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Comment by et1337 | Hacker News Reader

Another reason to use cursors is to avoid the problem of repeated or skipped elements caused by concurrent edits. If you use offsets and you’re on page 10, and someone deletes an item on page 1, the whole list shifts and you can accidentally skip an item on page 11. Likewise if someone adds an item on page 1 and you’re on page 10, one of the page 10 items will also show up on page 11.

Cursors elegantly sidestep these issues.

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One way to get around this is to drop the notion of page number. Instead do greater than last element on last page.

But this has the drawback of only working as long as the sorting field doesn't have many duplicates. You have to beware if timestamps are high resolution enough for your application or if you have 100+ entries with same family name.

Usually users are smart(lazy) enough to realize they got more than 100 results and will narrow down the search instead. When was the last time you looked beyond even the first 5 results on google yourself, i'd rather adjust the search term than go to page 2.

If your ability to query can manage sorting on two fields, this can be resolved by sorting on the field first, then the primary key as a secondary field.

If I'm sorting by last name, and I sort by last name, then id, I can save the cursor as last name + id, and be able to continue right where I left off.

> One way to get around this is to drop the notion of page number. Instead do greater than last element on last page.

That's the whole point of a cursor, isn't it?

And pagination just means querying the records to return a limited subset.

Depends how long the cursor is alive. Url containing ?date=lt:2012-12-27 is stateless, user readable and more likely to outlive ?cursorId=xyaxjeasu&page=2.

Now it might not be as accurate as having a cursor but it is a lot easier to implement, and still beats simply ?page=2 without cursor, which is the level you see in most apis today.

> Depends how long the cursor is alive.

The cursor is never invalidated. You can always get a valid response from any cursor, whether the resource exists or was already deleted. That's the whole point of using a cursor.

Perhaps your confusion lies in assuming that a cursor's life cycle is tied to a specific object. It isn't. A cursor means "get me whatever resources would immediately follow this resource". It matters nothing if the cursor exists or not. When you run the query, you will get exactly the resource which would follow the cursor. That's by design.

What if the last element on a page is deleted when you click "next"?

You use the sort key as the page selector. So if it's cities sorted by name, the query is "give me the first 20 cities with names after 'Constantinople'". If Constantinople gets the works, it doesn't matter, because there are still 20 cities whose names come after that lexicographically.

If it is updated, though (in this case, renamed to Istanbul), it can appear later once again, and newly inserted rows behind "Constantinople" will never appear.

I mean, these problems happen if you use offsets too, and my point is that they don't matter.

The most hardcore solution to those would be pre-computing pages but I doubt it makes sense to bring even more state to the search results.

Yes, pagination over a consistent view is a whole other problem! Either you store search results server side, in which case now you need to think about expiration and running out of space, or you are able to regenerate them on the fly somehow. A database which supports as-of queries might make that quite easy, but not many databases do.

> but not many databases do.

Really? The underlying storage engine of most databases stores all data necessary for an "as of" query. Simply ignore all data pages newer than the timestamp cutoff, and prevent the garbage collection of any page that could be part of such a query.

In fact thats the way [eg. postgres] can do a long-running query on a table that someone else is modifying. You in effect are looking at a snapshot of the table at the moment you started the query, even if it takes an hour to produce all the results.

> If Constantinople gets the works

Relevant song: https://youtube.com/watch?v=Z0JhC3LO0-8

A B C D

2 items per page

first page is A&B. Next query is "WHERE ID>'B'".

What's the problem?

Aa and Ab are inserted after loading the first page.

Sure, and then B comes by again. Which just means there are different use cases with different definitions of correct, and you need to specify yours.

That's an orthogonal problem that you need to define correctness for your application.

Because LIMIT...OFFSET will also arguably give you the wrong result. E.g. Aa is inserted. The user will see "B" both as the last item on the first page, and the first item on the second page.

Or with your example:

Aa Ab B C

Now "A" is inserted between page loads. The user will never see "A". What's right for your application? Maybe a notification saying "previous pages have gotten new items". Maybe not. It all depends.

Reddit can be annoying if you go page after page. As stories are bumped down you see them again. That, in my opinion, is a bug. But solving it requires something completely different from merely defining page boundaries.

> What if the last element on a page is deleted when you click "next"?

That makes absolutely no difference. You're still querying the database for elements that come after a value from a data type for which there is an order. You don't need that element to exist to run a comparison. For example, consider a timestamp-based query: you don't need an element with that specific date to exist to search for any element whose timestamp was taken after an arbitrary moment.

Though you can't avoid seeing an item just before it is deleted and skipping an item just before it is created. This isn't a huge problem but does mean that the list you end up with may not be a coherent view of the database. But if you require guarantees like that then you might be better of just using SQL directly.

Or just include everything in a single response, HTTPS was designed to handle arbitrary length payloads if I recall correctly. You might want to use a more streamable format than JSON though.

If you care about that kind of guarantee you might want to keep a connection open and stream new updates.

> If you care about that kind of guarantee you might want to keep a connection open and stream new updates.

You don't need that. You just need to get the collection resource to be cacheable and subsequently track your resources' state with conditional requests.

https://developer.mozilla.org/en-US/docs/Web/HTTP/Conditiona...

> use a more streamable format than JSON

Did you mean we should stream data over WebSocket or use HTTP/2 or do we need to do something different altogether?

AFAIK chunked transfer works specifically with HTTP 1.1, think Kubernetes for example uses this with JSON in a few prominent places for things like WATCH. But that setup is hardly ideal for the use case.

Check out MessagePack.

https://msgpack.org/

I don't think you need to do anything difficult you can just treat it as one big file, pretty sure online radio streams have been using this technique for ages. Though I'm not too sure about the particulars.

This is annoying to do with JSON though because you need to remember to close all your brackets, something like CSV is a lot easier because you can just write the header once and then just stream the data.

Edit: Looks like Python supports this using chunked transfer by simply providing the HTTP request data as an iterator.

I have question, lets say we have

page 1 elements: A B C D E

page 2 elements: F G H I J

so 5 elements in each page.

Suppose I'm on page 2. If I insert a new element Q and it gets pushed as first then page 1 will have Q A B C D. Now if I go back to page 1, I'll get A B C D E and also a token/pointer to go back one more time only to retrieve Q.

So while cursor solved the issues you mentioned, it still will have this case in which pagination gets broken. I'm interested how we can tackle this.

A cursor is either valid in a transaction, or will cache its results - either way it will give a consistent view of data:

https://www.postgresql.org/docs/13/sql-declare.html

These cursors are not database cursors. I too was confused by the bad terminology...

Ah, I assumed from the topic, term and discussion that this would be implemented via sql cursors. Apparently elastic search has a similar concept - but like sql cursors it cannot be stateless (because you cannot ask for Nth to Mth result of searching the collection as of state Z -if you don't want to capture/supply state Z).

This means any stateless pagination api is fundamentally broken. Please don't make promises you cannot keep.

For elastic: https://www.elastic.co/guide/en/elasticsearch/reference/curr...

Another approach to solving this issue is with a temporal database (standardized in SQL:2011); supporting a “valid time” in your query gives your user a consistent view of the database tables even if others are modifying it concurrently. It’s also handy if you want to keep audit records of the database (e.g. so users can see who/when/what changes were made).

https://en.wikipedia.org/wiki/SQL:2011

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