Common depictions of Buddhist monks often shows them living a very ascetic lifestyle. My sources for this are simply what I have seen in documentaries online. Do Buddhist monks live such an ascetic life style?
That's difficult to answer, without knowing what documentaries you're talking about.
Buddhist monastic rules say that monks have no personal possessions (except their robes and alms bowl and so on), never have or handle money, subsist on donations of food, etc.
For monks who want a lifestyle that's even more austere, there are the (optional) dhutanga practices.
I suppose that, in practice (in reality), the practices may vary: for example, the booklet called The Broken Buddha (2001) says that some monks in some countries aren't as (or at least don't seem to be as) 'ascetic' as the author (himself a monk) considers proper. It might be a mistake to generalize and assume that all monks are the same. Contrast the descriptions in The Broken Buddha with this description of Lessons in the Forest (from Ajahn Cha's forest monastery, which seems more ideal).
Also details of the monastic rules may vary if only slightly from one school/tradition to another (e.g. Theravada, Zen, Tibetan), as well (I presume) from one monastery (one abbot) to another.
If so, then why? Is this not going against the eight-fold path?
Before he realized his own enlightenment, the Buddha tried various practices including for example this:
"I thought: 'Suppose I were to take only a little food at a time, only a handful at a time of bean soup, lentil soup, vetch soup, or pea soup.' So I took only a little food at a time, only a handful at a time of bean soup, lentil soup, vetch soup, or pea soup. My body became extremely emaciated. Simply from my eating so little, my limbs became like the jointed segments of vine stems or bamboo stems... My backside became like a camel's hoof... My spine stood out like a string of beads... My ribs jutted out like the jutting rafters of an old, run-down barn... The gleam of my eyes appeared to be sunk deep in my eye sockets like the gleam of water deep in a well... My scalp shriveled & withered like a green bitter gourd, shriveled & withered in the heat & the wind... The skin of my belly became so stuck to my spine that when I thought of touching my belly, I grabbed hold of my spine as well; and when I thought of touching my spine, I grabbed hold of the skin of my belly as well... If I urinated or defecated, I fell over on my face right there... Simply from my eating so little, if I tried to ease my body by rubbing my limbs with my hands, the hair — rotted at its roots — fell from my body as I rubbed, simply from eating so little.
"People on seeing me would say, 'Gotama the contemplative is black.' Other people would say, 'Gotama the contemplative isn't black, he's brown.' Others would say, 'Gotama the contemplative is neither black nor brown, he's golden-skinned.' So much had the clear, bright color of my skin deteriorated, simply from eating so little.
After a while he decided that starving himself wasn't the right way (though the others who were with him didn't agree):
I thought: 'So why am I afraid of that pleasure that has nothing to do with sensuality, nothing to do with unskillful mental qualities?' I thought: 'I am no longer afraid of that pleasure that has nothing to do with sensuality, nothing to do with unskillful mental qualities, but that pleasure is not easy to achieve with a body so extremely emaciated. Suppose I were to take some solid food: some rice & porridge.' So I took some solid food: some rice & porridge. Now five monks had been attending on me, thinking, 'If Gotama, our contemplative, achieves some higher state, he will tell us.' But when they saw me taking some solid food — some rice & porridge — they were disgusted and left me, thinking, 'Gotama the contemplative is living luxuriously. He has abandoned his exertion and is backsliding into abundance.'
"So when I had taken solid food and regained strength, then — quite secluded from sensuality, secluded from unskillful mental qualities, I entered & remained in the first jhana: etc.
What he started to teach his first audience (which consisted of those five monks) he explained it as follows:
There are these two extremes that are not to be indulged in by one who has gone forth. Which two? That which is devoted to sensual pleasure with reference to sensual objects: base, vulgar, common, ignoble, unprofitable; and that which is devoted to self-affliction: painful, ignoble, unprofitable. Avoiding both of these extremes, the middle way realized by the Tathagata — producing vision, producing knowledge — leads to calm, to direct knowledge, to self-awakening, to Unbinding.
See also Why is an emaciated Buddha rarely represented in art?
If the depictions I have in mind are untrue then what is Buddhist monastery life like such that it balances indulgence and asceticism?
It is (or may be) to do with asceticism, if by "asceticism" you mean self-discipline, abstemious, etc.
But see this description of Renunciation: i.e. it's not that renunciation itself is necessarily rewarding, it's that sensual pleasures are seen to have drawbacks which a monk might want to be free of.
The kind of asceticism that's contrary to the Middle Way isn't the training to overcome sensuality-seeking, rather it's "self affliction" or (literally) "mortification of the flesh" i.e. killing your flesh.
Examples of "balancing indulgence and asceticism"
- Eat before noon (or eat once a day) ... a balance between "over-eating" and "starving"
- Drink non-alcoholic liquids any time ... a balance between "getting drunk" and "dehydrating"
- Wear monastic robes ... a balance between "adorning the body" and "going naked"
- Shaving the head ... a balance between "adorning the body" and "neglecting cleanliness"
- Minimal possessions
- Obeying monastic rules