index

Table of Contents

• Quickstart
• What problems can be solved
• Workflow
• General decisions
  • HIBF vs IBF
  • Choosing window and k-mer size
    • (w,k) minimisers vs (k,k) canonical k-mers
    • (w,k) minimisers
    • (k,k) canonical k-mers

Quickstart

What problems can be solved

• Approximate Membership Query (AMQ) based on k-mers
• Input are sequences. Also called samples/references/genomes/color. We use the term bin or user bin.
• Given a query sequence: Show all bins that possibly contain that sequence.
  • Either given a number of errors
  • Or given a percentage (e.g., 0.7 -> 70% of queried k-mers must be present in a bin)
• Does not model a colored de Bruijn graph:
  • ✔️ Given a k-mer: Tell to which color (bin) it belongs to
  • ❌ Given a color (bin): Show all k-mers that belong to this color (bin)

Workflow

• HIBF [raptor prepare] -> raptor layout -> raptor build -> raptor search
• IBF [raptor prepare] -> raptor build -> raptor search

General decisions

HIBF vs IBF

Recommendation: HIBF

Cases in which to consider the IBF:

• Evenly sized bins
• Small number of bins (≤ 128)

Click to see an HIBF / IBF comparison.

Note

The used data set is the worst case for the HIBF. In reality, the index size is usually smaller than the corresponding IBF, and build times of the HIBF are much closer to IBF build times.

Choosing window and k-mer size

(w,k) minimisers vs (k,k) canonical k-mers

	(k,k)	(w,k)
Index size	⬆️	⬇️
Runtime	⬆️	⬇️
RAM usage	⬆️	⬇️
Thresholding¹	Exact	Heuristic

¹ When searching with a given number of errors.

• (w,k) minimisers reduce the number of values to process by roughly \(\frac{w - k + 2}{2}\).
• (w,k) minimisers have a slightly lower accuracy than (k,k). However, the loss of accuracy mainly stems from false positives, not false negatives.

Recommendation: (w,k) with gentle compression (w-k=4)

Click to see the differences of (w,k) and (k,k) on different aspects.

(w,k) minimisers

Requirements:

• w > k
• w ≤ query length

Recommendation:

• w - k = 4
• w << query length

Examples:

• query length 100: w = 24, k = 20
• query length 250: w = 28, k = 24

Also see the figure in usage_w_vs_k_figure.

(k,k) canonical k-mers

Requirements:

• k ≤ query length
• k-mer counting lemma satisfied, when searching with a given number of errors.

Recommendation:

• k << query length

Examples (for two errors):

• query length 100: k = 20
• query length 250: k = 32

Depending on the number of errors that should be accounted for when searching, the kmer-size (k) has to be chosen such that the k-mer lemma still has a positive threshold.

K-mer counting lemma: For a given k and number of errors e, there are \(k_p = |p| - k + 1\) many k-mers in the pattern p and an approximate occurrence of p in text T has to share at least \(t = (k_p - k \cdot e)\) k-mers.

For example, when searching reads of length 100 and allowing 4 errors, k has to be at most 20 (100 − 20 + 1 − 4 · 20 = 1).

Furthermore, k shall be such that a random k-mer match in the database is unlikely. For example, we chose k = 32 for the RefSeq data set. In general, there is no drawback in choosing the (currently supported) maximum k of 32, as long as the aforementioned requirements are fulfilled.