This method accepts
+an `ArchiveProcessor` and returns an array mapping record names with record values to insert. Record values are
+either numeric metric values or `DataTable` instances, which get serialized and inserted as blobs.
+
+As for how they are created, well, there is no straightforward way to define how log aggregation is done.
+
+The current pattern in Matomo is to use the core `LogAggregator` class to query log data and loop through the result.
+If your plugin provides its own additional log tables, then the pattern is to define your own `Aggregator` classes
+to build and execute log aggregation SQL queries, and use those classes in your `RecordBuilders`.
+
+An example of this might look like:
+
+```
+public function aggregate(ArchiveProcessor $archiveProcessor): array
+{
+    $logAggregator = $archiveProcessor->getLogAggregator();
+
+    $report = new DataTable();
+
+    $query = $logAggregator->queryVisitsByDimension(['label' => 'config_browser_name']);
+    while ($row = $query->fetch()) {
+        $columns = [
+            Metrics::INDEX_NB_UNIQ_VISITORS => $row[Metrics::INDEX_NB_UNIQ_VISITORS],
+            Metrics::INDEX_NB_VISITS => $row[Metrics::INDEX_NB_VISITS],
+            Metrics::INDEX_NB_ACTIONS => $row[Metrics::INDEX_NB_ACTIONS],
+            Metrics::INDEX_NB_USERS => $row[Metrics::INDEX_NB_USERS],
+            Metrics::INDEX_MAX_ACTIONS => $row[Metrics::INDEX_MAX_ACTIONS],
+            Metrics::INDEX_SUM_VISIT_LENGTH => $row[Metrics::INDEX_SUM_VISIT_LENGTH],
+            Metrics::INDEX_BOUNCE_COUNT => $row[Metrics::INDEX_BOUNCE_COUNT],
+            Metrics::INDEX_NB_VISITS_CONVERTED => $row[Metrics::INDEX_NB_VISITS_CONVERTED],
+        ];
+
+        $report->sumRowWithLabel($row['label'] ?? '', $columns);
+    }
+
+    return [
+        'MyPlugin_myRecord' => $report,
+    ];
+}
+```
+
+This example queries the `log_visit` table, grouping by the `config_browser_name` column and aggregating visit metrics.
+Then, for each row of that query, it adds the metrics to a `DataTable` which is eventually returned.
+
+Most `aggregate()` methods will be more complicated than this, but hopefully it provides you with a general understanding
+of how they should work. We recommend looking at existing `RecordBuilder`s in Matomo as well to see what is possible.
+
+### Step four: decide whether you need to set custom row limits or aggregation operations
+
+At this point, the hard parts are over. The last two steps are just finishing touches.
+
+By default, Matomo does not limit the data that is inserted into archive tables. For reports that have a limited number
+of rows, like the `VisitorInterest.getVisitsByVisitCount` and `UserCountry.getCountry`, this is acceptable. But for reports
+with a variable number of rows, it's good practice to make sure the number of rows is capped.
+
+To set a limit, set the `maxRowsInTable` and `maxRowsInSubtable` properties in the constructor of your `RecordBuilder`.
+This can be hard-coded or it can come from configuration:
+
+```
+class MyRecordBuilder extends RecordBuilder
+{
+    public function __construct()
+    {
+        parent::__construct();
+        $this->maxRowsInTable = (int)Config::getInstance()->MyPlugin['datatable_archiving_maximum_rows'];
+        $this->maxRowsInSubtable = (int)Config::getInstance()->MyPlugin['datatable_archiving_maximum_rows_subtable'];
+
+        // we want to sort by the most important metric in our reports before we cut off rows
+        $this->columnToSortByBeforeTruncation = Metrics::INDEX_NB_VISITS;
+    }
+}
+```
+
+If you don't know what to use, you can set both values to `Config::getInstance()->General['datatable_archiving_maximum_rows_standard']`.
+
+Also note we set `columnToSortByBeforeTruncation` to make sure the rows with the least visits are the ones that get removed.
+
+Additionally, if your plugin provides metrics that should be aggregated together with an operation other than being `sum`-ed,
+you will need to set the `$columnAggregationOps` property:
+
+```
+class MyRecordBuilder extends RecordBuilder
+{
+    public function __construct()
+    {
+        parent::__construct();
+
+        // ...
+
+        $this->columnAggregationOps = [
+            'my_max_metric' => 'max',
+            'my_min_metric' => 'min',
+            'my_other_metric' => function ($thisValue, $otherValue) {
+                // custom aggregation logic here
+            },
+        ];
+    }
+}
+```
+
+Note that each of these settings can also be overridden for specific records by setting the relevant property
+on `Record` instances in your `getRecordMetadata()` method.
+
+### Step five: if your RecordBuilder is parameterized, implement the relevant event
+
+If your `RecordBuilder` is not parameterized then there's nothing else to do. You're done and Matomo will detect and use it.
+
+If it is parameterized, then there's still one thing left to do. Matomo will not be able to automatically create a `RecordBuilder`
+that takes parameters, so it must be added manually in the `Archiver.addRecordBuilders` event like so:
+
+```
+class MyPlugin
+{
+    public function registerEvents()
+    {
+        $hooks = [
+            'Archiver.addRecordBuilders' => 'addRecordBuilders',
+        ];
+        return $hooks;
+    }
+
+    public function addRecordBuilders(array &$recordBuilders): void
+    {
+        $idSite = \Piwik\Request::fromRequest()->getIntegerParameter('idSite', 0);
+        if (!$idSite) {
+            return;
+        }
+
+        $entities = StaticContainer::get(MyEntityDao::class)->getAllEntitiesForSite($idSite);
+        foreach ($entities as $entity) {
+            $recordBuilders[] = new MyRecordBuilder($entity);
+        }
+    }
+}
+```
+
+Here we create a `RecordBuilder` instance for every entity our plugin manages.
+
+---
+
+And that's it, your `RecordBuilder` is done.