How to Corrupt an SQLite Database File

The amount of engineering rigor that has gone into sqlite over the years boggles the mind. It&#x27;s a project that has been held up as a sterling example of successful software development many times, rightfully so. I especially like this part of their license, from the top of sqlite3.h<p><i></i>&quot;The author disclaims copyright to this source code. In place of <i></i> a legal notice, here is a blessing: <i></i> <i></i> May you do good and not evil. <i></i> May you find forgiveness for yourself and forgive others. <i></i> May you share freely, never taking more than you give.&quot;

I know SQLite &quot;does testing right&quot; and they rightly get credit for this.<p>They also &quot;do documentation right&quot; and this is another accessible, clear example of that.<p>It&#x27;s also all public domain. I&#x27;ve used their documentation as an example in the past when driving through documentation improvements and i&#x27;ll no doubt point to them again!

That&#x27;s why the D in ACID confused me for a long time. There&#x27;s no way to guarantee durability. They say &quot;durable&quot; but they mean &quot;durable, kinda, we think, we hope, if nothing is lying to us, if your storage array doesn&#x27;t lose power without a battery to flush the cache, if your system doesn&#x27;t power down while your drives are lying about their committed write status, or if your write succeeded then an elf ran away with your platters.&quot;

If you do happen to corrupt any SQLite database file, it can usually be repaired... unless it is on an Android phone and the app tries to re-open it before you can make a copy:<p>Android automatically deletes corrupt SQLite files!<p><a href="http://stackoverflow.com/questions/7764943" rel="nofollow">http:&#x2F;&#x2F;stackoverflow.com&#x2F;questions&#x2F;7764943</a>

Great stuff and all but can I ask why SQL folks make it hard for an application to get simple <i>read</i> access to a SQLite database? Especially, I have an application which wants to use the Firefox places-bookmarks-etc db file. I can copy the file to a different directory and then run a query on it but can&#x27;t open it in place for reading, for <i>just</i> reading. WTF is the problem with that? Is reading a file a way to corrupt it?

This also serves as a clear illustration of SQLite internals and DBMS internals in general without thick clots of rubbish jargon. Nice. If you&#x27;re interested in database implementation this neatly covers a multitude of topics.

I can tell you one of the easiest ways to corrupt an SQLite db file is to not let a library specifically designed for file operations, handle file operations. And of course, not using provided libraries for db access (E.G. PDO if you&#x27;re using PHP).<p>I ran a forum on SQLite a while back, and things worked extremely well as long as I didn&#x27;t pretend I knew better than the library.<p>There&#x27;s no good excuse to not use PDO and there hasn&#x27;t been one for years now. Before that, I&#x27;ve implemented a write queue, which seems redundant in retrospect, in case the file lock issue came about, but it never did even though I did hit the write queue a few times.<p>As for crashes, periodic snapshots of the db <i>and journals</i> (that&#x27;s very important) is usually the best way to avoid recovery issues.

I stumbled onto this article a couple of days ago when I was trying to find a library to integrate sqlite with a cloud service like dropbox. Does anyone know of one that has:<p>* Read<p>* Write<p>* Callback (to sort out paradoxes when two or more devices update the store independently offline and need to merge)<p>It&#x27;s possible to do this with CoreData, but poorly, with a high burden on the developer to learn the entirety of Apple&#x27;s APIs and no way to alert the user as to what it is doing under the hood, which causes the app to hang for minutes or even forever until the managed object context says it&#x27;s ready.

Tcl (the test harness) at work.

The message I got was that POSIX advisory locking needs to be ripped out.

On seeing this first thought was &quot;oh, another Ess Kyoo El person&quot; because reading a &quot;An See Kwul&quot; instead of &quot;A See Kwul&quot; just grated. Pardon my attempt at phonetic spelling.