update-bcd Architecture Notes

The update-bcd script iterates through entries from a local copy of browser-compat-data and compares them against a set of test results from a local copy of mdn-bcd-results to see if there should be any updates to the browser-compat-data repo.

The update-bcd script keeps an internal representation of the data from each iterated BCD file for purposes of comparing and applying possible transformations to the data.

Tracking the Update State of Each BCD Entry Iteration

As the script iterates through each BCD entry, an UpdateInternal object is created to represent the state of the data. This state object is shared across all of the script's sequential operation. The primary keys in this object are:

• path - the "path" of the BCD entry we're iterating through
• shared - a "shared" object that houses all the existing data from the local repositories, including the latest BCD data and the latest test result "support matrix".
• debug - a "debug" object, which builds a stack trace of any changes to the entry data.
• a set of "statement" objects that hold the existing and updated support statements for the BCD entry
  • allStatements - the set of existing support statements in the BCD entry
  • defaultStatements - a filtered set of existing "default" support statements in the BCD entry (statements without flags or browser prefixes)
  • inferredStatements - the set of support statements inferred from the test result "support matrix"
  • statements - the final set of support statements that represent the latest changes (if any) to the BCD entry

At a high level, this state object is built in three phases of operations per iteration:

1. First, we build the shared data object in the first set of provideShared.
2. Then, we collect the existing support statements in the following series of provide...Statements functions.
3. Finally, we make decisions about whether or not to update those statements in the last series of persist... functions.

Data Flow Examples

Here's a step-by-step explainer with JSON snapshots of this state object to show how it's built as we move through the chain of operations in the update method. Refer to this diagram for a visual representation of this data flow.

flowchart LR
    entries((BCD Entries))
    entries --> pathFilters

    subgraph one [" "]
        pathFilters{Match Optional\nPath Filters} -- Yes -->
        provideSharedBrowserMap[[provideShared\nBrowserMap]] -->
        hasBrowserMap{Has Browser Map?} -- Yes -->
        provideSharedSupport[["provideShared\n#quot;Support#quot;"]] -->
        provideSharedUnmodifiedSupport[["provideShared\n#quot;unmodifiedSupport#quot;"]]
    end

    provideSharedUnmodifiedSupport --> iterate
    pathFilters & hasBrowserMap -- No --> entries

    subgraph two [" "]
        iterate((Iterate Browsers\nper BCD Entry)) -->
        browserFilters{Match Optional\nBrowser Filters} -- Yes -->
        provideAllStatements[[provide\nAll Statements]] -->
        provideDefaultStatements[[provide\nDefault Statements]] -->
        detectDiffs{Detect Diffs in\nSupport Matrix vs. BCD} -- Yes -->
        provideInferredStatements[[provide\nInferred Statements]] -->
        tooManyInferred{Too Many\nInferred Statements?} -- No -->
        releaseFilters{Match Optional\nRelease Filters} -- Yes -->
        defaultOrInferred{Has Default or\nInferred Statements?}
    end

    defaultOrInferred -- Yes --> persistNonDefault
    browserFilters & detectDiffs & releaseFilters & defaultOrInferred -- No --> entries
    tooManyInferred -- Yes --> entries

    subgraph three [" "]
        persistNonDefault[[persist\nNon-Default\nStatements]] -->
        tooManyDefault{Too Many\nDefault Statements?} -- No -->
        hasVersionRemoved{Has #quot;Version\nRemoved#quot; Statement?} -- No -->
        inferredStatementsOutdated{Inferred Statements Outdated?} -- No -->
        persistInferredRange[[persist\nInferred Range]] -->
        persistAddedOverPartial[[persist\nAdded Over\nPartial]] -->
        persistAdded[[persist\nAdded]] -->
        persistRemoved[[persist\nRemoved]] -->
        clearNonExact[[clearNonExact]] -->
        stillHasStatements{Still has\nstatements?}
    end

    stillHasStatements -- Yes --> saveChanges[[Save Changes]] --> entries
    stillHasStatements -- No --> entries
    tooManyDefault & hasVersionRemoved & inferredStatementsOutdated -- Yes --> entries

expand("entry", ...)

Start iterating through entries in the BCD data and begin populating the state object.

{
  path: "api.AbortController",
  debug: {
    stack: [
      {
        step: "entry",
        result: {
          path: "api.AbortController",
        },
      },
    ],
  },
  shared: {
    bcd: {
      api: { ... },
    },
    entry: { ...api.AbortContoller entry data from bcd... },
  },
}
We exit early if the BCD Identifier path doesn’t match an optional path filter flag.
provideShared(“browserMap”, …)
Builds the browser “Support Matrix” data from the local test results filtered by the path key.
{
  path: "api.AbortController",
  debug: {
    stack: [
      {
        step: "entry",
        result: {
          path: "api.AbortController",
        },
      },
      {
        step: "provide_shared_browserMap",
        result: {},
      },
    ],
  },
  shared: {
    bcd: { ... },
    entry: { ... },
    browserMap: Map(2) {
      chrome => Map(4),
      safari => Map(3)
    }
  },
}
provideShared(“support”, …)
Get browser support data from BCD entry __compat data.

        
NOTE: This support key in the shared object is mutated at the end of the iteration if we’ve determined that there should be updates.

      
{
  path: "api.AbortController",
  debug: { ... },
  shared: {
    bcd: { ... },
    entry: { ... },
    browserMap: Map(2),
    support: {
      chrome: {
        version_added: "80",
      },
      safari: {
        version_added: null,
      },
    },
  },
}
provideShared(“unmodifiedSupport”, …)
Clones original support data. This key remains unmodified at the end of update.
{
  path: "api.AbortController",
  debug: { ... },
  shared: {
    bcd: { ... },
    entry: { ... },
    browserMap: Map(2),
    support: { ... },
    unmodifiedSupport: {
      chrome: {
        version_added: "80",
      },
      safari: {
        version_added: null,
      },
    },
  },
}
expand(“browser”, …)
Start iterating through browsers in BrowserMap Support Matrix test results per BCD entry. Gets versionMap support data from the BrowserMap by browser key.
{
  path: "api.AbortController",
  debug: { ... },
  shared: {
    bcd: { ... },
    entry: { ... },
    browserMap: Map(2),
    support: { ... },
    unmodifiedSupport: { ... },
    versionMap: Map(4) {
      "82" => null,
      "83" => true,
      "84" => true,
      "85" => true
    }
  },
  browser: "chrome",
}
Exit early here if browser doesn’t match the optional browser filter flag.
provideAllStatements
Gets all existing support statements from BCD.
{
  path: "api.AbortController",
  debug: { ... },
  shared: { ...},
  browser: "chrome",
  allStatements: [
    {
      version_added: "80",
    },
  ],
}
provideDefaultStatements
Gets existing un-flagged and un-prefixed statements from BCD. Exit if no default statements found. We also run an initial comparison of the test results from the versionMap against the defaultStatements to determine if there are possible updates. If not, exit early.
{
  path: "api.AbortController",
  debug: { ... },
  shared: { ... },
  browser: "chrome",
  allStatements: [...],
  defaultStatements: [
  {
      version_added: "80",
    },
  ]
}
provide(“inferredStatements”, …)
Infer support statements from local test results. Exits if more than 1 statement inferred or if version_added doesn’t match any optional release filters.
{
  path: "api.AbortController",
  debug: { ... },
  shared: { ... },
  browser: "chrome",
  allStatements: [...],
  defaulStatements: [...],
  inferredStatements: [
    {
      version_added: "≤83",
    },
  ],
}
persistIfNoDefault
Updates statements key with inferred statements (& existing un-flagged statements) when no default statements exist.
{
  path: "api.AudioContext.close",
  debug: {
    stack: [
      ...,
      {
      step: "provide_statements_nonDefault",
         result: {
          statements: [
      {
        version_added: "85",
      },
    ],
    reason: {
      step: "provide_statements_nonDefault",
      message: "api.AudioContext.close applied for chrome because there is no default statement",
      skip: true,
    },
  },
}
     ]
  },
  shared: { ... },
  browser: "chrome",
  allStatements: [],
  defaultStatements: [],
  inferredStatements: [
   {
      version_added: "85",
    },
  ],
  statements: [
    {
      version_added: "85",
    },
  ]
}

        
NOTE: Data that gets written into the statements key is ultimately used to mutate the original BCD data in the shared object when we exit the full entry loop, resulting in writes back to the BCD files.

      
Exits early if:

More than 1 default statement,
Default statement has version_removed data, or
BCD shows support for a newer browser version than there are test results for.

persistInferredRange
Persist inferred version range when BCD version was set to preview or when inferred range supersedes original data.
{
  path: "api.AbortController.abort",
  debug: { ... },
  shared: { ... },
  browser: "chrome",
  allStatements: [ ... ],
  defaultStatements: [
    {
      version_added: "85",
    },
  ],
  inferredStatements: [
    {
      version_added: "≤84",
    },
  ],
  statements: [
    {
      version_added: "≤84",
    },
  ],
}
persistAddedOverPartial
Sets statements support data to false if no inferred added data and only "partial implementation" in BCD data.
{
  path: "api.FakeInterface",
  debug: { ... },
  shared: { ... },
  browser: "chrome",
  allStatements: [ ... ],
  defaultStatements: [
    {
      version_added: "85",
      partial_implementation: true,
      impl_url: "http://zombo.com",
      notes: "This only works on Wednesdays",
    }
  ],
  inferredStatements: [
    {
      version_added: false,
    },
  ],
  statements: [
    {
      version_added: false,
    },
  ],
}
persistAddedOver
Updates statements key with inferred version_added data under following conditions:

it is different from existing value
is not a range
is not a boolean (if existing value is a string)
existing data does not contain partial_implementation

{
  path: "api.AbortController",
  debug: { ... },
  shared: {
    ...,
    unmodifiedSupport: {
      ...,
      safari: {
        version_added: null,
      }
    },
  },
  browser: "safari",
  allStatements: [ ... ],
  defaultStatements: [ ... ],
  inferredStatements: [
    {
      version_added: "≤13.1",
       },
  ],
  statements: [
    {
      version_added: "≤13.1",
    },
  ],
}
persistRemoved
Adds version_removed data to statements and optionally updates existing version_removed data if the inferred version_removed data is a string.
{
  path: "api.DeprecatedInterface",
  debug: { ... },
  shared: {
    ...,
    unmodifiedSupport: {
      ...,
      chrome: {
        version_added: null,
      }
    },
  },
  browser: "chrome",
  allStatements: [ ... ],
  defaultStatements: [ ... ],
  inferredStatements: [
    {
      version_added: "≤83",
      version_removed: "85",
    }
  ],
  statements: [
    {
      version_added: "≤83",
      version_removed: "85",
    },
  ],
}
clearNonExact
Overwrites and clears any statements with ranged data if optional exactOnly flag is set.
There is one final skip check after this step . If there are still statements at this point, then we include them in the final set of changes for BCD. We also run another comparison of the versionMap data from the test results against the updated defaultStatements to see if there are still possible updates that could have been made. If there are, then we log those with a warning that possible manual intervention may be required on the BCD entry.