Debugging Remote Cache Hits for Local Execution

This page describes how to investigate cache misses in the context of local execution.

This page assumes that you have a build and/or test that successfully builds locally and is set up to utilize remote caching, and that you want to ensure that the remote cache is being effectively utilized.

For tips on how to check your cache hit rate and how to compare the execution logs between two Bazel invocations, see Debugging Remote Cache Hits for Remote Execution. Everything presented in that guide also applies to remote caching with local execution. However, local execution presents some additional challenges.

Checking your cache hit rate

Successful remote cache hits will show up in the status line, similarly to Cache Hits rate with Remote Execution. In the standard output of your Bazel run, you will see something like the following:

    INFO: 7 processes: 3 remote cache hit, 4 linux-sandbox.

This means that out of 7 attempted actions, 3 got a remote cache hit and 4 actions did not have cache hits and were executed locally using linux-sandbox strategy. Local cache hits are not included in this summary. If you are getting 0 processes (or a number lower than expected), run bazel clean followed by your build/test command.

Troubleshooting cache hits

If you are not getting the cache hit rate you are expecting, do the following:

Ensure successful communication with the remote endpoint

To ensure your build is successfully communicating with the remote cache, follow the steps in this section.

Check your output for warnings

With remote execution, a failure to talk to the remote endpoint would fail your build. On the other hand, a cacheable local build would not fail if it cannot cache. Check the output of your Bazel invocation for warnings, such as:
```
 WARNING: Error reading from the remote cache:
```
or
```
 WARNING: Error writing to the remote cache:
```
Such warnings will be followed by the error message detailing the connection problem that should help you debug: for example, mistyped endpoint name or incorrectly set credentials. Find and address any such errors. If the error message you see does not give you enough information, try adding --verbose_failures.
Follow the steps from Troubleshooting cache hits for remote execution to ensure that your cache-writing Bazel invocations are able to get cache hits on the same machine and across machines.
Ensure your cache-reading Bazel invocations can get cache hits.

a. Since cache-reading Bazel invocations will have a different command-line set up, take additional care to ensure that they are properly set up to communicate with the remote cache. Ensure the --remote_cache flag is set and there are no warnings in the output.

b. Ensure your cache-reading Bazel invocations build the same targets as the cache-writing Bazel invocations.

c. Follow the same steps as to ensure caching across machines, to ensure caching from your cache-writing Bazel invocation to your cache-reading Bazel invocation.