Spectra Variable Initialization Strategies

---
name: spectra-variable-initialization-strategies
description: Use when when designing or configuring an MsBackend subclass (e.
license: CC-BY-4.0
metadata:
  edam_operation: http://edamontology.org/operation_3435
  edam_topics:
  - http://edamontology.org/topic_3520
  - http://edamontology.org/topic_0121
  tools:
  - Spectra
  - R
  - S4Vectors
  - R base
  techniques:
  - mass-spectrometry
  license_tier: open
  provenance_tier: literature
derived_from:
- doi: 10.3390/metabo12020173
  title: spectra
evidence_spans:
- The *Spectra* package defines an efficient infrastructure for storing and handling
  mass spectrometry spectra
- library(Spectra) library(IRanges)
- library(Spectra)
- return the **full** spectra data within a backend as a `DataFrame` object (defined
  in the `r Biocpkg("S4Vectors")`
claims: []
provenance:
  collection: https://w3id.org/holobiomicslab/asb-skill/collection/metabolomics/v2
  assembled_by: scripts/collect_metabolomics_collection.py
  sources:
  - build: coll_spectra
    doi: 10.3390/metabo12020173
    title: spectra
  dedup_kept_from: coll_spectra
schema_version: 0.2.0
attribution:
  generator: AgenticScienceBuilder
  original_doi: 10.3390/metabo12020173
  all_source_dois:
  - 10.3390/metabo12020173
  zenodo_doi: 10.5281/zenodo.20794027
  curators: []
  promoter: Louis-Félix Nothias
  sponsor: CNRS & Université Côte d'Azur
---

# spectra-variable-initialization-strategies

## Summary

Evaluate and select between eager pre-population and lazy on-demand filling of core spectra variables (@spectraVars slot) in MsBackend objects to optimize the memory–speed tradeoff. This skill surfaces the design choice between initialization timing and its consequences for object footprint and data access latency.

## When to use

When designing or configuring an MsBackend subclass (e.g. MsBackendTest, MsBackendMemory) and you need to decide whether to initialize the @spectraVars data frame with all core spectra variable columns (mz, intensity, rtime, scanIndex, precursorMz, precursorIntensity, acquisitionNum, msLevel) upfront, or populate them on-the-fly during spectraData() calls. Apply this skill if memory footprint and spectraData() access time are both constraints.

## When NOT to use

- If your MsBackend is read-only and never modifies @spectraVars after initialization (lazy filling adds unnecessary complexity).
- If spectra variables are extremely sparse and accessed infrequently (eager pre-population wastes memory with no speed benefit).
- If you are not designing a custom backend but simply using an existing MsBackend implementation (the tradeoff choice is already baked in).

## Inputs

- MsBackend subclass instance with @spectraVars slot (minimal or pre-populated)
- Core spectra variable column names (mz, intensity, rtime, scanIndex, precursorMz, precursorIntensity, acquisitionNum, msLevel, etc.)
- MS peak data (m/z and intensity values stored in NumericList slots or external files)

## Outputs

- Memory footprint measurement (object.size() in bytes) for pre-populated variant
- Memory footprint measurement for lazy-filled variant
- Execution time profile for spectraData() under each strategy
- DataFrame containing full spectra data with all core spectra variables populated
- Documented memory–speed tradeoff analysis and recommendation for backend configuration

## How to apply

Create two parallel MsBackend instances: one with @spectraVars pre-populated with all core spectra variable columns at initialization time, and one with minimal @spectraVars that will trigger fillCoreSpectraVariables() internally during spectraData() invocation. Measure in-memory object size using object.size() before and after calling spectraData(), and profile execution time of spectraData() for each variant. Pre-population increases memory overhead because all variables—including those with only missing values—are stored permanently in the object; on-the-fly filling avoids storing empty columns but incurs initialization cost at each spectraData() call. Document the measured memory footprint increase and access-time delta to inform backend design choices based on your use case (e.g., small, frequently-accessed objects favor pre-population; large, sparse spectra favor lazy filling).

## Related tools

- **Spectra** (R package providing MsBackend virtual class and spectraData() / fillCoreSpectraVariables() methods; required for backend implementation and profiling) — https://github.com/RforMassSpectrometry/Spectra
- **S4Vectors** (Provides DataFrame class used to store and return @spectraVars slot and spectraData() output)
- **R base** (Provides object.size() function for memory footprint measurement and system.time() for execution profiling)

## Examples

```
# Create MsBackendTest with pre-populated @spectraVars
backend_eager <- new("MsBackendTest", spectraVars = data.frame(mz=NA, intensity=NA, rtime=NA, scanIndex=NA, precursorMz=NA, precursorIntensity=NA, acquisitionNum=NA, msLevel=NA))
size_eager <- object.size(backend_eager)
time_eager <- system.time(df_eager <- spectraData(backend_eager))

# Create MsBackendTest with minimal @spectraVars (lazy filling)
backend_lazy <- new("MsBackendTest", spectraVars = data.frame())
size_lazy <- object.size(backend_lazy)
time_lazy <- system.time(df_lazy <- spectraData(backend_lazy))  # triggers fillCoreSpectraVariables()

message("Memory delta: ", size_eager - size_lazy, " bytes")
message("Time delta: ", time_eager[3] - time_lazy[3], " seconds")
```

## Evaluation signals

- Pre-populated @spectraVars object.size() is larger than lazy-filled variant by amount equal to total size of empty/sparse core variable columns.
- spectraData() execution time for pre-populated variant is consistently lower than lazy-filled variant (because no fillCoreSpectraVariables() call is needed).
- Both variants produce identical DataFrame output from spectraData() with all core spectra variables present and correctly populated.
- Lazy-filled variant's spectraData() shows measurable latency spike on first call (fillCoreSpectraVariables() overhead); subsequent calls may cache results.
- Memory delta between variants scales linearly with number of spectra and number of unfilled core variable columns.

## Limitations

- Memory measurement via object.size() captures in-memory R object size but does not account for external references (e.g. memory-mapped files or database connections managed by on-disk backends like MsBackendMzR).
- Execution time profiling is hardware- and R-session-dependent; results may not generalize across different machines or memory-constrained environments.
- The tradeoff assumes @spectraVars is read after initialization; if spectra variables are frequently modified or added dynamically, lazy filling may introduce unexpected state management complexity.
- fillCoreSpectraVariables() behavior and overhead are backend-specific; the measured cost depends on implementation details not exposed in the virtual class contract.

## Evidence

- [other] Pre-populating @spectraVars with all core spectra variables increases memory footprint because all variables, including those with only missing values, must be stored in the object, whereas on-the-fly initialization via fillCoreSpectraVariables() avoids storing empty columns but may be less efficient for data extraction.: "Pre-populating @spectraVars with all core spectra variables increases memory footprint because all variables, including those with only missing values, must be stored in the object, whereas"
- [intro] The `spectraData()` method should return the **full** spectra data within a backend as a `DataFrame` object: "The `spectraData()` method should return the **full** spectra data within a backend as a `DataFrame` object"
- [intro] Each `Spectra` object contains an implementation of such a `MsBackend` within its `@backend` slot which provides the MS data to the `Spectra` object.: "Each `Spectra` object contains an implementation of such a `MsBackend` within its `@backend` slot which provides the MS data to the `Spectra` object."
- [intro] slots = c(
             spectraVars = "data.frame",
             mz = "NumericList",
             intensity = "NumericList"
         ): "spectraVars = "data.frame",
             mz = "NumericList",
             intensity = "NumericList""