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+---
+layout: post
+title: Simple Parallel Bioinformatics Pipelines with find, basename, and xargs
+---
+
+# Big-Ass Servers and Data Parallelism
+
+A routine operation in bioinformatics is to process a lot of files on
+a so-called ["Big-Ass
+Server"](http://jermdemo.blogspot.com/2011/06/big-ass-servers-and-myths-of-clusters.html). In
+most cases, these have to be processed using the same tools, in the
+same way, making this a prime example of data-parallism. The unit of
+data divided across multiple cores is the file. 
+
+Note that there's very little opportunity for task-parallism in
+bioinformatics file processing pipelines. Consider a common task of
+most bioinformatics sequencing projects (resequencing, RNA-seq, etc):
+taking reads from many samples, running quality diagnostics, and
+running adapter and quality trimming. None of these steps can be done
+in a task-parallel fashion; they must be pipelined.
+
+# Find-Basename-Xargs
+
+Suppose in this example I have genomic sequencing data of 50 human
+individuals. All individuals' sequences are in the directory `seq/`
+and are semantically named in the format of
+`{lane-number}-{individual-id}.fastq`. In this example, suppose I just
+want to run each file through my adapter trimming program
+[scythe](github.com/vsbuffalo/scythe) in parallel, using four cores.
+
+For such tasks, I frequently use a design pattern I refer to as
+"find-basename-xargs" (FBX hereafter). I doubt this is novel, but I do
+refer to this specific design pattern frequently. I'll dissect an
+example.
+
+Our `seq/` directory contains many files we wish to trim with Scythe,
+in parallel. The first step of FBX is to use the `find` command to
+find all relevant files. In our case:
+
+    $ find seq -name "*.fastq"
+    seq/african-1.fastq
+    seq/african-10.fastq
+    seq/african-11.fastq
+    seq/african-12.fastq
+    # [...]
+
+Next, `basename` is used to drop the extension and `seq/` directory,
+which leaves us with only the identifying string (I think of this as a
+key... you'll see why in a few weeks). Having just the identifying key
+allows us to specify the output directory and any file suffixes. We
+use `basename` with `xargs` because we're processing each incoming
+line from stdin at a time (`-n1` specifes one argument will be taken
+at a time). The command results would look like:
+
+    $ find seq -name "*.fastq" | xargs -n1 -I{} basename {} .fastq
+    african-1
+    african-10
+    african-11
+    african-12
+    # [...]
+
+The argument `-I{}` specifes the replacement string. Since we're the
+last positional argument of `basename` is `.fastq`, this is necesary.
+
+Finally, we do the processing with `xargs`. GNU `parallel` also works,
+and provides nice additional features. Scythe takes some fixed
+arguments (adapter, prior) and file options dependent on the key
+(input file, output file). So to run Scythe, on each file in parallel
+and output the results to the trimmed directory, we'd use `xargs` with
+`-n1 -P4 -I{}`. Our exact command would be:
+
+    find seq -name "*.fastq" | \
+      xargs -n1 -I{} basename {} .fastq | \
+      xargs -n1 -P10 -I{} scythe -a adapters.fasta -p 0.4 -o trimmed/{}.fastq seq/{}.fastq
+
+Note that we re-specify the directories and extensions so Scythe can
+find and process the appropriate file.
+
+# Redirecting Output in Parallel and XBF Chaining
+
+I'm a stickler about gathering and analyzing statistics at all
+intermediary steps in a bioinformatics processing pipeline. Scythe
+will output summary statistics on found adapters in the reads, but it
+prints it out to stderr. With hundreds of files being processed,
+simple redirecting stderr to a file via `2>` is ineffective. Since
+`xargs` is calling the program, there's no way to redirect a specific
+Scythe calls' stderr to a specific file. 
+
+My work around this is to wrap a command call in a bash shell
+script. Our shell script would be incredibly simple (a better version
+would ensure directories exist):
+    
+    #!/bin/bash
+    set -o nounset
+    set -e
+
+    IN=$1
+    echo " started running scythe on file '$IN'..." 1>&2
+    scythe -a adapters.fasta -p 0.4 -o trimmed/$IN.fastq seq/$IN.fastq 2> stats/$IN-output.txt
+    echo " completed scythe on file '$IN'." 1>&2
+    echo $IN
+
+And we could call it in the same fashion.
+
+    find seq -name "*.fastq" | \
+      xargs -n1 -I{} basename {} .fastq | \
+      xargs -n1 -P10 bash run-scythe.bash
+
+A few notes: output message to stderr, not stdout. This allows us to
+*chain* the FBX pattern. When a file's processing is complete,
+execution will proceed to the echo line and send this file's key to
+the next step. This incredibly simple approach allows parallel steps
+to be pipelined. If we use Sickle to do quality-based trimming, the
+pattern is similar:
+
+    find seq -name "*.fastq" | \
+      xargs -n1 -I{} basename {} .fastq | \
+      xargs -n1 -P10 bash run-scythe.bash | \
+      xargs -n1 -P10 bash run-sickle.bash
+