It's not easy being a brick-and-mortar retailer today. There are just too many shops competing over a shrinking pool of customer traffic, and this imbalance is forcing many businesses to cut prices, leading to both weaker earnings and lower sales growth.

Costco's (COST 0.34%) low-price, membership-based operating model has helped it avoid the worst of those problems so far. But its latest operating results show that the business isn't immune from the broader industry slowdown.

Here's a look at the key trends that dominated the fiscal year that the warehouse giant just closed.

A customer roams the aisles of a warehouse retailer.

Image source: Getty Images.

1. Sales growth

Chart showing annual sales growth dipping to a 4% rate in 2017.

Chart by author. Data source: Costco filings.

Comparable-store sales growth of 4% was enough to keep Costco solidly ahead of its major retailing peers. Wal-Mart (WMT 1.00%) and Target (TGT -0.32%) are growing comps by less than 2% these days, and Kroger's (KR -0.44%) expansion pace has collapsed to below 1% from over 5% two years ago.

Still, Costco's sales improvement matches its worst result since the 2008 recession. The company just doesn't endure multiyear stretches of 5% or lower comps gains like the one shareholders are currently witnessing. That could explain why management is attacking the e-commerce sales channel with more urgency right now.

2. Net income

The beauty of Costco's subscription model is that earnings are tied to membership fees and so they tend to march higher even during periods of weaker sales growth. That's exactly what happened this past year, when membership fees rose to $2.85 billion from $2.65 billion in the prior year.

The spike allowed earnings to rise 14% to over $6 per share at a time when most rivals, including Wal-Mart, Target, and Kroger, are posting lower profits as they invest more heavily in price cuts.

TGT Net Income (TTM) Chart

TGT Net Income (TTM) data by YCharts.

The fee boost was helped along by an extra sales week in fiscal 2017 that won't occur next year. However, a recent annual fee increase should lift membership income by about $175 million next year and so investors have good reasons to expect the retailer's market-beating profit growth streak to continue.

3. Renewal rates

Chart showing renewal rates holding steady at 90% last year.

Chart by author. Data source: Costco filings.

Most subscription businesses would kill for a renewal rate of 90% or better. For Costco, though, its latest figure represents a potential warning sign. After all, last year marked the second straight year that renewal rates ticked lower after a five-year streak of gains. The renewal rate is Costco's single best metric to judge customer loyalty, and that's why any sustained drop would be a big concern for the business.

So far, management isn't concerned. They believe the renewal dip is just a lingering effect from last year's co-branded credit card switchover. Rates should begin improving again over the next few quarters, they told investors in early October.

Costco is probably right. After all, comparable-store sales improved in its most recent quarter as it booked an increase in customer traffic. If members were losing their attachment to the business, you'd expect both of those numbers to be trending lower. Still, investors will be watching renewal rates over the next year to see whether Costco has indeed lost some of its competitive advantage to e-commerce rivals.