How early should I build a defensive item as a mid laner?

I was playing as Yasuo against a Swain.

Up until level 3, we were doing the usual - trading, farming, etc. At level 3, we both had about half our respective max HPs. I went in for a trade seeing a possible kill opportunity when Swain was about to last hit a minion. Just as I did so, I was ganked by a Master Yi, and I then died because I wasn't map aware enough I guess.

Anyway, post this death, I bought a Spectre's Cowl to lessen Swain's poke on me knowing that I'd still do decent damage to him when it came to trades. I did this because I intended to play more passively.

Was this a right choice on Yasuo in this circumstance? In general, when is it advisable to build a defensive item as a mid laner?

Solution 1:

You usually want to avoid building defensive Items too early on midlaners since most champions that are played on this lane shine during midgame due to their damage output but if you do buy them as a first or second item.

However you won't be able to snowball every game and often you will find yourself in a situation where you risk getting killed without defensive items. The main problem here is the fact that you'll also need damage for the midgame so going for an item that provides defensive but little to no offensive value is usually a bad idea.

But then again defensive stats are often a lot stronger than offensive ones during laning phase and 1v1 situations.

Depending on the matchup there are a few Items you can take into consideration which boost your survivability and dueling potential in lane while not giving up too much damage/utility during midgame.

As an AP Midlaner

Seeker's Armguard is a great early defensive item on pretty much every AP champion. It becomes even more effective with every minion kill and it builds into Zhonya's Hourglass which is pretty much a must-buy on every AP Midlaner. Always get this item first if you're up against an AD midlaner as AP.
Abyssal Scepter provides you with extremely good offensive stats with some extra MR. The main problem about this item is that you have to get relatively close to the enemy to make good use of the offensive value. Buy the Negatron Cloak first if you're behind but if you're able to survive without it you should go for a Blasting Wand. If you are behind on long range mages you might wanna consider a different Item unless you're super behind but on assassins like Akali/Katarina/Diana this item is great.
Athenes Unholy Grail is The Abyssal Scepter for long range mages. The offensive value comes from the ability to spam abilites due to the mana regeneration and cooldown reduction. Keep in mind that this item is only really good if you're behind. If you have a lane advantage you should almost always go for Morellomonicon since it provides you with roughly the same mana regeneration but a lot more offensive value.
Rod of Ages works on almost every AP midlaner with Mana. HP is an extermely effective method to counter assassins and compared to the other items on this list, this is probably the best scaling one especially if you buy one of the other items aswell. Also the Catalyst gives you some extra Mana/HP on level up which allows you to stay in lane longer.

As an AD Midlaner

Hexdrinker might just be the best Item when it comes to defense combined with offense. The components are cheap, the AD and MR are decent and the magic damage absorbing shield is a ridiculous lifesaver during early-midgame. Don't hesitate to buy this item on any AD midlaner if you're facing an AP champion.
Quicksilver Sash will remove absolutely everything. When laning against a champion that has CC or DoTs it's a decent item to buy early. It's also good to buy this item against a Zed and although the magic resistance doesn't reduce his damage output, the active removes his ultimate which lowers his damage significantly. Again this item is one of the components for Mercurial Scimitar which also gives you a nice movementspeed boost.

Situational on every champion

Ninja Tabi/Mercury's Treads are usually built on AD champions but in extreme situations they are also good on AP midlaners. Ninja tabi will greatly reduce any damage you take from AD champions while Mercury's provide you with a decent amount of MR + Tenacity against CC.
Thornmail... Okay.. this is like really situational but in some situations it can win you a lane in no time. If you buy a thornmail against an AD midlaner you're losing against he will have close to zero kill potential on you but then again you don't have any killing potential either. This item is like a full-stop to start farming and you have to hope that he won't roam or outscale you with farm. In most cases a Ninja Tabi or Seeker's Armguard will do better though so only take this as a last resort.
Banshee's Veil is basically the equivalent of Thornmail just for AP. It blocks a spell to prevent burst which completely cripples most AP lanebullys but again, all of your aggressive potential vanishes if you buy this item.
Guardian Angel is extremely useful if you're teamfighting a lot. You should usually buy either the Armor or MR component during laning phase and finish the item once you have 2-3 Offensive items.

Solution 2:

In general, you should get defensive items as needed. Assuming you are playing a fairly typical mid hero, you are usually going to be focused on damage instead of survivability. Your decision will change based on the individual game. If you are beating your lane opponent, you should probably go for more damage (or attack speed, ability power, etc.) for the added gank/killing power.

If you are being beaten inlane, you should almost always get an early defensive item. Avoiding death is very important to prevent snowballing. An example would be an AP champ losing to a Zed. An early Seeker's Armguard will hurt your early game damage a lot, but you will make up for that by avoiding death early on which will make your late game much better.

Yasuo is a more specific champ because the usual build (in my league at least) is a glass cannon build. Still, if you are losing your lane, a basic defensive item (under 500 gold) will help you in the late game. For most mid champs that would be fine but Yasuo has a fairly weak early game compared to many midlaners so it may be better in the end to accept your weakness and go straight for a few items so you can get to mid/late game faster.

Spectre's Cowl may have been too expensive of an item, unless you play Yasuo as more of a bruiser type champ. Even if your goal is tanky with good damage, you probably could make do with just a small piece of a defensive item until your dps is better.

Solution 3:

There is no real rule of thumb. This decision depends on many factors:

How good can i play my champ?
Is the enemy able to oneshot me? (Ahri, Zed, LeBlanc)
Does my champ offer anything if built more tanky? (Utility or insane scaling/ high DoT?)
Does the defensive item I'm about to built make sense?

In the following, I'll try to answer all of these questions:

How good can i play my champ?

This is the most important aspect. You always have to keep in mind: Tankiness forgives mistakes, damage doesn't! If you are not that comfortable on your champ, you will most likely do a few mistakes which the enemy midlaner can punish heavily and set you back even further. On the other hand, more tankiness means less dmg, which means you will have to play around that factor.

Is the enemy able to oneshot me?

I think we all faced that situation at least once. You're happily farming your lane, until ...

Zed used death mark. 
It's very effective.
Whoever you plays fainted.

This is an extremely frustrating and also extremely dangerous situation. And there is also only one answer if you play an AP midlaner: Zhonyas Hourglass. This item offers a few advantages:

Good amount of AP
Protection from oneshot Champs or heavy AoE's
Good amount of armor (poke/all-in protection)
Mental aspect (a decent zed will not waste your ult on you if he knows you got a zhonyas, so you will be save from him)

As a AD Midlaner, you have QSS. But you shouldn't rush this since this only gives you defensive stats, which will set you back too, maybe even more than dying once or twice.

Does my champ offer anything if built more tanky?

This is also a pretty important aspect when thinking about defensive items early. Do you have enough impact with your basic scaling/utility to have any impact in a teamfight, even with less damage? A good example here is lulu. Even if you - somehow - manage it to completely fuck up the laning phase, you can build some tankiness and still have a massive impact in fights with your slows, shields, polymorphs and knockups. You can protect important members of your team and help them in that way. A Zed can't built tanky. Neither can a katarina. Rule of thumb: Mages can be built tanky, assassins can't (Except for ahri)

Does the defensive item I'm about to built make sense?

Yeah, you play Orianna and get stomped by a LeBlanc. Oh nice, Abyssal Scepter, AP and MR, nice, bought it - nope. Also, your choice with Spectres on Yasuo wasn't very smart since this item builts into nothing valuable for him. You can buy the Spirit Visage, yes, but that's not an item yasuo needs. The correct choice in your situation would have been hexdrinker.

Now i wanted to list the best defensive items, but that's already explained in the post of Jutschge.