kerneltime commented on code in PR #6482: URL: https://github.com/apache/ozone/pull/6482#discussion_r1571160333
########## hadoop-hdds/docs/content/design/overwrite-key-only-if-unchanged.md: ########## @@ -0,0 +1,191 @@ +--- +title: Overwriting an Ozone Key only if it has not changed. +summary: A minimal design illustrating how to replace a key in Ozone only if it has not changes since it was read. +date: 2024-04-05 +jira: HDDS-10657 +status: accepted +author: Stephen ODonnell +--- + +<!-- + Licensed under the Apache License, Version 2.0 (the "License"); + you may not use this file except in compliance with the License. + You may obtain a copy of the License at + + http://www.apache.org/licenses/LICENSE-2.0 + + Unless required by applicable law or agreed to in writing, software + distributed under the License is distributed on an "AS IS" BASIS, + WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. + See the License for the specific language governing permissions and + limitations under the License. See accompanying LICENSE file. +--> + + +Ozone offers write semantics where the last writer to commit a key wins. Therefore multiple writers can concurrently write the same key, and which ever commits last will effectively overwrite all data that came before it. + +As an extension of this, there is no "locking" on a key which is being replaced. + +For any key, but especially a large key, it can take significant time to read and write it. There are scenarios where it would be desirable to replace a key in Ozone, but only if the key has not changed since it was read. With the absence of a lock, such an operation is not possible today. + +## As Things Stand + +Internally, all Ozone keys have both an objectID and UpdateID which are stored in OM as part of the key metadata. + +Each time something changes on the key, whether it is data or metadata, the updateID is changed. It comes from the ratis transactionID and is generally an increasing number. + +When an existing key is over written, its existing metadata including the ObjectID and ACLs are mirrored onto the new key version. The only metadata which is replaced is any custom metadata stored on the key by the user. Upon commit, the updateID is also changed to the current Ratis transaction ID. + +Writing a key in Ozone is a 3 step process: + +1. The key is opened via an Open Key request from the client to OM +2. The client writes data to the data nodes +3. The client commits the key to OM via a Commit Key call. + +Note, that as things stand, it is possible to lose metadata updates (eg ACL changes) when a key is overwritten. + +1. If the key exists, then a new copy of the key is open for writing. +2. While the new copy is open, another process updates the ACLs for the key +3. On commit, the new ACLs are not copied to the new key as the new key made a copy of the existing metadata at the time the key was opened. + +With the technique described in the next section, that problem is removed in this design, as the ACL update will change the updateID, and the key will not be committed. + +## Atomic Key Replacement + +In relational database applications, records are often assigned an update counter similar to the updateID for a key in Ozone. The data record can be read and displayed on a UI to be edited, and then written back to the database. However another user could have made an edit to the same record in the mean time, and if the record is written back without any checks, those edits could be lost. + +To combat this, "optimistic locking" is used. With Optimistic locking, no locks are actually involved. The client reads the data along with the update counter. When it attempts to write the data back, it validates the record has not change by including the updateID in the update statement, eg: + +``` +update customerDetails +set <columns = values> +where customerID = :b1 +and updateCounter = :b2 +``` +If no records are updated, the application must display an error or reload the customer record to handle the problem. + +In Ozone the same concept can be used to perform an atomic update of a key only if it has not changed since the key details were originally read. + +To do this: + +1. The client reads the key details as usual. The key details can be extended to include the existing updateID as it is currently not passed to the client. +2. The client opens a new key for writing with the same key name as the original, passing the previously read updateID in a new field. Call this new field overwriteExpectedUpdateID. +3. On OM, it receives the openKey request as usual and detects the presence of the overwriteExpectedUpdateID. +4. On OM, it first ensures that a key is present with the given key name and having a updateID == overwriteExpectedUpdateID. If so, it opens the key and stored the details including the overwriteExpectedUpdateID in the openKeyTable. As things stand, the other existing key metadata copied from the original key is stored in the openKeyTable too. +5. The client continues to write the data as usual. +6. On commit key, the client does not need to send the overwriteExpectedUpdateID again, as the open key contains it. +7. On OM, on commit key, it validates the key still exists with the given key name and its updateID is unchanged. If so the key is committed, otherwise an error is returned to the client. + +Note that any change to a key will change the updateID. This is existing behaviour, and committing an rewritten key will also modify the updateID. Note this also offers protection against concurrent rewrites. + +### Alternative Proposal + +1. Pass the expected updateID to the rewrite API which passes it down to the relevant key stream, effectively saving it on the client +2. Client attaches update ID to the commit request to indicate a rewrite instead of a put +3. OM checks the update ID if present and returns the corresponding success/fail result + +The advantage of this alternative approach is that it does not require the overwriteUpdateID to be stored in the openKey table. + +However the client code required to implement this appears more complex due to having different key commit logic for Ratis and EC and the parameter needing to be passed through many method calls. + +The existing implementation for key creation stores various attributes (metadata, creation time, ACLs, ReplicationConfig) in the openKey table, so storing the overwriteExpectedUpdateID keeps with that convention, which is less confusing for future developers. + +In terms of forward / backward compatibility both solutions are equivalent. Only a new parameter is required within the KeyArgs passed to create and commit Key. + +If an upgraded server is rolled back, it will still be able to deal with an openKey entry containing overWriteUpdateID, but it will not process it atomically. + +### Scope + +The intention is to first implement this for OBS buckets. Then address FSO buckets. Review Comment: What is the additional complexity to do it for both? -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. To unsubscribe, e-mail: [email protected] For queries about this service, please contact Infrastructure at: [email protected] --------------------------------------------------------------------- To unsubscribe, e-mail: [email protected] For additional commands, e-mail: [email protected]
