# Replications

## Overview

Replications are the best way to use sling in a reusable manner. The `defaults` key allows reusing your inputs with the ability to override any of them in a particular stream. Both YAML or JSON files are accepted. When you run a replication, internally, Sling auto-generates many tasks (one per stream) and runs them in order.

See these pages for more details:

* [Structure](/concepts/replication/structure.md)
* [Modes](/concepts/replication/modes.md)
* [Source Options](/concepts/replication/source-options.md)
* [Target Options](/concepts/replication/target-options.md)
* [Columns & Constraints](/concepts/replication/columns.md)
* [Transformations](/concepts/replication/transforms.md)
* [Hooks](/concepts/hooks.md)

Here is a basic example, where all PostgreSQL tables in the schema `my_schema` will be loaded into Snowflake. The `my_schema.*` notation as the stream name is a feature possible **only in Replications**. Also notice how `defaults.object` uses [runtime variables](/concepts/replication/runtime-variables.md).

{% code title="replication.yaml" %}

```yaml
source: MY_POSTGRES
target: MY_SNOWFLAKE

# default config options which apply to all streams
defaults:
  mode: full-refresh
  object: new_schema.{stream_schema}_{stream_table}

streams:
  my_schema.*:

env:
  SLING_THREADS: 3
```

{% endcode %}

Another example:

{% code title="replication.yaml" %}

```yaml
source: MY_MYSQL
target: MY_BIGQUERY

defaults:
  mode: incremental
  object: '{target_schema}.{stream_schema}_{stream_table}'
  primary_key: [id]
  
  source_options:
    empty_as_null: false
    
  target_options:
    column_casing: snake

streams:
  finance.accounts:
  finance.users:
    disabled: true
  
  finance.departments:
    object: '{target_schema}.finance_departments_old' # overwrite default object
    source_options:
      empty_as_null: false

  finance."Transactions":
    mode: incremental # overwrite default mode
    primary_key: [other_id]
    update_key: last_updated_at
  
  finance.all_users.custom:
    sql: |
      select col1, col2
      from finance."all_Users"
    object: finance.all_users # need to add 'object' key for custom SQL

env:
  # adds the _sling_loaded_at timestamp column
  SLING_LOADED_AT_COLUMN: true 
  
  # if source is file, adds a _sling_stream_url column with file path / url
  SLING_STREAM_URL_COLUMN: true

  # parallel stream runs
  SLING_THREADS: 3

  # retry failing stream runs
  SLING_RETRIES: 1
```

{% endcode %}

We can use a replication config with: `sling run -r /path/to/replication.yaml`


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