5 ms·

Can't caching a page with varnish and memcache handle this?

deleted 11y ago
[deleted]
nothrabannosir 11y ago
They cache the bejeezus out of their pages. Problems come up when a lot of people want to edit a page, an inherently uncacheable operation.
- fleitz 11y ago
  It's possible to use stacks to 'cache' writes in scenarios like this. Writes to the same object go in the same stack, iterate over stacks, pop the first item, write it, clear the stack. It works miracles for ephemeral data like wikipedia edits. If you have extremely spikey load on servers, stacks are also a great replacement for queues, admit that during the deluge some portion of queries will timeout and go unanswered, instead of trying to process queries that are likely to timeout, simply process the first query on the stack and don't waste time processing the ones bound to fail.
  - chris_wot 11y ago
    Doesn't caching writes from multiple different servers potentially cause consistency and durability concerns? I know MongoDB still haven't marked the bug [1] reported by Kyle Kingsbury [2] that found stale reads on all consistency and write concern levels... 1. https://jira.mongodb.org/plugins/servlet/mobile#issue/SERVER-17975 https://jira.mongodb.org/plugins/servlet/mobile#issue/SERVER... 2. https://aphyr.com/posts/322-jepsen-mongodb-stale-reads https://aphyr.com/posts/322-jepsen-mongodb-stale-reads
    - fleitz 11y ago
      Yes, but this is wikipedia, the entire premise of it is eventual consistency. The idea being that the most recent update to a page is the correct one.
      - chris_wot 10y ago
        Good point.
untog 11y ago
So... did you read the article?
Washuu 11y ago
Varnish does handle the mass of logged out cached requests. However, because they were having such a high amount of page edits per second the cache in Varnish would only by valid for about a second. Then a flood of logged out users hit the servers at the same requesting the uncached page to be rendered. The PoolCounter extensions keeps the web servers under control and by throttling requests for page rendering.
- danielrhodes 11y ago
  Correct me if I am wrong, but I thought Varnish has support for limiting concurrent backend fetches to the same resource.
  - brianwawok 11y ago
    Across a cluster of varnishes? I think that limit is per varnish.
    - danielrhodes 11y ago
      No, definitely not across a cluster (although that would be quite nifty). Even on a single node that would reduce the thundering herd effect substantially.
      - chris_wot 11y ago
        Maybe that's a new feature request!
- seanp2k2 11y ago
  Why not just set the min TTL to a few minutes, or even a minute, for anonymous users? Is there enough usefulness in ~seconds of delay on article edits vs minutes to warrant a much more complex design?

deleted 11y ago

[deleted]

They cache the bejeezus out of their pages. Problems come up when a lot of people want to edit a page, an inherently uncacheable operation.

fleitz 11y ago
It's possible to use stacks to 'cache' writes in scenarios like this. Writes to the same object go in the same stack, iterate over stacks, pop the first item, write it, clear the stack. It works miracles for ephemeral data like wikipedia edits. If you have extremely spikey load on servers, stacks are also a great replacement for queues, admit that during the deluge some portion of queries will timeout and go unanswered, instead of trying to process queries that are likely to timeout, simply process the first query on the stack and don't waste time processing the ones bound to fail.
- chris_wot 11y ago
  Doesn't caching writes from multiple different servers potentially cause consistency and durability concerns? I know MongoDB still haven't marked the bug [1] reported by Kyle Kingsbury [2] that found stale reads on all consistency and write concern levels... 1. https://jira.mongodb.org/plugins/servlet/mobile#issue/SERVER-17975 https://jira.mongodb.org/plugins/servlet/mobile#issue/SERVER... 2. https://aphyr.com/posts/322-jepsen-mongodb-stale-reads https://aphyr.com/posts/322-jepsen-mongodb-stale-reads
  - fleitz 11y ago
    Yes, but this is wikipedia, the entire premise of it is eventual consistency. The idea being that the most recent update to a page is the correct one.
    - chris_wot 10y ago
      Good point.

untog 11y ago

So... did you read the article?

Washuu 11y ago

Varnish does handle the mass of logged out cached requests. However, because they were having such a high amount of page edits per second the cache in Varnish would only by valid for about a second. Then a flood of logged out users hit the servers at the same requesting the uncached page to be rendered. The PoolCounter extensions keeps the web servers under control and by throttling requests for page rendering.

danielrhodes 11y ago
Correct me if I am wrong, but I thought Varnish has support for limiting concurrent backend fetches to the same resource.
- brianwawok 11y ago
  Across a cluster of varnishes? I think that limit is per varnish.
  - danielrhodes 11y ago
    No, definitely not across a cluster (although that would be quite nifty). Even on a single node that would reduce the thundering herd effect substantially.
    - chris_wot 11y ago
      Maybe that's a new feature request!
seanp2k2 11y ago
Why not just set the min TTL to a few minutes, or even a minute, for anonymous users? Is there enough usefulness in ~seconds of delay on article edits vs minutes to warrant a much more complex design?