udf overview

Author: mamccrea

articles/stream-analytics/TOC.yml

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-  - name: User-defined functions
+  - name: Extend queries with functions
     items:
-    - name: Machine learning UDF
-      href: machine-learning-udf.md
-    - name: C# UDF
-      href: stream-analytics-edge-csharp-udf-methods.md
+    - name: Functions overview
+      href: functions-overview.md
     - name: JavaScript UDF
       href: stream-analytics-javascript-user-defined-functions.md
+    - name: JavaScript UDA
+      href: stream-analytics-javascript-user-defined-aggregates.md
+    - name: C# UDF
+      href: stream-analytics-edge-csharp-udf-methods.md
+    - name: Machine learning UDF
+      href: machine-learning-udf.md
   - name: Optimize your Stream Analytics job
     items:
     - name: Understand and adjust Streaming Units
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-      href: stream-analytics-javascript-user-defined-aggregates.md
     - name: High-frequency trading
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     - name: Process IoT streaming data

articles/stream-analytics/functions-overview.md

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+---
+title: User-defined functions in Azure Stream Analytics
+description: This article is an overview of 
+author: mamccrea
+ms.author: mamccrea
+ms.service: stream-analytics
+ms.topic: conceptual
+ms.date: 04/07/2020
+---
+
+# User-defined functions in Azure Stream Analytics
+
+The SQL-like query language in Azure Stream Analytics makes it easy to implement real-time analytics logic on streaming data. Stream Analytics provides additional flexibility through custom functions that are invoked in your query. The following code example is a UDF called `sampleFunction` that accepts one parameter, each input record the job receives, and the result is written to the output as `sampleResult`.
+
+```sql
+SELECT 
+    UDF.sampleFunction(InputStream) AS sampleResult 
+INTO 
+    output 
+FROM 
+    InputStream 
+```
+
+## Types of functions
+
+Azure Stream Analytics supports the following four function types: 
+
+* Javascript user-defined functions 
+* Javascript user-defined aggregates 
+* C# user-defined functions (using Visual Studio) 
+* Azure Machine Learning 
+
+You can use these functions for scenarios such as real-time scoring using machine learning models, string manipulations, complex mathematical calculations, encoding and decoding data. 
+
+## Limitations
+
+User-defined functions are stateless, and the return value can only be a scalar value. You cannot call out to external REST endpoints from these user-defined functions, as it will likely impact performance of your job. 
+
+Azure Stream Analytics does not keep a record of all functions invocations and returned results. To guarantee repeatability - for example, re-running your job from older timestamp produces the same results again - do not to use functions such as `Date.GetData()` or `Math.random()`, as these functions do not return the same result for each invocation.  
+
+## Diagnostic logs
+
+Any runtime errors are considered fatal and are surfaced through activity and diagnostic logs. It is recommended that your function handles all exceptions and errors and return a valid result to your query. This will prevent your job from going to a [Failed state](job-states.md).  
+
+
+## Next steps
+
+* [JavaScript user-defined functions in Azure Stream Analytics](stream-analytics-javascript-user-defined-functions.md)
+* [Azure Stream Analytics JavaScript user-defined aggregates](stream-analytics-javascript-user-defined-aggregates.md)
+* [Develop .NET Standard user-defined functions for Azure Stream Analytics jobs](stream-analytics-edge-csharp-udf-methods.md)
+* [Integrate Azure Stream Analytics with Azure Machine Learning](machine-learning-udf.md)
+