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".
Thanks,
Rich
From: jiangph <[email protected]>
To: [email protected]
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,
<<"hca">>)),,
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.
Parameters:
db –Database name
Request Headers:
Content-Type –application/json
Response Headers:
Content-Type –
application/json
Status Codes:
200 OK –Request completed successfully
404 Not Found –Requested database not found
Request:
GET /db/_deleted_dbs_info HTTP/1.1
Accept: application/json
Host: localhost:5984
Response:
HTTP/1.1 200 OK
Cache-Control: must-revalidate
Content-Type: application/json
{
"total_rows": 2,
"rows": [{
"deleted_when": "20200318.071532",
"info": {
"update_seq":
"0000019100b5992700000000",
"doc_del_count": 0,
"doc_count": 3,
"sizes": {
"external": 287,
"views":
0
}
}
}, {
"deleted_when": "20200318.071703",
"info": {
"update_seq":
"0000019105f0e29900000000",
"doc_del_count": 0,
"doc_count": 2,
"sizes": {
"external": 200,
"views":
0
}
}
}]
}
3) `PUT /{db}/_restore/{deletedTS}`
Restore a deleted database.
Parameters:
db –Database name
deletedTS - timestamp when database was deleted
Request Headers:
Accept –
application/json
text/plain
Response Headers:
Content-Type –
application/json
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
4xx
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.
Cheers
Peng Hui
[1]
https://urldefense.proofpoint.com/v2/url?u=https-3A__github.com_apache_couchdb_blob_master_src_couch_src_couch-5Ffile.erl-23L251&d=DwIFaQ&c=jf_iaSHvJObTbx-siA1ZOg&r=CDCq0vbFWjQXx1sCFm2-iYoMZQ4i0QQj2XmPZmLvZp0&m=Axkxn5LYrVZw0h2AT61RjMT83ZlljDikPgUfCzAql5E&s=91sTDh27SxPlA-ejydi3-mlMFm5qA-makaWpFPhqrlM&e=
[2]
https://urldefense.proofpoint.com/v2/url?u=https-3A__github.com_apache_couchdb_blob_prototype_fdb-2Dlayer_src_fabric_src_fabric2-5Ffdb.erl-23L182-2DL184&d=DwIFaQ&c=jf_iaSHvJObTbx-siA1ZOg&r=CDCq0vbFWjQXx1sCFm2-iYoMZQ4i0QQj2XmPZmLvZp0&m=Axkxn5LYrVZw0h2AT61RjMT83ZlljDikPgUfCzAql5E&s=44Jf80PJJPAsgg2LKZXYelRa8sWTxlm2CCtJN1JWOMg&e=
[3]
https://urldefense.proofpoint.com/v2/url?u=https-3A__activesphere.com_blog_2018_08_05_high-2Dcontention-2Dallocator&d=DwIFaQ&c=jf_iaSHvJObTbx-siA1ZOg&r=CDCq0vbFWjQXx1sCFm2-iYoMZQ4i0QQj2XmPZmLvZp0&m=Axkxn5LYrVZw0h2AT61RjMT83ZlljDikPgUfCzAql5E&s=5jLD7_hzgxzAhJP-NRqIbQuw2e8Q1vNeQQwAqXMsPBw&e=
<
https://urldefense.proofpoint.com/v2/url?u=https-3A__activesphere.com_blog_2018_08_05_high-2Dcontention-2Dallocator&d=DwIFaQ&c=jf_iaSHvJObTbx-siA1ZOg&r=CDCq0vbFWjQXx1sCFm2-iYoMZQ4i0QQj2XmPZmLvZp0&m=Axkxn5LYrVZw0h2AT61RjMT83ZlljDikPgUfCzAql5E&s=5jLD7_hzgxzAhJP-NRqIbQuw2e8Q1vNeQQwAqXMsPBw&e=
>
[4]
https://urldefense.proofpoint.com/v2/url?u=https-3A__docs.couchdb.org_en_stable_api_database_common.html-23delete-2D-2Ddb&d=DwIFaQ&c=jf_iaSHvJObTbx-siA1ZOg&r=CDCq0vbFWjQXx1sCFm2-iYoMZQ4i0QQj2XmPZmLvZp0&m=Axkxn5LYrVZw0h2AT61RjMT83ZlljDikPgUfCzAql5E&s=WkBi398mDF995FmYkRw0DeqwQSMtCD4xYLmPwntrRkI&e=
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