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[DataCache] "ready" CacheEvent and Related Questions

From: Joseph Pecoraro <joepeck02@gmail.com>
Date: Thu, 10 Dec 2009 13:33:59 -0500
Message-Id: <636EDB80-C9F4-440C-B6B9-178C444873CA@gmail.com>
To: public-webapps@w3.org
In the DataCache API, the "ready" event is the most appropriate place to call swapCache() and update the effective cache. However, the spec outlines a special case for the ready event, which I cannot understand.

- 4.2.2.4 Completing a Transaction
http://dev.w3.org/2006/webapi/DataCache/#complete-transaction
This is the only time the "ready" CacheEvent is queued. There is a special clause, which no other CacheEvent has. It is the "except for the cache host passed to these steps" which concerns me.

[[
7. For each cache host associated with a data cache in cache group, except for the cache host passed to these steps, queue a task to fire an event at the cache host with the name ready, which does not bubble, is not cancelable, and which uses the CacheEvent interface.
]]

Is there a specific reason that the cache host passed into the steps is ignored? If my understanding of that is correct, then given a scenario with multiple cache hosts (window, worker worker); if a user creates a cache transaction on the window's cache host, it would never get the "ready" event. This concerns me.

I would have expected this to be either:

  1. normal queue CacheEvent
  2. queue 'ready' CacheEvent only on the given cache host



Some other related issues I have with CacheEvents are:

- Event Names and Cache Host Ambiguity
I think "ready" and "error" are overly generic names for events fired on the "cache host". Maybe better names would be "cache-ready" or "cache-error". The specification is also ambiguous on what exactly the "cache host" should be. Claiming:

  - it is a Document or Shared Worker
  http://dev.w3.org/2006/webapi/DataCache/#cache-host

  - it is a Window or WorkerUtils object
  http://dev.w3.org/2006/webapi/DataCache/#opening-cache

Since "Window" is used often throughout (including IDLs), I have sided with that.



- 4.1.1. Examples
http://dev.w3.org/2006/webapi/DataCache/#examples
The first example seems to be out of date for a number of reasons.

[[
  var uri = ...
  var cache = window.openDataCache();
  document.body.addEventListener('onready', function(event) {
    event.cache.swapCache();
    ... // take advantage of the new stuff in the cache
  });

  cache.transaction(function(txn) {
    txn.capture(uri);
    txn.finish();
  });
]]

The issues I see are:

  - CacheTransaction.finish does not exist, I raised this point earlier
  - document.body.addEventListener why document.body and not the cache host
  - addEventListener('onready') I would expect this to just be 'ready' or a better name

I would have expected the example to look more like the following:

----
  var uri = ...
  var cache = window.openDataCache();
  window.addEventListener('cache-ready', function(event) {
    event.cache.swapCache();
    ... // take advantage of the new stuff in the cache
  });

  cache.transaction(function(txn) {
    txn.capture(uri);
    txn.commit();
  });
----


Cheers,
Joseph Pecoraro
Received on Thursday, 10 December 2009 18:34:33 GMT

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