fix: remove time series before adding it

[olavloite]

Adding the same time series multiple times will cause an error in OpenCensus. A time series could be added multiple times for the same Database client id if a database client was invalidated, for example because if was created for a database that did
not exist.

This works for OpenCensusImpl 0.26 but not for 0.24, but that seems to be the case both with and without this change. With this change the problem is solved for 0.26. In version 0.24 OpenCensusImpl will always throw an error if you try to add a metric with the same name twice.

Fixes #202

[codecov]

Codecov Report

Merging #766 (5dc62bc) into master (3d9689c) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master     #766   +/-   ##
=========================================
  Coverage     85.00%   85.00%           
  Complexity     2562     2562           
=========================================
  Files           143      143           
  Lines         14007    14016    +9     
  Branches       1338     1338           
=========================================
+ Hits          11906    11915    +9     
  Misses         1538     1538           
  Partials        563      563           

Impacted Files	Coverage Δ	Complexity Δ
...ain/java/com/google/cloud/spanner/SessionPool.java	88.92% <100.00%> (+0.09%)	71.00 <0.00> (ø)

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[sebright]

@olavloite Thanks for fixing issue #202! I tested this PR with my application, and it worked with OpenCensus 0.26.0 as well as 0.28.0.

[sebright]

If I understand correctly, the issue with increasing client IDs could still occur if a different exception were thrown from

java-spanner/google-cloud-spanner/src/main/java/com/google/cloud/spanner/SpannerImpl.java

Line 225 in c99294b

SessionPool.createPool(

. It might help to also make a separate change to the error handling in SpannerImpl.getDatabaseClient.

[olavloite]

If I understand correctly, the issue with increasing client IDs could still occur if a different exception were thrown from

java-spanner/google-cloud-spanner/src/main/java/com/google/cloud/spanner/SpannerImpl.java

Line 225 in c99294b

SessionPool.createPool(

That is correct.

. It might help to also make a separate change to the error handling in SpannerImpl.getDatabaseClient.

I'm somewhat reluctant to do that. As I see it, the real problem here is not that the client IDs are increasing, but the fact that createPool was throwing an exception. It is a method that should normally not throw any exceptions, and adding additional error handling in SpannerImpl#getDatabaseClient(DatabaseId) might mask other unexpected errors. Note that if the user has specified for example an invalid database name, createPool will not throw an exception. Instead, that error will be delayed until the user tries to get a session from the pool. Such errors that are caused by invalid input arguments will not cause the problem with ever-increasing client IDs.

[sebright]

That makes sense. I didn't realize that createPool was designed to not throw exceptions.

[thiagotnunes]

@olavloite Just for my understanding, won't this remove historical data (time series data) if the user redeploys their application?

[olavloite]

@olavloite Just for my understanding, won't this remove historical data (time series data) if the user redeploys their application?

Good question. It won't be a problem after a redeploy, as these time-series are all in-memory on the local client, so these data structures will be all empty after a redeploy. But I'll take an extra look to check that it won't be a problem if a SessionPool is used for a while, then invalidated because of for example a deleted database, and then a new one with the same name is created.

[olavloite]

@olavloite Just for my understanding, won't this remove historical data (time series data) if the user redeploys their application?

I've been stepping through the OpenCensus implementation, and my conclusion based on that is:

1. Removing a TimeSeries or other metric that has not already been added in the current local deployment is a no-op. This means that in the normal case where a new SessionPool is created, the removeTimeSeries(..) calls are no-ops.
2. Removing a TimeSeries that has been previously added in the current JVM, will remove the local data structure from memory. This means that any data that has not yet been pushed to the server, will be lost. It will not remove any data on the server.
3. How much metrics that might be lost, depends on the exporter and how often this exports data to the server.

It is however important to realize that any in-mem sampling data will only be lost in the case that:

1. A SessionPool is created and can operate normally.
2. The SessionPool is invalidated because the underlying database or instance is deleted.
3. A new SessionPool is created in the same JVM for the same database that was deleted in step 2.

[thiagotnunes]

Thanks for the analysis @olavloite. LGTM