14 Jan 2014 joolean   » (Journeyer)


Happy new year, everyone. I've just released gzochi version 0.5. Get it here!

As part of the fixes that went into this version, I made several adjustments to the error-handling behavior of the data storage layer, mostly to enable better communication about the result of a query to the database and the ensuing changes in transaction state. Prior to this, I'd taken the approach that any "failure" in data access -- such as a transaction deadlock -- could be indicated by the return value of the data access function, in part to smooth out differences in API between the various storage engines that gzochi supports. This had the fairly obvious disadvantage that it was impossible to tell the difference between a lookup for a non-existent key and an error (well, I figured, application code just shouldn't be doing that), and also that there was no way at all to indicate an error for functions that return void; and so after tracking down and fixing enough hard-to-fix bugs, I decided to fix the behavior of this API. In C, your options for returning multiple bits of information to a caller or distinguishing between, say, an "empty" response and an error are limited. You can make the type of data you return more complex by wrapping it inside of a data structure that also includes metadata about the invocation; or you can pass pointers to "outvalues" as arguments to your function, and have the callee modify them to indicate errors or other out-of-band responses. I like this latter approach because it allows you to preserve for the most part the intended interface between the function and its caller. You can, after all, allow people to pass NULL for those outvalues if they don't care about anything besides the return value. It does require, however, that your function reliably handle the intricacies of checking whether the outvalues are non-NULL and possibly allocating storage for them. GLib's GError type and its associated helper functions and macros are very convenient here. Pass a GError ** to your function and use g_set_error to set it conditionally.

The problem I was trying to solve with the foolishness above still exists, though: gzochi still supports several different storage engines (or, at least, it will -- support for GDBM was removed in this release; future versions will support HamsterDB) and each supported database has its own set of error codes and ways of returning them. So I created a kind of error code independence layer to convert implementation-specific values to values that are part of that layer. For example, in BerkeleyDB:


Back to the release: There's also a new "managed" data structure that employs the principles behind Project Darkstar's ScalableList and provides SRFI-44 collection semantics (one of the more contentious SRFI discussions I've read); and enhancements to the web monitoring console. Like I said, go get it!

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