This is an LSST Data Management Technical Note giving a high-level overview of SQuaRE’s principles, and our current vision for delivering a flexible, sustainable and evolving capability to support software development, DM subsystem construction, commissioning, operational and scientific activities; and to do so consistently with its funded responsibilities and resources in the areas of Quality Assurance, Integration & Test and Level 3 tools.
For a historical background on the origin of this approach see Appendix: Historical Background.
This document also contains a list of areas for improvement of the controlling documentation. As these issues are addressed, this document will be updated to remove those sections.
THIS IS AN EARLY DRAFT
This is an early draft published in order to assist the DM Pipelines WG in its rework of LDM-151. It is incomplete, unreviewed and certainly not baselined.
There are three main documents that reference the QA system:
- LSE-63 has been as described as the requirements document for the QA work, but as it title describes, it mostly sets out a framework for doing QA. Section 3 has a number of high-level functional and design requirements for the QA system that are in general highly compatible with the approach outlined in this document.
- LDM-151 has section 8 relating to QA WBSes. Contrary to the stated scope of the document, it doesn’t confine itself to the scientific aspects of the design but frequently descends into software design and implementation.
- LDM-230 has two brief references to the QA system that architecturally unclear as to scope and to which system falls their implementation.
All three would require some level of change to be consistent with the approach described in this document, a new requirements flow-down, re-plan, etc. See Areas lacking definition for a list of areas particularly benefiting from updating.
In our approach, we have been driven by the following principles:
- Focus on the ability to service ad-hoc requests whenever possible (eg. rather than just provide a fixed metric, allow user to provide code that implements their own metric); this reduces the cost of change in-house as well as externally
- Write once, use many: service our requirements with a evolving system based on modular components that can be re-used in multiple contexts
- Humans are expensive, computers are cheap: Prioritise minimising human time over compute time when they are in conflict
- Don’t reinvent the wheel: Leverage commodity software, platforms and computing whenever possible
- Drive improvements to the stack architecture when these would facilitate stack use, re-use, maintainability and/or usability
- Encourage alternative implementations that fit will well within a single architectural model to service different requirements, to demonstrate interface generality and avoid lock-in, to optimise developer velocity
- Deploy-anywhere engineering (deployments use configuration management to allow them to be scaled or replicated elsewhere)
An evolving roadmap¶
Our technical roadmap involves an expanding capability based based on previously released components with a common thread through all of SQuaRE’s deliverables, from simple CI functionality to a full-featured L3-type capability that allows execution of ad-hoc code on data:
See Appendix: Historical Background for the full motivation to this approach. It is important to note that the evolution left to right is cumulative; the previous functionality still exists, but the capabilities of the system are expanded.
(The light bulb symbols indicate progression as of end of X16).
At the current design iteration, we are working towards our SQuaSH architecture strawman:
Consistent with our design principles, we initiated a change in the command-line task layer of the stack in order to separate the environment from the science code, thus allowing us to re-use the same science code in multiple environments, while also being able to optimise some aspects of the architecture to different use cases (for example, allowing that code to be executed simply from the command line, or in an intelligent work-flow system at scale).
This work was aided by our previous iteration, the CI system design. It uses tooling we developed to, for example, deploy stack-ready instances for executing stack code (sqr-002). It also uses the log monitoring system (codename: panopticon - sqr-007 in preparation) which is also going to be used for CI, and the UI (at this iteration a dashboard - see sqr-009 ), later also the SUI) has elements in common with our documentation infrastructure (sqr-006).
For the database, we re-used (and are proposing some changes) to the SDQA tables in the LSST Database Schema (sqr-008) and we are prototyping using metrics developed for KPM calculation, validate_drp (dmtn-008).
The initial production version will support simple data processing runs on static data sets (which motivated our Git-LFS service, see sqr-001) in order to publish a “QA CI” service to developers that detects regressions in the codebase and will use primarily HSC data since the stack is well-supported for that instrument and we have shared developers that will benefit from our QA functionality.
Areas lacking definition¶
The following areas would benefit from input from a Working Group (whether the current Pipelines WG or a future one) or similar input. We are primarily concerned in uncovering uncaptured requirements and making the documentation consistent with itself and with the technical roadmap.
- While we believe we are highly consistent with LSE-63 in its spirit, a joint WG/SQuaRE examination of LSE-63 clarifying intent/scope in certain requirements listed in Section 3 is necessary.
- It is clear to us that QA as we astronomers mean it is intrinsically linked with a number of calibration aspects. We are not aware of any documentation addressing this. LDM-151 seems like the right place to capture such requirements.
- We consider it very likely that in order to perform useful forensics, a number quantities beyond the data and its metadata will need to be exposed for co-analysis. We understand that much of this data is going to be available in the Engineering Facilities Database but we have neither audited whether that is sufficient not whether the interfaces and APIs to the EFD are suitable for our intended use. Again, LDM-151 seems like the right place to make sure this is captured.
- Commissioning has a symbiotic relationship with QA; QA tooling can great assist commissioning while commissioning can greatly inform QA efforts. We are working with the LSST Commissioning scientist on understanding his requirements. These will probably result in a new document.
- In a high-data-throughput environment it is not sufficient to detect QA issues - a system has to be as self-repairing as possible. We anticipate that not only does the QA system consume data, but that it also could publish services or data that can be used by other systems to automatically take corrective action (be it to reject data, to repair data, to extrapolate missing data, to page personnel etc). This seems out of scope for the construction given the controlling documentation except for a brief reference in LDM-230, but we would still be interested to identify interfaces that we could provide as part of our resourced development that could be useful downstream for such purposes.
- Level 3
There are a number of aspects about what has been termed “Level 3” processing. Some aspects relate to a provision of compute capacity. Others relate to the ability to allow the community to be able to interact with the data in custom ways. We believe the approach outlined in this document fits in with a “bring your code to the data” approach to Level 3 that would allow to science users to leverage our architecture enabling ad-hoc code execution on compute capacity that they may have access to (eg. university clouds, commodity clouds, NSF clouds).
An overall clarification of the current scope for Level 3 is due; we have worked with the DM Interfaces Scientist to understand how our work can be extended naturally in that direction, but we could benefit from a more formal definition in that area. This is especially the case as it is not unlikely that significant development of Level 3 is likely to happen in Operations, and we want to be well set up to deliver a system that can be extended in that direction. A new document is likely to be required.
- The project has so far been somewhat contradictory in its position on whether the LSST stack is intended as a user-level product. It is fair to say that while this is a goal that has been both publicly promised and that is dear to the heart of DM developers (not to mention of great service to the astronomical community), we have failed to qualify (never mind quantify) the architectural and resource implications of such a commitment. We would like to see some clearly stated requirements in this area. LDM-151 could be the relevant document.
Appendix: Historical Background¶
The Data Management sub-system work during LSST construction is peformed against a Work Breakdown Structure around which (largely) institutional teams have been resourced to do design, development and integration work centered on specific areas of technical activity.
The particular area of activity for the AURA/LSST (aka Tucson) team was Science QA, and the WBS structure at the beginning of LSST construction was as follows:
|02C.01.02||Science Data Quality Integration and Test|
(prior to Construction start, Calibration Products was also included, but later was moved to Princeton’s WBS).
The Tucson team took the approach that a key aspect of ensuring the eventual data quality would be to ensure the quality for the software that produced it (in the FDR, this was referred to as “Level 0 QA”, Docushare 15232). We therefore developed a strong interest in supporting activities that are well-understood to be associated with good code, such as:
- continuous integration
- low-friction development workflows
- low-friction documentation workflows
- regression testing
- portability testing
- supporting good coding practices (code review etc)
- keeping dependencies up to date
- monitoring and forensics
- communication tools
- low-impact release management
We also became concerned about the agility of the codebase, in particular in minimising the cost of change. Our experience is that science data quality does not end upon the discovery of a problem, but rather at the deployment of a solution.
We therefore sought and obtained the developer support WBS. However we did not increase our resources to cover these activities, for reasons including:
- We believed our solutions to these problems would increase productivity in other areas important to science quality, in particular pipeline development and testing
- We had staff with good velocity in these areas
- We did not wish to disadvantage the institution that previously held the developer support WBS
But most critically, we saw that by leveraging recent commodity software developments we could develop a single technical roadmap that expands, in an on-going fashion, a set of capabilities that can be applied to all of our areas of acitivity: developer support, QA assessment, commissioning/intergration support, and even functionality consistent with Level 3 goals. As a result, we intended for the effort in many areas of “developer support” to be recouped further into construction by being re-used in the QA and L3 context. This document outlines a technical roadmap designed to do exactly that.
In order to distinguish this approach from the previous approach of servicing QA goals as a largely standalone application (SDQA, which was intended as a re-factoring of the R&D application PipeQA), we have given this architecture the codename SQuaSH (for Science Quality AnalysiS Harness). However this is just an implementation choice; it does not affect the goals or capabilities of the system.
The SQuaRE “origin story”¶
This approach to supporting software quality as a way of ensuring product quality, with a focus on a devops engineering approach that beats down the operational demands of deployed services through architectural and implementation choices, is a current practice often called Site Reliability Engineering. The change of the name of the group from the Survey Science Group to SQuaRE (Science Quality and Reliability Engineering) was intended to convey our focus on sustainable software solutions to service science quality and scientific utility demands. It was not intended to lessen our focus on science quality evaluation (which is the primary focus of our deliverables).
|||There is some difference of opinion as to what Level 3 “really means” at this point, see later discussion.|