Is it actually unimportant if a chat message is dropped? It seems damn important to me, what use is a chat app if someone sends you an important message and you never receive it? I could see that being true for something like anonymized logs where you are only going to be looking at it in aggregate, but just silently ignoring chat messages really doesn't seem acceptable to me.
Well, in practice, it's not too uncommon to send chat messages or SMS messages that just vanish, or arrive out of order, or arrive the day after they were sent. People do not, then, say that SMS is completely useless; instead, they accept that once in a while a message won't get through, and that they should call if it's important.
I'm not saying it's not at all important that chat messages actually get sent, and if it happens every single day to a user, then they might well look for alternatives, but it's not of the same importance as losing a banking transaction. If accepting that occasional writes will be dropped on the floor allows you to get your product out in October instead of December, that could be an acceptable tradeoff. Certainly not every use case is like this, but some are.
I mean is it desirable behaviour? Would you not want a chat program which never dropped your messages? If that's the case, we should work towards it, not accept a 1 of 1000 messages lost because "there are more important things to worry about". The most important feature should be prioritized over less important ones, but it should not make us forget them - they should be as good as possible as well.
I guess what I'm trying to say is this: You cannot ignore all the other features except the biggest one.
Not really, in my experience. Losing chat messages or logs is among the worst things a chat application can do.
I take your point though, but I think consistency is still one of the most important attributes for anything that is going to store data. Why even use a database, if your data matters so little? Just throw it into memory or memcache.