Add sample application and README file

[JatinSanghvi]

Checking the PR branch in forked repo: https://github.com/JatinSanghvi/keda-external-scaler-azure-cosmos-db (including the README file) might provide a better sense of the changes that were made

Checklist

• Commits are signed with Developer Certificate of Origin (DCO)

Fixes

• Addresses issue Provide GitHub Action to publish official image on GHCR #9 (need review to confirm)

• Scales targets based on count of Cosmos DB container's partition-ranges with non-zero pending items. Existing implementation scales according to the estimate of items pending to be processed across all partitions, which is incorrect. Cosmos DB architecture limits the degree of parallelism of "processing new changes to the container" to the "number of logical partition ranges" (More information). The logical partition ranges is dependent on the physical partitions, which is turn depends on combination of container data storage size and container throughput (More information). For small containers with low throughput provisioned, we expect the scale targets to not scale beyond 1.

• Fixes generation of metric names. Existing implementation generates the metric name in method GetMetricSpec but uses metric name from request metadata in method GetMetrics. If metric name is not specified in ScaledObject definition, returning metric name in GetMetricSpec won't make the metric-name part of scaler metadata when GetMetrics is called. On other hand, if the metric name is supplied by user in ScaledObject definition, GetMetricSpec would override the name, and KEDA won't be able to get the metrics for the generated metric-name when calling GetMetrics. New implementation provides consistent behavior across methods. All methods will reuse the metric-name if provided by user, else generate one based on other 'required' properties in ScaledObject.

• Bases metric name generation based on lease container properties. The current metric-name generation logic will create same metric name if two different lease containers are created to monitor the same container.

• Uses standard names as in Cosmos DB documentation for trigger properties. Connection instead ConnectionString (matches with other KEDA scalers), ContainerId instead of CollectionName and ProcessorName instead of LeaseCollectionPrefix.

Other changes

• Adds sample producer and consumer projects with steps in README file to test scaling of target application between 0, 1 and 2 replicas.
• Refactors code and updates unit tests.

Closes #2
Closes #3
Closes #9

[JatinSanghvi]

Tagging @pragnagopa, @TsuyoshiUshio, @divyagandhisethi, @ealsur, @lpapudippu

[JatinSanghvi]

@ahmelsayed, @tomkerkhove - Can you please review the PR?

[JatinSanghvi]

@ahmelsayed, @tomkerkhove - Gentle reminder for PR review.

[tomkerkhove]

I just got back from holiday and will review asap

[tomkerkhove]

I did a first skip but have a few high-level remarks:

• The README does not provide documentation on what the scaler itself does and how it helps the customer. You should provide guidance on what it is, how to deploy and how to configure a ScaledObject
• The scaler project was renamed, what was the reasoning for this as the previous naming convention (CosmosDb.Scaler) was a lot better imo
• The repo needs a bit more structure, can you please introduce the following folders please?
  • src/ for the external scaler code
  • samples/ for the sample code and README to provide guidance how to run the sample
• We should explicitly document that this is purely for Changefeed and not the rest

[JatinSanghvi]

@tomkerkhove - Please review the updated PR when you have time.

• Added main README file with scalar overview, deployment instructions and trigger metadata details. Moved earlier README for sample app to sub-directory.
• Introduced separate GitHub actions for different triggers to avoid confusion and ensure the built package versions do not overlap.
• Updated namespace names from Keda.CosmoDbScaler to Keda.CosmoDb.Scaler.
• Updated folder structure. The sample app is moved to /src/Scaler.Demo as that made more sense to me and helped retaining all project references in the solution file.

[tomkerkhove]

LGTM, a big improvement - Thanks a ton! End-users can now better understand how it works and if it's a fit for them.

Just made a remark on streamlining the SO metadata but rest looks fine 👌

[tomkerkhove]

@pragnagopa @ealsur Please take a final look

[pragnagopa]

LGTM !

[tomkerkhove]

Ha, I have missed the last commit @JatinSanghvi.

Would it be possible to change ghcr.io/kedacore/keda-cosmosdb-scaler back to ghcr.io/kedacore/keda-external-scaler-azure-cosmos-db please?

This is important for our scaler approach where people need to understand that it's an external scaler and follow the convention. THank you!

[pragnagopa]

This is important for our scaler approach where people need to understand that it's an external scaler and follow the convention. THank you!

@JatinSanghvi - please use same name in mcr as well.

[JatinSanghvi]

@JatinSanghvi - please use same name in mcr as well.

Sure @pragnagopa, I was going to use the same name to avoid chance of confusion.