Databases
StackOverflow2013
Postsdbo
PO
 

Note that there are some explanatory texts on larger screens.

plurals
  1. PO
    primarykey
    data
    text
    <p>There doesn't seem to be an elegant way to do this in .NET, although I have improved the algorithm thanks to @RobV's suggestion of a loop. Here is the final solution I settled on.</p> <p>It is immune to the 'orphaned reference' bug that seems to be typical of the standard pattern followed by @Steven's answer.</p> <pre><code>using System; using System.Collections.Generic; using System.Text; using System.Threading; namespace ImageResizer.Plugins.DiskCache { public delegate void LockCallback(); /// &lt;summary&gt; /// Provides locking based on a string key. /// Locks are local to the LockProvider instance. /// The class handles disposing of unused locks. Generally used for /// coordinating writes to files (of which there can be millions). /// Only keeps key/lock pairs in memory which are in use. /// Thread-safe. /// &lt;/summary&gt; public class LockProvider { /// &lt;summary&gt; /// The only objects in this collection should be for open files. /// &lt;/summary&gt; protected Dictionary&lt;String, Object&gt; locks = new Dictionary&lt;string, object&gt;(StringComparer.Ordinal); /// &lt;summary&gt; /// Synchronization object for modifications to the 'locks' dictionary /// &lt;/summary&gt; protected object createLock = new object(); /// &lt;summary&gt; /// Attempts to execute the 'success' callback inside a lock based on 'key'. If successful, returns true. /// If the lock cannot be acquired within 'timoutMs', returns false /// In a worst-case scenario, it could take up to twice as long as 'timeoutMs' to return false. /// &lt;/summary&gt; /// &lt;param name="key"&gt;&lt;/param&gt; /// &lt;param name="success"&gt;&lt;/param&gt; /// &lt;param name="failure"&gt;&lt;/param&gt; /// &lt;param name="timeoutMs"&gt;&lt;/param&gt; public bool TryExecute(string key, int timeoutMs, LockCallback success){ //Record when we started. We don't want an infinite loop. DateTime startedAt = DateTime.UtcNow; // Tracks whether the lock acquired is still correct bool validLock = true; // The lock corresponding to 'key' object itemLock = null; try { //We have to loop until we get a valid lock and it stays valid until we lock it. do { // 1) Creation/aquire phase lock (createLock) { // We have to lock on dictionary writes, since otherwise // two locks for the same file could be created and assigned // at the same time. (i.e, between TryGetValue and the assignment) if (!locks.TryGetValue(key, out itemLock)) locks[key] = itemLock = new Object(); //make a new lock! } // Loophole (part 1): // Right here - this is where another thread (executing part 2) could remove 'itemLock' // from the dictionary, and potentially, yet another thread could // insert a new value for 'itemLock' into the dictionary... etc, etc.. // 2) Execute phase if (System.Threading.Monitor.TryEnter(itemLock, timeoutMs)) { try { // May take minutes to acquire this lock. // Trying to detect an occurence of loophole above // Check that itemLock still exists and matches the dictionary lock (createLock) { object newLock = null; validLock = locks.TryGetValue(key, out newLock); validLock = validLock &amp;&amp; newLock == itemLock; } // Only run the callback if the lock is valid if (validLock) { success(); // Extremely long-running callback, perhaps throwing exceptions return true; } } finally { System.Threading.Monitor.Exit(itemLock);//release lock } } else { validLock = false; //So the finally clause doesn't try to clean up the lock, someone else will do that. return false; //Someone else had the lock, they can clean it up. } //Are we out of time, still having an invalid lock? if (!validLock &amp;&amp; Math.Abs(DateTime.UtcNow.Subtract(startedAt).TotalMilliseconds) &gt; timeoutMs) { //We failed to get a valid lock in time. return false; } // If we had an invalid lock, we have to try everything over again. } while (!validLock); } finally { if (validLock) { // Loophole (part 2). When loophole part 1 and 2 cross paths, // An lock object may be removed before being used, and be orphaned // 3) Cleanup phase - Attempt cleanup of lock objects so we don't // have a *very* large and slow dictionary. lock (createLock) { // TryEnter() fails instead of waiting. // A normal lock would cause a deadlock with phase 2. // Specifying a timeout would add great and pointless overhead. // Whoever has the lock will clean it up also. if (System.Threading.Monitor.TryEnter(itemLock)) { try { // It succeeds, so no-one else is working on it // (but may be preparing to, see loophole) // Only remove the lock object if it // still exists in the dictionary as-is object existingLock = null; if (locks.TryGetValue(key, out existingLock) &amp;&amp; existingLock == itemLock) locks.Remove(key); } finally { // Remove the lock System.Threading.Monitor.Exit(itemLock); } } } } } // Ideally the only objects in 'locks' will be open operations now. return true; } } } </code></pre> <p>Consuming this code is very simple:</p> <pre><code>LockProvider p = new LockProvider(); bool success = p.TryExecute("filename",1000,delegate(){ //This code executes within the lock }); </code></pre>
    singulars
    1. This table or related slice is empty.
    1. POBetter solution to multithreading riddle?
      singulars
      1. PTQuestion
    1. PTAnswer
    1. USLilith River
    1. USLilith River
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. POBetter solution to multithreading riddle?
      singulars
      1. PTQuestion
    1. This table or related slice is empty.
    1. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTAcceptedByOriginator
    1. This table or related slice is empty.
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload