Book Resilience And Replenishment

Spread and depth describe a book at a single instant. Resilience describes what happens next: after a trade removes liquidity, do fresh limit orders arrive quickly to refill the gap, or does the book stay damaged for a while? Two stocks can have identical spread and depth right now and completely different resilience — one snaps back in milliseconds, the other takes minutes, and that difference matters enormously for anyone trading in size.

A worked comparison

Stock A: touch is 50.00/50.02, 500 shares on each side. A 500-share market buy sweeps the full offer, moving the touch to 50.00/50.03 (the next level up). Watching the tape afterward: within 40 milliseconds, three new sell orders totalling 480 shares arrive at 50.02, and the touch snaps back to 50.00/50.02. Depth restored, spread restored — high resilience.

Stock B: same starting book, same 500-share sweep, same move to 50.00/50.03. But new sell orders trickle in slowly — 100 shares after 2 seconds, another 150 after 8 seconds — and the touch doesn't fully recover to 50.02 for over 20 seconds. A second market buy arriving 5 seconds after the first would, in Stock A, meet a fully healed book; in Stock B, it walks even further up because the damage from the first trade hasn't repaired yet. Low resilience means trades in quick succession compound each other's impact instead of each facing a fresh book.

Why it matters. Execution algorithms that slice a large order into pieces rely on resilience to avoid walking their own book repeatedly: send a slice too soon after the last one, in a low-resilience name, and it lands on a book that hasn't healed, paying impact twice for what should have been separate, independent trades. This is exactly why participation-rate algorithms throttle down in illiquid names — not because the depth is thin at any one moment, but because it stays thin after being touched.