Add source table migration guide

docs/guides/materialize_source_table.md

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+---
+page_title: "Source versioning: migrating to `materialize_source_table` Resource"
+subcategory: ""
+description: |-
+
+---
+
+# Source versioning: migrating to `materialize_source_table` Resource
+
+In previous versions of the Materialize Terraform provider, source tables were defined within the source resource itself and were considered subsources of the source rather than separate entities.
+
+This guide will walk you through the process of migrating your existing source table definitions to the new `materialize_source_table` resource.
+
+## Old Approach
+
+Previously, source tables were defined directly within the source resource:
+
+### Example: MySQL Source
+
+```hcl
+resource "materialize_source_mysql" "mysql_source" {
+  name         = "mysql_source"
+  cluster_name = "cluster_name"
+
+  mysql_connection {
+    name = materialize_connection_mysql.mysql_connection.name
+  }
+
+  table {
+    upstream_name        = "mysql_table1"
+    upstream_schema_name = "shop"
+    name                 = "mysql_table1_local"
+  }
+}
+```
+
+The same approach was used for other source types such as Postgres and the load generator sources.
+
+## New Approach
+
+The new approach separates source definitions and table definitions. You will now create the source without specifying the tables, and then define each table using the `materialize_source_table` resource.
+
+## Manual Migration Process
+
+This manual migration process requires users to create new source tables using the new `materialize_source_table` resource and then remove the old ones.
+
+### Step 1: Define `materialize_source_table` Resources
+
+Before making any changes to your existing source resources, create new `materialize_source_table` resources for each table that is currently defined within your sources. This ensures that the tables are preserved during the migration:
+
+```hcl
+resource "materialize_source_table" "mysql_table_from_source" {
+  name           = "mysql_table1_from_source"
+  schema_name    = "public"
+  database_name  = "materialize"
+
+  source {
+    name = materialize_source_mysql.mysql_source.name
+    // Define the schema and database for the source if needed
+  }
+
+  upstream_name        = "mysql_table1"
+  upstream_schema_name = "shop"
+
+  ignore_columns = ["about"]
+}
+```
+
+### Step 2: Apply the Changes
+
+Run `terraform plan` and `terraform apply` to create the new `materialize_source_table` resources. This step ensures that the tables are defined separately from the source and are not removed from Materialize.
+
+> **Note:** This will start an ingestion process for the newly created source tables.
+
+### Step 3: Remove Table Blocks from Source Resources
+
+Once the new `materialize_source_table` resources are successfully created, you can safely remove the `table` blocks from your existing source resources:
+
+```hcl
+resource "materialize_source_mysql" "mysql_source" {
+  name         = "mysql_source"
+  cluster_name = "cluster_name"
+
+  mysql_connection {
+    name = materialize_connection_mysql.mysql_connection.name
+  }
+}
+```
+
+This will drop the old tables from the source resources.
+
+### Step 4: Update Terraform State
+
+After removing the `table` blocks from your source resources, run `terraform plan` and `terraform apply` again to update the Terraform state and apply the changes.
+
+### Step 5: Verify the Migration
+
+After applying the changes, verify that your tables are still correctly set up in Materialize by checking the table definitions using Materialize’s SQL commands.
+
+During the migration, you can use both the old `table` blocks and the new `materialize_source_table` resources simultaneously. This allows for a gradual transition until the old method is fully deprecated.
+
+## Automated Migration Process (TBD)
+
+> **Note:** This will still not work as the previous source tables are considered subsources of the source and are missing from the `mz_tables` table in Materialize so we can't import them directly without recreating them.
+
+Once the migration on the Materialize side has been implemented, a more automated migration process will be available. The steps will include:
+
+### Step 1: Define `materialize_source_table` Resources
+
+First, define the new `materialize_source_table` resources for each table:
+
+```hcl
+resource "materialize_source_table" "mysql_table_from_source" {
+  name           = "mysql_table1_from_source"
+  schema_name    = "public"
+  database_name  = "materialize"
+
+  source {
+    name = materialize_source_mysql.mysql_source.name
+    // Define the schema and database for the source if needed
+  }
+
+  upstream_name        = "mysql_table1"
+  upstream_schema_name = "shop"
+
+  ignore_columns = ["about"]
+}
+```
+
+### Step 2: Modify the Existing Source Resource
+
+Next, modify the existing source resource by removing the `table` blocks and adding an `ignore_changes` directive for the `table` attribute. This prevents Terraform from trying to delete the tables:
+
+```hcl
+resource "materialize_source_mysql" "mysql_source" {
+  name         = "mysql_source"
+  cluster_name = "cluster_name"
+
+  mysql_connection {
+    name = materialize_connection_mysql.mysql_connection.name
+  }
+
+  lifecycle {
+    ignore_changes = [table]
+  }
+}
+```
+
+- **`lifecycle { ignore_changes = [table] }`**: This directive tells Terraform to ignore changes to the `table` attribute, preventing it from trying to delete tables that were previously defined in the source resource.
+
+### Step 3: Import the Existing Tables
+
+You can then import the existing tables into the new `materialize_source_table` resources without disrupting your existing setup:
+
+```bash
+terraform import materialize_source_table.mysql_table_from_source <region>:<table_id>
+```
+
+Replace `<region>` with the actual region and `<table_id>` with the table ID. You can find the table ID by querying the `mz_tables` table.
+
+### Step 4: Run Terraform Plan and Apply
+
+Finally, run `terraform plan` and `terraform apply` to ensure that everything is correctly set up without triggering any unwanted deletions.
+
+This approach allows you to migrate your tables safely without disrupting your existing setup.
+
+## Importing Existing Tables
+
+To import existing tables into your Terraform state using the manual migration process, use the following command:
+
+```bash
+terraform import materialize_source_table.table_name <region>:<table_id>
+```
+
+Ensure you replace `<region>` with the region where the table is located and `<table_id>` with the ID of the table.
+
+> **Note:** The `upstream_name` and `upstream_schema_name` attributes are not yet implemented on the Materialize side, so the import process will not work until these changes are made.
+
+## Future Improvements
+
+The Kafka and Webhooks sources are currently being implemented. Once these changes, the migration process will be updated to include them.