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Kenta Takeuchi
Kenta Takeuchi

Posted on • Originally published at bmf-tech.com

Stuck with ReadOnlyTransaction in Go's Spanner Client

#googlecloudplatform #spanner #tips

This article was originally published on bmf-tech.com.

Overview

I made some notes about the issues I encountered while using ReadOnlyTransaction from pkg.go.dev - cloud.google.com/go/spanner.

What Happened?

I was writing batch processing code that handles tens of thousands of records split across multiple requests. I wrote the processing using ReadOnlyTransaction as follows:

for {
    // 〜略〜

    // c is *spanner.Client
    iter := c.ReadOnlyTransaction().Query(ctx, stmt)
    defer iter.Stop()

    // 〜略〜
}
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At first glance, it seemed fine, so I ran the batch process, but a problem occurred where processing would stop after exceeding a certain number of records.

Cause

The Spanner Go client has a session management mechanism, but due to a missing transaction termination process, the session pool was exhausted, causing requests to time out. Internally, it seemed that the session management mechanism was blocking the execution of ReadOnlyTransaction.

Solution

I changed it to call the transaction termination process:

for {
    // 〜略〜

    // c is *spanner.Client
    tx := c.ReadOnlyTransaction()
    defer tx.Close()
    iter := tx.Query(ctx, stmt)
    defer iter.Stop()

    // 〜略〜
}
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Without the transaction termination process, a new session is generated every time a transaction is executed, leading to exhaustion of the session pool. The reason processing stopped at the same record count each time was likely due to hitting the limit of SPANNER_SESSION_POOL_MAX_OPEND. When calculated, it adds up.

Countermeasures

Aside from reading the documentation properly, there are also tool-based solutions available.
github.com - gcpug/zagane

Additionally, monitoring the cloudspanner session count in GCP might also be a good idea.

References

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