kv Index.Walk function matches the foreign key by prefix only

[stuartcarnie]

What

When the kv store searches the index, it utilises a forward cursor matching by prefix:

influxdb/kv/index.go

Lines 192 to 193 in 15b9531

	cursor, err := indexBucket.ForwardCursor(foreignKey,
	WithCursorPrefix(foreignKey))

The cursor will return all keys that have a matching prefix, in the following loop, including those keys which are longer:

influxdb/kv/index.go

Lines 206 to 208 in 15b9531

	for ik, pk := indexCursor.Next(); ik != nil; ik, pk = indexCursor.Next() {
	keys = append(keys, pk)
	}

For example, if the foreignKey is hello and the index contains the keys hello, hello-world and hello-there, the loop will gather them all.

In most cases, the index is typically the result of combining 16-byte IDs, so this is not a problem. However, for dbrp, it creates an additional orgID + database foreign key:

influxdb/dbrp/service.go

Lines 70 to 76 in b274e15

	byOrgAndDatabase: kv.NewIndex(kv.NewIndexMapping(bucket, indexBucket, func(v []byte) ([]byte, error) {
	var dbrp influxdb.DBRPMappingV2
	if err := json.Unmarshal(v, &dbrp); err != nil {
	return nil, err
	}
	return indexForeignKey(dbrp), nil
	}), kv.WithIndexReadPathEnabled),

Creation of DBRPs fail:

1. Create bucket1

2. Create a dbrp mapping for bucket1 with db jenkins-aws, rp never:

   curl -H 'Content-type: application/json' --header "Authorization: Token $INFLUX_TOKEN" -XPOST "$INFLUX_HOST/api/v2/dbrps" --data-binary '{"bucket_id": "<BUCKET1_ID>","database": "jenkins-aws","default": true, "organization_id": "<org-id>", "retention_policy": "never"}'

3. Create bucket2

4. Attempt to create a dbrp mapping for bucket2 with db jenkins, rp never:

   curl -H 'Content-type: application/json' --header "Authorization: Token $INFLUX_TOKEN" -XPOST "$INFLUX_HOST/api/v2/dbrps" --data-binary '{"bucket_id": "<BUCKET2_ID>","database": "jenkins","default": true, "organization_id": "<org-id>", "retention_policy": "never"}'

   Fails with the following error:

   another DBRP mapping with same orgID, db, and rp exists
   

[GeorgeMac]

Hey @stuartcarnie 👋

Sorry, definitely a gap in the documentation here. This index is currently a non-unique many to one secondary index.

resource >- association
dashboard >- org

And so the index is structured like so:

indexprefix/foreign_key/primary_key -> primary_key

bucketsbyorgv1/org_foo/bucket_one -> bucket_one
bucketsbyorgv1/org_foo/bucket_two -> bucket_two
bucketsbyorgv1/org_foo/bucket_three -> bucket_three
bucketsbyorgv1/org_foo/bucket_four -> bucket_four

Such that we can have multiple entries per foreign key. Meaning we could retrieve the many resources for a particular foreign key in a single range / cursor with a prefix lookup.

As this was originally designed around creating a URM by User index.

The Walk function visits each key / value pair in the source bucket. Not the index bucket. You are effectively walking the source, based on entries in the index:

influxdb/kv/index.go

Lines 218 to 229 in 15b9531

	values, err := sourceBucket.GetBatch(keys...)
	if err != nil {
	return err
	}
	for i, value := range values {
	if value != nil {
	if cont, err := visit(keys[i], value); !cont || err != nil {
	return err
	}
	}
	}

I have thought about adding support for a secondary index with a uniqueness constraint. In that scenario walk should find one item. Or perhaps a better name might be Find or similar. Infact it could just be another type which also consumes the IndexMapping type. The mapping type describe a source and index bucket name and a function for retrieving the associated index foreign key, for sure when backfilling and unpopulated index.

[stuartcarnie]

@GeorgeMac thanks for the reply! I do understand the many to one feature. There is a bug or unexpected behaviour, as described in the issue, that because Walk matches the foreign key using a prefix match, it will find any keys with that prefix.

So, back to the dbrp example, if a user adds a mapping to a database named jenkins-aws, the indexes foreign key would be <OrgID>jenkins-aws. The next attempt to add a mapping to a database named jenkins fails as the lookup (a prefix match) for jenkins finds <OrgID>jenkins-aws.

It seems it is expected behaviour as a number of other tests fail when I fix that issue. It seems we might need a WalkExact API to match the foreign key exactly.

[GeorgeMac]

ooooooh I seee 😂 now I am with you. Took me long enough. I added a comment to your draft PR, should be fixable in Walk.