title: Is ngx.shared.DICT safe to use?
body: <p>I'm using <code>HttpLuaModule</code> to perform some actions against requests. The way I do it is I setted up <code>ngx.shared.dict</code> like so:</p>
  
  <pre><code>lua_shared_dict foo 10m;
  </code></pre>
  
  <p>And then I use <code>rewrite_by_lua</code> to get and set values to and from this dictionary. </p>
  
  <p>My question is:</p>
  
  <p>During the high load when hundreds requests needs to be processed every second. Is it safe to use this dictionary? For example in my script this dictionary gets updated every 60 seconds from <code>Redis</code>. If I set an expiration time for every single key:value pair in dictionary. And then suddenly all connections needs to access this key, but it's not there, then all of them will perform a Redis call to retrieve data and update dictionary. is that true? Or may be it was designed for some other use cases?</p>