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35 lines
2.3 KiB
Markdown
35 lines
2.3 KiB
Markdown
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# Design motivation
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## Separate from django database setting, QuerySet and migration system
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ClickHouse SQL and DML language is near to standard, but does not follow it exactly ([docs](https://clickhouse.tech/docs/en/introduction/distinctive_features/#sql-support)).
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As a result, it can not be easily integrated into django query subsystem as it expects databases to support:
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1. Transactions.
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2. INNER/OUTER JOINS by condition.
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3. Full featured updates and deletes.
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4. Per database replication (ClickHouse has per table replication)
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5. Other features, not supported in ClickHouse.
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In order to have more functionality, [infi.clickhouse-orm](https://github.com/Infinidat/infi.clickhouse_orm)
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is used as base library for databases, querysets and migrations. The most part of it is compatible and can be used without any changes.
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## Sync over intermediate storage
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This library has several goals which lead to intermediate storage:
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1. Fail resistant import, does not matter what the fail reason is:
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ClickHouse fail, network fail, killing import process by system (OOM, for instance).
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2. ClickHouse does not like single row inserts: [docs](https://clickhouse.tech/docs/en/introduction/performance/#performance-when-inserting-data).
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So it's worth batching data somewhere before inserting it.
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ClickHouse provide BufferEngine for this, but it can loose data if ClickHouse fails - and no one will now about it.
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3. Better scalability. Different intermediate storages may be implemented in the future, based on databases, queue systems or even BufferEngine.
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## Replication and routing
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In primitive cases people just have single database or cluster with same tables on each replica.
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But as ClickHouse has per table replication a more complicated structure can be built:
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1. Model A is stored on servers 1 and 2
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2. Model B is stored on servers 2, 3 and 5
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3. Model C is stored on servers 1, 3 and 4
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Moreover, migration operations in ClickHouse can also be auto-replicated (`ALTER TABLE`, for instance) or not (`CREATE TABLE`).
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In order to make replication scheme scalable:
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1. Each model has it's own read / write / migrate [routing configuration](routing.md#clickhousemodel-routing-attributes).
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2. You can use [router](routing.md#router) like django does to set basic routing rules for all models or model groups.
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