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Share files with the host: BLAST, a bioinformatics example

Overview

Teaching: 10 min
Exercises: 10 min
Questions
Objectives

  • Learn how to mount host directories in a container

  • Run a real-world bioinformatics application in a container

Access to host directories

Let’s start and cd into the root demo directory:

$ cd $SC19/demos

What directories can we access from the container?

First, let us assess what the content of the root directory / looks like outside vs inside the container, to highlight the fact that a container runs on his own filesystem:

$ ls /

bin  boot  dev  etc  home  lib  lib64  media  mnt  opt  proc  root  run  sbin  scratch  shared  srv  sys  tmp  usr  var

Now let’s look at the root directory when we’re in the container

$ singularity exec library://ubuntu:18.04 ls /

bin  boot  data  dev  environment  etc	home  lib  lib64  media  mnt  opt  proc  root  run  sbin  singularity  srv  sys  tmp  usr  var

In which directory is the container running?

For reference, let’s check the host first:

$ pwd

/home/ubuntu/sc19-containers/demos

Now inspect the container (HINT: you need to run pwd in the container)

Solution

$ singularity exec library://ubuntu:18.04 pwd

/home/ubuntu/sc19-containers/demos

Host and container working directories coincide!

Can we see the content of the current directory inside the container?

Hopefully yes

Solution

$ singularity exec library://ubuntu:18.04 ls

03_blast    03_blast_db 04_trinity  05_gromacs  06_openfoam 07_lolcow   08_rstudio  09_python

Indeed we can!

How about other directories in the host?

For instance, let us inspect $SC19/_episodes.

Solution

$ singularity exec library://ubuntu:18.04 ls $SC19/_episodes

ls: cannot access '/home/ubuntu/sc19-containers/_episodes': No such file or directory

Host directories external to the current directory are not visible! How can we fix this? Read on…

Bind mounting host directories

Singularity has the runtime flag --bind, -B in short, to mount host directories.

The long syntax allows to map the host dir onto a container dir with a different name/path, -B hostdir:containerdir.
The short syntax just mounts the dir using the same name and path: -B hostdir.

Let’s use the latter syntax to mount $SC19/_episodes into the container and re-run ls.

$ singularity exec -B $SC19/_episodes library://ubuntu:18.04 ls $SC19/_episodes

01-containers-intro.md    03-bio-example-mounts.md  05-gpu-gromacs.md         07-mpi-openfoam.md        09-ml-python.md           11-docker.md
02-singularity-intro.md   04-writable-containers.md 06-build-intro.md         08-gui-rstudio.md         10-workflow-engines.md    12-other-tools.md

Now we are talking!

If you need to mount multiple directories, you can either repeat the -B flag multiple times, or use a comma-separated list of paths, i.e.

-B dir1,dir2,dir3

Also, if you want to keep the runtime command compact, you can equivalently specify directories to be bind mounting using an environment variable:

$ export SINGULARITY_BINBPATH="dir1,dir2,dir3"

Mounting $HOME

Depending on the site configuration of Singularity, user home directories might or might not be mounted into containers by default. We do recommend avoid mounting home whenever possible, to avoid sharing potentially sensitive data, such as SSH keys, with the container, especially if exposing it to the public through a web service.

If you need to share data inside the container home, you might just mount that specific file/directory, e.g.

-B $HOME/.local

Or, if you want a full fledged home, you might define an alternative host directory to act as your container home, as in

-B /path/to/fake/home:$HOME

Running BLAST from a container

We’ll be running a BLAST (Basic Local Alignment Search Tool) example with a container from BioContainers. BLAST is a tool bioinformaticians use to compare a sample genetic sequence to a database of known sequences; it’s one of the most widely used bioinformatics tools.
This example is adapted from the BioContainers documentation.

We’re going to use the BLAST image biocontainers/blast:v2.2.31_cv2, which we previously downloaded in $SIFPATH. Let’s verify it’s there:

$ ls $SIFPATH/blast*

/home/ubuntu/sc19-containers/demos/sif/blast_v2.2.31_cv2.sif

Run a test command

To begin, let us run a simple command using the downloaded image $SIFPATH/blast_v2.2.31_cv2.sif, for instance blastp -help, to verify that it actually works:

Solution

$ singularity exec $SIFPATH/blast_v2.2.31_cv2.sif blastp -help

USAGE
  blastp [-h] [-help] [-import_search_strategy filename]
[..]
 -use_sw_tback
   Compute locally optimal Smith-Waterman alignments?

Now, the demos/03_blast demo directory contains a human prion FASTA sequence, P04156.fasta, whereas another directory, demos/03_blast_db, contains a gzipped reference database to blast against, zebrafish.1.protein.faa.gz. Let us cd to the latter directory and uncompress the database:

$ cd $SC19/demos/03_blast_db
$ gunzip zebrafish.1.protein.faa.gz

Prepare the database

We then need to prepare the zebrafish database with makeblastdb for the search, using the following command through a container:

$ makeblastdb -in zebrafish.1.protein.faa -dbtype prot

Try and run it via Singularity.

Solution

$ singularity exec $SIFPATH/blast_v2.2.31_cv2.sif makeblastdb -in zebrafish.1.protein.faa -dbtype prot

Building a new DB, current time: 11/16/2019 19:14:43
New DB name:   /home/ubuntu/sc19-containers/demos/03_blast_db/zebrafish.1.protein.faa
New DB title:  zebrafish.1.protein.faa
Sequence type: Protein
Keep Linkouts: T
Keep MBits: T
Maximum file size: 1000000000B
Adding sequences from FASTA; added 52951 sequences in 1.34541 seconds.

After the container has terminated, you should see several new files in the current directory (try ls).
Now let’s proceed to the final alignment step using blastp. We need to cd into demos/03_blast:

$ cd ../03_blast

Run the alignment

and then adapt the following command to run into the container:

$ blastp -query P04156.fasta -db $SC19/demos/03_blast_db/zebrafish.1.protein.faa -out results.txt

Note how we put the database files in a separate directory on purpose, so that you will need to bind mount its path with Singularity. Give it a go with building the syntax to run the blastp command.

Solution

$ singularity exec -B $SC19/demos/03_blast_db $SIFPATH/blast_v2.2.31_cv2.sif blastp -query P04156.fasta -db $SC19/demos/03_blast_db/zebrafish.1.protein.faa -out results.txt

The final results are stored in results.txt:

$ less results.txt

                                                                      Score     E
Sequences producing significant alignments:                          (Bits)  Value

  XP_017207509.1 protein piccolo isoform X2 [Danio rerio]             43.9    2e-04
  XP_017207511.1 mucin-16 isoform X4 [Danio rerio]                    43.9    2e-04
  XP_021323434.1 protein piccolo isoform X5 [Danio rerio]             43.5    3e-04
  XP_017207510.1 protein piccolo isoform X3 [Danio rerio]             43.5    3e-04
  XP_021323433.1 protein piccolo isoform X1 [Danio rerio]             43.5    3e-04
  XP_009291733.1 protein piccolo isoform X1 [Danio rerio]             43.5    3e-04
  NP_001268391.1 chromodomain-helicase-DNA-binding protein 2 [Dan...  35.8    0.072
[..]

We can see that several proteins in the zebrafish genome match those in the human prion (interesting?).

Key Points

  • By default Singularity mounts the host current directory, and uses it as container working directory

  • Map additional host directories in the containers with the flag -B

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