A HealthOmics ETag (entity tag) is a hash of the ingested content in a sequence store. This simplifies data retrieval and processing while maintaining the content integrity of the ingested data files. The ETag reflects changes to the semantic content of the object, not its metadata. The specified read set type and algorithm determine how the ETag is calculated. The ETag calculation doesn't alter the actual file or genomic data. When the file type schema of the read set permits it, the sequence store updates fields that are linked to data provenance.
Files have a bitwise identity and a semantic identity. The bitwise identity means that the bits of a file are identical, and a semantic identity means that the contents of a file are identical. Semantic identity is resilient to metadata changes and compression changes as it captures the content integrity of the file.
Read sets in HealthOmics sequence stores undergo compression/decompression cycles and data provenance tracking throughout an object's lifecycle. During this processing, the bitwise identity of an ingested file may change and is expected to change each time a file is activated; however, the semantic identity of the file is maintained. The semantic identity is captured as a HealthOmics entity tag, or ETag that's calculated during sequence store ingestion and available as read set metadata.
When the file type schema of the read set permits it, the sequence store updates fields are linked to data
provenance. For uBAM, BAM, and CRAM files, a new @CO or Comment tag is added to the header.
The comment contains the sequence store ID and ingestion timestamp.
Amazon S3 ETags
When accessing a file using the Amazon S3 URI, Amazon S3 API operations may also return Amazon S3 ETag and checksum values. The Amazon S3 ETag and checksum values differ from the HealthOmics ETags because they represent the file's bitwise identity. To learn more about descriptive metadata and Objects, see the Amazon S3 Object API documentation. Amazon S3 ETag values can change with each activation cycle of a read set and you can use them to validate the reading of a file. However, don't cache Amazon S3 ETag values to use for file identity validation during the file's lifecycle because they don't remain consistent. In contrast, the HealthOmics ETag remains consistent throughout the read set's lifecycle.
How HealthOmics calculates ETags
The ETag is generated from a hash of the ingested file contents. The ETag algorithm family is set to MD5up by default, but it can be configured differently during sequence store creation. When the ETag is calculated, the algorithm and the calculated hashes are added to the read set. The supported MD5 algorithms for file types are as follows.
-
FASTQ_MD5up – Calculates the MD5 hash of an uncompressed, complete FASTQ read set source.
-
BAM_MD5up – Calculates the MD5 hash of the alignment section of an uncompressed BAM or uBAM read set source as represented in the SAM, based on the linked reference, if one is available.
-
CRAM_MD5up – Calculates the MD5 hash of the alignment section of the uncompressed CRAM read set source as represented in the SAM, based on the linked reference.
Note
MD5 hashing is known to be vulnerable to collisions. Because of this, two different files might have the same ETag if they were manufactured to exploit the known collision.
The following algorithms are supported for the SHA256 family. The algorithms are calculated as follows:
-
FASTQ_SHA256up – Calculates the SHA-256 hash of an uncompressed, complete FASTQ read set source.
-
BAM_SHA256up – Calculates the SHA-256 hash of the alignment section of an uncompressed BAM or uBAM read set source as represented in the SAM, based on the linked reference, if one is available.
-
CRAM_SHA256up – Calculates the SHA-256 hash of the alignment section of an uncompressed CRAM read set source as represented in the SAM, based on the linked reference.
The following algorithms are supported for the SHA512 family. The algorithms are calculated as follows:
-
FASTQ_SHA512up – Calculates the SHA-512 hash of an uncompressed, complete FASTQ read set source.
-
BAM_SHA512up – Calculates the SHA-512 hash of the alignment section of an uncompressed BAM or uBAM read set source as represented in the SAM, based on the linked reference, if one is available.
-
CRAM_SHA512up – Calculates the SHA-512 hash of the alignment section of an uncompressed CRAM read set source as represented in the SAM, based on the linked reference.
