Pbid Tom Live

Power BI Desktop — Live TOM / ADOMD Operations

Local Analysis Services proxy inside Power BI Desktop. Enables TOM (metadata) and ADOMD.NET (DAX query) connections from PowerShell. Does not work for thin reports, Direct Lake models, or remote XMLA endpoints. For those use a remote XMLA / Power BI REST approach — see fabric-tmdl for semantic-model authoring and fabric-tmdl-api for Fabric semantic-model definition REST call patterns (createItemWithDefinition, updateDefinition).

Note (April 2026): Direct Lake on OneLake gained calculated-column / calculated-table preview support. The local msmdsrv proxy still does not bind to Direct Lake models — author and edit those columns through the remote XMLA endpoint (or the Desktop UI) and use TOM there. Local TOM / ADOMD scripting in this skill remains scoped to Import / DirectQuery models.

Prerequisites

Packages extract to $env:TEMP\tom_nuget\...\lib\net45\. Load via Add-Type -Path. If TOM calls fail with missing type / compatibility level errors, swap to the newer Microsoft.AnalysisServices (.NET 8) package.

Discovering Running PBID Instances

Each open .pbix/.pbip spawns one msmdsrv.exe on a random localhost port. When multiple are running, connect to each, read $server.Databases[0].Name, and ask the user which to target.

Watch (verified July 2026, preview): The Power BI Desktop Bridge is a preview IPC surface hosted inside the Desktop process — a named pipe pbi-desktop-bridge-<processId> speaking JSON-RPC 2.0 (Content-Length framing), enabled by default under Options → Preview Features → "Enable external tool access to Power BI Desktop through secure local APIs". Discover methods with bridge.manifest (call it first; the surface changes per Desktop version, and unsupported methods return -32601 MethodNotFound). This could eventually supersede the msmdsrv port-discovery + AS-proxy dance in this skill — pipe name keys off the PID (no msmdsrv.port.txt / netstat needed), and one Desktop window = one bridge. Not there yet: it's preview with an unstable API, currently oriented toward report-authoring / agentic scenarios, and does not replace TOM metadata + ADOMD DAX access covered here. Track the manifest and GA status before relying on it. Re-verify the Learn page before acting on this note.

Connection Strings

$basePath = "$env:TEMP\tom_nuget\Microsoft.AnalysisServices.retail.amd64\lib\net45"
Add-Type -Path "$basePath\Microsoft.AnalysisServices.Core.dll"
Add-Type -Path "$basePath\Microsoft.AnalysisServices.Tabular.dll"

$server = New-Object Microsoft.AnalysisServices.Tabular.Server
$server.Connect("Data Source=localhost:$port")
$db = $server.Databases[0]
$model = $db.Model

Always $server.Disconnect() when done. Only $model.SaveChanges() persists modifications; without it edits are discarded. Ctrl+Z in PBI Desktop cannot undo TOM saves.

Executing DAX via ADOMD.NET

Add-Type -Path "$env:TEMP\tom_nuget\Microsoft.AnalysisServices.AdomdClient.retail.amd64\lib\net45\Microsoft.AnalysisServices.AdomdClient.dll"
$conn = New-Object Microsoft.AnalysisServices.AdomdClient.AdomdConnection
$conn.ConnectionString = "Data Source=localhost:$port"
$conn.Open()

$cmd = $conn.CreateCommand()
$cmd.CommandText = "EVALUATE SUMMARIZECOLUMNS('Date'[Year], ""@Total"", [Total Sales])"
$reader = $cmd.ExecuteReader()
while ($reader.Read()) {
	for ($i = 0; $i -lt $reader.FieldCount; $i++) {
		"$($reader.GetName($i)) = $($reader.GetValue($i))"
	}
}
$reader.Close(); $conn.Close()

DAX rules over ADOMD:

• Fully-qualify columns: 'Sales'[Amount], never [Amount]
• Measures unqualified: [Total Revenue]
• String literals: double quotes, escape "" inside PowerShell here-strings
• Prefer SUMMARIZECOLUMNS over summarize; prefer DIVIDE; never IFERROR
• Reader returns fully-qualified column names. Access by index via GetName($i) / GetValue($i) — short-name indexing silently returns blanks
• Use ExecuteNonQuery() for TMSL commands; ExecuteReader() only for EVALUATE

Validating DAX Before Saving

$testExpr = "SUM('Sales'[Amount]) / COUNTROWS('Sales')"
$cmd.CommandText = "EVALUATE ROW(""@Test"", $testExpr)"
try { $cmd.ExecuteReader().Close(); "VALID" } catch { "INVALID: $($_.Exception.Message)" }

For table expressions wrap in COUNTROWS(). For RLS filter expressions wrap in CALCULATETABLE(ROW("@OK",1), <filter>).

Refreshing the Model

Three equivalent methods:

Refresh dependencies: dimensions first, facts second, then a calculate to recompute DAX. Or issue full on the database and let the engine order it. Refreshes are synchronous — bump shell timeouts to 300000ms+ for large models.

VertiPaq / Storage Engine DMVs

All run via ADOMD $conn.

Filter out internal tables with WHERE LEFT(DIMENSION_NAME,1) <> '$'. High cardinality = large dictionary = memory hotspot.

SELECT DIMENSION_NAME, SUM(USED_SIZE + DICTIONARY_SIZE) AS TotalBytes, SUM(RECORDS_COUNT) AS Rows
FROM $SYSTEM.DISCOVER_STORAGE_TABLE_COLUMN_SEGMENTS
WHERE LEFT(DIMENSION_NAME,1) <> '$'
GROUP BY DIMENSION_NAME
ORDER BY TotalBytes DESC

Query Listener (Capturing Visual DAX)

Poll $SYSTEM.DISCOVER_SESSIONS once per second. When SESSION_LAST_COMMAND changes and starts with DEFINE|EVALUATE|VAR , record it. Timings come from SESSION_LAST_COMMAND_ELAPSED_TIME_MS and SESSION_CPU_TIME_MS.

Ignore internal traffic: <Subscribe>, <ImageSave>, <Batch><Discover>, MDSCHEMA_*, SELECT * FROM $SYSTEM.*.

Visual query anatomy:

Limits: SESSION_LAST_COMMAND is truncated for very large queries; intermediate queries between polls are missed (use 200-500ms interval for busy reports); thin-report queries are invisible.

EVALUATEANDLOG Debugging

EVALUATEANDLOG(<Value>, [<Label>], [<MaxRows>]) wraps a DAX expression, returns it unchanged, and emits intermediate results as a JSON trace event (DAXEvaluationLog, event class ID 135). PBI Desktop only; passthrough in Service / SSAS / Azure AS.

Setup via TOM Trace API: dot-source scripts/setup-evaluateandlog-trace.ps1 — assumes a connected $server (see "Discovering Running PBID Instances" above) and leaves $trace, $evalEvents, and $job in scope for the caller. The script body is the synchronized-ArrayList pattern; the rationale, cache-clear requirement, and FE/SE timing quirks are below.

Critical: Register-ObjectEvent -Action runs in a separate runspace; $global: does not cross. Use a synchronized ArrayList via -MessageData.

Always clear the VertiPaq cache before each debug query; cached results skip EVALUATEANDLOG:

$server.Execute('{ "clearCache": { "object": { "database": "' + $db.Name + '" } } }') | Out-Null

Events arrive asynchronously. Use a persistent trace across queries, record $evalEvents.Count before each call, and Start-Sleep -Seconds 2 before reading. Some calls emit no event due to engine optimizations. COUNTROWS(EVALUATEANDLOG(table)) is a known broken pattern.

JSON payload fields:

Cleanup: $trace.Stop(); Unregister-Event $job.Name; $trace.Drop().

Performance Profiling (FE vs SE Split)

Equivalent to DAX Studio's Server Timings. Subscribe to QueryEnd, VertiPaqSEQueryEnd, VertiPaqSEQueryCacheMatch.

Formula Engine time = QueryEnd.Duration - Σ VertiPaqSEQueryEnd.Duration.

Take 6-12 samples, compare medians not means, discard the first cold-cache run as warm-up. If ranges overlap significantly, the delta is noise.

DAXLib Package Patterns

daxlib.org is a registry of DAX UDF (user-defined function) packages. Requires compatibility level 1702+ — installing upgrades CL automatically and is irreversible; confirm with user before running on models below 1702.

CLI (daxlib): search | info | versions | functions | download (standalone); add | update | remove | installed (require --port <PORT> and .NET 8 SDK). Updates replace all functions matched by DAXLIB_PackageId annotation; user-created functions are untouched.

SVG measures from DaxLib.SVG return data:image/svg+xml;utf8,.... Set the column's data category to "Image URL" so Power BI renders inline in Table/Matrix/Card/Multi-row Card.

Calendar Column Groups (Compatibility Level 1604+)

Declarative date hierarchy + time intelligence mapping. A Calendar lives on a date table and contains one or more CalendarColumnGroup entries, each mapping a column to a TimeUnit enum. With calendars defined, DATESYTD / DATESMTD / SAMEPERIODLASTYEAR / DATEADD automatically adapt to the declared hierarchy.

Rules:

• Calendar names must be unique across the entire model, not just the table
• Each calendar draws columns from one host table only
• Never repeat a time unit within the same calendar
• A column mapped to a unit in one calendar must map to the same unit in every other calendar it participates in
• Each column group has one primary column (sort key) + optional associated columns (display labels). If Year Month is sorted by Year Month Number, the Number column is primary
• Only one TimeRelatedGroup per calendar — combine IsWeekend, Season, RelativeMonth, etc. into it
• Week-based (ISO / 4-4-5) calendars: always pair ISO weeks with ISO year; map Period to the Month time unit

$cal = New-Object Microsoft.AnalysisServices.Tabular.Calendar
$cal.Name = "Gregorian Calendar"
$g = New-Object Microsoft.AnalysisServices.Tabular.CalendarColumnGroup
$g.TimeUnit = [Microsoft.AnalysisServices.Tabular.TimeUnit]::Month
$g.PrimaryColumn = $dateTable.Columns["Year Month Number"]
$g.AssociatedColumns.Add($dateTable.Columns["Year Month"])
$cal.CalendarColumnGroups.Add($g)
$dateTable.Calendars.Add($cal)
$model.SaveChanges()

Transactions and Rollback

SaveChanges() is a single atomic commit — any validation failure rolls back the whole batch. For pre-commit inspection, use $model.UndoLocalChanges() to discard pending modifications since the last save. There is no explicit begin/commit on the local AS proxy.

Finding the Source File Path

TOM does not expose .pbix/.pbip path. Most reliable source is PBI Desktop's User.zip → Settings.xml → FileHistory entries (ordered by lastAccessedDate). The first entry is usually (but not guaranteed to be) the currently-open file. Confirm with user when ambiguous.

Fallbacks: non-Store install exposes path in Get-Process PBIDesktop | Select MainWindowTitle. Win32_Process msmdsrv.exe command line reveals the workspace path but not the source file.

Reference

• Microsoft Learn: Tabular Object Model (TOM) — introduction
• Microsoft Learn: Install, distribute, and reference TOM (assembly split, NuGet, dependencies)
• Microsoft Learn: External tools in Power BI Desktop
• Comprehensive MS Learn link bundle (TOM/AMO/ADOMD APIs / TMSL refresh / DMVs / trace events / external-tools / compatibility levels / remote XMLA / DAXLib): references/REFERENCE.md

Gotchas