Consistency

This page explains which Google Cloud Storage operations are strongly consistent and which are eventually consistent. In the case of cacheable, publicly readable objects, you control the degree to which operations on the objects are consistent.

Strongly consistent operations

Cloud Storage provides strong global consistency for the following operations, including both data and metadata:

  • Read-after-write
  • Read-after-metadata-update
  • Read-after-delete
  • Bucket listing
  • Object listing
  • Granting access to resources

When you upload an object to Cloud Storage, and you receive a success response, the object is immediately available for download and metadata operations from any location where Google offers service. This is true whether you create a new object or overwrite an existing object. Because uploads are strongly consistent, you will never receive a 404 Not Found response or stale data for a read-after-write or read-after-metadata-update operation.

In addition, when an upload request succeeds, it means your data is replicated in multiple data centers. The latency for writing to Cloud Storage's globally consistent, replicated store may be slightly higher than for a non-replicated or non-committed store. This is because a success response is returned only when multiple writes complete, not just one.

Strong global consistency also extends to deletion operations on objects. If a deletion request succeeds, an immediate attempt to download the object or its metadata will result in a 404 Not Found status code. You get the 404 error because the object no longer exists after the delete operation succeeds.

Bucket listing is strongly consistent. For example, if you create a bucket, then immediately perform a list buckets operation, the new bucket appears in the returned list of buckets.

Object listing is also strongly consistent. For example, if you upload an object to a bucket and then immediately perform a list objects operation, the new object appears in the returned list of objects.

For buckets, while metadata updates are strongly consistent for read-after-metadata-update operations, the resulting configuration changes may take time to propagate. For example, if you enable object versioning on a bucket, you should wait at least 30 seconds before deleting or overwriting objects.

Eventually consistent operations

The following operations are eventually consistent:

  • Revoking access from resources

It typically takes about a minute for revoking access to take effect. In some cases it may take longer.

As an example of behavior that can arise from eventual consistency, if you remove a user's access to an bucket, this change is immediately reflected in the metadata for the bucket; however, the user may still have access to the bucket for a short period of time.

Cache control and consistency

Cached objects that are publicly readable might not exhibit strong consistency. If you allow an object to be cached, and the object is in the cache when it is updated or deleted, the cached object is not updated or deleted until its cache lifetime expires.

The cache lifetime of an object is defined by the Cache-Control metadata associated with the object. The Cache-Control metadata can be set using a Cache-Control request header included in the initial upload of the object, or in a subsequent update to the metadata of the object. Because you control the Cache-Control metadata, you also control the degree to which cached objects are consistent. Moreover, while requests for the object can include their own Cache-Control header, these headers are ignored by Cloud Storage if they're set to avoid cached content.

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Cloud Storage Documentation