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File Formats
vg has a variety of formats, some unique to the world of graphs and others directly analogous to familiar legacy formats. We explain some of them here.
If you are looking for a more comprehensive list of different file types, see File Types.
Protocol Buffers are used by many of the vg file formats. There are a few things that frustrate first-time viewers that we'll address here. First, protobuf is binary, so you can't just use less to view it; use vg view and the relevant flags.
In addition, when a protobuf field has a value of 0 or is unset, it won't show up in the output. This can make parsing a bit of a pain if you aren't aware.
Protobuf specifies only the encoding of a single message or object at a time, of a limited maximum size. This is usually 64 megabytes, and although vg raises this limit, it cannot be made arbitrarily large. Consequently, VG's Protobuf-based formats, such as .vg for graphs and .gam for read alignments, need to hold multiple messages per file. For graphs, each message is a partial Graph, and for reads, each message is an Alignment.
These messages are strung together using a framing format, implemented in libvgio. A file consists of a series of groups. Each group starts with a count, serialized as a Protobuf varint64, and then has that many messages, with each message being prefixed with its length as a varint64. The first (and sometimes only) message in a group, instead of actually being a message, can instead be a type tag: a string describing the type of the objects in the group (usually the name of the Protobuf type). This is intended to prevent one Protobuf-based file type from accidentally being decoded as another. This is a real concern: Protobuf serialized data consists entirely of filed numbers and field values, with no type information, and people often insert the wrong file into vg tools.
The whole stream of groups can be optionally block-GZIP compressed.
The backing structures of vg are Protobuf messages. Protobuf is a schema language that allows us to easily develop our data model, create C++ source code for it, and serialize/deserialize it to and from disk. It is binary data (and often compressed), so don't bother trying to look at it with less - we suggest using the vg view command to check out a graph in GFA or even as a pdf using graphviz.
The .vg format is the actual graph object, just stored in a file. We use the graph for things like vg view, and we can modify it with vg mod. We often operate on graph indices though for the sake of efficiency.
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The 'reference graph' : We use the phrase reference graph to describe a graph containing a reference genome and variation to that reference (i.e. a FASTA and a VCF).
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The 'flat graph': A flat graph contains only a reference genome. It's 'flat' in the sense that because there are no bubbles; the graph itself resembles pearls on a string.
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The 'augmented graph': We use this to describe a graph which has had reads mapped to it, then had all paths implied by those reads inserted into the graph. This means if you mapped the same reads to an augmented graph every read would (in theory) map perfectly - all variation and error included in the reads is now included in the graph. We use the augmented graph as the basis for
vg genotype. -
The 'sample graph': A variation graph containing variation from one or several samples. These may be from an assembly of several genomes, for example, or a single flat graph augmented with the reads from a single sample.
XG can be thought of as a succinct (i.e. lightweight or low-memory) representation of the graph it indexes. The xg index contains the paths of the graph and the nodes and edges, though not their sequences; the structure of the graph is conserved but the actual sequence is removed.
XG uses succinct data structures to accomplish this, and the translation between the graph and these structures is a bit complicated so we'll leave its description to elsewhere in the wiki. However, the basic idea is this - you can perform queries like getting the distance between two nodes, or get adjacent nodes, or find the in degree of a node, all using just the xg index of a graph. You'll use the xg index for mapping reads but also for a variety of functions that leverage its speed compared to operating on the raw graph.
The GCSA/GCSA2 index is equivalent to the .sa file produced by bwa index. This contains a suffix array which allows quick lookup of where specific sequences are in the graph. This allows us to map reads against the graph, and this is where you'll see the GCSA index come up.
GAM is vg's BAM equivalent, providing the relevant information to determine where a sequence (e.g. a read) mapped in the graph. It supports paired end reads by interleaving them in the GAM. It is again usually in binary (and compressed) form, but a quick vg view will expose what it looks like.
GAM can also be transformed into JSON records, with the following characteristics:
- One record (i.e. one read) per line, which is one JSON object.
- All fields set to false / 0 / NULL do not appear in JSON output.
GAM is great, but there is not really an implicit order of sequences in the format. We use the GAM index to query reads without having to loop over the entire GAM for each query. For example, say we'd like a way to get all the reads mapping to a specific position or sort reads by position like BAM does. We can accomplish these with the GAM index.
Under the hood, the GAM index is a RocksDB database. RocksDB is a disk-based key-value store (essentially a map). It's nice because it keeps us from having to put a GAM in memory, which would cause serious problems for most machines, but unfortunately it can be a little slow since we have to go to and from disk.
DOT is a standard format used by graphtools, a package for graph visualization designed by AT&T labs a long, long time ago. It's kind of human readable, but it's best used as an intermediate between vg and a PDF or SVG file.
GFA is a format that's designed to allow data interchange between programs which operate on sequence graphs. These might include assemblers; pangenomics programs like vg; toolkits for manipulating the format like RGFA, gfatools and gfakluge; or visualization programs like Bandage. By allowing interoperability between these different programs GFA enables some really cool workflows - imagine performing a de novo assembly, outputting it as a graph in GFA, then using vg to map reads against it before visualizing with Bandage. GFA is a big step toward building an entire community and ecosystem of graph-based tools.
GFA is sort of human readable; check out the docs for a proper schema description.
The vg tool can understand plain GFA and rGFA. It also supports some conventions not specified in the GFA standard.
One of these is an RS tag on the GFA header, which vg interprets as a space-separated list of "reference samples". GFA paths and walks associated with these samples will be interpreted by vg as reference-sense paths, and will, for example, be used as default targets for "surjection" to output aligned reads in linear-reference formats. For example, a GFA header of H VN:Z:1.1 RS:Z:GRCh37 GRCh38 would specify that the GRCh37 and GRCh38 samples should be interpreted as embedded linear references, while any other samples should be interpreted as observed haplotypes.
Another convention is support for a PanSN-style format for P line path names. A path named GRCh38#0#chr1 will be interpreted as belonging to sample GRCh38, phase/haplotype 0, and contig chr1. This allows P and W lines to be used more interchangeably, and allows P lines to be associated with samples (which may in turn appear in an RS tag).
There is a lot of information contained in reads mapped to a graph. We can use mappings to a graph as the basis
for machine learning algorithms using vg vectorize. In essence, a mapping to the graph describes some information about the genotype of a sample. We can associate specific mappings (e.g. a causal SNP) with class labels (e.g. phenotypes) and train learning algorithms to recognize reads supporting specific labels based on their mappings.