Wednesday, October 18, 2006

Disappearing Images #2

In my previous post, in this series, Disappearing Images, I suggested that this is caused by an MTU setting problem.

In thinking further, I think that the nature of the photos problem fits both a DNS problem, and an MTU setting problem.It's apparently random, by multiple factors
  • Blog.
  • Photo within blog.
  • Reader of the blog.
  • Time.


From reading questions by Blogger, I am guessing that they are focusing on the blogs, and the photos. I think that we need to diagnose the DNS and MTU factors, since we are the ones affected.

Bothe DNS and MTU involve 3 groups of factors.
  • The reader (you, or your friends), and client computer.
  • The network, including routers and servers between the readers, and the Blogger servers.
  • The Blogger servers themselves.


The problem is seemingly random by time and by person - some people will see the problem, while others won't, even when viewing the same blog. No two computers in the world, including what network activity they are involved in, how they are connected to the network, and what blogs they are accessing, will ever be the same. So two people, even when accessing the same blog simultaneously, will get different results.

The problem is seemingly random by photo within blog - any one person will see some photos, but not others. This is caused by cache issues. Any time that you view a blog, and you have ever viewed any post previously, some of the photos will be in cache. Some photos may be in cache, but be expired. Different photos may have been added to the blog at different times, will have different cache expiry status, and may or may not be displayed if there is a problem at that time.
  • Any photos that are not in cache won't be retrieved and won't be displayed.
  • Any photos that are in cache, but are expired, won't be retrieved and won't be displayed.
  • Any photos that are in cache and are not expired, will be displayed.
  • If there is no problem at that time, all photos, regardless of cache status, will be displayed.


DNS also is cache sensitive. At any time, any DNS server in the world may need to provide the IP address of photos1.blogger.com, so a customer can retrieve a photo. The Blogger DNS server will be called upon, to provide that IP address. If you, or any other user of the same DNS server, needs that address, its availability will be subject to similar possibilities.
  • If the address is not in cache, any photos that are needed won't be retrieved and won't be displayed.
  • If the address is in cache but expired, any photos that are needed won't be retrieved and won't be displayed.
  • If the address is in cache and not expired, any photos that are needed will be retrieved and displayed.
  • If there is no problem at that time, all photos, regardless of DNS status, will be retrieved and displayed.


There are 3 factors that affect the status of any item in a cache - be it a DNS cache, or Temporary Internet files.
  • Activity. With a more active computer, client or server, items expire sooner.
  • Age. The oldest items expire sooner from cache .
  • Cache size. With a smaller cache, items expire sooner.
Here, too, every computer in the world will be different.

The bottom line here? My gut feel is that there's one or more servers, somewhere in the Blogger or Google complex, that are intermittently bad. Given all of the above possibilities, the intermittent period doesn't have to be too small, or too irregular. It doesn't take too much imagination to see any one server being down for hours, if not days, and causing the above problems.

With all of that said, I would like to diagnose whether DNS or MTU are involved on the client end. Any diagnosis will require the involvement of the folks seeing the symptoms, when the symptoms are seen. I will discuss what needs to be done next, in my next article in this series, Disappearing Images - 3.

(Edit 10/26 15:30): Blogger equipment replacement today apparently involved changes to the DNS infrastructure.

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2 comments:

Trish said...

All I know is, it's driving me nuts having my images come and go...!!

Justen said...

Hi!

My images have (apparently) permanently disappeared. I've sent four or five Blogger help requests, with no response since. I tried calling Google. I've now blogged about it.

http://justen.blogspot.com/

Someone at Blogger, please wake up!