Add Bookmark support for mango json queries

[garrensmith]

This adds a bookmark that is sent with each query and can be used to
continue a query from a specific key. This will allow users to paginate
mango queries.

Checklist

• Code is written and works correctly;
• Changes are covered by tests;
• Documentation reflects the changes;

[willholley]

testing a basic case where no results are returned, I get a response:

{"warning":"no matching index found, create an index to optimize query time",
"docs":[
],
"bookmark": "nil"}

However, passing "nil" back as a bookmark returns an error:

$ curl 'http://127.0.0.1:15984/test/_find' -H 'Content-Type:application/json' -d '{"selector":{"id":"willnotmatch"},"bookmark":"nil"}'
{"error":"invalid_bookmark","reason":"Invalid bookmark value: \"nil\""}

[willholley]

suggest adding a pagination test where each document emits the same value into the index to ensure progress is made using startkey_docid.

[garrensmith]

Thanks @willholley I've fixed the issue and added another test.

[tonysun83]

This part got me thinking a bit. What is a valid vs invalid bookmark? So when unpacked, a json bookmark is a start_key and end_key range, and a text bookmark is a node+shard range. So the composition is slightly different. So a text index bookmark should be considered invalid for a json index bookmark right? When I tried that particular scenario, the bookmark passed correctly Here's a sample text bookmark: "g2wAAAACaAJkAA5zdGFydGtleV9kb2NpZG0AAAAkZTkwMDAwMWQtYmM0OC00OGE2LTliMWEtYWM5YTFmNWQxYTAzaAJkAAhzdGFydGtleWsAAStq" .

If you did it the other way around...substitute a json index bookmark into a text index bookmark, it fails with invalid_bookmark.

Wondering if we should be more consistent/strict and and verify both types of bookmarks?

[tonysun83]

missing new line

[garrensmith]

I don't quite follow? Do you want me to remove that new line or add another?

[tonysun83]

weird, it showed no new end of line. just normal end of file new line is fine

[tonysun83]

whitespace change

[garrensmith]

@tonysun83 thanks for the review. I've make some fixes based on your feedback

[tonysun83]

+1 after that fixing that weird end of line issue

[anuragvohraec]

This adds a bookmark that is sent with each query and can be used to
continue a query from a specific key. This will allow users to paginate
mango queries.

How exactly is bookmark implmented? [I have no knowledge of Erlang, but have written a couchdb client in dart]. Is it base64 enocded of json of last index[or doc] sent to user ?

[garrensmith]

@anuragvohraec the code is here https://github.com/apache/couchdb/blob/master/src/mango/src/mango_json_bookmark.erl
We basically take the startkey and startkey_docid and convert to erlang binary and then to base64