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drafts/postgres_cdc.md

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+# Postgres CDC 
+
+## What is CDC ? 
+
+CDC stands for Change Data Capture. It is a mechanism that enables the replication of a database.
+That is we listen to changes on the tables of the database so that we can replicate them into 
+another database.
+
+This is used in data engineering pipelines to extract data from sources and to replicate
+them into the datawarehouse, data lake or lakehouse for example.
+This way it is possible to do analysis over these data without impacting the transactionnal
+database, which is used by another software as its primary storage.
+
+The other advantage of replicating the database is that it can be stored in another way.
+For example it is possible to store the resulting mirrored database as parquet files,
+or any columnar storage format, to speed up analytics queries.
+
+## Replication in Postgres
+    
+The database needs some configuration to enable a replication sufficient for a CDC data pipeline.
+
+First, in the `postgres.conf` file the three following lines shall be added :
+- `wal_level=logical`
+- `max_replication_slots=10`
+- `max_wal_senders=10`
+
+Here follows a quick explanation of what each of these parameters mean :
+
+### wal_level
+
+WAL stands for Write Ahead Logs. These are the logs written by postgres to 
+record all operations on the database.
+By default the level is `replica`, which is .... [TODO]
+but for CDC we need the highest level `logical`. This level records every transaction
+happening is the database, at the point that we can literally reconstruct the database
+from the logs; which is exactly what CDC is trying to achieve.
+
+### max_replication_slots
+
+Here comes another concept : the replication slots.
+These are ... .[TODO]
+Naturally, all WAL are not kepts forever, hence we need to configure replication slots
+so that unread WAL are not destroyed before our CDC pipeline has had the chance to read them.
+
+### max_wal_senders
+
+[TODO]
+
+## Publications
+