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106 lines
5.0 KiB
Markdown
106 lines
5.0 KiB
Markdown
# Synchronization
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## Design motivation
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Read [here](motivation.md#sync-over-intermediate-storage).
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## Algorithm
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<!---  --->
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1. [Celery beat](https://docs.celeryproject.org/en/latest/userguide/periodic-tasks.html) schedules `django_clickhouse.tasks.clickhouse_auto_sync` task every second or near.
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2. [Celery workers](https://docs.celeryproject.org/en/latest/userguide/workers.html) execute `clickhouse_auto_sync`.
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It searches for `ClickHouseModel` subclasses which need sync (if `Model.need_sync()` method returns `True`).
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2. `django_clickhouse.tasks.sync_clickhouse_model` task is scheduled for each `ClickHouseModel` which needs sync.
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3. `sync_clickhouse_model` saves sync start time in [storage](storages.md) and calls `ClickHouseModel.sync_batch_from_storage()` method.
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4. `ClickHouseModel.sync_batch_from_storage()`:
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* Gets [storage](storages.md) model works with using `ClickHouseModel.get_storage()` method
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* Calls `Storage.pre_sync(import_key)` for model [storage](storages.md).
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This may be used to prevent parallel execution with locks or some other operations.
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* Gets a list of operations to sync from [storage](storages.md).
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* Fetches objects from relational database calling `ClickHouseModel.get_sync_objects(operations)` method.
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* Forms a batch of tuples to insert into ClickHouse using `ClickHouseModel.get_insert_batch(import_objects)` method.
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* Inserts batch of tuples into ClickHouse using `ClickHouseModel.insert_batch(batch)` method.
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* Calls `Storage.post_sync(import_key)` method to clean up storage after syncing batch.
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This method also removes synced operations from storage.
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* If some exception occurred during execution, `Storage.post_sybc_failed(import_key)` method is called.
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Note, that process can be killed without exception, for instance by OOM killer.
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And this method will not be called.
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## Configuration
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Sync configuration can be set globally using django settings.py parameters or redeclared for each `ClickHouseModel` class.
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`ClickHouseModel` configuration is prior to settings configuration.
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### Settings configuration
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* [CLICKHOUSE_CELERY_QUEUE](configuration.md#clickhouse_celery_queue)
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Defaults to: `'celery'`
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A name of a queue, used by celery to plan library sync tasks.
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* [CLICKHOUSE_SYNC_STORAGE](configuration.md#clickhouse_sync_storage)
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Defaults to: `'django_clickhouse.storages.RedisStorage'`
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An [intermediate storage](storages.md) class to use. Can be a string or class.
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* [CLICKHOUSE_SYNC_BATCH_SIZE](configuration.md#clickhouse_sync_storage)
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Defaults to: `10000`
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Maximum number of operations, fetched by sync process from [intermediate storage](storages.md) per sync round.
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* [CLICKHOUSE_SYNC_DELAY](configuration.md#clickhouse_sync_storage)
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Defaults to: `5`
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A delay in seconds between two sync rounds start.
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### ClickHouseModel configuration
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Each `ClickHouseModel` subclass can define sync arguments and methods:
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* `django_model: django.db.models.Model`
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Required.
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Django model this ClickHouseModel class is synchronized with.
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* `django_model_serializer: django.db.models.Model`
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Defaults to: `django_clickhouse.serializers.Django2ClickHouseModelSerializer`
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[Serializer class](models.md#serializers) to convert DjangoModel to ClickHouseModel.
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* `sync_enabled: bool`
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Defaults to: `False`.
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Is sync for this model enabled?
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* `sync_batch_size: int`
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Defaults to: [CLICKHOUSE_SYNC_BATCH_SIZE](configuration.md#clickhouse_sync_storage)
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Maximum number of operations, fetched by sync process from [storage](storages.md) per sync round.
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* `sync_delay: float`
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Defaults to: [CLICKHOUSE_SYNC_DELAY](configuration.md#clickhouse_sync_storage)
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A delay in seconds between two sync rounds start.
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* `sync_storage: Union[str, Storage]`
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Defaults to: [CLICKHOUSE_SYNC_STORAGE](configuration.md#clickhouse_sync_storage)
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An [intermediate storage](storages.md) class to use. Can be a string or class.
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Example:
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```python
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from django_clickhouse.clickhouse_models import ClickHouseModel
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from django_clickhouse.engines import ReplacingMergeTree
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from infi.clickhouse_orm import fields
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from my_app.models import User
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class ClickHouseUser(ClickHouseModel):
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django_model = User
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sync_enabled = True
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sync_delay = 5
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sync_batch_size = 1000
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id = fields.UInt32Field()
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first_name = fields.StringField()
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birthday = fields.DateField()
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visits = fields.UInt32Field(default=0)
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engine = ReplacingMergeTree('birthday', ('birthday',))
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```
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## Fail resistance
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Fail resistance is based on several points:
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1. [Storage](storages.md) should not loose data in any case. It's not this library goal to keep it stable.
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2. Data is removed from [storage](storages.md) only if import succeeds. Otherwise import attempt is repeated.
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3. It's recommended to use ReplacingMergeTree or CollapsingMergeTree [engines](models.md#engines)
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instead of simple MergeTree, so it removes duplicates if batch is imported twice.
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4. Each `ClickHouseModel` is synced in separate process.
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If one model fails, it should not affect other models.
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