title: Forcing web api consumers to accept new fields in responses
body: <p>I'm creating v2 of an existing RESTful web api.</p>
  
  <p>The responses are JSON lists of objects, roughly in the form:</p>
  
  <pre><code>[
    {
       name1=value1,
       name2=value2,
    },
    {
       name1=value3,
       name2=value4,
    }
  ]
  </code></pre>
  
  <p>One problem we've observed with v1 is that some clients will access fields by integer position, instead of by name. This means that if we decide to add fields to the response (which we had originally considered a compatibility-preserving change), then some of our client's code breaks, unless we add the fields at the end. Even then, other clients code breaks anyway, because they will fail in some way when they encounter an unexpected attribute name.</p>
  
  <p>To counter this in v2, we are considering randomly reordering the fields in every response. This will force clients to index fields by name instead of by position.</p>
  
  <p>Additionally, we are considering adding a randomly-named field to every response. This will force clients to ignore fields they don't recognize.</p>
  
  <p>While this sounds somewhat barmy, it does have the advantage that we will be able to add new fields, safe in the knowledge that this isn't breaking any clients. This means we can issue compatible updates to v2.1, v2.3, etc at the same URL, and that means we will only have to maintain &amp; support a smaller number of API versions.</p>
  
  <p>The alternative is to issue compatibility-breaking v3, v4, at new URLs, which means that we will have to maintain &amp; support many incompatible API versions, which will stretch us that little bit thinner.</p>
  
  <p>Is this a bad idea, and if so, why? Are there any other similar ideas I should think about?</p>
  
  <p><strong>Update:</strong> The first couple of responses are pointing out that if I document the issue (i.e. indicate in the docs that fields may be added or reordered) then I am no longer to blame if client code breaks when I subsequently add or reorder fields. Sadly I don't think this is an appropriate option for us: Many dozens of organisations rely on the functionality of our APIs for real-world transactions with substantial financial impact. These organisations are not technically oriented - and the resulting implementations at the client end cover the whole spectrum of technical proficiency. We already <em>did</em> document that fields may get added or reordered in the docs for v1, and that clearly didn't work, because now we're having to issue v2 because many clients, due to lack of time or experience or ability, still wrote code that breaks when we add new fields. If I were now to add fields to the interface, it breaks a dozen different company's interfaces to us, which means they (and us) are bleeding money every minute. If I were to refuse to revert the change or fix it, saying "They should have read the docs!", then I will soon be out of the job, and rightly so. We may attempt to educate the 'failing' partners, but this is doomed to fail as the problem gets larger every month as we continue to grow. My question is, can I systemically head the whole issue off at the pass, preventing this situation from ever arising, no matter what clients try to do? If the techniques I suggest would work, why shouldn't I use them? Why isn't everyone else using them?</p>