docs: clarify locking mechanism requirement for S3

docs/usage/writing/writing-to-s3-with-locking-provider.md

@@ -1,7 +1,10 @@
 # Writing to S3 with a locking provider
 
-A locking mechanism is needed to prevent unsafe concurrent writes to a
-delta lake directory when writing to S3.
+Delta lake guarantees [ACID transactions](../../how-delta-lake-works/delta-lake-acid-transactions.md) when writing data. This is done by default when writing to all supported object stores except AWS S3. (Some S3 clients like CloudFlare R2 or MinIO may enable concurrent writing without a locking provider, refer to [this section](#enabling-concurrent-writes-for-alternative-clients) for more information).
+
+When writing to S3, delta-rs provides a locking mechanism to ensure that concurrent writes are safe. This is done by default when writing to S3, but you can opt-out by setting the `AWS_S3_ALLOW_UNSAFE_RENAME` variable to ``true``.
+
+To enable safe concurrent writes to AWS S3, we must provide an external locking mechanism.
 
 ### DynamoDB
 DynamoDB is the only available locking provider at the moment in delta-rs. To enable DynamoDB as the locking provider, you need to set the ``AWS_S3_LOCKING_PROVIDER`` to 'dynamodb' as a ``storage_options`` or as an environment variable.
@@ -43,8 +46,15 @@ Here is an example writing to s3 using this mechanism:
 ```python
 from deltalake import write_deltalake
 df = pd.DataFrame({'x': [1, 2, 3]})
-storage_options = {'AWS_S3_LOCKING_PROVIDER': 'dynamodb', 'DELTA_DYNAMO_TABLE_NAME': 'custom_table_name'}
-write_deltalake('s3a://path/to/table', df, 'storage_options'= storage_options)
+storage_options = {
+    'AWS_S3_LOCKING_PROVIDER': 'dynamodb',
+    'DELTA_DYNAMO_TABLE_NAME': 'custom_table_name'
+}
+write_deltalake(
+    's3a://path/to/table',
+    df,
+    storage_options=storage_options
+)
 ```
 
 This locking mechanism is compatible with the one used by Apache Spark. The `tablePath` property, denoting the root url of the delta table itself, is part of the primary key, and all writers intending to write to the same table must match this property precisely. In Spark, S3 URLs are prefixed with `s3a://`, and a table in delta-rs must be configured accordingly.
@@ -71,12 +81,30 @@ choose to set the `AWS_S3_ALLOW_UNSAFE_RENAME` variable to ``true`` in order to
 You need to have permissions to get, put and delete objects in the S3 bucket you're storing your data in. Please note that you must be allowed to delete objects even if you're just appending to the deltalake, because there are temporary files into the log folder that are deleted after usage.
 
 In AWS, those would be the required permissions:
+
 - s3:GetObject
 - s3:PutObject
 - s3:DeleteObject
 
 In DynamoDB, you need those permissions:
+
 - dynamodb:GetItem
 - dynamodb:Query
 - dynamodb:PutItem
 - dynamodb:UpdateItem
+
+### Enabling concurrent writes for alternative clients
+
+Unlike AWS S3, some S3 clients support atomic renames by passing some headers
+in requests.
+
+For CloudFlare R2 passing this in the storage_options will enable concurrent writes:
+
+```python
+storage_options = {
+    "copy_if_not_exists": "header: cf-copy-destination-if-none-match: *",
+}
+```
+
+Something similar can be done with MinIO but the header to pass should be verified
+in the MinIO documentation.

python/deltalake/writer.py

@@ -208,9 +208,9 @@ def write_deltalake(
     For higher protocol support use engine='rust', this will become the default
     eventually.
 
-    A locking mechanism is needed to prevent unsafe concurrent writes to a
-    delta lake directory when writing to S3. For more information on the setup, follow
-    this usage guide: https://delta-io.github.io/delta-rs/usage/writing/writing-to-s3-with-locking-provider/
+    To enable safe concurrent writes when writing to S3, an additional locking
+    mechanism must be supplied. For more information on enabling concurrent writing to S3, follow
+    [this guide](https://delta-io.github.io/delta-rs/usage/writing/writing-to-s3-with-locking-provider/)
 
     Args:
         table_or_uri: URI of a table or a DeltaTable object.