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POI'm confused about HTTP caching
 

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  1. POI'm confused about HTTP caching
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    <p>I've been thinking about batch reads and writes in a RESTful environment, and I think I've come to the realization that I have broader questions about HTTP caching. (Below I use commas (",") to delimit multiple record IDs, but that detail is not particular to the discussion.)</p> <p>I started with this problem:</p> <h2>1. Single <code>GET</code> invalidated by batch update</h2> <pre><code>GET /farms/123 # get info about Old MacDonald's Farm PUT /farms/123,234,345 # update info on Old MacDonald's Farm and some others GET /farms/123 </code></pre> <p>How does a caching server in between the client and the Farms server know to invalidate its cache of <code>/farms/123</code> when it sees the <code>PUT</code>?</p> <p>Then I realized this was also a problem:</p> <h2>2. Batch <code>GET</code> invalidated by single (or batch) update</h2> <pre><code>GET /farms/123,234,345 # get info about a few farms PUT /farms/123 # update Old MacDonald's Farm GET /farms/123,234,345 </code></pre> <p>How does the cache know to invalidate the multiple-farm <code>GET</code> when it sees the PUT go by?</p> <p>So I figured that the problem was really just with batch operations. Then I realized that any relationship could cause a similar problem. Let's say a farm has zero or one owners, and an owner can have zero or one farms.</p> <h2>3. Single <code>GET</code> invalidated by update to a related record</h2> <pre><code>GET /farms/123 # get info about Old MacDonald's Farm PUT /farmers/987 # Old MacDonald sells his farm and buys another one GET /farms/123 </code></pre> <p>How does the cache know to invalidate the single GET when it sees the PUT go by?</p> <p>Even if you change the models to be more RESTful, using relationship models, you get the same problem:</p> <pre><code>GET /farms/123 # get info about Old MacDonald's Farm DELETE /farm_ownerships/456 # Old MacDonald sells his farm... POST /farm_ownerships # and buys another one GET /farms/123 </code></pre> <p>In both versions of #3, the first GET should return something like (in JSON):</p> <pre><code>farm: { id: 123, name: "Shady Acres", size: "60 acres", farmer_id: 987 } </code></pre> <p>And the second GET should return something like:</p> <pre><code>farm: { id: 123, name: "Shady Acres", size: "60 acres", farmer_id: null } </code></pre> <p>But it can't! Not even if you use <code>ETag</code>s appropriately. You can't expect the caching server to inspect the contents for <code>ETag</code>s -- the contents could be encrypted. And you can't expect the server to notify the caches that records should be invalidated -- caches don't register themselves with servers.</p> <p>So are there headers I'm missing? Things that indicate a cache should do a <code>HEAD</code> before any <code>GET</code>s for certain resources? I suppose I could live with double-requests for every resource if I can tell the caches which resources are likely to be updated frequently.</p> <p>And what about the problem of one cache receiving the <code>PUT</code> and knowing to invalidate its cache and another not seeing it?</p>
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    1. USJames A. Rosen
    1. USJames A. Rosen
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    1. COHaving just built a rapid response ajax web application, I'm quite interested in how this question is going to be answered. +1 and Star!
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      1. USKarl
    2. COMaybe what I'm looking for is an "invalidates-other" header that the server can add when returning from the PUT, POST, or DELETE. Alas, it doesn't seem to exist.
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      1. USJames A. Rosen
 

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