{"id":36,"date":"2006-12-13T02:10:58","date_gmt":"2006-12-13T06:10:58","guid":{"rendered":"http:\/\/www.coactus.com\/blog\/?p=36"},"modified":"2007-04-11T07:59:35","modified_gmt":"2007-04-11T11:59:35","slug":"validation-considered-harmful","status":"publish","type":"post","link":"http:\/\/www.coactus.com\/blog\/2006\/12\/validation-considered-harmful\/","title":{"rendered":"Validation considered harmful"},"content":{"rendered":"

We don’t get a lot of enjoyment about trampling sacred cows here at Coactus, honest we don’t. But we see so much bad practice out there – more so recently<\/a> – that we feel compelled to speak out.<\/p>\n

Today’s sacred cow is document validation, such as is performed by technologies such as DTD<\/a>s, and more recently XML Schema<\/a> and RelaxNG<\/a>.<\/p>\n

Surprisingly though, we’re not picking on any one particular validation technology. XML Schema has been getting its fair share of bad press<\/a>, and rightly so, but for different reasons than we’re going to talk about here. We believe that virtually all<\/em> forms of validation, as commonly practiced, are harmful; an anathema to use at Web scale. Specifically, our argument is this;<\/p>\n

\nTests of validity which are a function of time make the independent evolution of software problematic.\n<\/p><\/blockquote>\n

Why? Consider the scenario of two parties on the Web which want to exchange a certain kind of document. Party A has an expensive support contract with BigDocCo that ensures that they’re always running the latest-and-greatest document processing software. But party B doesn’t, and so typically lags a few months behind. During one of those lags, a new version of the schema is released which relaxes an earlier stanza in the schema which constrained a certain field to the values “1”, “2”, or “3”; “4” is now a valid value. So, party A, with its new software, happily fires off a document to B as it often does, but this document includes the value “4” in that field. What happens? Of course B rejects it; it’s an invalid document, and an alert is raised with the human adminstrator, dramatically increasing the cost of document exchange. All because evolvability wasn’t baked in, because a schema was used in its default mode of operation; to restrict rather than permit.<\/p>\n

Just because it only makes sense for a field in a document to contain the values “1”, “2”, or “3” today, does not necessarily mean “4”, “0”, or “9834” won’t be valid tomorrow. Similarly, just because a document doesn’t now contain a field named “Blarg”, it doesn’t mean it won’t later. A good rule of thumb in document design is to avoid making assumptions about what won’t<\/em> be there in the future, and a rule of thumb for software is to defer checking extension fields or values until you can’t any longer.<\/p>\n

On the Web, you need to be able to process messages from the future.<\/p>\n

P.S. if you’re wondering what time-in<\/em>dependent validity looks like, we’ll cover that at a later date (check the tags of this post for a hint).<\/p>\n","protected":false},"excerpt":{"rendered":"

We don’t get a lot of enjoyment about trampling sacred cows here at Coactus, honest we don’t. But we see so much bad practice out there – more so recently – that we feel compelled to speak out. Today’s sacred cow is document validation, such as is performed by technologies such as DTDs, and more […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[7,3],"tags":[],"_links":{"self":[{"href":"http:\/\/www.coactus.com\/blog\/wp-json\/wp\/v2\/posts\/36"}],"collection":[{"href":"http:\/\/www.coactus.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/www.coactus.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/www.coactus.com\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"http:\/\/www.coactus.com\/blog\/wp-json\/wp\/v2\/comments?post=36"}],"version-history":[{"count":0,"href":"http:\/\/www.coactus.com\/blog\/wp-json\/wp\/v2\/posts\/36\/revisions"}],"wp:attachment":[{"href":"http:\/\/www.coactus.com\/blog\/wp-json\/wp\/v2\/media?parent=36"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/www.coactus.com\/blog\/wp-json\/wp\/v2\/categories?post=36"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/www.coactus.com\/blog\/wp-json\/wp\/v2\/tags?post=36"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}