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From Richard Ellis <>
Subject Re: [DISCUSS] soft-deletion
Date Thu, 19 Mar 2020 17:23:24 GMT
This is a nice proposal, but I have a few comments/questions regarding the 
proposed API:

## API

1)  `DELETE /{db}`

I wonder if there should be some consideration of the status code here. 
Currently for a db delete a 200/202 is returned depending if quorum is 
met. Do FDB transactional semantics mean that is likely to change at all?
I can't really decide if a 202 always makes more sense for a soft-deletion 
(in the sense that the server has accepted the request to delete the 
database, but won't really do it for some time). On the other hand as soon 
as the database is in a soft-deleted state the {db} paths will return 404 
because the database is not operable at that path, so perhaps there isn't 
really any change.

2)  `GET /{db}/_deleted_dbs_info`

It isn't shown in the example, but I'm assuming as this is a view-like 
response that the database name is the `key` for the row similarly to 
_dbs_info endpoint. Is that correct?

Would there be any advantage to using a standard timestamp format 
(specifically ISO 8601, which I think should be used for all new internet 
date/time representations). Compared to the examples shown this would 
probably be more user friendly for people to identify the actual time 
associated with instances and would have improved interoperability with 
other systems interacting with the API that might want to display the 
deleted date/time. Having said that I think the additional complexity of 
ISO8601 would be less useful as the identifier for restore operations, 
which would make me lean towards the UUID proposed by Nick for that part.

3) `PUT /{db}/_restore/{deletedTS}`

It seems that this is basically identical in structure to the create 
operation. Are we expecting down the line a more complex set of options, 
different response structure, or different permission to a normal db 
create that warrants having a new endpoint? Given there is precedent for 
using a `partitioned=true` parameter to create a database in a "special" 
way, is there a good reason not to follow the same pattern here? 
e.g. PUT /{db}?restore=timestamp/uuid

That would maintain the symmetry of the PUT/DELETE operations on the /{db} 
path. What I mean by this is that other `PUT /{db}/foo/bar` path 
operations always create sub-resources of a database (i.e. docs, 
design_docs, attachments etc) and 404 in the cases that the database does 
not exist. It seems strange to me to use a path structure similar to those 
that has different semantics because it doesn't 404 when the {db} part 
doesn't exist. Consider an equivalent could be the expectation that the 
`PUT /{db}/{doc_id}` operation would create the database if it doesn't 
exist instead of returning 404. Additionally usually the PUT verb is used 
for creating resources at the path of the request, but that is not the 
outcome of this call.

I also wonder a little bit about the use of "restore" here. I guess there 
maybe should be a little consideration of whether there might be some 
future backup/restore functionality and whether it be able to restore via 
the same method or not. If not then calling this "restore" might cause 
confusion with any future backup/restore API, say, compared to calling 
this something like "undelete".


From:   jiangph <>
Date:   18/03/2020 12:05
Subject:        [EXTERNAL] [DISCUSS] soft-deletion

Hi Everyone,

Instead of automatically and immediately removing data and index in 
database after a delete operation, soft-deletion allows to restore the 
deleted data back to original state due to a “fat finger”or undesired 
delete operation, up to defined periods, such as 48 hours.

In CouchDB 3.0, soft-deletion of database is implemented in [1]. The 
.couch file is renamed with the .<timestamp>.deleted.couch file after 
soft-deletion is enabled, and such file can be changed back to .couch for 
the purpose of restore. If restore is not needed and some specified period 
passed, the .<timestamp>.deleted.couch file can be deleted to achieve 
deletion of database permanently.

In CouchDB 4.0, with the introduction of FoundationDB, the data model and 
storage is changed. In order to support soft-deletion, we propose below 
solution and then implement them. 

## Proposed Data model change

In CouchDB 4.0, directories and indirection access in FoundationDB are 
already used to better build data model. One key/value pair is used to 
build reference from Dbkey to DbPrefix. All other key/value pairs are 
based on DbPrefix instead of DbKey. This decouples the direct relationship 
between DBName and data in this database. The current implementation for 
`DBKey -> DBPrefix` is in [2]. So you can see below information in 
FoundationDB using fdbcli, etc.

{?ALL_DBS, DbName} -> {?DBS, DbName}
{?DBS, DbName, other part of key} -> <value>

To support soft-deletion, especially allowing one database to be 
deleted/re-created multiple time, we need to use different DbPrefix for 
the same DbKey/DBName. The proposed change is to use a unique value 
allocated via High Contention Allocator(HCA) algorithm in [3].

    DbPrefixAllocator = erlfdb_hca:create(?ERLFDB_EXTEND(DbId, 
    DbPrefix = erlfdb_hca:allocate(DBPrefixAllocator, Tx),
    erlfdb:set(Tx, DbKey, DbPrefix),

The data in FoundationDB looks like:

{?ALL_DBS, DbName} -> <unique key allocated by hca>
{<unique key allocated by hca>, other part of key} -> <value>

Using HCA algorithm, it can acquire one unique key quickly while avoiding 
conflicting. The more important, it is shorter enough to save space 
because `DBPrefix` exists in almost every key/value pair for database.

## Soft-deletion, restore and permanent-deletion

Once database is soft-deleted, the only action is to change `DBKey -> 
DBPrefix` pair. All other data for this data is not changed. In order to 
give clear namespace management, the proposal is to move DBkey from 
`?ALL_DBS` to `?DELETED_DBS`. The timestamp when database was deleted is 
added to `DBKey` so that we can know when the data in this database can be 
permantenly restored. The `DBKey -> DBPrefix` pair is changed to 

{?DELETED_DBS, DbName, TimeStamp} -> <unique key allocated by hca>

There is a background task to clear the ranges eventually. Depending on 
setting on how long the soft-deleted database will be kept, such as 48 
hours, the background task will check `DELETED_DBS` namespace, and find 
eligible key/value pairs, and delete data associated with this `DBPrefix` 
and then delete DbKey/DbPrefix pair finally.

Overtime, it is possible that database can be deleted several times. The 
`_deleted_dbs_info` endpoint is proposed to list information about all 
deleted instances for the specified database, including deletion 
timestamp, document counts and disk size, etc. This allows users to 
identify which one to be restored. Also it also provides information for 
billing. In given period, such as 48 hours, the deletion times of same 
database is most likely limited, the design API is to list all instances 
in one time using GET method with query parameter. 

After deciding which instance to be restored, users can use the `_restore` 
endpoint with `deletedTS` to restore database. The underlying logic is to 
change `DBKey -> DBPrefix` back to 

{?ALL_DBS, DbName} -> <unique key allocated by hca>

Considering the sensitive actions, the `_deleted_dbs_info` and `_restore` 
endpoints are supposed to be an admin-only endpoints only to allow granted 
user to restore the database.

##view index and search index

Although view index and search index is based on the `DBPrefix`, the 
change of value of `DBPrefix` doesn't have impact on storage and search of 
view index and search index because `DBPrefix` is one opaque value. If 
database is soft-deleted, the `DBkey -> DBPrefix` pair is changed so that 
any access to view index and search index will be blocked with `"Database 
does not exist."` error. This is expected. 

The only thing we need to care is to stop all indexing or pending requests 
for soft-deleted database.

## API

1)  `DELETE /{db}`

There is no change on this endpoint [4] to send DELETE against one 
database. The soft-deletion is triggered once 
[couchdb][enable_database_recovery] is set to true in configuration file.

2)  `GET /{db}/_deleted_dbs_info`

    returning basic information of all deleted instances for the specified 
database, including when the instance was deleted.

        db –Database name

    Request Headers:
        Content-Type –application/json

    Response Headers:

        Content-Type –

    Status Codes: 

        200 OK –Request completed successfully
        404 Not Found –Requested database not found


    GET /db/_deleted_dbs_info HTTP/1.1
    Accept: application/json
    Host: localhost:5984


    HTTP/1.1 200 OK
    Cache-Control: must-revalidate
    Content-Type: application/json
                 "total_rows": 2,
                 "rows": [{
                                 "deleted_when": "20200318.071532",
                                 "info": {
                                                 "doc_del_count": 0,
                                                 "doc_count": 3,
                                                 "sizes": {
 "external": 287,
                 }, {
                                 "deleted_when": "20200318.071703",
                                 "info": {
                                                 "doc_del_count": 0,
                                                 "doc_count": 2,
                                                 "sizes": {
 "external": 200,

3) `PUT /{db}/_restore/{deletedTS}`

    Restore a deleted database. 

        db –Database name
        deletedTS - timestamp when database was deleted

    Request Headers:

        Accept –

    Response Headers:

        Content-Type –
            text/plain; charset=utf-8

    Response JSON Object:

        ok (boolean) –Operation status. Available in case of success
        error (string) –Error type. Available if response code is 4xx
        reason (string) –Error description. Available if response code is 

    Status Codes: 

        200 Restored –Database restored successfully 
        400 Bad Request –Invalid database name or deleted timestamp
        401 Unauthorized –CouchDB Server Administrator privileges required
        412 Precondition Failed –Database already exists
What do you think of that? Any questions or thoughts on this? Once again a 
big acknowledgment to Nick and Paul who helped with initial design and 
provide consultation on this.

Peng Hui






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